A handbook for emc testing and measurement

IET Electrical Measurement Series 8

A Handbook for

EMCTesting and Measurement

David Morgan

Contents

Foreword1 Nature and origins of electrom.agnetic com.patibility1.1 Definitions of electromagnetic compatibility1.2 Visualising the EMI problem

1.2.1 Sources of EMI1.2.2 EMI coupling to victim equipments1.2.3 Intersystem and intrasystem EMI

1.3 Historical background1.3.1 Early EMC problems1.3.2 Early EMC problems with military equipment1.3.3 The cost of EMC1.3.4 Serious EMI problems

1.4 Technical disciplines and knowledge areas within EMC1.4.1 Electrical engineering1.4.2 Physics1.4.3 Mathematical modelling1.4.4 Limited chemical knowledge1.4.5 Systems engineering1.4.6 Legal aspects of EMC1.4.7 Test laboratories1.4.8 Quality assurance: total quality management1.4.9 Practical skills

1.5 Philosophy of EMC1.6 References

2 EMC standards and specifications2.1 The need for standards and specifications

2.1.1 Background2.1.2 Contents of standards2.1.3 The need to meet EMC standards

2.2 Civil and military standards2.2.1 Range of EMC standards in use2.2.2 Derivation of military standards2.2.3 Derivation of commercial standards2.2.4 Generation of CENELEC EMC standards

2.3 UK/European commercial standards2.3.1 UK standards relating to commercial equipment2.3.2 Comparing tests2.3.3 European commercial standards2.3.4 German standards

2.4 US commercial standards2.4.1 US organisations involved with EMC2.4.2 FCC requirements2.4.3 Other US commercial standards

2.5 Commercial EMC standards inJapan and Canada2.5.1 Japanese EMC standards2.5.2 Canadian EMC standards

2.6 Product safety2.6.1 Safety of electrical devices2.6.2 Product safety2.6.3 Radiation hazards to humans2.6.4 Hazards of electromagnetic radiation to ordnance

2.7 ESD and transients2.7.1 ESD (electrostatic discharge)2.7.2 Transients and power line disturbances

2.8 US military EMC standards2.8.1 MIL STD 461/462/4632.8.2 MIL-E-6051 D2.8.3 Other US military standards

v

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VI A HANDBOOK FOR EMC TESTING AND MEASUREMENT

2.9

2.102.11

UK military standards2.9.1 Service and establishment-specific standards2.9.2 Project-specific standards2.9.3 DEF STAN 59-41 (1.988)Following chaptersReferences

313133343636

3 Outline of EMC testing3.1 Types of EMC testing

3.1.1 Development testing3.1.2 Measurement to verify modelling results3.1.3 Preconformance test measurements3.1.4 Conformance testing3.1.5 Conforrnance test plan

3.2 Repeatability in EMC testing3.2.1 Need for repeatability and accuracy3.2.2 Accuracy of EMC measurements3.2.3 Implications of repeatability of EMC measurements

3.3 Introduction to EMC test sensors, couplers and antennas3.3.1 EMC sensor groups3.3.2 Conduction and induction couplers3.3.3 Radiative coupling EMC antennas

3.4 References

4 Measurem.ent devices for conducted EMI4.1 Introduction4.2 Measurement by direct connection

4.2.1 Line impedance stabilisation network4.2.2 10 flF feed through capacitor4.2.3 RF coupling capacitors4.2.4 Distributed capacitance couplers4.2.5 High-impedance RF voltage probes4.2.6 Directly connected transformers

4.3 Inductively coupled devices4.3.1 Cable current probes4.3.2 Current injection probes4.3.3 Close magnetic field probes4.3.4 Surface current probes4.3.5 Cable RF current clamps4.3.6 Magnetic induction tests

4.4 References

5 Introduction to antennas5.1 EMC antennas5.2 EMC antenna basics

5.2.1 Arbitrary antennas5.2.2 EMC antennas

5.3 Basic antenna parameters5.3.1 Gain5.3.2 Aperture5.3.3 Transmitting antenna factor5.3.4 Receiving antenna factor5.3.5 Antenna phase centre5.3.6 Mutual antenna coupling5.3.7 Wavefield impedance5.3.8 Near-field/far-field boundary5.3.9 Beamwidth5.3.10 Spot size5.3.11 Effective length5.3.12 Polarisation5.3.13 Bandwidth5.3.14 Input impedance

5.4 References

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CONTENTS VB

6 Antennas for radiated emission testing 866.1 Passive monopoles 86

6.1.1 Construction 866.1.2 Performance 87

6.2 Active monopoles 886.2.1 Advantages 886.2.2 Disadvantages 88

6.3 Tuned dipoles 896.3.1 Introduction 896.3.2 Practical tuned dipoles 906.3.3 Commercial EMC tuned dipoles 916.3.4 Radiated emission testing 91

6.4 Electrically short dipoles 926.4.1 Special short calibration dipoles 926.4.2 Roberts dipoles 926.4.3 Small nonresonant dipoles 936.4.4 Microscopic dipole probes 93

6.5 Biconic dipoles 946.5.1 Introduction 946.5.2 Commercial biconic antennas 946.5.3 Use of biconic antennas 95

6.6 Wideband antennas 966.6.1 Introduction 966.6.2 Log-periodic antenna 96

6.7 Log-periodic dipole antenna 966.8 Conical log-spiral antenna 986.9 Horn antennas 1006.10 Ridged guide horn antennas 1026.11 Reflector antennas 1036.12 Magnetic field antennas 105

6.12.1 Introduction' 1056.12.2 Passive loops 1056.12.3 Active loops 1066.12.4 Loop calibration 1066.12.5 Magnetic field susceptibility tests 107

6.13 References 108

7 Use of antennas for radiated susceptibility testing 1107.1 Introduction 110

7.1.1 Types of antennas used in susceptibility testing 1107.1.2 Standards requiring immunity tests 110

Free-field antennas7.2 Tuned halfwave dipoles III7.3 Biconic dipoles III7.4 Log-periodic dipoles 1127.5 Conical log-spiral antennas 1137.6 Horn antennas 1137.7 Parabolic reflector antennas 1147.8 Radiated immunity field strength requirements 114

7.8.1 Requiremen ts for commercial products 1147.8.2 Requirements for civil aircraft 1147.8.3 Military requirements 115

7.9 E-field generators 1157.9.1 Construction 1157.9.2 Practical devices 116

7.10 Long wire lines 1187.10.1 Advantages 1187.10.2 Use in testing military equipment 118

Bounded-wave devices7.11 Parallel-plate line 119

7.11.1 Properties 1197.11.2 Line impedance 1197.11.3 Construction 119

VUI A HANDBOOK FOR EMC TESTING AND MEASUREMENT

7.11.4 Complex lines7.11.5 Field uniformity and VSWR7.11.6 Use in screened room

7.12 TEM cells7.12.1 Basic construction7.12. 2 Crawford cell performance7.12.3 Wave impedance in TEM cell7.12.4 Field distortions in TEM cell7.12.5 Other uses of TEM cells7.12.6 Asymmetric TEM cells

7.13 GTEM cells7.13.1 Description7.13.2 Typical construction7.13.3 Power req uiremen ts7.13.4 GTEM cells used for emission testing7.13.5 Pulse testing

7.14· References

8 Receivers, analysers and measurement equipment8.1 Introduction

8.1.1 Outline of equipment8.1.2 Groups of equipment

Instrumentation for emission testing8.2 EMI receivers

8:2.1 Design requirements8.2.2 Selectivity and sensitivity8.2.3 Detectors8.2.4 Commercially available EMI receivers

8.3 Spectrum analysers8.3.1 Introduction8.3.2 Analyser types8.3.3 Analyser operation

8.4 Preselectors and filters8.4.1 Preselectors8.4.2 Bandlimiting filters

8.5 Impulse generators8.5.1 Description8.5.2 Design8.5.3 Use of impulse generators

8.6 Digital storage oscilloscopes8.6.1 Advantages of digi tal oscilloscopes8.6.2 Typical waveforms to be measured8.6.3 Recording injected pulses for immunity testing8.6.4 Digital transient recorder architecture

8.7 AF IRF voltmeters8.8 RF power meters8.9 Frequency metersInstrumentation for susceptibility testing8.10 Signal sources

8.10.1 Signal synthesisers8.10.2 Signal sweepers8.10.3 Tracking generators

8.11 RF power amplifiers8.11.1 Introduction8.11.2 Specifying an amplifier8.11.3 RF amplifiers - conclusions

8.12 Signal modulators8.12.1 Modulation requirements8.12.2 Built-in modulators8.12.3 Arbitrary waveform generators

8.13 Directional couplers, circulators and isolators8.13.1 Amplifier protection devices

121121122123123123124124125126126126126127127128128

130130130130130130130132133134134134134135136136136137137137138139139139140140141141142142142142143143144144145147147147147148148148

8.13.2 Directional couplers8.13.3 Hybrid rings, circulators and isolators8.13.4 Protection devices conclusion

8.14 Automatic EMC testing8.14.1 Introduction8.14.2 Automated emission testing8.14.3 Automated susceptibility testing8.14.4 In the future?

8. 15 References

9 EMC test regitnes and facilities9.1 Introduction

9.1.1 Main test regimes9.1.2 Special testing

9.2 EMC testing in screened chambers9.2.1 Enclosed test chambers9.2.2 Standard shielded enclosures9.2.3 RF anechoic screened chambers9.2.4 Mode-stirred chambers9.2.5 Novel facilities

9.3 Open-range testing9.3.1 Introduction9.3.2 Test site9.3.3 Testing procedures9.3.4 Site calibration9.3.5 Measurement repeatability9.3.6 Comments on open-site testing

9.4 Low-level swept coupling and bulk current injection testing9.4.1 Introduction9.4.2 Low-level swept coupling9.4.3 Bulk current injection

9.5 References

10 Electrotnagnetic transient testing10.1 Introduction

10.1.1 Transient types10.1.2 Continuous and transient signals

10.2 Fourier transforms10.2.1 Introduction10.2.2 The transform10.2.3 Introducing phase10.2.4 Fourier transform expressions10.2.5 Impulse response10.2.6 Convolution10.2.7 Advantages of time-domain manipulation

10.3 ESD-electrostatic discharge10.3.1 Introduction10.3.2 The ESD event10.3.3 Types ofESD10.3.4 ESD-induced latent defects10.3.5 Types of ESD test10.3.6 Number of discharges per test10.3.7 ESD test voltage levels10.3.8 Assessing EDT performance

10.4 Nuclear electromagnetic pulse10.4.1 Introduction10.4.2 Types ofNEMP10.4.3 Exoatmospheric pulse generation10.4.4 NEMP induced currents10.4.5 NEMP testing

10.5 Lightning impulses

CONTENTS IX

148150151151151152152152152

154154154154154154155159163164165165165165167168171171171172175176

179179179179180180180181182182184184185185185187188188191191192192192193193194195201

x A HANDBOOK FOR EMC TESTING AND MEASUREMENT

10.5.1 Lightning environment10.5.2 Defining the discharge10.5.3 Effects on equipment

10.6 Transients and general power disturbances10.6.1 Importance of power transients10.6~2 Examples of power supply immunity standards10.6.3 Summary

10.7 References

201202204205205205206207

11 Uncertainty analysis: quality control and test facility certification 20911.1 Introduction 20911.2 Some definitions 20911.3 Measurement factors 21011.4 Random variables 211

11.4.1 Student's t-distribution 21311.5 Systematic uncertainty 21311.6 Combining random and systematic uncertainties 21411. 7 Uncertainties in EMC measurements 214

11. 7.1 Contributions to measurement uncertainty 21411.7.2 Identification of uncertainty factors 21511. 7.3 Estimation of uncertainty values 21611.7.4 Estimate of total uncertainty 218

11.8 Test laboratory measurement uncertainty 21811.8.1 NAMAS 21811.8.2 NAMAS and measurement uncertainty 21811.8.3 Limits and production testing 219

11.9 NAMAS requirements for laboratory accreditation 21911.9.1 Requirements for accreditation 21911.9.2 Advantages of laboratory accreditation 220

11.10 References 221

12 Designing to avoid EMC problem.s 22312.1 Intrasystem and intersystem EMC 223

12.1.1 Intrasystem EMC 22312.1.2 Design for formal EMC compliance 224

12.2 System-level EMC requirements 22812.2.1 Top-level requirements 22812.2.2 Determining EMC hardening requirement 22812.2.3 Simple coupling models 22912.2.4 Susceptibility hardening case study 23112.2.5 Emission suppression requirement 23312.2.6 System hardening flow diagram 23312.2.7 Subsystem apportionment and balanced hardening 23312.2.8 Staff support for EMC 235

12.3 Specific EMC design techniques 23612.4 References 236

13 Achieving product EMC: checklists for product developtnent and testing 23813.1 Introduction

13.1.1 Chapter structure13.1.2 Example adopted13.1.3 Personal computers and information technology

13.2 Information about EMC13.2.1 Customer sources13.2.2 Regulatory authorities13.2.3 Industry sources13.2.4 Equipment, component and subsystem suppliers13.2.5 Professional bodies and conferences13.2.6 EMC consultants and training13.2.7 Electronics and EMC technical press

238238238238238238239240240240241241

13.3 Determining an EMC requirement13.4 Developing an approach to EMC design

13.4. 1 Process flow chart13.4.2 EMC strategy13.4.3 Immunity first?13.4.4 Example of EMC design process

13.5 Technical construction file13.5.1 Routes to compliance options13.5.2 Circumstances requiring the generation of a technical file13.5.3 Contents of a technical file13.5.4 Report from a competent body13.5.5 Testing or technical file?

13.6 Self certification13.6.1 Need for an in-house facility13.6.2 Gradual development13.6.3 Estimates of facility cost13.6.4 Turnkey facilities

13.7 Conclusion13.8 References

Appendix 11.1 Signal bandwid th definitions1.2 UK EMC legislation (up to 1January 1996)1.3 European EMC standards1.4 German decrees and standards1.5 US EMC regulations and standards1.6 German, North American and Japanese EMC standards1.7 Electrical safety and electromagnetic radiation1.8 Military EMC standards1.9 Compendium of EMC and related standards

Appendix 22.1 Modulation rules

Appendix 33.1 NAMAS-accredited laboratories3.2 Competent bodies3.3 EMC consultancy and training3.4 Useful publications on EMC

Index

CONTENTS Xl

241242242242243243244244245245246246246246247248248248249

250250252254259261262264266271

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278278280282283

285

During the latter part of the 1980s the worldwideinterest in electromagnetic compatibility hasgrown significantly in the electronics industry,particularly in the commercial and domesticequipment sectors. The field of electromagneticcompatibility is a dynamic one, with continuousevolution and change. This has accelerated in the1990s with the introduction of the EuropeanCommunity EMC directive which applies toalmost all electrotechnical equipment. TheDirective attempts to harmonise regulations inmember states concerned with EMC after 1992(with a transition period up to 1996). Equipment(including imports) must meet the new EMC standards before they can be legally offered for salewithin the EC.

This book is devised in the form of a handbookconcerned with most aspects of electromagneticcompatibility and addresses in a single volumemany of the technical and managerial issueswhich will be of concern to designers and manufacturers of commercial and military electronicequipment. The primary focus of the "book is onEMC testing and discusses the numerous testmethods in relation to the EMC standards forwhich they are required.

Understanding EMC is important to producereliable interference-free products and understanding EMC testing is the key to demonstratingthat well designed equipment meets legislativeand contractual requirements, including the newEC regulations.

EMC grew in importance in the field of militaryelectronics over a 20-year period from abolit themid 1960s, and therefore has been a precursor tothe developments in the civil sector. Thus manyof the design techniq ues and some test methodshave been introduced into the field of commercialelectronics EMC from this military background.Other test methods have developed entirely inregard to commercial equipment and have nocounterpart in military EMC testing. This bookcontains examples of many commercial and military EMC tests and shows how difficult it can beto compare results from various tests whichpurport to measure the same quantity.

Although the recent surge in interest in EMChas been spurred in the commercial electronicsindustry by the introduction of the EC Directiveon EMC, the interest of military electronics manufacturers in EMC is strong and still growing.Many of the larger test facilities in the UK whichcarry out testing of commercial and military electronics are operated by military/aerospace companies. For these reasons this book devotes a proper

XIU

Foreword

proportion of space to discussing test methods andspecifications which relate to military equipmentEMC, and includes a brief section on nuclear electromagnetic pulse effects in a chapter concernedwith electromagnetic transients such as electrostatic discharge and lightning.

Electromagnetic compatibility engineering is amultidisciplinary activity involving electrical engineers, physicists, chemists, systems and mechanical engineers and has quality, marketing andlegal implications in the management of electronic product development. This book has therefore been written with the aim of being accessibleto both technical staff and management. Forexample, it has detailed technical sections onbasic antenna theory and EMC testing practice aswell as sections oriented towards management/project control, concerned with regulations andstandards, and the EMC planning aspects ofproduct development.

1'he emphasis lies on EMC testing. Within thetext, test specifications, test methods and testequipment are discussed in some detail. I t wouldbe cumbersome and impractical to explain eachtest method in detail for the enormous range ofcivil and military EMC specifications in use todaythrough the world. Therefore the technical material on testing has been constrained to presentingthe details of a number of generic types of testand has been constructed around a description ofthe physical principles and mechanisms whichunderlie the measurements.

Information is presented for the followinggeneric types of test:

Radiated emissionRadiated susceptibilityConducted emissionConducted susceptibili tyTransient emissions (mainly conducted)Transient susceptibility (including electrostaticdischarge)

The couplers, sensors and antennas used to makethese measurements are treated in groups definedby the physical coupling mechanism they employ.For example, conducted emission devices aredivided into direct connection probes, inductivecouplers and capacitive couplers. In this way,some physical meaning and order is brought tothe construction and operation of the extensiverange of antennas and sensors used in EMCtesting.

Although it contains considerable technicaldetail the material in this book has also beenchosen to give the reader who may be coming to

XIV A HANDBOOK FOR EMC TESTING AND MEASUREMENT

the subject of electromagnetic compatibility for thefirst time a broad view of the range of topicscovered by the s.ubject. The text thereforeincludes sections on topics such as

The nature of electrical interferenceStandards and specificationsDeveloping system and subsystem EMC requIrementsEMC design techniq.uesThe nature of EMC testingTest methodsMeasurement uncertaintyQuality control in test laboratoriesLaboratory accreditationAchieving EMC in new products

While the reader is encouraged to adopt astraightforward reading path*, this material hasbeen organised to facilitate the selection and useof particular chapters in the manner of a handbook for day-to-day use. Chapters 6 and 7 forexample contain useful antenna formulas whichmay need to pe .accessed quite often by a practising EMC test engineer.

Where appropriate, small sections of the texthave been devoted to reviewing the theory whichunderlies arguments which are to follow. This hasbeen done in Chapter 5 with some basic conceptsof antennas, and in Chapter 10 where an expla-

*It may be appropriate for readers who are new to EMCto read Chapter 12 on EMC design after Chapter 2(EMC standards). Chapter 12 contains a number ofconcepts and insights into the causes and solutions ofEMC problems which may help the reader with the restof the book. The chapter on EMC design has been positioned towards the end of the book so as not to disruptthe progression from the introduction followed by EMCstandards directly to the main focus of the book onEMC testing.

Acknowledgem.ents

Extracts from British Standards are reproducedwith the permission of BSI. Complete copies canbe obtained by post from BSI Sales, LinfordWood, Milton Keynes, MK14 6LE.

Figures 5.12, 5.14 and 8.34 are from the book'Reference data for engineers: radio, electronics,computer and communications' edited by Mac

nation of the mathematical relationship betweenthe frequency and time domains is given to enablethe reader to understand transient testing and tobe able to visualise the frequency content of waveforms.

For ease of reading, long lists or tables of information have been put into appendices at the endof the book. This is particularly so for Chapter 2on EMC standards where lists of standards, equipment classes and tests would otherwise break upthe flow.

Numerous examples of EMC standards fromaround the world are included in Chapter 2.These range from the earliest historical examplesto the recently issued and proposed EuropeanNorm standards. EMC standards and testmethods are changing rapidly and whilst everyattempt has been made to include the most up-todate information, new standards are bound to beissued and supersede existing ones.

I t is not possible to cover in a single book theenormous range of information which constitutesthe current body of knowledge on EMC. The aimhere has been to provide an introduction to EMCand particularly to EMC testing in the form of auseful day-to-day handbook. There are manyother published texts, some in up to 12 volumes,which deal in detail with the design aspects ofEMC. These are referenced in this text. With theexpansion of interest in EMC a number of usefulbooks have been written since 1990 which reflectthe dynamic nature of the topic. They are listedin Chapter 13 along with information on the lEEdistance learning project on EMC.

I t is hoped that readers find this handbookhelpful, informative and easy to read and use. Ifit helps to stimulate an interest in this fascinatingsubject and direct the reader to sources of moredetailed information it will have achieved itsobjective.

E. Van Valkenburg. Published by SAMSPublishing, a division of Prentice HallComputer Publishing. Used by permission ofthe publisher.

The publishers are grateful to MarconiInstruments Ltd. for help in the cover design ofthis book.

Chapter 1

Nature and origins ofelectrom.agnetic com.patibility

1.1 Definitions of electroIllagneticcOIllpatibility

The formal definition of electromagnetic compatibility, as given in the InternationalElectrotechnical Vocabulary (IEC 50) is: 'theability of a device, equipment or system tofunction satisfactorily in its electromagneticenvironment without introducing intolerable electromagnetic disturbances to anything in thatenvironment' [1 J. A similar definition cited byDuff [2J is given as: 'the ability of equipmentsand systems to function as intended, withoutdegradation or malfunction in their intendedoperational electromagnetic environments.Further, the equipment or system should notadversely affect the operation of~ or be affectedby, any other equipment or system'.

Electromagnetic interference (EMI) can beviewed as a kind of environmental pollutionwhich can have consequences that arecomparable to toxic chemical pollution, vehicleexhaust emissions or other discharges into theenvironment. The electromagnetic spectrum is anatural resource which has been progressivelytapped by man over the last 100 years. Most ofthe development has taken place in the last 50years with the advent of public service broadcasting, point-to-point and mobile communications etc. which has brough t great economic andsocial benefits. The spectrum is now almost fulland it is proving difficult to satisfy the pressuresfor new uses of this resource. Modern life hascome to depend heavily on systems that use theelectromagnetic spectrum and its protection is inthe interests of us all. For this reason unwarrantedelectromagnetic interference represents a realeconomic and social threat which can even resultin injury or death.

Unfortunately, electromagnetic interferencecannot be smelled, tasted or seen by either the layperson who purchases electronic products or bythe corporate technical manager who has tosupervise the design of the latest electronicproduct and get it to the marketplace as fast aspossible, for the lowest possible cost. There has,therefore, been a tendency to deny that EMI is aproblem in the modern world and to argue that

the costs which are associated with achieving electromagnetic compatibility (EMC) need not beborne. Some of these wider issues are exploredlater, but for now another definition of thisfascinating and wide ranging concept is examined.

Keiser [3J defines EMC in this way: 'electricaland electronic devices can be said to be electromagnetically compatible when the electrical noisegenera ted by each does not interfere wi th thenormal performance of any of the others. EMC isthat happy situation in which systems work asintended, both within themselves and in theirenvironment' .

Electromagnetic in terference ind ucesundesirable voltages and curren ts in the circui ts ofthe victim equipment. This can cause audiblenoise in radio receivers and spots, snow or loss offrame synchronisation on TV pictures. Whenvital communications links, computer installationsor computer driven industrial process controlequipment is the victim equipment, more seriousconsequences can occur.

Interference can reach the victim system by twobasic routes: conduction along cables, and electromagnetic radiation. This chapter examines typicalsources of EMI and discusses the technical basis ofelectromagnetic compatibility within anequipment, and between the equipment and itsenvironment in terms of conducted and radiatedinterference paths.

1.2 Visualising the EMI probletn

1.2.1 Sources of EMI

Any electrical or electronic device that haschanging voltages and currents can be a source ofEM!. If the culprit equipment has no cablesconnecting it to the outside world, for example abattery powered electric shaver, then theinterfering energy generated by sparking withinthe electric motor can only travel as an electromagnetic wave. If the shaver is mains powered,both radiated noise and interference conductedalong the cable into the mains wiring are possible.This is illustrated in Figure 1.1 where a mainspowered shaver and a washing machine are both

2 A HANDBOOK FOR EMC TESTING AND MEASUREMENT

HOUSE MAINS WIRING INTERFERENCE NOISE CURRENTSEMI CURRENTS FLOWINTO OTHER BUILDINGS

TV receiveris victim

) ) JIndirect radiation fromcurrents in mains cables

\Motors, switchesrelays, etc.

o eo

\\DIRECT RADIATION FROMELECTRIC SHAVER (ALSORADIATES OUTSIDE

BU)D1NGlJ )

Mains power'waveform

"~,.

,. Motor noise

\

~ '-)\~,V /

/

CULPRIT 1 CULPRIT 2ELECTRIC SHAVER WASHING MACHINEMOTOR

Figure 1.1 Illustration of simple EMI problem

causing interference to a TV picture in an adjacentroom. Radiation is emitted not only directly fromthe shaver or washing machine motor, but alsofrom the mains wiring which is carrying theconducted radio frequency (RF) noise.

Generally, the faster the rate of change of voltageor current in the culprit equipment, the wider is thespectrum of RF interference produced. The greaterthe magnitude of the noise voltages or currents thegreater will be the conducted and radiatedemissions. Therefore electric motors, whichgenerate high voltages and currents with fastrisetimes as the inductive coils in the rotor areswitched by the commutator, are good examples ofparticularly powerful sources of EMI.

Other common sources of electromagnetic interference are given in Table 1.1

These sources can be grouped in terms of usageas shown in Figure 1.2. They may be sources ofcontinuous or transient interference as shown inFigure 1.3. Continuous sources include radiotransmitters and emissions from RF heaters forexample where the signal is an uninterruptedcarrier, but also include pulsed systems such asradars and the emissions from digital computerswhich have wide but stable RF spectra. Theemissions from these sources are best measuredand analysed in terms of their spectral contentusing narrow bandwidth scanning receivers orspectrum analysers.

Table 1.1 Common sources of EM!

Powerline arcing" and corona discharge

Automobile ignition systemsFluorescent lightingSwitched-mode power suppliesPortable electric generatorsStatic or rotary power convertersAny appliance using brush commutator motorAir conditioning equipmentComputer equipment and peripheralsEquipment with switches and relaysDiathermy medical equipmentArc-welding setsHigh-voltage neon signsLight dimmersMicrowave ovensCB radioRadar transmittersBroadcast transmittersAtmospherics (noise from lightning around theworld)Whistlers, chorus and hiss from the magnetosphereNearby lightning stormsPrecipitation static noiseDisturbed and quiet radio noise from the sunCosmic radio noise

NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY 3

SOURCES OF EMI

-BROADCAST STATION-RADAR-CBRADIO-AMATEUR RADIO-ELFNLF NAVIGATION/COMS-MOBILE Tx-REMOTE CONTROL DOOROPENING TRANSMITTERS-POINT TO POINT HF COMS.-IONOSPHERIC SCATTER

RADAR AND COMS.

- PORTABLE -AUTOMOBILES - WORKSHOP -DIELECTRIC - MICROWAVE OVENSPOWER GENS. & VEHICLES MACHINES HEATERS - LIGHT DIMMERS

- STATIC & ROTARY -TRACTION - COMPRESSORS -AIR CONDITIONING - PERSONAL COMPUTERSCONVERTERS POWER - ROTARY SAWS - COMPUTERS - MIXERS/BLENDERS

- RECTIFIERS POWER - BALL MILLS -FLUORESCENT - VACUUM CLEANERS- TRANSMISSION CONVERSION RF HEATERS LIGHTS - WASHING MACHINES

LINE NOISE -ELECTRIC - ULTRASONIC LASER SYSTEMS - HAIR DRYERS- POWER FAULTS MOTIVE CLEANERS NEON DISPLAYS - ELECTRIC MODELS- CONTACTORS POWER - WELDERS - MEDICAL EQUIP. - FRIDGESIFREEZERS

-IGNITION -SPARK EROSION -PROCESS CONTROL - SHAVERSSYSTEMS - CRANES I FANS X RAY MACHINES -THERMOSTATSMOBILE Tx. - OVENS IKILNS

STATIC NOISE

}'igure 1.2 (;roups oj EMI sources

ATMOSPHERICS SOLAR·NOISE LIGHTNING COSMIC RADIO BACKGROUND NOISE

SOURCES OF TRANSIENT EMI(COMPOSED OF SEPARATE PULSES)

Sources of transient emISSIons includelightning, nuclear electromagnetic pulse, powerline faults, switch and relay operation, etc. Theyare characterised by single or intermi tten toccurrence at unpredictable times with nosignificant time pattern. Often, the signals are ofshort duration and hence have a wide signalbandwidth. I t is easier to measure, record andanalyse such signals as a waveform in the timedomain. rrhe advent of wide-bandwidthtransient-capture waveform digitisers and fasttransformation algorithms has only recently ledto the ability to view easily the spectrum of asingle fast transient.

The details of EMC test equipment and testtechniques that are capable of measuring the widerange of interference signals from these EMI

SOURCES OF EMI

SOURCES OF CONTINUOUS EMI(CONTINUOUS SPECTRUM OF NOISE)

sources are the main concerns of this book andare to be found in Chapters 4 to 11.

ExaInples of the extent of the frequencyspectrum, repetition rates and signal amplitudesfrom some typical EMI sources are given inI'able 1.2 which contains both intended andunintended sources of EM radiation.

Examples of approximate field strengths fortypical broadcast transmitters in the UK aregiven in Figure 1.4 and for other sources inFigure 1.5. These data with regard to emissionlevels and freq uencies for potential sources ofElYlI allow the appreciation of the potential scaleof the problem if such sources can couple tosensitive victim equipment which may only beable to tolerate a few microvolts or millivolts ofunwanted

1.3 Sources oj~ continuousand transient interference

• BROADCAST STATIONSHIGH POWER RADAR

- ELECTRIC MOTOR NOISEFIXED & MOBILE COMMUNICATIONSCOMPUTERS, VDUs & PRINTERS etc.

• AC I MULTIPHASE POWER RECTIFIERSHIGH REPETITION RATE IGNITION NOISESOLAR AND COSMIC RADIO NOISE

BEST MEASURED AND ANALYSED IN THE-FREQUENCY DOMAIN- - (SPECTRUM)

- LIGHTNING- NUCLEAR EMP- POWER LINE FAULTS (SPARKING)- SWITCHES AND RELAYSELECTRIC WELDING EQUIPMENT

- LOW REPETITION RATE IGNITION NOISEELECTRIC TRAIN POWER PICK-UP ARCING

- HUMAN ELECTROSTATIC DISCHARGE

BEST MEASURED AND ANALYSED IN THE-TIME DOMAIN- - (WAVEFORM)


Table 1.2 Frequencies and noise levels from typical interference sources

Source type Comments

Mains disturbances Double exponential transients with risetimes of 1 J.1S and fall times of 50 J.1S atapprox.10kV

100 kHz ringing waveform with 0.5 J.1S rising edge

Power dips up to 100 ms long

Power frequency harmonics up to 2 kHz

Unintended radiators

Switches and relays Transients with risetimes of a few ns and levels up to 3 kv producing frequenciesinto the VHF band

Commutator motors

Human electrostaticdischarge

Switchingsemiconductors

Produce frequencies up to 300 MHz at repetition rates of up to 10 kHz

I-IOns risetime30-200 ns fall time ampli tudes up to 15 kV

Risetimes from 20 to 1000 ns at rep rate of kHz to 10 MHz for voltages up to300V

Switched-mode powersupplies

Digital logic

Industrial and medicalequipment

Produce continuous spectrum of noise from kHz to 100 MHz

Circuits produce continuous noise up to 500 MHz

Metals heating in 1-199 kHz range.Medical equipment operates from13-40 MHz. Using high power-hundreds of watts

Intended radiators

Broadcast stations

Other RF transmittersincluding radar

See Figure 1.4

See Figure 1.5

1000

-r - 100

SHORT WAVE T-r---l..- T UHF Vim

T TUHF TVTELEGRAPHY MEDIUM WAVE VHF TV

1---L }VHFl 10LONG WAVE RAIO

I I I I I 110k 100k 1M 10M 100M 10

FREQUENCY OF TRANSMITIER (Hz)

120 .... --a. ...... .....~ .... .... ....

Figure 1.4 Field 180

strengths in thevicinity of broadcast 170

transmitters

160~

dBuV/m

150 -

140 -

130

Reproduced by permission of' BAe Dynamics Ltd.

5

100m IRADAR

400m

NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY

1000 Figure 1.5 Fieldstrengths in thevicinity oj other sources170

180

CB Tx ON VEHICLES100160

/ -AMATEUR TRANSMITIERS @ 10m -

150 _/ ~_/ VIm

CB Tx (100w @ 10m)dBuV/m

140 f f 10RF HEATERS~ LAND MOBILE BASE STATION

130 TRANSMITIERS @ 10mCBTx 4w @ 10m

120

1M 3M 10M 30M 100MFREQUENCY Hz

300M 10 Reprod uced by permission of BAeDynamics Ltd,

1.2.2 EMI coupling to victim equipments

An EMI problem can only exist if a source ofEMI, a culprit, is able to exchange electromagnetic energy with a receptor or victim equipment.Thus the general EMI problem can berepresented by these three componen t~ as show.nin Figure 1.6. The coupliI).g can be VIa metalhcconductors such as power or signal cables, if theyexist, or more generally by electromagneticradiation from one equipment to the other.

Energy exchange will take place between th.eelectromagnetic wavefield produced by the culprItequipment, and metallic conductors attached tothe victim equipment which will have RFcurrents induced in them. If the primary couplingmode is radiative but the receptor has a cableattached to it (but not connected to the culprit)currents can be induced into the cable that willthen flow directly into the victim, even if it isprotected from direct radiation by a highperformance shielded case. See Figure 1.7a.

Equally, if the primary coupling bet:ve.enculprit and victim is conducted, but the vIctImhas a good filter connected to the input cable,the currents in the cable can still radiate EMenergy which may then couple into the victim ifit has no electromagnetic screening. SeeFigure 1.7b. This situation demons't:ates thatboth radiative and conducted couphng paths,whether direct or indirect, must be addressed inparallel if the EMI energy is to be prevente?from reaching the victim equipment. ThIsargument leads directly to .the. combi~ed

application of shielding and filterIng In practIcalsolutions to EMI problems. Common receptorsor victim equipments are listed in Table 1.3. Thelist contains both intended and unintendedreceivers.

Electromagnetic compatibility engineering iscomposed of four basic topics as shown inFigure 1.8 related to the combinations. of source/victim and radiated/conducted couphng. EMCstandards and specifications often use thefollowing initials to describe these four elements:

Figure 1.6 Three components oj an EMC problem

SOURCE OF EMI(Culprit equipment)

SOURCE

RADIATION

CONDUCTION

COUPLING

, RECEPTOR OF EMI(Victim equipment)

RECEPTOR

RE - radiated emission from a culpritRS - radiated susceptibility of a victimCE - conducted emission from' a culpritCS - conducted susceptibility of a victim

Because conducted and radiated transient EMI,including electrostatic discharge (ESD) aremeasured using different techniques, they aresometimes represented as an additional fifth group,


Shield prevents direct radiative couplingfrom culprit to victim~ "Radiated EMI from culprit

/

VICTIM SYSTEM CULPRIT SYSTEMInput connector

RF CURRENTS CONDUCTEDINTO VICTIM

\ lamps \

Induced RF EMIcurrent into cable

~CABLE ACTS ASAN UNWANTED IlANTENNA"

"-TO OTHER EQUIPMENT

""-,

~-.

(aj

(NO DIRECTLY RADIATED EMI)

CULPRIT SYSTEMlamps

///~POOR or NO SHIELD Radiation reaches PCB /'

~ f /Radiation from the Cable1--- /l

I V~C~\STIEM ( :.L (L -T \~ \ ,,(source of RF currents)

CIRCUITS DIRECTLY EXPOSED TO RtATE~-:MI \~ High le'el of CONDUCTED EMI

\ \ GOOD FILTER STOPS CONDUCTED EMI

(bj

Figure 1.7 (aj Radiated to conducted EMI (b j Radiation Jrom cable carrying EM1 currents

Table 1.3 E'xamples of victims oj EMI

Intended receivers

Radio receivers 0.0 1-1 f1V sensitivityBroadcast receiversTV receiversMobile communication receiversMicrowave relay systemsMobile telephonesAircraft communication receiversN avigation aidsRadar receivers

[inintended receivers

Aircraft engine control systemsAircraft flying surface con troIsWeapons systems, guided missilesElectronic ships systems inc steering gearVideo recording/playback equipmentComputer equipmentIndustrial process control systemsSignalling systemsMedical electronic instrumentsHeart pacemakersBiological tissueOrdnance and explosive fuses

as in Figure 1.8. RF hazards to fuels, explosives,electrically detonated ordnance, and RF hazardsto biological tissue (including humans) can also beencompassed within the general concept of electromagnetic compatibility.

EMC testing is carried ou t in all these areas,though the measurement techniques and instrumentation vary widely depending on theparticular test. For RE and RS tests theequipment under test (EDT) is placed at somedis tance (usually 1 or 3 m) from a range ofantennas that are used either to measure the lowlevel EM emissions from the equipment, or togenerate high field strengths at the equipment forsusceptibility testing.

Conducted emission (CE) tests involve the use ofspecial coupling transformers or current probes tosense the level ofRF current being conducted awayfrom the EDT along its cables. The conductedsusceptibili ty (CS) tests require RF current to beinjected into cables either using coupling transformers and probes or by direct connection.

The main body of this text is concerned withexplaining these test methods and examining indetail the physical principles and operatingpractice associated with the types of antennas,current probes and other sensors used.


CONDUCTED SUSCEPTIBILITY (CS)

20Hz to 400MHz @ -2OdB VA

CONDUCTED EMISSIONS (CE)

20Hz to 100 MHz CABLES

100MHz -4OGHz ANTENNACABLES

(LEVELS DOWN TO -120dBA)

RADIATED SUSCEPTIBILITY (RS)

10Hz to 40 GHz

1 - 200V/m (CW)

SAFETY OF ELECTRO-EXPLOSIVE

DEVICESSAFETY OF NON-IONISING

RADIATION ON BIOLOGICAL___________...:------------1--------1 TISSUE

RADIATED EMISSIONS (RE)

10 Hz to 40 GHz

E FIELD 14kHz - 400HzH FIELD 10Hz - 30MHz(LEVELS DOWN TO

-110dBV/m)

NUCLEAR ELECTROMAGNETIC

PULSE50kV/m RS (10/400ns)

100 A (DAMPED SINUSOID)

MAINS DISTURBANCES

AND LIGHTNING10kV TRANSIENTS100ms DROPOUTS

HARMONIC DISTORTION

HUMAN ESD

15kV TRANSIENTSDIRECT & RADIATED

(5/30ns waveform)

Figure 1.8 Examples of EMG) and related activities

1.2.3 Intersystem and intrasystem EMI

EMC activity can also be differentiated in terms ofthe level at which it is applied. The widest levelconcerns the compatibility between a system ofinterest and all other systems with which it couldinteract, including the general EM environment.This is called intersystem EMC and can involvefor example, frequency planning, equipmentsiting, antenna sidelobe suppression and theimposition of operating restrictions includingtiming constraints. In the military worldoperational restrictions might apply to theminimum space between aircraft or the minimumdistance at which they may approach a groundbased transmitter.

The interaction of commercial civilian systemswith broadcast receivers and the general EMenvironment is regulated largely by voluntarytrade agreements or government laws. Theadvent of the widespread use of desktop digitalcomputers has forced the introduction oflegislation to control the radiated and conductedemissions from such equipments in order toprotect the broadcast spectrum.

Intrasystem EMC is concerned with the selfcompatibility of the system of interest. It relies onthe premise that if each individual unit within thesystem is required to emit less EMI than any ofthe units would be susceptible to, plus a marginfor safety, then when the units are assembled as awhole the system will be electromagneticallycompatible. The overall system EMCrequirement must be determined and apportionedto each subsystem or unit. The detailed designwork is then carried out at the level of circuit and

board design, cable design, choice of I C technologies etc. to meet the unit level EMC requirement.

Military equipment designers are required tofollow the guidelines given in military EMCdesign handbooks and to meet the unit levelEMC testing limits given in MIL STD 461/2/3 orDEF STAN 59-41, for example.

The relationship between intersystem andintrasystem EMC is shown in Figure 1.9. Apictorial representation of a typical intersystemEMI situation is given in Figure 1.10 showingpossible conducted and radiated coupling pathsbetween various systems. A similar representation,but of intrasystem EMI, is given in Figure 1.11using a transmitter with a case-mounted antennaas an example.

1.3 Historical background

1.3.1 Early EMC problems

EMI problems and EMC solutions are not new.Jolly [4], quoted by Braxton [5], tells of how ataround the turn of the century GugielmoMarconi had been contracted to build demonstration models of his new wireless telegraph setsfor the Bri tish, French and Americangovernments. When he had installed severalequipments on board ships and at land sites, theusers complained that they could only operateone station at a time. I t was thereforediscovered by accident tha t freq uencymanagement was important in communicationsystems, as the spectrum from the crude transmi tters overlapped. Marconi had to return to theinstallations and attempt to make them tunable


Followequipf1!ent specifications - ego MIL STD 461 Follow system requirements - ego MIL STD 6051 DIEC 801 FCC part 15/18

SUBSYSTEM 1EMI CONTROL

SUBSYSTEM 2 SUBSYSTEM n COMPATIBILITYWITH OTHERKNOWNSYSTEMS INOPERATIONALENVIRONMENT

REGULATIONS SUSCEPTIBILITYON EMISSIONS TO GENERALTO GENERAL CABLE-BORNEENVIRONMENT INTERFERENCE

& TRANSI ENTS

IMMUNITYTO

LIGHTNINGEFFECTS &POSSIBLY

NEMP

Figure 1.9 Intersystem and intrasystem EMC

to avoid co-channel interference. This isprobably the first example of an EMC 'fix'applied to an existing equipment. Many peoplehave followed Marconi's example in theintervening years.

1.3.2 Early EMC problems with militaryequipment

While the International Special Committee onRadio Interference (CISPR) has been tackling

SYSTEM 2

DISTURBANCES ON POWER-LINE

RADAR & COMMUNICATIONSTRANSMISSIONS FROM MOVING

VE~~ ~

PICK-UP ON TELEPHONE LINES

Susceptibility to andemissions of noise oncomms. lines (

Susceptibility to andemission of mains noise

SYSTEM· 1

LIGHTNING (DIRECT AND INDIRECT EFFECTS)

POWER LINES

TELEPHONE LINES

Susceptibility toand emission ofmains-borne

noise {;

----- -NATURAL & COSMIC ______________

RADIO & ·STATIC· NOISE /-----

Figure 1.10 Intersystem EMI


SLOTS &APERTURESIN SUBSYSTEM& SYSTEM CASES

POWER~LINE ~IX:

POTENTIAL RFLEAKAGE INCASES LEADSTO PICK UPINSIDE SYSTEM

ADEQUATE RF & SAFETY GROUNDS

RF CONTROLLED INTERNALTO SUBSYSTEM BYSCREENING & FILTERING

~INTERNAL UNWANTEDRADIATION PICKED UPON POWER & SIGNALCABLES

FILTER

POWER SUPPLY

MAINS POWER LEAD

POSSIBLE RF LEAKAGE FROM Tx ANTENNA ------------=M(AoLINCINTO BOX via CONTROL PANEL ANTENNA "'-

\

(Example of a transmilfer with a case mounted antenna) ~ ~

SUBSYSTEM A SUBSYSTEM BFRONT Conducted EMIPANEL TRANSMITIER......... OSCILLATOR DOUBLER PACONTROLS CONTROL STAGE

(Shield apertures if CIRCUITRY Signal & Datapossible) lines

POSSIBLECONDUCTEDNOISE

EXTERNAL RADIATIONINDUCES CONDUCTEDEMI INTO MAINS CABLES(DISTURBS POWERSUPPLY REGULATIONWITHOUT FI LTERS )

Figure 1.11 Example oj'intrasystem EM!

the intersystem problems of interference tobroadcast receivers since 1934, the majority ofearly interest in intrasystem EMC has been withregard to military equipment. Early examples ofEMC problems aboard military aircraft havebeen recorded [6, 7J and illustrate the problemsof the day, as follows.

1944 B29: Radiated and power cable conductednoise to HF and VHF communicationsequipment.1947 B50: EMI problems due to poor groundingand bonding.1950 B50, B47, C97: Problems with DC power busnOIse.1954 B52: Interference from radar to communications equipment.1958 B52: Problems with coupling between the400 Hz power system and navigation and bombaiming equipment. In this case the problems wereso acute that production was stopped while cureswere found. Eventually modifications to bothcabling and equipment proved effective.

A similar history exists for naval equipment [8].1939-45: Metric radar causes interference to HFcommunications.1945-55: During this period there was a greatincrease in the use of servo equipment to controlguns for example. New 10 f.1V sensitivityintercoms and tactical radio communicationswere now using voice channels with microphonesdistributed throughout the ship. New frequency

bands were used, sonar, radar and new navigational electronics were fitted. All these changesmade for a more complex use of the electromagnetic spectrum and resulted in increased interference between systems.1955-65: The change from DC commutator powergeneration to 440 V three-phase 60 Hz resulted inan improvement in the ship board EMenvironment, but at the same time the changefrom lead clad cables to plastic sheathed onesremoved the natural screening offered by theolder type.1965 onward: The introduction of semiconductortechnology greatly increased the number andsophistication of electronic equipments.Introduction of digital computers and automatictelegraphy all required greater vigilance inspectrum management and the control of spuriousemissions from these devices.

1.3.3 The cost of EMC

Development programmes on early ICBMs such asAtlas, Thor and Titan suffered delays owing toEMC problems. The cost of EMC remedies andprogramme delays ran into many millions ofdollars. A subsequent, more complex missiledevelopment included an EMC controlprogramme which cost an estimated $3.5M butresulted in minimal problems and no delays to theproject. This shows, on a grand scale, that when asensible EMC policy is adopted and implemented

10 A HANDBOOK FOR EMC'TESTING AND MEASUREMENT

early in product development considerable costbenefit can result.

It is clear that failure to address EMC designissues can result in major budget overspends andprogramme delays. The cost of implementing anEMC control programme into productdevelopme.nt is generally estimated at about 5%of the development cost. With careful design andchoice of components and materials, theadditional cost element on each equipment soldcan often be small, and sometimes almostnegligible in the case of large production runs.

1.3.4 Serious EMI problems

As most EMC activity until the early 1980s wasassociated with military hardware, it is to beexpected that problems would be associated withthis type of technology. One of the mostpublicised incidents to military equipment inwhich EMI was implicated was that whichoccurred on the USS Forestall off Vietnam inJuly, 1967. It was reported that RF energy from ahigh powered ship's radar coupled into the firingcircuits of a aircraft-mounted missile rocketmotor, which ignited and fired the weapon into anumber of other armed aircraft on the carrierflight deck. The resulting explosions and firekilled 134 people and caused $72M of damagenot counting the 27 lost aircraft. More recently(around 1980), interference to avionics from aground based transmitter was implicated in amili tary aircraft crash in Germany.

EMC activity in the nonmilitary world hasconcentrated on the need to keep the electromagnetic spectrum free from interference to enablecomlTIunications systems to operate efficiently andto minimise the upset caused to radio andtelevision broadcast reception.

The last 40 years have seen a dramatic increasein the number of licensed radio services. In theUSA it is reported [9J that since 1950 the numberof broadcast stations has quadrupled to 11,000radio and 1,400 TV stations. The FCC (FederalCommunications Commission) has also licensed2.7 million mobile and fixed systems using 12million transmitters [10].

With the widespread introduction of digitalcomputing technology and microprocessorcontrolled products into the commercial, industrialand domestic environments in the early 1980s theneed for government agencies to act to control thepossible explosion in EMI was overwhelming.

By this time, EMC was firmly established in theindustries designing military electronic equipmentas an accepted part of the design and manufacturing/quality process. Many early EMCengineers working on military products had

persuaded programme managers over thepreceding ten years that the costs associated withEMC engineering were affordable in order toproduce equipment which not only met formalspecifications, but also worked well in the field.

In the late 1980s and early 1990s engineers anddesigners working in the civil electronics fieldfaced a similar situation [5, 11, 12] and thecommercial electronics business learned that goodEMC design is important if their products are tocompete successfully in national, regional andglobal markets.

1.4 Technical discil?lines andknowledge areas wIthin EMC

This section considers briefly the wide range oftechnical skills that have a part to play in thefield of EMC. Each skill or knowledge areadiscussed below is involved in solving EMCproblems. 1'he task of the product managerinvolved with EMC is to blend these various skillstogether to engineer robust EMC solutions intothe product at an affordable cost.

1.4.1 Electrical engineering

Electrical and electronic engineers are expected tobe knowledgeable in the following areas relevantto EMC design and installation practice:

Analogue and digital circuit designSemiconductor device technologyTransient suppression devices and circuitsCircui t board designComponent selection: operating limits and

reliabili ty /cos tEM aspects of mechanical design (propagation

through slots, holes, joints etc), grounding andbonding impedances

Power generation, distribution and switching systemsElectrical safety and lightning protection filters

and surge arrestorsGrounding techniques, single/multipointDifferential and common mode cable couplingTransmission line theoryScreening theory a~d shielding designInterface circuit designData bus and interface circuit designOptoisolation techniquesRadiation from cables and slots in screensFourier transforms between freq uency and time

domainsUse of sophisticated RF test equipmentPrinciples of RF receivers and transmittersBasic antenna theoryRadiowave propagation theory (near-field ,effects

being of particular interest)

1.4.2 Physics


1.4.4 Limited chemical knowledge

The physics of electromagnetic energy exchangebetween RF currents and waves is very importantin understanding the complex processes that occurin a real EMI situation. The manner in which RFcurrents flow in and around the surfaces ofconducting and nonconducting complex structuresof the victim equipment determines to a largeextent the nature of an EMI problem. Developingand implementing the means to contain, absorb ordivert such currents in a harmless way is the coreof good EMC design practice.

The physical equations derived by Maxwellgoverning EM waves and their interaction withmatter form the basis for a real understanding ofEMI problems and their solutions. Such equationscan be solved generally using large 3D finiteelement, finite difference, or boundary elementcomputer codes which can predict the current flowpatterns in a complex structure when illuminatedby an external electromagnetic wavefield.

The physics of EM wave propagation in boththe near and far fields must be considered by theEMC engineer, along with standing-wavephenomena and the performance of radioabsorbent materials in large shielded testingchambers if meaningful EMI measurements are tobe made and explained. Understanding howEMC test antennas behave in the near field,possibly amid multiple test chamber reflections, isa difficult topic which represents a significantchallenge to the physicist involved in EMCmeasurement.

1.4.3 Mathematical modelling

Large projects that require EMC to be consideredat all levels of development often make use ofextensive computer models and the EMC designeror manager should be familiar with the differenttypes used. These can include

(i) Models of physical processes, such as RFcurrent distributions on structures due toimposed EM field, wavefield to transmissionline coupling, lumped and distributed filterperformance in circuits with arbitrarysource and load impedances.

(ii) Models of intersystem and intrasystemcompatibility matrices to identify potentialEMI problems because of unwantedfrequency matches, noisy culpritequipments, oversensitive victims, or a highlevel of coupling owing to the closeproximity of subsystems.

(iii) Programme management software that canbe used to monitor and control an extensiveEMC activity.

Occasionally, good, apparently simple and costeffective solutions to an EMI problem are ruledout because of concern about the chemistry of theproposed design. Corrosion of RF gaskets due tocontact of dissimilar metals in damp, salt-laden orcorrosive atmospheres can be a real problemwhich not only renders some EMI solutions void,but can also result in serious damage to theequipment cases or containers.

1.4.5 Systems engirleering

To be successful, EMC must be considered early,when a contract to develop and supply electronicequipment is being negotiated. Failure to do socan result in incorrect bid pricing and aneventual failure to comply with the contractualEMC requirements. The customer requirementsmust be interpreted and reflected in a systemEMC specification which is then apportioned toeach subsystem or element of the design. If thesetasks are carried outit is then possible to considera balanced hardening design of the system whichhelps to ensure the most cost-effective route to electromagnetic compatibili ty.

1.4.6 Legal aspects of EMC

EMC requirements on products stem in part fromlegal req uirements by government agencies formanufacturers not to produce electronicequipment that will pollute the electromagneticspectrum. Failure to comply with such laws canresult in possible litigation by customers, othersuppliers, and of course, the official agencies.Under the new EC harmonisation directive ECI89/336 within Europe it becomes a criminaloffence to sell equipment which does not meet thecommunity EMC requirements as set out in theharmonised or national (technically equivalent)standards.

Corporate legal advice on EMC regulation isbeginning to be sought in the USA [10J and it isinadvisable for EMC design or test engineers toventure a direct opinion on the legal implicationsof EMC technical matters. They should, however,be in a position to advise management andlawyers about the multitude of regulations andstandards which relate to EMC throughout theworld , particularly in those countries or tradingblocks into which their products are being sold.

1.4.7 Test laboratories

Often the formal proof of meeting an EMCrequirement is obtained from a test In anaccredited laboratory. This can be made within


the company that is developing the product or byan outside specialist EMC test house. Clearly, thetest engineers must comprehend the EMCstandards and specifications, the philosophybehind the testing approach, the detailedtechnical methods used in testing and the qualitycontrol requirements which govern testprocedures and test house management.

I t follows that it is also in the in teres ts ofdesigners and project managers to have a goodunderstanding of all these aspects of EMC testingto be able to interact meaningfully with the testengineers and understand the results which areobtained. Providing such information is one of theprincipIe aims of this book.

1.4.8 Quality assurance: total qualitymanagement

EMC test engineers and managers of test housesmust be acutely aware of the need for discretion,impartiality and q uali ty assurance in the conductof their business. The EMC facility manager isrequired to hold delegated ,quality authority froma senior company manager to ensure that thelaboratory is run in a manner which satisfies thenational laboratory accreditation scheme(NAMAS in the UK).

1.4.9 Practical skills

Effective EMC designers often have first-handpractical engineering experience of equipmentdevelopment and solving tricky RF problems withrelatively meagre resources. Experienced radioamateurs tend to have a good understanding ofbasic RF engineering, which incidentally, is beingtaught less each year in university courses. Radiohams often display a flair for EMC engineering,offering cheap effective design solutions perhaps asa result of using novel techniques.

The skills mentioned above are req uired byindividuals and teams in pursuing the goal of electromagnetic compatibility for electrical andelectronic products being developed in today'scommercial world of competitive costs and tighttimescales. Many of the issues and technical topicstouched on so far are amplified in the rest of thisbook with the aim of giving the reader an appreciation of design practices and a good awarenessof EMC testing techniques.

1.5 Philosophy of EMC

Engineering of any kind can be viewed as thecreative process of defining, organising anddistilling out of a range of possible outcomes thedesired system performance. I t IS by the

application of consistent design constraints andaccurate manufacturing processes that electronicequipments with specific properties are produced.

EMC engineering is concerned with identifying,understanding and managing the normally uncontrolled and often unexpected transfer of electromagnetic energy from device to device such thatthe desired product performance goals are notimpaired. EMC engineering can therefore beviewed as the application of an extra set ofcontrols or constraints to the design, manufacture,installation, operation and maintenance of thesystem in question, which ensures that theequipment performs only those functions for whichit was designed, and does not respond to anyspurious signals resulting from EM interference.

This means that EMC engineers attempt todesign-out all spurious system responses, leavingthe product design engineers to design-in thewanted performance. I t is the role of thoseconcerned with EMC to ensure that the secondbroad element of system design, the 'notengineering' is carried out cost-effectively inconjunction with the usual design process if theproduct is to avoid being compromised by EM!.

Perhaps the essential difference between the twotypes of engineering is that the conventionaldesigner is concerned in great detail with only arelatively !1arrow range of specific product relatedissues whereas the EMC engineer, or designerwith responsibility for EMC, is concerned with allpossible external electromagnetic influences onthe proposed system.

Not all good electronics designers take to EMCengineering: the subject seems to suit individualswho enjoy lateral thinking and have a wideinterest in electronics, RF engineering and abroad knowledge of the many skills outlinedearlier. However, most electronics designers canbecome competent EMC practitioners withappropriate training.

The next chapter looks in detail at the range ofEMC requirements and specifications usedaround the world with which EMC managers,designers and test engineers must be familiar.This is the essential starting point from which todevelop an understanding of the technical factorsthat are involved in EMC design and testingaimed at meeting these specifications.

1.6 References

JACKSON, G.A.: 'The achievement of electromagnetic compatibility'. ERA report 90-0106, ERATechnology, Leatherhead, Surrey, UK

2 DUFF, W.G.: 'Fundamentals of electromagneticcompatibility'. Interference Control' Technologies,Inc.

NATURE AND ORIGINS OF ELECT'ROMAGNETIC COMPATIBILITY 13

3 KEISER, B.: 'Principles of electromagnetic compatibility' (Artech House, 1987, 3rd edn.)

4 JOLLY, W.P.: 'Marconi' (Constable, London, 1972)5 BRAXTON, T.E.: 'Selling EMC in a large organi

sation'. Proceedings of IEEE symposium on EMC,1988, pp. 447-451

6 'Nature and characteristics of EMC'. USAF generalinformation document AFSC DH 1~4, section IB,chap. 1

7 MORGAN, D.: 'An introduction to EMC'.Presented at lEE fourth vacation school on RFelectrical measurernents, University of Lancaster,july 1979

8 FIELD, j.G.C.: 'Electromagnetic compatibility inwarship design'. Proceedings of IERE symposiumon EMC, April 1978

9 FCC Public Notice 2519, April 199010 FCC 54th annual report, fiscal year 198811 STAGGS, D.M.: 'Ethics within the corporate

structure'. Proceedings of IEEE symposium onEMC, 1990, pp. 526-528

12 HILLIARD, D.E., DESOTO, K. E. andFELKERT, A.D.: 'Social and economic implications of EMC: A broadened perspective'.Proceedings of IEEE symposium on EMC, 1990,pp. 520-525

Chapter 2

EMC standards and specifications

2.1 The need for standards andspecifications

2. 1. 1 Background

As the use of electronic equipment grew and theneed to allocate and protect the electromagneticspectrum for communications became moreimportant, there arose the requirement to developEMC regulations to ensure that an uncontrolledsituation did not develop. Governments soughtlegislation through appropriate administrativedepartments, giving force to sets of standardswhich ensured that electromagnetic compatibili tywas managed properly in the design and use ofcertain categories of electronic equipment.

The style and content of standards were usuallycharacteristic of the nation which introducedthem, although in many cases they were based onthe, work of international bodies such as CISPR(International Special Committee on RadioInterference). This loosely co-ordinated nationalapproach led to problems in the commercial tradeof electronic equipment across national boundariesboth within Europe and with the USA.

The task of developing standards for the controlof EMC can be said to have begun in 1934 withthe formation of CISPR [1 J. The name CISPR isderived directly from the French, ComiteInternational Special des PerturbationsRadioelectriques, and was formed by several in ternational organisations coming together to institutea joint comnlittee to specify measurement methodsand limits of radio frequency interference. Since1950 CISPR has been a special committee underthe sponsorship of the IEC (InternationalElectrotechnical Committee) whose role is to issueinternational standards.

CISPR has made considerable progress indeveloping methods of measurement and limits todeal with interference to communicationsequipment. More recently, the appearance ofnonradio receiver interference phenomena led tothe involvement of the IEC. In 1982 it wasreported [2] that some 65 of the 200 committeeswere concerned in part with problems of EMC.For example, technical committee TC77 wasconcerned with power distribution networks,while TC18 produced the first EMC standard forelectrical installations in ships (IEC pub. 533).Medical electronic equipment was covered by

14

TC62 and industrial process and con trolequipment was the responsibility of TC65.

2.1.2 Contents of standards

The standards produced strive to ensure electromagnetic compatibility by requiring equipmentdesigners to consider the subject as part of thedesign process from the earliest possible stage inthe development of the product concept. Theyusually contain a section devoted to the definitionof relevant technical terms used in the documentand often specify the requirements for

planning and project management of EMCtest methods and specific test equipmentspecified limits which must be metspecification for acceptable EMI measurementreceivers are also referred to in these standards.

As a good example of a well constructed standardone may refer to UK defence standard DEFSTAN 59-41 [3J which is issued in five parts:

1 General requirements2 Management and planning procedure3 Technical requirements, test methods and limits4 Open-site testing5 Requirements for special EMC test equipment

(draft) .

EMC standards usually undergo a process ofevolution and updating to meet the developingneeds of industry and society. They are producedafter extended consultation between theregulatory authority and the supplying industryand other interested parties such as nationalstandards institutes. They contain a list ofdefinitions of words, phrases and technical termsrelated to EMC and present carefully orderedinformation with the aim of providing generalguidance [4J and sometimes specific instruction inorder to demonstrate compliance with theassociated specification limits. An EMC specification usually contains numerical details andgraphical representations of limits for measurableparameters such as radiated field strength orconducted interference current.

2.1.3 The need to meet EMC standards

With the development of the EC as a tradingentity, directives have been issued to harmonise

product standards in many fields. In the UKdemonstrating compliance with EMC standardsand related specifications for new and importedelectronic products is a legal requirement from1996 onward. .

EMC specifications are also invoked withincommercial cantracts for the purchase of largeequipment and military systems. They can gobeyond. demonstrating compliance with the basiclegal requirements for EMC and containadditional requirements that are specific to theequipment and the environment in which it willbe operated. In this case, demonstration ofcompliance is normally required by the purchaser.

Whether a manufacturer is satisfying the basiclegal requirement or a more involved contractualone, the demonstration of compliance may be inthe form of a technical dossier and/or an EMCtest report. Such reports are normally producedby an independent test house, or certain specifications permit the supplier to self certify theequipment. In either case the measurements mustbe made in strict accordance with the testmethods described in the specification and the testlaboratory will be operated within strict qualityguidelines and accredited by external q uali tyauthorities such as NAMAS [5].

2.2 Civil and tnilitary standards

2.2.1 Range of EMC standards in use

T'here are a large number of EMC standards andassociated specifications in use in the world todaycovering an enormous variety of electrical, electromechanical and electronic equipment in variousindustrialised nations. Table 2.1 [1] contains a listof standards covering commercial equipmentwhich are, or have been, in force in a selection ofindustrial coun tries. I t is inappropriate to discussthem all in detail in a book on EMC testing butsome insights maybe gained into the nature ofthese EMC standards and the relationshipsbetween them by grouping them under theheadings military standards and civil standards,as used in Europe, the USA, and other industrialnations. This section explores the nature ofmilitary and civil standards and specifications.Subsequent sections examine specific illustrativeexamples from the three groups.

2.2.2 Derivation of military standards

The substantial differences between military andcivil or commercial EMC standards and specifications are due to both equipment requirementsand the environments in which they operate [6].Generally, mili tary requirements are more wide

EMC STANDARDS AND SPECIFICATIONS 15

ranging and stringent than commercial ones, andthus more difficult and expensive to meet. Asmilitary standards cover more aspects of electromagnetic compatibility over wider frequencyranges than do commercial ones, many of theexamples of measurement techniques discussedare taken from military standards.

The aim of one set of military EMC standards isto ensure that mission success is not compromisedby poor intrasystem control of spurious electromagnetic energy. Additional standards controlintersystem EMC / ensure that individualsystems do not compromise each other'sperformance in operation. _

Standards are imposed by military procurementauthorities in the form of contractual conditionswhich companies tendering to supply equipmentsmust meet. T'hey may insist on a contractordemonstrating his EMC project management,design, development and measurement capabilities prior to awarding a can tract.

In the fu ture, cantracts for large commercialsystems may also adopt _similar req uirements asthe need to achieve good management of EMCbecomes more important.

The concept that underlies intrasystem EMCstandards is that if each identifiable electronicsbox and each electronic subsystem meets thestandard then full system level EMC will beachieved.

Writing specifications and defining test methodsfor measuring spurious and unintended emissionsand unwanted responses is difficult anddocuments are written not in terms of wantedperformance (as with most product design specifications) but by stating what may not be allowedup to a given level.

A concept that is helpful in achieving systemlevel EMC by specifying box-level limits is that ofthe source-victim margin. Each electronics box isconsidered both as a source of electromagneticinterference, either conducted or radiated andalso as a potential victim of such interference. SeeFigure 2.1. The frequency, amplitude andmodulation characteristics of interference sourcesand victims within each box are usually different.A matrix may be drawn up showing each box ina rank or file as a source and a victim. Any matchin terms of frequencies, levels and modulationcharacteristics identifies the possibility of a systemlevel EMC failure. See Figure 2.2.

The key concept with these box-level militaryEMC specifications is to set the limits of emissionsand susceptibilities such that there is a margin ofsafety between them. If this margin is achieved itis impossible for matches to be made in the cellsof the matrix in Figure 2.2 and system levelcompatibility is ensured. Safety margins of 6dB


Table 2.1 Examples oj electromagnetic compatibility regulationsIstandards oj various countries

Country Ignition systems RF equipment Household electric Radio and Fluorescent lamps Solid state IT & EDPincluding ISM applicances TV/video and luminaires controls equipment

CISPR PUB 12 PUB II PUB 14 PUB 13 PUB 15 PUB 22

European norm (72/245/EEC) EN55011 EN55014 EN55013 EN55015 EN55022EN60555-213 EN55020

Australia AS2557 AS2064/2279 AS I044/2279 ASI053 AS2643 ASI054 EN55022

Austria OVE-F65 OVE-F61/62 OVE-F61/62 OVE-F64 OVE-F61/62 o V E- F61/62/55022

Belgium RCI960/74/76 RCj6.6.1966 RC/1978/1983 Royal Dec. 1960 RC1978f1983 EN55022

Brazil Relevant CISPR PUBS also IEC/TCn standards

Canada SOR 75-629 SOR/75629 CSAC I08-5-4 SOR/75-629 CSACI08-5 CSA22-4 CSACI08-8CSAI08-4 CSA22-1/3/5 CSACI08-5-2 SOR/83-352 VDI054

CSAC235

Czech. CSN34-2875 CSN34-2865 CSN34-2860 CSN34-2870 CSN34-2850

Denmark M04-j78 M05689 M0416/NAHR3/2 M0396/NAHR4 M0416/NAHR5 NAHRI M0416/568/396M03/83

Finland PUB '1'35-65 PUB '1'33-86 PUB '1'33-86 T33-86/NAHR4 PUB '1'33-86 PUB '1'33 PUB '1'33-86

France C91-103 C91-102 NFC70-100 C91-104 NFC91-100 NFC91-100 NFC91-022EN55011 82/499/EEC C91-110 82/500/EEC EN55022

EN55014 EN60555 EN55015EN50006 82/499/EECEN6055.1

Germany 72/245/EEC VDEOl60 82/499/EEC EN55020 82/500/EEC VDE0875 VDE0871VDE0879 VDE0750 EN55014 VDE0871 EN55015

VDE0871 VDE087.1/6/7 VDE0872 VDE0875VDE0838 VDE0875EN60555 VDE0838

Italy 72/245/EEC CISPRII EN55014 CEIII0-3/4 82/500/EEC EN5502282/499/EC EN60555 EN55015CEIl 10-1 CEIII0-2CEIn-1EN50006EN60555

Japan CISPRI2 RERART65 EA&MCLAW EA&MCLAW EA&MCLA\V VCCIJR'rC/MP'r JRTC73/74 JRTC73/74/75 J R'!'C 71 /74/75 JR'!'C73/ 74/75 CISPR22

M P'f1970/71 &82 MPT1970/71CISPRI3

Netherlands 72/24·5/EEC RCIOPREP 82/499/EEC NENJOOl3 82/500/EEC RC/N.PNEPNENI00I2 NENIOOII RC402/1984 EN60555 RC401/1984 CISPR22

EN55011 EN55014 EN55015 EN55022VDENI975 NEN-EN50006

EN60555

New Zealand CISPR 12 RFS4·9-1 RFS49-1 RFS49-1 CISPR15 ClSPR22NFCCOM AS2279 CISPR13

BS5406

Norway Regs for motor NEMK0662/82 NEMK031/83 NAHR4 NEMK031/83 NEt\.1K031/83 NEMK0662/82vehicles 1969 PUB. NO. EN55014 NEMK0661/77 EN55015 (CISPRI4) NEMK0661/n43/6317 80POOS NEN67-76 LET-NO.32/83 NEMK0665-168 NR33/83CISPRI2 CISPRII EN50006 EN60555 NR32/83

EN60555 EN55022

South Africa R2862-1979 R2862-1979 R2862-1979 R2862-1979 SABS R2862-1979 SABS(CISPRI6) CISPRII (CISPRI6) (CISPRI6) (CISPRI6) (CISPR22)

(CISPRI6)

Spain UNE20505 UNE20506 UNE20507 RC2704/l982 UNE20510 EN55022UNE20503 CISPRII EN55014 UNE20511 82/500/EECCISPR12 EN60555 EN55015(CISPR16)

United BS833 BS4809 82/499/EEC 82/449/EEC 82/500/EEC BS800 Pt3 BS6527Kingdom (CISPRI6) BS6662 EN55014 BS905 BS5394 EN5S022

EN75-31 BS800 EN60555 EN55015BS4941 BS727 BS800/1983BS4999 BS5406 BS6345ERP (SERIES) EN60555

USA SAEJ551C FCC Ptl8 FCC Ptl5 FCC Pt2 FCC Pt15.J FCC Ptl5JFCC MP-5 ANSIC63-2 FCC Ptl5/C FCC MD-4 FCC MP-4MDS2010004 ANSIC63-4NEMA ICS-2 NEMA WD2-1970I EEE518-1982MIL STD461/2

MULTIPLE INTERACTIONS TAKE PLACEBETWEEN ALL UNITS


2.2.3 Derivation of commercial standards

VICTIMS OF EMI

Building attenuation

/(

Building = Emission level limitattenuation at distance0- 10 dB 3 -30 meters

---d-----3, 10, 30 meters

E =300 - 1000 ~ V1m

/

E dB ~ Vim - Required SIN +( Signal to noise)

20 - 40dB

Regulations, standards and specifications relatingto commercial electronic equipment are aimed atcontrolling pollution of the electromagneticspectrum and protecting radio communications.The key intention is to limit the intersystemradia ted or conducted interference emissions fromequipment to a level which does not causeproblems for radio and television reception.

An increasing number of commercial standardsaddress the susceptibility or immunity ofelectronic equipment to electromagnetic threats.However, the majority are concerned with thecontrol of unwanted emissions from theequipment. Commercial specifications are oftenlimited to conducted interference limits below30 MHz and radiated limits only above 30 MHz[6], since conduction along power or signal cablesis more likely at the lower frequencies andradia tion is the more significant energy transportmechanism at the higher frequencies.

The underlying assumption which determinesspecification levels for commercial equipment isthat allowable emissions will be kept below thestrength which will degrade radio and televisionreception based on practical receiving signallevels. Assumptions are made as to the probabledistance separating a TV receiver and a radiatingequipment such as a personal computer in aresidential situation. This is taken to be between 3and 30 m. I t is then possible to calculate the levelof an interfering signal at test distances of 3, 10and 30 m for an acceptable received signal-tonoise ratio. See Figure 2.3. Interference limits atthe specified test distances are then issuedtogether with careful test methods designed todetermine whether an equipment meets them.

The characteristics of appropriate measurementequipment are also specified, often based onCISPR standards. These documents may bereferenced in the EMC regulations such as those

p'(gure 2.3 Regulatory model for radiated emissions fromelectrical equipment

SYSTEM

t I

UNIT3SOURCE 3 (f, I, m)

VICTIM (f, I, m)

Each unit is a source and a victim withparticular frequency level and modulationcharacteristics

SUB-SYSTEMS or UNITS(1 • n)

V1 V2 V3 V4 V s Vn

S1 X

S2 X

S3 X

S4 X

85 X

Sn X

Figure 2.2 Intrasystem interaction matrix

Figure 2.1 Intrasystem-source-victim

~wu.o(f)w()0:::::>o(f)

are used in the derivation of MIL STD 1541(USAF) [7] for the limits for power and signalcable interference levels, with a higher 20 dBmargin for ordnance circuits.

Established military specifications such as MILSTD 461 C (USA) [8] are often tailored to thespecific requirements demanded by the procurement of a particular item of equipment, such as anaval aircraft. Without the ability to tailor generalmilitary EMC requirements it would be impossibleto achieve electromagnetic compatibility for aunique system performing a specialist role in aparticular environment at an affordable cost.

In the case of commercial electronic equipment,specifications cover particular categories of relatedproducts. This approach does not involveincurring the unjustifiable costs imposed bymeeting a general wide ranging specification.VDE 0871 [9] is an example of a commercialspecification concerned only with ISM(industrial, scientific and medical equipment).


required by the FCC in the USA e.g. FCC part15J [10], the FTZ (FernmeldetechnischeZentralamt or Central TelecommunicationsOffice) in Germany e.g. VDE 0871/0875 [11] andin the UK by the DTI Radi.o InvestigationService [12,13] with specifications such asBS4809, BS800, BS6527, etc.

EMC control and its economic impact onindustry and commerce is achieved.

Standards are continually developing andevolving and EMC designers and test engineersshould keep themselves well informed of proposedintroductions of and changes to EN standards.

2.2.4 Generation of CENELEC EMCstandards

2.3 UK/European com.m.ercialstandards

Table 2.2a EMC emission standards

Table 2.2b EMC immunity standards

There are also two widely used British Standardswhich relate to the susceptibility or immunity ofcommercial equipment to interfering signals.These are given in Table 2.2b.

Broadcast receiversIndustrial processcontrol

Applicabili ty

EN55020HD481

ECequivalent

BS905 pt 2BS6667

BritishStandard

British EC Applicabili tyStandard equivalent

BS3GI00 Equipment for use inpt 4 sec 2 aircraftBSG229 Environment for aircraft

equipmentBS800 EN55014 Household appliancesBS6527 EN55022 Information technology

equipmentBS905 pt 1 EN55013 Radio and TV receiversBS4809 EN55011 Industrial, scientific and

medicalBS5394 EN55015 Fluorescent lamps and

lightingBS5406 EN60555 Electrical supply

networksBS833 Vehicles ignition systemsBS1597 Interference suppression

in marine systems

2.3.1 UK standards relating tocommercial equipment

In the UK, EMC standards have been issued formany years by BSI. Examples of widely usedEMC emission standards are listed in Table 2.2atogether with equivalent EC harmonisedEuropean Norm (EN) standards whereappropriate.

CISPR StandardsIEC, ECMA, CEPT

Other interestedboards

Figure 2.4 Process ojgenerating EN standards byCENELEC

EN Documents

This section briefly considers the process by whichthe harmonised European Norm EMC standards(discussed in Section 2.3) are produced. TheseEN standards are now the most important for ECmanufacturers and importers of electrical andelectronic equipment. The European electricalstandards committee is known as CENELEC andhas been given a mandate by the EC to overseethe harmonisation of national standards and toprepare new ones when necessary. Technicalcommittee TC 11 0 has the responsibility for EMCstandards. EN standards are usually based onexisting CISPR documents which may be familiaras they are similar to some British standards andGerman VDE emissions regulations.

A simplified process of generating EN standardsby CENELEC is shown in Figure 2.4.Government departments, trade associations andother interested bodies can stimulate andinfluence the appropriate British Standardscommittee to represent their interests via thenational committee to CENELEC. Existingstandards may be used or modified to generate anew draft EN standard. Once published forcomment, the interested parties will reflect theirviews through the committee structure. A numberof revisions may occur before the technical detailsand an agreed balance between the need for

There are two other widely used UK standardswhich contain immunity limits:

BS3G 100 pt. 4 sec. 2 'Equipment for use inaircraft' also contains limits for equipment susceptibility.NW0320 (National vVeights and MeasuresLaboratory) is concerned with 'Weighing andmeasuring equipment immunity to electricaldisturbances' and sets limits for electromagneticradiation, magnetic induction fields, electrostaticdischarge, power line transients and radiatedinterference.

The characteristics of special test receivers whichare used to make interference measurements asrequired by the emission standards are themselvessubject to another British Standard: BS727 'Radiointerference measurement apparatus'. Thisstandard is in line with the requirements ofCISPR publication 16.

2.3.2 Comparing tests

E.ach emission or immunity standard specifiesparticular test methods and limits and in generalit is difficult to read across from one standard toanother. For example, the radiated emissionmeasurements in BS3G 100 are conducted with amonopole, a bow tie (broadband dipole) and alogarithmic conical spiral antenna connected to apeak measuring receiver covering a frequencyrange from 150 kHz to 1 GHz. All measurementsare made inside an RF damped screened chamberto eliminate ambient noise, and the EUT(equipment under test) carefully set out on the


surface of a solid metal ground plane as in Figure2.5 1 m from the antennas.

The technical details of antenna types, peakdetectors, narrow/broadbandwidths and screenedrooms are all addressed and explained in theappropriate chapters. As an example, the radiatedemission field-strength limits for this standard areshown in Figure 2.6. (For an explanation of theterms narrowband and broadband/referred to inthis Figure see Appendix 1.1.)

This BS3G 100 test may be contrasted with thearrangements for measuring radiated emissionsfrom data processing and electronic officeequipment as specified in BS6527/EN55022 wherethe E UT is placed at 2. distance of 30 or 10m(depending on whether the equipment is forcommercial or domestic use) from a measuringantenna on an open field test site. See Figure 2.7for a plan of the test site. No screening of ambientsignals is available on an open test site other thanthat afforded perhaps by nearby hills. Such afacility will preferably have been located in aradio quiet environment but background/ambientRF signals inevitably intrude on the measurementwhich can slow down the testing while they areidentified and marked.

The specified measurement antenna to be usedin open range testing is a resonan t lengthbalanced dipole for frequencies above 80 MHzand fixed at 80 MHz resonant length forfrequencies below 80 MHz. The need to manuallyadjust the dipole at each test frequency is timeconsuming and slows down the test. Broadbandantennas such as the bow tie and log conicalspiral used in BS3G 100 are sometimes permittedto speed up the measurement.

Powerinputs

~90 em

\

Secondary power lines

to load NORM~LOR SIMULATEDINPUT & OUTPUT CIRCUITS

Equipmentunder test

Monopole antenna14 kHz - 30 MHz

Metal platenot less than30 cm square

Log conical spiral antenna200 MHz - 1 GHz

Figure 2.5 Typical radiated emission test corifiguration Reproduced by permission of BSI


Reprod uced by permission of BS I

Reproduced by permission of BSI

2.3.3 European commercial standards

In the early 1970s there was a wide diversity ofnational regulations in Europe, some of whichwere related to CISPR standards but many wereparticular to a given country. This situation ledto problems with free trage in electronic goodsacross national boundaries and gave rise tovarious public purchasing and subsidy policieswhich distorted fair competition [12].

With the signing of the Single European Act in1985 governments were committed to a singleEuropean market which was to be achievedprogressively by the end of December 1992. Oneof the main tasks was to remove technicalbarriers to free trade and this was to be achievedby harmonising technical standards whichproducts must meet within all member countries.

E:rvIC standards came to be harmonised underDirective 89j336jEEC of the 3rd May 1989 [14](amended by 91j263jEEC and 92j31jEEC).These are complex and legalistic documentscontaining many articles dealing with issues suchas definitions, applicability, release for sale,recognition of special measures in member states,relevant national EMC standards and declarationof conformity. The scope of the directive includesalmost all electrical and electronic appliances,equipment, systems and installations which arebrought into service within the community. Thedirective has been implemented in UK law by theElectromagnetic Compatibility Regulations 1992(SI2372).

From October 1992 manufacturers of electricaland electronic products governed by theregulations have the choice of either following theEuropean Community regime, or continuing tocomply with the existing national 'legislation inforce' on the 30th June 1992 in the member statesin which the product is to be marketed [15].Existing UK legislation on EMC is listed inAppendix 1.2.

radiated emission field strength limits for BS6527are shown in Figure 2.8.

A quick comparison of these two standardsclearly shows the difficulties that will beencountered in trying to read across betweenmeasurements made under different standards,even within the same country. Great care shouldbe exercised in comparing standards, particularlyif the comparison is being made between UK andother national standards for the purpose ofcertifying compliance when exporting equipmentabroad. It is partly to overcome such problemsthat the EC has pressed forward with aprogramme of harmonisation of standardsincluding those relating to EMC.

-go.0

"850 ~

40 N

~

30 E:>

20 .:

~10 .8o

co'"C

1000: 61

1.0 10

FREQUENCY MHz

Class B limits for the field strength of radiated spurious signalsin the frequency range 30 MHz to 1000 MHz

Test distance ( F ) Frequency range Quasi-peak limits

m MHz dB (~V/m)

10 30 to 230 30

10 230 to 1000 37

Jooe------- major diameter = 2F--------l

Volume above earth to be free of reflecting objects

Figure 2.8 BS6527 radiated emission limits foropen site testing

------T-----...,.,....- I -- ..........

/// minordiamet

1

er=V3F ........ ,"'-

/ ~I \

I j( F=10-30m.---7E: )

\ RECEIVING AERIAL 1 EUT (equipment /~, under test ) /, /, //

...........--.-- ----,...,-/- ---

Boundary of area defined by an ellipse

Figure 2.7 Plan view of open range radiation test site

Other differences between BS6527 andBS3G 100 relate to the movement of themeasurement antenna to obtain maximumreceived interference signal strength and the useof a quasipeak detector in the receiver. The

Class A limits for the field strength of radiated spurious signalsin the frequency range 30 MHz to 1000 MHz

Test distance ( F ) Frequency range Quasi-peak limits

m MHz dB (~V/m)

30 30 to 230 30

30 230 to 1000 37

Reproduced by permission ofBSI

Figure 2.6 BS3G100 radiated emission limits

;- 60

~ 50gc=. 40

~ 30::l.

i 20

~ 10co'"C

From 1st January 1996 most electrical andelectronic products made or sold in the UK(including imports) must meet the requirementsof the EC directive on EMC. Failure to complywill become a criminal offence and the productwill be prohibited from sale in the EC.

Most electrotechnical products are subject tothese regulations; there are however a number ofgeneral and specific exclusions which are listed inAppendix 1.2.

I t is envisaged that demonstration bymeasurement that the equipment conforms to arelevant harmonised EMC standard will be thenormal means of complying with the directive.For some equipments it may be appropriate tosubmit a 'technical file' or for radio communication transmitting apparatus by EC-typeexamination.

There will be cases where a manufacturerconsiders that it is inappropriate for equipment tobe assessed against such a standard. I t is possiblethat no suitable standard exists. In such circumstances the assessment of compliance with thedirective shall be by means of the production of atechnical file and the involvement of a competenttechnical body designated by the Department ofTrade and Industry, possibly a NAMASaccredited EMC design/test house.

The directive permits the manufacturer todemonstrate compliance by self certification in itsown test laboratories. Such facilities wouldusually have EMC accreditation by a bodyrecognised for the certification of such laboratories. In the UK this would be a NAMASapproval.

Listed in Table A1.3.1 in Appendix 1.3 are someof the European EMC standards, their applicability and equivalent national or otherstandards. Reference is also made to the closestequivalent US standard. I t is, however,dangerous to assume an exact read across to theUS FCC standards as many detailed differencesexist in test methods frequency ranges and limits.I t will be evident from comparing the two partsof the Table that there are no current equivalentFCC EMC immunity standards in the USA,though these migh t be introduced for informationtechnology equipment.

Tables Al.3.2 and Al.3.3 of Appendix 1.3contain lists in number order of EC ENstandards [16, 17] which have been referencedin the official journal of the EuropeanCommunities and are therefore notified for usein the self certification route to compliance withthe EC EMC directive.

Table A 1.3.4 [16] lists a number of proposedproduct specific EMC standards on whichCENELEC is working (in 1993). The committee


aims to introduce these standards before the ECEMC directive becomes mandatory in 1996.

The EMI limits and test methods recommendedby CISPR have been adopted as the basis formany EN standards, European countries nationalstandards and EMC standards throughout theworld. Table A1.3.5 [1] in Appendix 1.3 lists therelevant CISPR documents. CISPR standardscall for a quasipeak (QP) measurement of interference as this yields a result which is proportional tothe subjective annoyance effect experienced byradio broadcast listeners.

The characteristics of the quasipeak detector asspecified by CISPR publication 16 are given inTable 2.3.

Table 2.3 Characteristics of CfSPR EMf meters

Frequency range

10 kHz- 150 kHz- 30 MHz-150 kHz 30 MHz 1 GHz

Electrical chargetime constan t, ms 45Electricaldischarge time 500 160 550constant, ms6 dB detectorbandwidth, kHz 200 9 120Meter timeconstant, ms 160 160 100Predetectionoverload factor, dB 24 30 43.5Postdetectionoverload factor, dB 12 12 6

Similar specifications for EMI measurIngreceivers exist in the USA but they are notidentical to those of CISPR. The equivalent USANSI (American National Standards Institute)C63.2 for measurement equipment contains avalue for electrical discharge time constant of theq uasipeak detector in the freq uency range150 kHz to 30 MHz of 600 ms rather than theCISPR 160 ms.

2.3.4 German standards

Prior to the introduction of the EC EMC directive,many designers and exporters of electricalequipment paid particular attention to themandatory national requirements which existed inWest Germany. The VDE (German Institute ofElectrical Engineers) standards called up in thenational laws on EMC, promulgated by Vfg(Decree Verfugung) and issued by BPM (Deutsche


FCC Class B

FCC Class A----.r--Narrowband conducted emissions

100

90

> 70::1.to'0

50

30.01

80 I

70~ Radiated emissions at 3 meters -FCC Class A

E50~ • FCC Class B ->

::1.to'0

301- -I-

10 I10 100 1000

FREQUENCY MHz

0.1 1 10 100FREQUENCY MHz

Figure 2.9 FCC limits for radiated and conductedemzsszons

class B limits (see Figure 2. 10), the VDE is moreparticular about the testing and accompanyingdocumentation for equipment tested to class Astandards. This is the opposite of the FCCapproach where class B products require FCCauthorisation and class A products are selfcertified for compliance [18].

In Germany, equipment which is in the lessstringent class A had to be tested by the VDEitself or at a VDE approved laboratory andadditional paperwork was submitted. Productswhich fall within the more stringent class B canbe self certified. Manufacturers may thereforechoose to put their commercial eq uipment intoclass Band suppress their emissions to meet thelower levels to save test time and money by selfcertification and bring products to the market inthe shortest possible time.

Manufacturers who sell both to the USA andEurope, including Germany may opt to meetthe class A limits of the FCC (easier to test) forthe USA market and VDE 0871 class B limitswhich are easier to test and market in Europe.

Lohbeck [18] gives an excellent precis of themain differences between FCC part 15j and theFTZ (VDE 0871 and 0875) standards. See TableAl.4.2 in Appendix 1.4.

For VDE 0871/0875 regulations:

Bundespost) are amongst the most stringent in theworld. Therefore demonstrating compliance withthe VDE limits would almost always assuretechnical compliance in most other countries.

Table Al.4.1 [18] in Appendix 1.4 lists theimportant Vfg decrees issued by the BPM and therelated VDE standards. In Germany the FTZ(Central Telecommunications Office) have therole of EMC adluinistration. This is comparablewith that of the FCC in the USA.

Because EMC standards are not yet fullyharmonised at an international level there havebeen many cases where a product which meetsthe FCC limits for a particular category ofequipment has subsequently failed to meet theappropriate VDE limits. This can be due to theequipment designer failing to understand fully thedifferences which exist, both in the- generalapproach and in specific test methods between theFCC and VDE standards. It is thereforeinteresting to examine some of the similarities anddifferences between these two important EMCstandards as the FCC rules apply throughout theUSA and the VDE specifications havecontributed to the basis for the harmonisedEuropean standards.

There is one major feature which is common toboth the FCC and VDE standards. Equipment isdivided into different classes depending partly onoperational use. For the US FCC part 15jregulations, classes are defined as:

The applicable limits are for emission only and aredivided into conducted emissions below 30 MHzand radiated emissions above 30 MHz. Theselimits are given in Figure 2.9.

Since the class A limits are less stringent than the

Class A: equipment which is used solely incommercial environments where separationsbetween equipment will normally be tens ofmetres and the professional equipment operatorswill have some incentives to position theirequipments so as to cause the minimum interference to neighbours.Class B: equipment which can be used in either acommercial or a residential environment.

Class A: Normally for commercial equipmentrequiring a test by VDE and individual permitfrom FTZ/ZZF (RFI registration office).Class B: General equipment for unrestricted distribution after self certification.Class C: A special on site test provision for large'one of a kind' installations.


VDE Class B

100 r__---__---~r__---......__---_

Industrial organisations concerned with EMCstandards and measurement within the USinclude:

equipment and systems for use in aerospacevehicles.NCMDRH, the National Center for MedicalDevices and Radiological Health, is concernedwith the safety aspects of both ionIsIng andnonionising radiation produced by electrical andelectronic products.NBS is renamed the National Institute of Scienceand Technology (NIS1-'), concerned withmetrology in general, of which EMCmeasurement is a part.NVLAP, the National Voluntary LaboratoryAccreditation Programme, is administered byNIST and provides confidence that EMC testingcarried out by laboratories in the USA withinthe progralume are to the desired q uali tystandard.

100

VDE Class B

VDE Class AElectric field

0.1 1 10

FREQUENCY MHz

Narrowband conducted emissions

" "" " ,

---, VDE Class A._----

0.01

40

80

80 _--------r--------_

60

60

>::l

CD"0

E>et 40"0

2.4 US commercial standards

2.4.1 US organisations involved withEMC

2.4.2 FCC requirements

The involvement of the FCC in the realm of EMCbegan in 1934 when the Communications Act ofthat year gave the FCC the authority to imposerules and regulations on industrial, commercialand consumer devices which could radiate electromagnetic energy. The significant parts of the rulesand regulations with regard to EMC are set outin title 47, parts 15, 18 and 68 of the US Code ofFederal Regulation (also known as FCC docket20780). A list of the parts relevant to EMC isgiven in Table A1.5.1 of Appendix 1.5.

FCC part 15j covers any device that intentionally generates and uses electrical energy in excessof 9 kHz or 9,000 pulses per second for

EIA, the Electronics Industries Association, ISconcerned with equipment EMI.IEEE is concerned with EMC standards.SAE, the Society of Automotive Engineers, plays aspecial role in respect of both air and land vehicleEMC. For example, SAE Practice ARP-937relates to jet engine EMI and applies to electricalsystems and accessories, ignition systems,actuators, fuel con troIs, solenoids, servo con trol,electrical alternators, etc. This SAE documentcovers conducted and radiated emission andconducted susceptibility requirements.R TCA, the Radio Technical Commission forAeronautics, issues documents on the airborneequipment environment, which includes sectionson EMC and lightning.ANSI, the American National Standards Institute,is concerned with measurement techniquesincluding those for EMC.

1000

41

30

100FREQUENCY MHz

0'~140625Z~A~25109375

06375078125 ± 001

0625 .

.01

10

20

wI"-(l')LO00, 'g~00

120-....~---

103.5100

91.5

80

~ 63.560

::l 51.5CD"0 40

20

0.1 1.0 10

FREQUENCY MHz

Figure 2.10 VDE 0871 conducted and radiated limits

FCC, the Federal Communications Commission, isresponsible for spectrum allocations outside thegovernment sector.NTIA, the National Telecommunications andInformation Agency, has a committee namedIRAC (Interdepartmental Radio AdvisoryCommittee) which is responsible for spectrumallocations within the federal government sector.NASA is concerned with the EMC aspects of


computation, control, operations, transformations,recording, filing, sorting, storage, retrieval ortransfer of data. The rules encompass computingdevice peripherals but exclude transmitters andreceivers and devices covered by other FCCregulations, computing devices used in transportation vehicles [19J, and control or power systemsused in public utilities and a range of other ISMand household equipment that are covered byother regulations.

FCC part 15 regulations address only emissions;however, the laws provide for susceptibilitytesting as well. The FCC has chosen not tomandate susceptibility or immunity limits forcommercial equipment as it prefers to leave thisresponsibility to the manufacturer whom itsupposes to have a self interest in the compatibility of its equipment with the EM environmentin which it is to be operated.

The FCC has five administrative arrangementsfor dealing with equipment that emits nonionisingradiation including both intended andunintended emissions:

(i) Type acceptance is based on informationand test data supplied by the manufactureror licensee to the FCC, which may thenchoose to test the item.

(ii) Type approval is granted when anequipment has passed the specified FCCtest. Tests are carried out by the FCC.

(iii) Certification is similar to type acceptance,however, no licence is held by the user.

(iv) Verification, when the FCC carries out spotchecks to ensure that the manufacturer'sself testing has been suitably carried out.

(v) Notification. In this procedure the FCC maynot require detailed data to accompany anapplication.

The applicable arrangement will depend on thenature of the equipment for which EMCclearance is being sought.

2.4.3 Other US commercial standards

A list of some examples of the standards preparedby bodies other than the FCC is given inTable 2.5.2 of Appendix 2.5.

2.5 Cornm.ercial EMC standards inJapan and Canada

These two countries are examples of importantproducers and markets for electronic productsand both have laws or voluntary regulationsgoverning EMC.

2.5.1 Japanese EMC standards

Japan has had the Electrical Appliance andMaterial Control Law for many years which isconcerned with regulating EMI from householdelectrical appliances, electrical tools, fluorescentlamps, radio and TV receivers. The radioequipment regulations, article 65, is concernedwith radio frequency equipment including ISM.The measurement equipment and techniques aredefined by the JRTC (Japanese Radio TechnicalCouncil) of the Ministry of Posts andTelecommunications (MPT).

In 1985 the Japanese Electronic IndustryDevelopment Association, the Japan BusinessMachine Makers Association, the ElectronicIndustries Association of Japan and theCommunications Industry Association of Japancame together at the request of the MPT to formthe Voluntary Control Council for Interferenceby Data Processing Machines and ElectronicOffice Machines, known as the VeeI [24]. Thegoal of the VCCI is

'To take voluntary control measures againstelectronic interference from data processingequipment and electronic office machines, andthereby contribute to the development of asocially beneficial and responsible state- of affairsin the realm of electronic data processingequipment inJapan'.

Membership is not limited to Japanese companiesand is open to foreign organisations. Someoverseas manufacturers of electronic equipmenthave found membership of, and compliance with,the VCCI beneficial in competing in the Japanesemarket place. Members must test the product forconformance and submit a report in Japanese toVCCI before placing the equipment on sale.When the report is registered a certificate ofacceptance is awarded to the company for thatproduct.

There are two classes of equipment specified inthe VCCI standards:

Class 1: information technology equipment (ITE)used in industrial and commercial applicationsClass 2: ITE used in residential situations.

The limits applied to the two classes for equipmentmanufactured after December 1989 are equal tothose specified in CISPR 22 [25J. The VCCIlimits for radiated emissions are given in Figure2.11 and those for conducted emissions in Figure2.12. These VCCI limits are similar to the FCCand VDE limits but there are differences infrequency coverage, test methods and thearrangement of the equipment under test. Alimi ted comparison of the test methods set out in


Narrowband conducted emissions

100------------..------..........

the three standards has been made by Hoolihanet al. [24J, see Table A1.6.1 of Appendix 1.6.

10 100 1000FREQUENCY MHz

Figure 2.11 VCC! radiated emission limits

100Narrowband conducted emissions

80 VCCI Class 1 Quasi-peak limits

>::t VCCI Class 1 Mean limits

CD 60'U

40

0.1 1 10 100

FREQUENCY MHz

Communications announced that they intendedto regulate radio frequency electromagnetic noiseemissions from digital electronic devices. All suchdevices manufactured or imported after 31stJanuary 1989 must have been tested and shownto comply with the new regulations [26J andcarry an identifying mark to that effect.

Under these new regulations products aredivided into two groups: class A, industrial andclass B, residential. The class B limits are set at alower level of tolerable emissions than class A andin this respect are the same as the FCC and VDEstandards. As in the USA there are some productswhich contain digital devices which are exemptfrom the regulations. For Canada these include

Transportation vehiclesPublic utility or industrial plantTest equipment in industrial, medical or

commercial environmentsCertain motor driven domestic appliancesSome medical equipment and monitorsSome central office telephone equipmentSystems using radio transmitters and receivers.

The Canadian EMC regulations are referred to bythe publication number SOR/88-475 [27]. Theassociated testing procedures are given in CSAstandard C 108.8-M 1983 [28J. The testingprocedure is similar to that defined by the FCC,but it is not identical [29J. The CanadianDepartment of Communications (DOC) makesthe important stipulation that a product testedand shown to be compliant with the FCCregulations need not be retested. The FCC reportwill be accepted as proof of compliance providedthat a note is attached indicating that the resultsare considered to be satisfactory proof ofcompliance with the Canadian regulations.Compliance with the DOC regulations will beverified by the Canadian authorities if acomplaint of interference caused by the product isreceived, investigated and subsequently confirmed.

The applicable limits for radiated emissions30 MHz-1 GHz and the conducted interferencelimits from 450 kHz-30 MHz are given in TableA1.6.2 of Appendix 1.6.

100

'- VCCI Class 2 Quasi-peak limits"'-. I

, ......-----V-C~C..I Class 2 Mean limits

IRadiated emissions at 3 meters

~ -- VCCI Class 1_

- VCCI Class 2. _

- -- -:~ -

I

0.1

20

80

1 10

FREQUENCY MHz

Figure 2.12 VCC! conducted emission limits

80

40

>::t

CD 60'U

E 60

>::t

CD'U 40

2.5.2 Canadian EMC standards

Canada has legal requirements for EMC coveringvehicle ignition systems, radio frequencyequipment including ISM and TV receiversunder regulation SOR/75-629 1975. Voluntarycompliance to CSAC108.5 standard has beensought for products such as household electricalappliances and fluorescent lamps. In January1987 the Canadian Department of

2.6 Product safety

2.6.1 Safety of electrical devices

Electrical product safety and the safety of humansand ordnance devices when exposed to highpower electromagnetic radiation is not theprimary subject of this book. However, thesetopics are related to EMC, and often both safetyand EMC specifications must be met simultaneously in a single equipment design.


Table 2.4 Industries involving use oj electromagneticenergy

is human. There are diverse opinions with regardto safe levels of exposure. In the West the generallyaccepted criteria for exposure to CW irradiationare based on the thermal effects generated withinthe body by the absorption of RF energy. Theapproach in the former USSR to human exposureseems to be based on the effects of RF energy onthe central nervous system, which are stated tooccur at between one to two orders of magnitudelower power density than for thermal effects [32].

There are little data available in any country onthe effects of intense short pulses or on chronicexposure at low levels. Concerns are beginning tobe expressed about safe levels of low frequencymagnetic fields at power line frequencies [33]. l'hebiological effects of exposure to electromagneticfields are likely to become a significant technical,legal and social issue in the 1990s. In a modernind ustrial society many millions of people mayrou tinely be exposed to RF radiation of some kindduring their occupation. Typical categories ofworkers are listed in Table 2.4 [34]. Table A1.7.2 of

Occasionally, the design that gives the best EMCresul ts might be in contravention of the electricalsafety rules. This can occur, for example, whenconsidering the optimum grounding or earthingpolicy for an equipment where a safety earth canact as an effective radiating antenna.

2.6.2 Product safety

From the manufacturer's point of view there are anumber of legal and commercial advantages to begained by designing and testing equipment tomeet the widest range of electrical and electromagnetic safety regulations which exist world wide. Hecan thereby ensure that his equipment is operableand reliable within the boundaries of recognisedminimum safety standards [30J. Safety agencyperiodic audits of approved products will alsoencourage manufacturers to maintain theirproduct safety quality control during production.

Probably the most important benefit to begained by meeting recognised safety standards isthe almost certainly reduced liability judgementsthat may be made against a manufacturer in theevent of safety litigation. For example, inGermany when equipment carries a separateVDE or TUV [31J laboratory safety mark (seeFigure 2.13), then liability of the manufacturer inthe German courts is minimal. When theapproving agency mark is applied to the product,this places the burden of proof on the end usernot the manufacturer.

Figure 2.13 Product safety mark

In the USA, the Underwriters Laboratories andin Canada the Canadian Standards Association arethe recognised agencies for safety testing ofelectrical products. Typical national and international electrical product safety standards are givenin 1'able A1.7.1 of Appendix 1.7 and EMCengineers should be familiar with these technicalrequirements as design measures which ensureproduct safety will always override those for electromagnetic compatibility if a choice has to be made.

2.6.3 Radiation hazards to humans

A special case of electromagnetic compatibilityexists when the potential receptor or victim system

Sector

Industrial

Medical

Military

Research andscientificapplications

Communications

Application

Food preparation bymIcrowaves

India rubber processingAdhesive processingHeating or fusion of metal and

crystals

DiathermyMagnetic resonance imagingComputer imaging of x-raysUltrasound proceduresH ypothermy-sensi tising

malignant tumours

CommunicationsRadio navigationAviationPreparation of precooked

foodsWeapons guidance and

triggering devicesRadar systemsRadio communications

Food processingPlasticsChemical applicationsMetal stress tests

Radio stationsPortable transmittersSatellite hookups


types. In Germany, the standard for RF ignitionhazards is VDE 0848 (DIN57848 issued by theGerman standards institute). There is also an international standard issued as IEC 79-2, 'Electricalapparatus for explosive gas atmospheres'.

The RF ignition of ordnance (HERO) is asafety-cri tical feature of military equipmentwhich contains electrically triggered ini tia tors(EEDs). They may be used to activate switches,separation fastenings and set off explosives orrocket motors. Accidental ignition is clearly aserious issue bu t if the explosive mixture isheated and not detonated by currents inducedinto the firing fuse in the EED this can renderthe explosive chemicals unresponsive to asubseq uent real firing pulse. This is calledd udding and is also a concern for the safeoperation of the system.

In the UK, the pertinent regulations posted asOrdnance Board (OB) Proc.41273 [47], are nowsuperseded by OB Proc.42413 March 1986. EMCengineers are often called on to design and testEED circuits as part of the wider system and theyshould be familiar with the appropriate regulations.

10·3 1--__--'- L..---'-_........_L----L__----'

0.1 10 100 1GHz 100Hz

FREQUENCY MHz

Figure 2.14 Comparison of electromagnetic waveexposure standards

10-2

1Q3------IUK MoD(1982)

I STANAG 2345 r- '-..-- ..\ HUMAN BODYl:fSARMV"1L-------.., RESONANCE REGION

: INEW(1982~ANSI STANDARDI (WORKERS AND

1()2 US AIR : I POPULATION)

FOR~~~.~.~.~~~.~.~. ~.~11 I US NAVY

E NIOSHT -\ llj--:r;;EvIOUS ANSI STANDARD~ 101 STANDARDS ~\. :' .....tl'J:.::lbln.:r!:._._._i::n:.lI:'l:'l:l'\I.lII'I:Ill'I:__.::nt.

E (WORKERS) "'~ EEC

~ \. ART. 5 /'7~ \. / .... /~ 1 INIRC (1988>" \\..... _ ..... _ ..... _ ..... /' .... -I ,r-~ \ NRPB(1989) / /

ffi " '-- __-0/~ ~~--'0. 10-1 SOVIET WORKER UK NRPB (1986)

STANDARD PUBLIC ACCESS5 HOURS/DAY

Appendix 1.7.7 contains examples of standardsregulating human exposure to RF energy.

A comparison of the limits contained withinsome of these standards is given in Figure 2.14including those proposed by INIRC (1988). Safelevels of exposure were recommended by theNRPB (1986) in a consultative document whichwas revised in 1988 and issued as NRPB-GS 11.The recommended reference levels are gIven InTable A1.7.3 of Appendix 1.7.

2.6.4 Hazards of electromagneticradiation to ordnance

The history ofRF ignition hazards goes back to theFirst World War when experiments showed thatcotton bales could be ignited by RF powerinduced into the restraining metal bands [41]. Thehigh-power RF hazard has long been recognisedin the USA and studied by a variety of agencies[42-45]. Work in the UK by Excell [46] assisted inupdating BS4992 (1974) 'Guide to protectionagainst ignition and detonation initiated by radiofrequency radiation'. BS4992 has now beensuperseded by BS6657 'Prevention of inadvertentinitiation of EEDs (electro explosive devices) byRF radiation', and BS6656 'Flammableatmospheres and RF radiation'. In BS6657 acomprehensive list is given of minimum safedistance for EEDs from transmitters employingvarious powers, polarisations and modulation

2.7 ESD and transients

2.7.1 ESD (electrostatic discharge)

With the conjunction of the spread of digitalinstrumentation, process control equipment andgeneral information technology into officebuildings and industrial plants fitted withsynthetic materials and h umidity controlled airconditioning, a particular type of transient EMIbecame a problem [48]. Digital equipment isinherently sensitive to short high-level widebandwidth voltage transients caused by thedischarge of electrostatic energy which can easilybuild up on people, chairs and tables etc. inenvironments with abundant insulators (plasticsand manmade fibres). These transients can causedigital data disruption, microprocessors to changefunction and even to lock up. ESD and fasttransients are becoming a more important part ofEMC as the need increases to understand andcontrol this phenomenon across a wide range ofequipment.

The measurement procedure defined in BS6667Part 2 (and in IEC80 1 Part 2) recommends testlevels of between 2 and 15 kV using a speciallydesigned spark discharge probe. See Figure 2.15.The inten tion is to simula te the discharge ofstatic electricity built up on an equipmentoperator into sensitive circuits via conductingmetalwork.


Figure 2.15 Typical ESD test Reproduced by permission of' BSI

2.8.1 MIL STD 461/462/463

INTERCONNECTINGCABLE

emIssIon "7I. Conducted

¥ h susceptibilIty

POWER CABLE

for military electronic equipment have beendeveloped around them. Because of this, many ofthe examples of the wide range of EMCmeasurement techniques discussed in this bookare described with reference to these standards.Some of the EMC tests included in thesestandards are not yet required for commercialelectronic equipment. With the inevitable proliferation of equipment in the future, and theconsequent increase in the importance of EMC,adaptations of some of these tests which currentlyapply only to military systems may becomerelevant to commercial equipment.

MIL STD 461/2/3 documents and theirsuccessors are issued by the US Department ofDefence and are now used around the world.MIL STD 461 A entitled 'Electromagnetic interference characteristics requirements forequipment' and first issued in August 1968 aimsto ensure that interference control is consideredand incorporated into the design of equipmentand subsystems. I t also provides a basis forevaluating the electromagnetic compatibility ofthese equipments and subsystems when operatedin a complex electromagnetic environment. Thestandard is concerned with both radiated andconducted emissions and susceptibility of anequipment as illustrated in Figure 2.16.

The associated standards are MIL STD 462,'Measurement of electromagnetic interference characteristics', first issued in July 1967, and MIL STD463, 'Definitions and systems ofunits for electromagnetic interference technology'. These can be appliedto single or multiservice procurements and arecalled up in the equipment specification andcontract. Compliance with the requirements ofthese standards does not guarantee that theequipment will not suffer from any EMC problems,but it should ensure that they are minimisedwithout incurring excessive costs for over protection.

PROTECTIVECONDUCTOR

/

EARTH REFERENCE PLANEPLANE

5 ns ± 30%50ns15 ms

300ms

CLAMPING DEVICE

I ESD GENERATORI PROBE

MAINS OUTLETWITH EARTHTERMINAL ~

2.8 US tnilitary EMC standards

Immunity to fast transients due to switching onmains supplies is also increasingly important withthe widespread introduction of semiconductorsand digital microprocessors into domestic,commercial and industrial equipment. It is onlyrelatively recently that international agreementhas been obtained with regard to suitable testmethods and limits. The methods call fortransients to be induced into the equipment mainsleads directly or via a distributed capacitance.The characteristics of the transients which mustbe injected are given in IEC 801 part 4 and havea risetime of only 5 ns. The regulation specifies

RisetimePulse lengthBurst durationBurst periodRep. frequency(within burst) 5kHz

(for amplitude up to 1 kV)2.5 kHz (above 2 kV)

There are also standards which specify thelimitations of disturbances caused in electricitysupply networks by domestic and otherequipment; BS 5406 (1988) or IEC 555 areexamples. The practising EMC engineer must beaware of these power line transients and ESDspecifications and may be called on to design andtest systems to which they are applicable.

2.7.2 Transients and power linedisturbances

These standards are among the most formal,technically comprehensive and widely used of allEMC standards. Many other tests, particularly

f-'igure 2.16 Unit- or box-level en7ission andsusceptibility testing (MIL SYD 461)

Reprod uced by permission of' ICT Inc.

MIL-STD 461A1st August 1968

NOTICE 1 CORRECTIONS2nd February 1969

NOTICE 2 (AIR FORCE)20th March 1969

NOTICE 3 (AIR FORCE)1st May 1970

NOTICE 4 ( ARMY)9th February 1971

NOTICE 5( MOBILE ELECTRIC POWER)

6th March 1973

NOTICE 6( CORRECTIONS TO NOTICES)

3rd July 1973

Figure 2.17 MIL SYD 461A and revision notices

Reproduced by permission or ICI' Inc.

MIL STD 461A has been the subject of manyrevisions and amendments over the years and thisis illustrated in Figure 2.17. Similarly, revisionshave taken place to the testing methods and limitsin MIL STD 462. See Figure 2.18. Many of theamendments have been concerned with procurements for a single service (army, navy or airforce).

The test req uirements are organised logicallyand can be represented as shown in Figure 2.19 inthe form of a flow chart or tree showing thesuccessive subdivisions into emissions and susceptibility, then conducted and radiated mechanismsfor each, and so on. Not all of these tests need beperformed on all equipment which falls within the

MILSTD46231st July 1967

NOTICE 1 CORRECTIONS1st August 1968

NOTICE 2 ( AIRFORCE )1st May 1970

NOTICE 3 (ARMY)9th February 1971

Figure 2.18 MIL SYD 462 and noticesReproduced by permission or rCT Inc.


general regulations. Equipments are grouped intoclasses and further divided into subclasses. SeeTable A1.8.1 of Appendix 1.8. Only certain testsare carried out within a given class as illustratedin Table A1.8.2 of Appendix 1.8.

In 1980 a major revision to MIL STD 461A wasissued as MIL STD 461B. It was the outcome ofasignificant change in the philosophy whichunderpinned the intentions of the regulations. Allequipment purchased for the US military wasrequired to meet EMC specifications derived fromthe equipment type or class and the intendedinstallation and criticality of the equipment to'mission success'. Thus MIL STD 46IB issectioned into parts 1 to 10 which correspond tothe types of installations in which the equipmentwill be used. See Figure 2.20. The relationshipbetween the equipment classes and subclasses tothe numbered sections or parts of 46IB is shownin Table A1.8.3 of Appendix 1.8.

The inclusion of the impact of installation andmission aspects in addition to the equipmentclasses is a further attempt to ensure enhancedoperational electromagnetic compatibility, butagain at a reasonable cost, by tailoring therequirement for only certain tests as appropriate.In some cases, further tailoring will be necessaryfor specific missions such as navy carrier aircraft[49] which have to operate in close proximity topowerful radar and satcom antennas. In suchsituations field strengths for the RS03 test mayneed to be increased to 200 V 1m or more toensure operational EMC.

There are many differences between MIL STD461A and 461B which are too numerous to bedescribed in detail in this book. However, some ofthe major changes to tests are listed in short formin Table A1.8.4 of Appendix 1.8. Some of thetests introduced into MIL STD 46IB are onlyapplicable to procurements for particular servicesand readers should consult the Standard forfurther detail.

A second upgrade and revision of this series ofmilitary standards arose in 1986 with the introduction of MIL STD 461C. The new standardretains the same format as 46IB and introduces anumber of modifications [50]. The mostimportant addition is that concerned with testsdesigned to ensure survival of militaryequipments against EMP (electromagnetic pulse).The following tests have been added and areapplicable to installations listed in parts 2, 4, 5and 6 of the document:

CSIO: pulse injection onto equipment pinsCS 11: pulse injection onto interconnecting cables

The characteristics of the damped sinusoidto be injected are given in Figure 2.21.


MIL STO 461Specification test requirements

CS 03 30 Hz - 10 GHzIntermodulation

CS 04 30 Hz - 10 GHzSignal rejection

CS 05 30 Hz - 10 GHzCross modulation

CS 07 Squelch circuitCS 0830 Hz -100Hz

(signal rejection)

CE 01 30 Hz - 20 kHzCE 03 20 kHz - 50 MHzCE 05 30 Hz - 50 MHz

RS 03 10 kHz - 10 GHzRS 04 10 kHz - 30 MHz

(parallel plate)

RS 01 30 Hz - 30 kHzRS 02 Magnetic induction

Electric andelectromagnetic

Transmitter: key upTransmitter: key downReceiver: LO leakage

CE 02 30 Hz - 20 kHzCE 04 20 kHz - 50 MHzCE 05 30 Hz - 50 MHz

RE 02 14 kHz - 10 GHzRE 03 10 kHz - 40 GHz

spurious & harmonicRE 05 150 kHz - 1 GHz

vehicles ~ enginesRE0614kHz-1 GHzoverhead power lines

Electric andelectromagnetic

Figure 2.19 MIL STD 461 test specifications Reprod uced by permission or ICT Inc.

Figure 2.20 MIL STD 461Band 462 (notices 4 and 5)

MIL STD 461 B1st April 1980

PART 1 - GENERAL

PART 2 - AIRCRAFT & GSE

MIL STD 46231 st July 1967

NOTICE 1 - CORRECTIONS1st August 1968

NOTICE 2 - ( AIR FORCE)1st May 1970

PART 3 - SPACECRAFT & GSE NOTICE 3 - ( ARMY)9th February 1971

Reprod uced by permission or ICT Inc.

PART 4 - GROUND FACILITIES

PART 5 - SURFACE SHIPS

PART 6 - SUBMARINES

PART 7 - NON-CRITICAL GROUND AREA

PART 8 - TACTICAL & SPACE VEHICLES

PART 9 - ELECTRIC POWER

PART 10 - COMMERCIAL

NOTICE 4 - INTERIM NEMP(CS 09 TEST) 1sf April 1980 (Navy)

NOTICE 5 - INTERIM NEMP(CS 10 & RS 05) 5th January 1983 (Navy)

Figure 2.22 US MIL Syn 461C NEMPsusceptibility

0.01 ""--_-"'-__...&.-_--'-__....

0.01 0.1 100

FREQUENCY MHz

Figure 2.21 Limits for CS10 and CS11 NEMPcurrent injection


2.8.3 Other US military standards

A list of relevant EMC related US standards canbe found in Appendix 1.9.

2.9 UK tnilitary standards

electrical and electromagnetic interferencephenomena. The aim of the standard is to ensurea compatible total system under operationalcondi tions and therefore is concerned with

The system electromagnetic environluentLightning protectionStatic electricityGrounding and bonding.

It suggests guidelines for generating procedures todefine system compatibility demonstration tests byusing the normal system performance indicatorsto reveal EMC failures. The system is testedunder real operational load conditions and interference is injected into it at critical points and ata level which is 6 dB (or greater) than the worstcase levels created by the system or experienced inthe environment. The aim is to prove that amargin of compatibility exists over and above thelevel required for fault-free operation during asimulated mission.

The imposition of MIL-E-6051 D on a system isnot contingent on the individual equipmentswithin it having been designed or tested to MILSTD 461/2/3. I t requires a contractor to preparedetailed procedures and test plans to demonstratecompatibility at system level. Clearly, such acontractor could make considerable use of theequipment test results from MIL STD 461/2/3 inunderstanding the electromagnetic characteristicsof these equipments. One may initially choose tomodel EMC at system level using computer codessuch as IEMCAP [53J to predict ranked systemlevel incompatibilities (starting with safety criticalcircuits [54J) and thereby to better define a costeffective detailed test plan for satisfying MIL-E6051D.

Without such computation and subsystem testwork, meeting this requirement to demonstrate afinal check on complex system level compatibilityunder operational conditions (simulated) cart belengthy and expensive in its preparation andexecution.

I mtlXnnnII n1\ A _

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IIIIIII II I

--~-----~--------------------I I

~ 10E

100

II

Q) 1j5

8

II 0.1c::'0..

RS05: pulsed E-field (only for mission-criticalequipments located outside of an intentionally hardened area). The E-field pulseshape to be used is shown in Figure 2.22.

A summary of the test identifiers, short titles,applicability to parts of 461 C and detailed notesis given in the form of a requirements tree inFigure 2.23 [51].

The process of continuing to modify and updatethis very important set of military standards iscontinuing. A triservice committee has been setup to further revise MIL STD 461/462/463 withone of the aims being to eliminate the controversial area of defining and measuring narrow (CW)and broadband (impulse) signals [51 J, as referredto in Appendix 1.1.

50

45

40

35a:~ 30LU:E 25-~ 20

15

10

5

0-----1---""'-----------------'---1-t r-I-td-i------tf-------i~5 X 10-9 550 x 10-9

~ 38 X 10-9 LIMIT FOR RS 05

2.8.2 MIL-E-6051D2.9.1 Service and establishment-specificstandards

This standard is entitled 'Systems electromagneticcompatibility requirements'. It is applicable to allitems of equipment within a definable system, theperformance of which may be influenced by

Military interest in EMC has been growing significantly in the UK since the mid 1960s following theintroduction of compact, complex, semiconductorbased equipment into platforms for all three

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Figure 2.24 RRE 6405 conducted emission limits

FREQUENCY MHzLIMITS FOR HIGH FREQUENCY NARROWBAND CONDUCTED VOLTAGE

N

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RCE04

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all three services. This was achieved by the use ofdifferent limits in relation to land, shipboard andair service equipments. See Table A1.8.7 ofAppendix 1.8. For example, the limits for RCE03where a line impedance stabilisation network(LISN) is used in the measurement of RFconducted voltages show the progressive relaxationof the limits from grade L. See Figure 2.24.

Many aspects of this comprehensive and cleardocument have been carried over to DEF STAN59--41 which is the current central UK militaryE~1C standard. It too covers equipment for land,shipboard and air use with various limits basedon equipment classes.

2.9.2 Project-specific standards

With the aim of economically achieving the bestpossible EMC performance, many large militaryprojects have generated comprehensive EMC specifications specifically for that project. They makereference to the general EMC standards whichexist but tailor the limits and tests to help ensuregood E~IC performance of the system with respectto its particular operational mission andenvironment. The generation of project specificEMC requirements can also be of value inharmonising the views of sections of industry whichcommonly operate with national EMC defence

services. Early specifications such as FVRDE 2051section 4 [55J, which was concerned with radiointerference from military vehicles, have beenprogressively superseded by more modernstandards MVEE 595 and specific amendments[56J in the same way as the US MIL STD 461/2/3. Where possible these standards now form partof the central (nonproject specific) UK standardDEF STAN 59-41.

Each service tends to generate standards whichare designed to produce cost-effective EMC forparticular types of platform, weapons systems andequipment which must function together in theparticular EM environment in which that serviceoperates. For example, MVEE 595 dividesmilitary vehicles into two classes: FFR, vehiclesfitted for radio, and nonFFR, vehicles withoutradio. In this way, the interference limits can bemore relaxed for nonFFR vehicles so thatacceptable electromagnetic compatibility can beachieved economically without compromising theperformance of a particular system.

Where appropriate, military standards refer toBritish Standards such as BS 727 (RFI measuringequipment) and BS833 (vehicle ignition) forexample.

The navy has produced a set of standards forequipment on board ships at sea which takes intoaccount issues such as screening offered by metalhulls below deck and the need to use electrochemically compatible metals for equipment boxes,access doors and RFI gaskets where these areexposed to a corrosive salt atmosphere.

Examples of navy standards are

NWS 3 (amendments 1-4) 1981: Electromagneticcompatibility of naval electrical equipmentsNWS 1000 part 1 Chap. 5 1986. 'Electromagneticcompatibility-design guide'NES 1006 1988 [5 7J : 'RF environment andacceptance criteria for naval stores containingEEDs' (supersedes NWS 6)

In 1974 the Royal Radar Establishment produceda comprehensive EMC standard RRE 6405 [58Jto cover the EMC performance of RRE (laterRSRE, now Defence Research Agency) controlledprojects. I t comprised three parts:

1: GuideTerminology and definitionsControl and test plansRelated documents

2: Requirements/limits3: Measurement techniques

This standard specified equipment classes (TableA1.8.5 of Appendix 1.8) and contained a comprehensive set of limits and test methods (TableA1.8.6 of Appendix 1.8) which could be applied to


standards, when they form multinational collaborative ventures for large aerospace or defence projects.

Examples of project-specific specifications are

SPP-90003 Tornado IDSSPP-90203 Tornado ADVEA98 QO 10J EH 101 helicopterSPE-J-000-E 1000 European fighter aircraft

National project-specific EMC requirements arealso produced, e.g.:

rrS1527TS 1727

Ptarmigan} communications systemsWavel

standard also does not cover immunity orprotection from lightning strikes and is notconcerned with spurious signal outputs, spectralpurity, IF rejection or intermodulation productsin connection with communications or othersensitive receiver equipment.

Part 2 of the standard sets out the requirementsfor management and planning procedures, whichare mandatory in order to ensure that potentialinterference problems are properly addressed asearly as possible in the life of a project. Specificactions are required:

2.9.3 DEF STAN 59--41 (1988)

This standard was first issued in March 1971 andwas revised as Issue 2 in August 1979. Until Issue3 in 1986, individual establishment EMC specifications were invoked for land, shipboard and airapplications. Issue 3 of DEF STAN 59-41 wasintended to stand alone and contained many testmethods and limits which were differentiated byequipment class, service use and grade. The June1988 issue of DEF STAN 59--41 is in five parts:

1: General requirements2: EMC management and planning procedures3: Technical requirements, test methods and

limits4: Open site testing5: Technical requirements for special EMC test

equipment

The standard outIines typical electromagneticenvironmental requirements for equipment usedin the services but the limits may be tailored tomeet the needs of specific projects. I t covers allequipments for military use that may give rise toor may suffer from electromagnetic interference.I t is invoked, wherever practicable, in all designs,contracts etc. for equipment for military use.

Specific requirements for projects may begenerated based on DEF STAN 59-41 byconsidering

Selection of appropriate types of testModification of the scope of certain tests as

appropriateModification of test limits to match the environ

mental requirements of the projectConsideration of transients or other infrequent

emISSIons.

The standard does not directly cover issuesconnected with explosive, nuclear effects (otherthan EMP), hazards to personnel or ignition offuels by electromagnetic energy. Suitablereferences to other documents are made. rrhe

An EMC co-ordinator should be appointed forthe project.An EMC working group should be formed fora complex project.A suitable control plan must be developed.EMC technical policies for screening, filtering,grounding and bonding should be established.Appropriate test plans must be prepared.Suitable, test facilities should be available.Configuration control must establish the buildstandard of all items for test.

Part 3 of the standard defines the equipment typesthat are subject to certain tests, see Table 2.5.

Table 2.5 DEF STAN 59-41 equipment types

Type 1 All equipments fitted with electroniccomponents

Type 2 Motors, genera tors and electromechanical selectors (wi thout electroniccontrol)

Type 3 Relays, solenoids and transformers

The tests applicable to each equipment type areindicated in Table 2.6. The application of DEFSTAN 59-41 tests are further tailored byintroducing the grading of limits in relation toservice environment. See Table 2.7.

Table 2.6 Equipment type and tests

Tests Type 1 Type 2 Type 3

All tests Y N NAll emission and N y N*imported spike testsImported and exported N N Yspike tests only

*DRE02 test at power frequency is required if EDT isfed with AC power.

A list of DEF STAN 59-41 test methods and theirapplicability to a service is given in Table Al.8.8of Appendix 1.8.


INPUT / OUTPUTTEST GEAR

150 MHz,20

10 MHz 150 MHz

100 MHz, 40 CLASS 0

100 MHz, 25 CLASS C

100 MHz, 10 CLASS B

~~~~-4 CLASS A & E

FILTERING

................

................ ,

III

EUT II

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100 Hz 1kHz

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FREQUENCY

-20'--_..a.-._--'-__L...-_....I-_--I.__....I

100 Hz 1kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz

LIMITS FOR SHIP USE

FREQUENCY

-20'--_--'-_---'__-.1-__.1.--_.........__.......

100 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz

LIMITS FOR LAND SERVICE USE

ZQ_130(J)<S;.~:::i.110::2:_WfJ) 900"0

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}-'igure 2.27 DE}' S TA,N 59-41 (Part 3) /2 ( DGYE02)

Reprod ueed by permission or H MSO

}'igure 2.26 Test lnethod DGYE02: signal/control linecurrent 20 Hz-l00 MHzDEI? STAN 59-41 (Part 3) /2

GROUND PLANES

MAIN SCREENED ROOMcomplete with RF absorber

TEST ROOM AREAcleared of any unnecessaryequipment and personnel

FEEDTHROUGH CONNECTORS ORFEEDTHROUGH FILTERS

FEEDTHROUGHCONNECTOR

EUT MONITORINGEQUIPMENTOR TEST SET

}zgure 2.25 Suggested layout .for screened room complexDEF STAN 59-41 (Part 3) /2

EMC TEST EQUIPMENT( sig. gen., power amps.,receiver, etc.)

DEF STAN 59-41 has many test method andequipment layout features in common with MILSTD 462. It could be considered somewhat inadvance of the current US MIL SPEC in respectof its employment of wideband bulk currentinjection tests for conducted susceptibility DCS02,DCS04 & DCS05.

Use Grade Detail

Air One grade All test methodsShip Above decks

Below decksLand Class A Equipment within 2 m of

comms. antennaClass B 2-15 m of comms. antennaClass C 15-100 m of comms.

antennaClass D > 100 m from comms.

antennaClass E Shielded equipmentClass F Civil standards are

adequate

The tests specified in DEF STAN 59-41 arecomprehensive, but great care has been taken tomake them as straightforward as possible toimplement in an effort to eliminate some of theinconsistency that may be associated with EMCmeasurements. T ~sts are to be carried out in largeshielded chambers equipped with radio absorbentmaterial (RAM) if possible. See Figure 2.25.

Table 2.7 Grading oj limits

Reproduced by permission or HMSO Reproduced by permission or HMSO


The adoption of fixed receiver bandwidths foremission measurements means that the problemsnormally associated with the interpretation ofbroadband and narrowband signals have beeneliminated. A future revision of MIL STD 461/2/3 may also adopt this approach.

An example of the test layout for DCE02 can beseen in Figure 2.26 with the associa ted limits forconducted emissions appropriate to air, shipboardand land use in Figure 2.27.

A list of examples of some widely used UK EMCcivil and military standards can be found inAppendix 1.9.

2.10 Following chapters

This book has been prepared with the aim ofguiding the reader from a simple introduction toEMC in Chapter 1, through issues relating toregulations and standards in Chapter 2, to adetailed consideration of the sensors, antennas andmeasuring apparatus used in EMC testing whichare discussed in subsequent chapters. In this way,readers with some familiarity of the basics of EMCcan progress to the main body of information onEMC testing in the most direct way.

An introduction to design techniques for EMChas been provided in Chapter 12 to complement'the emphasis on EMC testing. Much has beenwritten in detail elsewhere on EMC design andthe reader is referred to appropriate references inthe text of Chapter 12. Readers new to EMC andwho wish to cover the basic elements of designbefore continuing with the details of EMC testingin Chapter 3, may read Chapter 12 now and thenreturn to Chapter 3.

2.11 References

MERTEL, H.K.: 'International and European RFIregulations'. EMACO Inc. 7562 Trade St., SanDiego, CA 92121, USA, also section 10/2 IEEEconference on EMC) 1982

2 SHO\VERS, R.M.: 'Wide scope of IEC work onelectromagnetic compatibility problems', Trans.South Afr. Inst. Electr. Eng.) Jan. 1982

3 DEF STAN 59-41 (parts 1-4). Ministry of DefenceDirectorate of Standardisation, Glasgow, UK

4 KEISER, B.: 'Principles of electromagnetic compatibility' (Artech House, 1987, 3rd edn.)

5 NAMAS Executive, National Physical Laboratory,Teddington, Middlesex, TWll 01 W, UK

6 GERKE, D.: 'A fundamental review of EMIregulations', R.p'. Design, April 1989, pp. 57-62

7 MIL STD 1541 'Electromagnetic compatibilityrequirements for space systems'. US Department ofthe Air Force, Washington DC, October 1973

8 MIL STD 461 C 'Electromagnetic emISSIon andsusceptibility requirements for the control of electromagnetic interference'. US Department of Defense,Washington DC, August 1986

9 VDE 0871 Equivalent to EN55011, CISPRll,BS4809. Similar to FCC Title 47 pt. 18. Obtainablein UK from BSI, Milton Keynes, MK14 6LE

10 FCC Title 47 parts 15,18 and 68 of the US code ofFederal Regulations, Federal CommunicationsCommission, Washington DC

11 VDE 0875 'Regulation for the radio interferencesuppression of electrical appliances and systems'.English translation available from BSI, MiltonKeynes, MK 14 6LE

12 WHITEHOUSE, A.C.D.: 'EMC Regulationswithin Europe', Electron. Commun. Eng. J.) March/April 1989, pp. 57-60

13 Radiocommunications Division of the Departmentof Trade and Industry, Waterloo Bridge House,Waterloo Road, London SE1 8UA, UK

14 89/336/EMC 'EC council directive on the approximation of the laws of the member states relating toelectromagnetic compatibility', OJ!. J. Eur.Communities) L 139/19, 23.5.89

15 'Electromagnetic compatibility product standards,UK Regulations' Department of Trade andIndustry and Central Office of Information, INDYJ1896NE.40M, April 1993

16 'EMC directive, technical report part 2', NewElectronics) May 1993

17 'EMC and the European directive', one day seminarpapers. Assessment Services and Hewlett Packard,Nov. 1991 (Assessment Services, Tichfield, Hants,UK, Hewlett Packard, Stoke Gifford, Bristol, UK)

18 LOHBECK, D.: 'West German RFI laws andregulations'. Interference Technology EngineersMaster 1988, pp. 302-342

19 'Uriders tanding the FCC regulations concerningcomputing devices'. OST Bulletin 62, FCC Officeof Science & Technology, Washington DC, May1984

20 Standard J -551 C, 'Measurement of electromagneticradiation from motor vehicles (20-1000 MHz)'.Society of Automotive Engineers, Warrendale, PA,USA

21 Practice APR-93 7, 'Jet engine electromagneticinterference test requirements and test methods'.Society of Automotive Engineers, Warrendale, PA,USA

22 Report AIR-1147, 'EMI on aircraft from jet enginecharging'. Society of Automotive Engineers,Warrendale, PA, USA

23 ANSI C63: 1980 'Specifications for electromagneticinterference and field strength instrumentation,10kHz-10GHz'. ANSI, 1430 Broadway, NewYork, NY 10018, USA

24 HOOLIHAN, D.D., JOHNSON, J.W. andWEBBER, C.L.: 'Compliance with JapaneseStandards creates opportunities'. InterferenceTechnology Engineers Master 1988, pp. 291-344

25 'Limits and methods of measurement of radio interference characteristics of information technologyequipment'. CISPR publication 22, 1985, first edn.

26 'New radio interference regulations amendment 28/9/89'. Canada Gazette part 2, vol. 122 no. 20

27 'Radio interference regulations amendment'. SOR/88-475 Canadian government publishing centre,Department of Supply and Services, Ottawa,Ontario KIA 059, Canada

28 C108.8-M1983, Canadian Standards Association,178 Rexdale Boulevard, Rexdale, Ontario M9WlR3, Canada

29 MA.Y, C.R.: 'EMI and amended Canadian radiointerference regulations'. Interference TechnologyEngineers Master 1988, pp. 84-90

30. LESCHAK, D.: 'Safety and regulatory compliance'.Interference 1'echnology Engineers Master 1989,pp. 325-354

31 TUV, TUV Rheinland of North Anlerica Inc., 108Mill Plain Road, Danbury, CT 06811, USA

32 DUFF, W.G.: 'Fundamentals of electromagneticcompatibility'. Interference Control TechnologiesInc., Gainsville, Virginia, USA

33 SMITH, C.W. and BEST, S.: 'Health hazards inelectrical environment (Dent, London)

34 CIPOLLONE, E., BEVACQUA, F. and AMICI, S.:'Electromagnetic field measurements for radiationhazards evaluation'. Interference TechnologyEngineers Master 1989, pp. 190-288

35 ANSI standard C95.1, 'Safety levels with respect tohuman exposure to radiofrequency electromagneticfields, 300 kHz-100 GHz'. American NationalStandards Institute, 1982, New York, USA

36 'Advice on the protection of workers and membersof the public from the possible hazards of electricand magnetic fields with freq uencies below300 GHz'. National Radiological Protection Board,(IIMSO, 1986)

37 'Guidance as to restrictions on exposures to timevarying electromagnetic fields and the 1988 recommendations of the international nonionisingradiation committee', NRPB-GSII (HMSO, 1988)

38 DEF STAN 05-74/1: 1989 'Guide to the practicalsafety aspects of the use of RF energy'.Directorate of Standardisation, 65 Brown St.,Glasgow G2 8EX

39 INIRIC 'Interim guidelines on the limits ofexposure to radiofrequency electromagnetic fields inthe frequency range from 100 kHz to 300 G Hz' ,Health Phys.) 1984, 46, pp.975-984. Updated inHealth Phys.) 1988, 54, p. 115

40 EEC articles 4 and 541 LE ROY, G.A.: 'Sur les incendies provoques par les

ondes hertziennes', Comptes Rendus 1991, 168,pp. 244-247

42 'Control of high-frequency radar equipment'.


National Fire Codes vol. 10 Transportation,Boston. National Fire Protection Association, 1972

43 'Electromagnetic radiation hazards'. GroundElectronics Engineering Installation Agencystandard T.0.31z-10-4, 1970

44 'Naval shore electronics criteria: EMC & EMradiation hazards'. Naval ElectronicCommand, doc. Navelex 0101, 106, 1971

45 Hazards of EM radiation to fuel (HERF). N avseaOP3565/Navair 16-1-529, vol. 1, Chap. 7

46 EXCELL, P.S.: 'Radio frequency ignition hazards',Hazard Prevention, May/June 1984

47 OB procs. 41273 and 42413. Ordnance Board,Empress State Building, Lillie Road, London SW6ITR

48 'The achievement of electromagnetic compatibility'.ERA report 90-0106, Feb. 1990, ERA Technology,Cleeve Rd., Leatherhead, Surrey, UK

49 SIEFKER, R.G.: 'Tailoring MIL STD 461B fornaval avionics applications'. Proceedings of IEEEconference on EMC, pp. 387-395

50 SIKORA, P.A.: 'A comparison of MIL STD 461Cto the previously issued MIL STD 461B'.Interference Technology Engineers Master 1988,pp. 72-111

51 Electro-Metrics Company, 100 Church St.,Amsterdam, New York 12010, USA

52 GOLDBLUM, R.D.: MIL STD 461/2/3 revisionprocess. Interference Technology Engineers Master,1990, pp. 208-357

53 'Intra-system electromagnetic compatibility analysisprogram (IEMCAP)'. Rome Air I)evelopmentCenter, USAF

54 LEE. G. and ELLERSICK, S.D.: 'Methodology fordetermination of circuit safety margins for MIL-E6051 EMC system test'. Proceedings of IEEEsymposium on EMC) 1985, pp. 502-510

55 'Radio interference from vehicles'. Fighting VehicleResearch and Development Establishment,FVRDE 2051 section 4. (FVRDE is now MilitaryVehicles and Engineering Establishment,Chobham, Surrey)

56 'Electromagnetic interference and susceptibilityrequirements for electrical equipments and systemsin military vehicles'. MVEE 595, 1975

57 'Radio frequency environment and acceptancecriteria for naval stores containing EEDs'. NES1006, 1988, Procurement Executive, MoD, DeputyController Warships, Foxhill, Bath, UK (restricteddocument)

58 'Requirements for electromagnetic compatibility ofelectronic equipments'. RRE 6405, 1974, RoyalRadar Establishment, DRA, Malvern, Worcs., UK

Chapter 3

Outline of EMC testing

3.1 Types of EMC testing

3.1.1 Development testing

A key element in cost-effective EMC design andcertification of a civil or military equipment orproduct is careful programme planning. TheEMC aspects of the project must be considered atall stages and may require some design and costcompromises to be made. The particular solutionsarrived at will of course be determined by specificproject conditions, but it is vital that the existenceand timing of decisions involving electromagneticcompatibili ty are known and as far as possibleplanned for.

To this end, it is advantageous to be able tosubmit key components, circuits, boards, cableforms and hardware such as cases or racks, toquick and simple tests during the design process.In this way it is often possible to differentiatemore easily between the EMC attributes ofcompeting designs than by calculation orcomputer modelling (which may be inappropriatefor small projects).

Simple tests carried out on the bench, withperhaps a signal generator, oscilloscope andspectrum analyser, can help to build a sounddesign based on a number of small tests devised toresolve EMC queries as they arise. This activityalso allows the designer or EMC engineer to buildup a broad picture of the 'electromagneticlandscape' of the system being developed. Thiswill help to put into context particular EMCissues which rnay arise during the design task.

For example, it will be possible to balance theeffort and programme funding devoted to casescreening, compartment filtering and screenedcable design, to ensure that no one feature is overdesigned and too costly for the EMC performancewhich will be achieved as a whole. These simplebench tests can be carried out in minutes ratherthan hours and many will not require the use of ascreened room, although setting up such tests onan RF ground plane in an electrically quietcorner of the laboratory can be an advantage.

Measurements are made at close range withinexpensive simple E- and H-field sensors, smallcurrent probes or high-impedance RF voltageprobes. They need not be accurately calibrated totest house standards but it should be possible to

38

relate the measurements made to field, current orvoltage standards to within a few dB. In manycases it is sufficient to make comparisons betweentwo alternative items for test and check theamplitude and spectral characteristics of emissionsor immunity to resolve a design question.

I t is an advantage for the electrical andmechanical designers of the product to beinvolved with these simple bench tests, perhapswith assistance from an EMC engineer, as theygain first-hand knowledge of the reasons whyEMC issues must be taken into account andsometimes take precedence over straightforwarddesign preferences.

On large projects where there may be an EMCcontrol board and specialist EMC design engineersin both the prime contractor's organisations and inmany of the subcontractor firms, these simple testsshould be carried out as a routine matter, havingbeen specified in the EMC control plans which callfor tests at all the various levels of development.These control plans may require exploratory teststo be carried out that are neither simple nor quickto perform. For example, it may be that sophisticated measurements need to be made to gatherinformation about the RF induced currents on anexisting system or installation, as a guide to theEMC design features of a new type.

Whatever the scope and scale of thesepreliminary tests, they will help to build up apicture of the electromagnetic behaviour of theequipment and give an insight into the optimumdesign solu tions.

3.1.2 Measurement to verify modellingresults

On large projects, or in cases where it is inappropriate or prohibitively expensive to test acritical EMC design feature, it may be necessaryto model the situation in a computer to gainenough information about options and possiblesolutions to 'enable the design to progress. Themodels used may be relatively simple andinexpensive to acquire and run. The results andlimitations of simple models are usually wellunderstood and data are used only as a guide.This is the case for exampIc when calculating thecapacitative crosstalk between parallel wires [1, 2Jor the transfer impedance of shielded cables [3].

In some situations, however, large models (suchas numerical electromagnetic code, NEC [4])may be complex and the limitations on thevalidity of the results may be difficult todetermine. In these cases it is necessary toconstruct careful experiments based on specificand simple situations where the results can becalculated analytically and the measured,calculated and modelled results can be comparedand any modelling errors estimated. In this way itis possible to gain confidence that the complexcomputer model being used is well behaved andits predictions are meaningful.

An example of such a situation is where a systemin a complex metal case is being tested in an RFanechoic chamber and it is required to assess thesurface currents induced in the casing by planewave free-space illumination [5]. In such asituation it is possible to create a patch model ofthe system casing for input to NEC and model itsbehaviour. One might also model a simple shortfa t dipole antenna of similar proportions to themetal case, calculate the induced current patternusing an analytical approach [6J and makecareful measurements in a high performance RFanechoic chamber.

The aim is to assess the agreement between theanalytical solution and the measurements for thesimple case and then to compare these with thecomputer model prediction and estimate the sizeand nature of any model errors. The model maythen be run with the adopted patch configurationof the system casing with increased confidence.

A great deal of reliance can be placed oncomputer models that are used to support theEMC design of large projects and it is vital thatthe predictions are questioned and tested wheneverpossible by comparison with measurement.

3.1.3 Preconformance test measurements

It is best practice to build on the developmenttesting which takes place during a project, bychecking the EMC performance of completesubsystems or prototype equipments against thespecification by testing them in a semiformal wayusing the test methods associated with thespecified standards. This practice requires accessto an appropriate test house or suitable companyfacility. Consequently, the test can sometimes beexpensive and is reserved for confirming EMCdesign progress at key stages of developmentwhere screening, grounding, filtering, cable andstructure policies are to be fixed.

Preconformance test checks need not attemptto cover all the specified tests listed in thestandard. They may concentrate only onemiSSions or immunity or a wide range of tests

OUTLINE OF EMC TESrrING 39

may be carried out for example on safety orperformance critical items.

If these tests are performed by an external testhouse then it is vi tally important to select alaboratory which is not only competent to carryout the testing using the specified methods, butone which can also analyse the resulting data andinterpret them to give clear guidance to thecustomer for improving the design should this berequired. These semiformal tests are best carriedout in the company's own facilities if they areavailable, as there is no substitute for hands-onexperience by designers and EMC engineers inassessing equipment performance against thespecified standards which the final product mustmeet.

Simple development and preconformance testfacilities can be constructed for a few tens ofthousands of pounds. Well appointed EMC testfacili ties wi th large semianechoic chambers cancost around a million pounds to build but theycan usually be hired for a few thousand poundsper day.

Both large and medium-sized projects make useof development models which are dedicated EMCtest beds [7J on which a variety of assessmentscan be made during the design phase and a goodreference database can be constructed. TheseEMC physical models can then be subject to thesemiformal tests towards the end of developmentwith increased confidence that the preproductionand production items submitted for test will meetthe EMC requirements.

3.1.4 Conformance testing

The final stage of testing formally demonstrateswhether the equipment or system will meet theEMC limits set out in the standard against whichit was designed. The contractor should haveconfidence that the equipment will meet theselimi ts, as preconformance tests on criticalsubsystems and iterative design testing will havecontributed to a full EMC database on theequipment in line with the EMC control plan forthe project. Conformance testing should be forconfirmation of compliance; there should be nosurprises at this late stage.

The exact circumstances surrounding conformance testing will depend on a number of factors:

(i) The constraints on the conformance teststation imposed by the appropriate nationallaw governing the registration, sale andoperation of the equipment being submittedfor test. In the past, some countries havespecified the use of central national testlaboratories for certain conformance tests.

40 A HANDBOOK FOR EMC TESTING AND MEASUREMENT'

Functional and physical description of the itemto be tested detailing its purpose, method ofoperation, modes of operation, interfaces toexternal objectsPerformance and modes of operation should beidentified as

criticalmission or performance criticalreversionary or backup modesself test/check/calibration modes

Statement of the objectives of the conformancetestStatement of the standard or specificationagainst which the item shall be tested and alist of tests to be performedStatement of any tests not to be performed, orlimitations of tests to be performedList of applicable documents with regard to

the EUT, e.g. operating manuals etc.reference to addi tional information in thecontrol plan

rrime schedule for the tests with daily test goals1-'est layouts including EUT position,orientation, details of cable layouts andbonding/grounding arrangementsCriteria for determining the locations of EUTmonitoring points and methods of monitoring(immunity tests)Applicable limits of performance degradation(for immunity tests)Description, methods of connection, positionand operation of any associated testequipment needed to function the EUI' In a

customer approved) test plan. 1'he plan will havebeen called for by the project EMC control plan.The size, content and style of the test plan willvary depending on the type of conformance test.For example test plans for equipment being testedto MIL STD 461 must be prepared in accordancewith DI-EMCS-80201 [8J and it is difficult togeneralise with regard to the contents of possibleconformance test plans.

They can be written by the equipment designauthority contractor or can be subcontracted toan EMC consultant, who as an independent thirdparty, will liaise with the test house on the contractor's behalf. Or test plans can be written by theselected test house as part of the testing contract.In any event, the plan will contain informationfrom both the equipment contractor and the testhouse in order that planned, accurate andtrouble-free testing can be conducted quickly forthe rninimum cost.

Test plans should include as a minimum thefollowing information to be supplied by theequipment design authorityj.contractor or the testhouse:

Whatever the equipment, specification, testmethod or test it is important to work

to a preprepared where necessary,

3.1.5 Conformance test plan

The conditions con tained in the contract tosupply the equipment may have a sectiondevoted to the EMC. Conditions withregard to conformance or acceptance testsmay be laid down in such a section. I t ispossible that the procuring authority willspecify the conformance test house to beused. I'his can sometimes be his own facility.The type of conformance testing undertakenwill depend on the specification to be met,e.g. system level or equipment level, and onthe size and complexity of the equipmentunder test. Not all certified test houses willbe able to cover every specification for bothcivil and military use and may not be ableto accomrnodate large, distributed orcomplex E-UT (equipment under test). Ingeneral, test houses have facilities that areei ther directed to testing civil/commercialequipment or to military equipment.

with the rapid expansion of the need foras a result of harmonisedEC

regulations applied to almost all electronicmilitary and aerospace test facilities have

more active in the commercial field. Evenfor tests on military systems, few industrial"r>.'n-lr'.':lYllP." would have all the facilities to carry outacceptance of for example a new aircraft

without the support of the procurement orauthorities to provide a flight test range

f>r'\p.r·--:\t-l,n.Yl<:l1 environment for mission electroYrl'lcrr\""r'lI' environment simulation.

to the specialisations and limitationswhich most test houses will have, the selection ofa will depend on the civil or

nature the product and its size. Testfacilities m.ay only be able to support limited'-'.....,"'~~,'J-'--'-'-'IJ of for conformance testing:

Commercial equipment (small < 1 m cube)Commercial equipment (large or distributed)Commercial equipment (as installed)Military equipment (small < 1 m cube)Military equipment or distributed)Military

safe serviceworthiness)

acceptance trials)

1'he technical details of EMC test equipment andconformance rnethods applicable to these

1-L>r.. r."V",.o.0 of are discussed in detail in later

representative manner to include all E UTstimuli, instrumentation, electrical services,cooling and hydraulic suppliesElectromagnetic and physical req uirements fora conformance test site (open range orchamber testing) with special reference toEUT size, weight, disposition and specialrequirements (e.g. Dynamometer/rolling roadfor vehicle testing).

In particular, the selected test house mustcontribute statements with regard to

Any modifications or limitations applied to thetest requirements set out by the EUT manufacturer/contractor for the test to be carried outThe necessary approvals from a nationalquality organisation to carry out the specifiedconformance tests1'he state of calibration of all equipment to beused (including screened rooms) and thesource of such calibrationTest equipment, including sensors, receiversand recorders (type and serial numbers) to beused in the tes tTest methods to be employed and relevantproceduresStanding orders and safety proceduresExperience of testing staff who carry out theworkAn agreed test programme planThe nature and content of test reports (e.g. forreports on MIL STD 461 tests these should bein conformance with MIL STD 831)Delivery date for the test report, andAn estimate for EUT setup and stripdowntimes and consequent facility occupation(usually charged at a lower daily rate than fora test day).

Test houses are usually booked well in advanceand it is prudent to fix a date and duration forthe testing and then stick to it. Setting the testdate can be difficult as the manufacturer willhave to balance a desire to set an early date to getthe product to the market quickly, against thepossibility of delays through development overruns which could cause him to miss his slot andperhaps not get another for some months. If slotsare missed and the selected test house is full,manufacturers may have to change test house andmodify the test plan.

The manufacturer has to make the test datejudgment in the light of dates available atsuitable test houses. If the equipment is large andcomplex and is to be thoroughly tested to aninvolved specification (usually for a militaryproject), there may be very few test facilitieswhich can accommodate the equipment and there

OU1'LINE OF EMC TESrfING 41

will be limited scope in setting the test date.This explains why many of the larger and more

established test houses are within militaryequipment or aerospace manufacturers, as theyneed the assurance of being able to test accordingto the dictates of their project programmes ratherthan by the availability of external test facilities.

As EMC testing becomes routine in thecommercial sector, larger electronics corporationsare setting up similar extensive in-house testfacilities. The number of test facilities which cancarry out testing of commercial electronicequipment to the European EN standards or theUS FCC regulations is increasing all the time, butthere is predicted to b~ a shortfall in the mid1990s in the UK when the full impact of EECharmonisation Directive EEC/89/336 is felt acrossthe whole of the electronics industry.

If the manufacturer can justify the return oninvesting in an in-house EMC test capability thenmost regulations and specifications permit selfcertification of an equipment. A small in-housefacility may start with development testing andgrow to meet the company's developingrequirement for EMC conformance testing.

3.2 Repeatability in EMC testing

3.2.1 Need for repeatability and accuracy

The basis of the scientific method is founded on thetheoretical postulation and experimental verification of propositions. Thus the need forexperiments and measurements to be conductedin a well ordered, accurate and repeatablemanner is fundamental if hypotheses are to beconfirmed or denied. These experiments must beable to be repeated by others, and only when thishas been done is a consensus established withregard to the validity of the theorem or notion.

In the case of practical engineering, the need forcareful test rnethods which can be carried out toyield accurate and repeatable results is just asimportant as in basic science. Indeed, in thepractical world of near market design,development and sale of electronic productswhich are covered by national and internationallaws (including those relating to EMC andelectrical safety), it is vital that accurate wellfounded test data can be generated to support therelease of a product. Failure to certify the electromagnetic compatibility of a product to arecognised standard is not only an impediment tosuccessful sales but can be an infringement of thelaw, for which individuals within a company canbe held responsible.

EMC conformance measurements can influencethe success or failure of a product and thereby affect


the prosperity of the company which is developingthem. For electronic engineers engaged on projectsboth large and small, understanding EMC testmethods and the limits of accuracy which may beexpected is becoming ever more important.Accurate EMC measurements are essential for

Engaging in disputes over product liabilityEnsuring system or product functionality in itsintended EM environment, andCost-effective EMC in product development.

It is an unfortunate fact that some EMC measurements are difficult to undertake simply andrepeatably. Increasingly in situations wherelegislation is involved and litigation may turn onEMC test data, every effort must be made bymembers of the EMC community to define,specify, understand and work to agreed testmethods which yield the most reliable results.

Many years of continuous effort by engineersand regula tory committees go into developingand ilnproving test methods in the light of realworld measurement experience to arrive at testmethods which are technically meaningful,economic to conduct and are as reliable,repeatable and accurate as possible. Such testmethods should be regarded with respect andfollowed carefully as they embody the collectiveexperience of many people. They should not,however, be followed blindly, and an effort shouldbe made to understand how the tests wereoriginated and developed and where they are stilldifficult to perform, or may be inadequate orinaccurate in some way.

EMC symposia held throughout the world havesections devoted to new or improved testtechniques with a view to improving accuracyand repeatability. Such technical developmentaccounts, in part, for the continuous changes toEMC standards and specifications.

3.2.2 Accuracy of EMC measurements

Measurements of fundamental quantities of mass,length and time have reached levels of extremeaccuracy with the advent of lasers and atomicclocks. For example, time and frequency can bemeasured accurately to about 1 part in ten to thepower 13. In everyday engineering somequantities can be measured in the work place to afraction of a percent regularly and without effort.

Unfortunately, EMC measurements ofquantities such as electric and magnetic fieldstrengths, power density and wave impedance,surface and cable currents over a very widefrequency range from Hz to GHz, are not particularly easy to conduct or to repeat. When wavefieldmeasurements are made in relation to equipments

being tested which do not have known electromagnetic characteristics, e.g. radiation pattern, farfield distance, etc. and are conducted in imperfectsurroundings such as screened enclosures whichgive rise to multiple signal reflections, then considerable errors can arise.

A full discussion of EMC measurement errors isgiven in Chapter 11 and follows the detaileddescription of the various probes, sensors andantennas used in testing. In this section apreliminary indication is given as to the sourceand Inagnitude of some of these errors such thatEMC measurement accuracy may be comparedwith more familiar measurements.

Consider a typical EMC measurement system asshown in Figure 3.1. The sources of EMCmeasurement error will depend on

• The accuracy of the measurement meter,usually an EMI receiver, spectrum analyser,power meter or other sensitive calibratedmeasuring device.

• The nature of the sensor or probe connected tothe meter. It is important to know whatquantities it actually measures, whether itloads or disturbs the quantity being measuredand whether its output is an average overspace Qr time. Such questions apply particularly to wavefield measuring sensors orantennas. This area of measurementuncertainty has prompted considerableresearch by national standards bodies,academics and practising EMC engineers intodefining exactly the characteristics of specificantennas, what they actually measure in agiven test situation, and how to calibrate them.

• The connection of the sensorIantenna to thereceiver or measuring meter can play asignificant part in introducing errors into themeasurement. Anomalies of up to 10 dB havebeen observed with regard to the movementof antenna cables on open test sites [9].

• The surroundings in which the test is being madehave a major impact on wavefield measurements. EMC tests are usually carried out in

a laboratory with a ground plane on a benchfor conducted EMI testsa screened chamber, possibly lined with radioabsorbent material (RAM) for militaryradiated emISSIon and susceptibility tests.Errors have been reported of 17 dB in measurements between standard antennas in RAMlined chambers [10Jan open site, sometimes referred to as an OATS(open-air test site) for commercial radiated andconducted emission tests. Errors of around10 dB have been reported concerning just thecalibration of open sites [11, 12J

OUTLINE OF EMC TESTING 43

RADIO ABSORBENT MATERIALHELPS TO MINIMISE REFLECTIONS

000

<i 0

o

MEASURING METER - EMI RECEIVEROR SPECTRUM ANALYSER

CALI BRATION ERRORS -

DRIFT ERRORSEQUIPMENT FAULTS

+EXTERNAL SIGNALS REACH ANTENNAAND CONFUSE MEASUREMENT

INTERACTION BETWEEN ANTENNAAND CABLE

C.tt8(

;

12 0 ;:'-1.1::'!V0r:

I-t(

UNBALANCED CURRENTS

FLOWING ON MEASUREMENT CABLE

SCREENED ROOM TEST CELL

~SUREMENTSENSOR(A BICONIC ANTENNA INTHIS EXAMPLE)

~ METALLIC FLOOR - GROUND PLANE

~ #i\, ~: ~ ANTENNA IS PHYSICALLY LARGE AND

MUTUAL COUPLING OF ANTENNA ~ \ ~, AVERAGES THE FIELD STRENGTHTO ITS IMAGE CHANGES ITS " I,'CALIBRATION t ELECTRICAL IMAGE OF ANTENNA

IN GROUND PLANE'1\

I I \I I \

I I \',I ;~4I~

EUT AND CABLE PLACEMENT UNKNOWN RADIATION FIELD

UNKNOWN NEAR FIELD I FAR FIELDDISTANCES FOR RADIATED EMISSIONCOMPONENTS

Figure 3.1 Possible sources of measurement uncertainty in a measurement oj'radiated emitted jield strength

the actual installation where the equipment isto be used. This is usually confined to largesystems which may occupy a whole building,such as a central computing facility or atelephone exchange. Problems encountered incarrying out such testing have been reportedand discussed [13J.

In each of these locations the surroundings willaffect and sometimes dominate the accuracy ofEMC measurements which are made, particularlyradiated emission and susceptibility measurements. Multiple reflections from the inside of ascreened room have been reported as leading toerrors at some frequencies of up to 50 dB in fieldstrengths [14J.

The radiated-field measurement errors arefurther complicated by the interaction of themeasuring meter with the connecting cable andthe antenna, which manifests itself as a VSWRmismatch. The cable also acts as a parasiticelement to the receiving antenna and disturbs thefield being measured [12]. I t can also act as anactive element if the antenna or sensor isunbalanced and a significant common-modecurrent flows on the outer of the connectingcoaxial cable [9J. Thus the construction of dipoleantenna baluns with low leakage and goodbalanced transformer properties is important inminimising interactions between the antenna,cable and receiver [15-1 7].

Significant errors can be introduced intoradiated EMI measurements by the interaction of

the antenna and the ground plane at an open siteor the walls of a shielded chamber. Such effectscan be understood in terms of the mutualimpedance between the test antenna and theelectrical images set up by the conducting planes.Monopole antenna input impedances can varywildly with frequency by an order of magnitudeor more inside shielded chambers [18-20J.

Such variations will not be allowed for in theantenna calibration factor and will appear as ameasurement error. Because the frequencydependent antenna impedance variation willinteract with the mismatched antenna cable, themeasured field strength against frequency will bea function of the particular antenna at aparticular location in the screened room and witha particular connecting cable which is laid out ina particular way.

• One of the largest sources of error is thatintroduced by the configuration of the EUTitself. The precise layout of cables whichconduct RF currents from the energisingsource (the E UT) will significantly affect boththe amplitude spectrum and the spatial distribution of current along the cables. This inturn will affect the field radiated from it.Changes in cable length or shape will affectthe cable resonance and the nature of theconducted and radiated signals particularly atfrequencies around cable resonances wherechanges of 10 dB have been reported forvarious ways of bundling the same cable [21 J.


3.2.3 Implications of repeatability ofEMC measurements

From this overview of some of the issues whichdetermine the accuracy of EMC measurements itis clear that radiated measurements in particularare generally

(i) more complicated than some more commonmeasurements of physical dimensions suchas mass, time, pressure, or direct voltage orcurrent

(ii) the measurements cannot be easily isolatedfrom the surroundings in which they arebeing made

(iii) the full nature of the object beingmeasured (in the case of an EDT) is notknown and it is far too time consumingand uneconomic to investigate it fully suchthat the most appropriate measurementsof maximum radiated field strength, forexample, may be made over a wide rangeof freq uencies

(iv) the measurements are consequently far lessaccurate than those for more commonquantities. In general it may be supposedthat conducted current measurements canbe made with a repeatable accuracy ofaround 6-10 dB, but radiated measurements can contain errors in excess of 20 dB.White [22J indicates that under somecircumstances total measurement uncertaintyat some frequencies can be as high as ±40 dBfor radiated EMC tests.

With this level of uncertainty in the repeatabilityof these costly EMC measurements and the legal!financial importance of being able to prove thatan equipment meets the national and international EMC standards before it may be sold orimported into a country, it is clear that pressurefrom government standards and regulatory bodiestogether with industrialists and engineers willgradually result in developing higher accuracyEMC test methods.

There are those, like T.]. Dvorak in Switzerland[12J, who believe that: 'A real improvement wouldmean adopting entirely new concepts based onrecent advances in inverse modelling and moderncomputer technology or even to completelyabandon the measurement of radiated fields infavour of conducted measurements which stillhave a large developing potential'.

I t is conceivable that electronic products of thefuture will be released against specification limitswhich are computer derived from standardcalibrated source current distributions whichsimulate the EDT. The EDT conducted current

patterns would be measured and a computerwould calculate the full 3D wavefield (includingradial components in the near field) as a functionof distance which would be compared with a newtype of specification.

As the need for more accurate results intensifiesin the coming years and the cost of manpowerintensive testing increases while the cost ofcomputer modelling decreases, there may be apowerful incentive to move towards Dvorak'stechnically superior computer simulation methodsfor EMC clearance rather than relying onmeasurement. For the present, EMC testing withall its difficulties and uncertainties will continueto be carried out worldwide on an increasinglywide range of electrical and electronic products asthe importance of EMC grows.

3.3 Introduction to EMC testsensors, couplers and antennas

3.3.1 EMC sensor groups

Sensors and coupling devices for EMC testing canbe grouped basically according to whether the RFcoupling is by conduction and induction or byradiation. In some cases the same couplingdevices can be used for signal injection andemission measurement. I t is convenient to groupprobes, sensors, couplers and antennas based oncoupIing mechanism and their use in emissions orsusceptibility measurement as shown:

Conduction and inductionConducted susceptibility (CS) signal injectionConducted emission (CE) signal measurement

Radiative couplingRadiated susceptibility (RS) antennasRadiated emission measurement (RE) antennas

3.3.2 Conduction and induction couplers

As an example, an artificial composite conductedEMC test arrangement (CE and CS) is shown inFigure 3.2 where the conduction and inductioncouplers used in a wide range of emission andsusceptibility tests have been illustrated. Thedirect conduction couplers include

10/lF low-inductance RF feedthrough capacitorsLine-impedance stabilisation networks (LISN)Wideband RF capacitor, used in conjunctionwith a cable break-out boxHigh-impedance wideband voltage probes(oscilloscope probes)Direct ESD injection.


10uF FEEDTHROUGH CAPACITOR _TO STRIP °RFo FROM MAINS SUP?LY

r".-----......~DISTRIBUTED CAPACITIVE

CURRENT PROBE MEASUREMENT COUPUNG WIREOR CURRENT INJECTION ONON POWER UNES

~ ~~ALWINDING AR.OUND EUT BOX~ MAGNETIC INDUCTION TEST

/' -INDUCTIVE or CAPACITIVE CABLE CLAMP RF GROUND PLANE

Figure 3.2 RF coupling techniques usedJor conducted emitted (CE) and conducted susceptibility (CS) testing

COUPLING DEVICE DEVICE FREQUENCY COVERAGE(Power line frequencies)

SPIRAL WINDING ( BOX) (SJ: 'ke Injection)SPIRAL WINDING (CABLE) (Pm er frequ.) (Sp kes)10 uF CAPACITORUSNsAUDIO TRANSFORMERSTORROIDAL CURRENT PROBES

INDUCTIVE CLAMPCAPACITIVE CLAMPDIRECT CAPACITOR INJECTIONESD PROBEHIGH IMPEDANCE VOLTAGE

PROBE

Figure 3.3 Examplesoj coupler frequen(Ycoverage for typicalconducted EMCtesting

10 100 1k 10k 100k 1MFREQUENCY Hz

10M 100M 1G

Induction couplers include

Cable and surface current probesSpiral induction windings for boxesSpiral induction windings for cablesDistributed capacitance parallel wiresCapacitive and inductive cable clampsIndirect ESD to nearby conducting plane.

There are a large number of sensors and probeswhich are commonly used to make EMCmeasuremen ts by using direct connection or closespaced induction. The approximate frequencycoverage of each type of couplerIsensor is shownin Figure 3.3. The technical details of thesedevices is discussed in subsequent chapters wheretheir use in carrying out typical EMC tests isexplored.

3.3.3 Radiative coupling - EMC antennasFigure 3.4 shows a collection of typical standardantennas used for radiated emission and susceptibility testing. The approximate frequency

coverage of these antennas is given In Figure 3.5.They include the following types:

H-field loop antennas for measuring the magneticfield component of an EM waveSmall shielded loop antenna (155 mm dia.)Large shielded loop antenna (0.5 m dia.)E-field monopole antennas for measuring theelectric component of the wavefield (with respectto an RF ground plane)1 m effective height rod antennaSmall battery powered ~ensor with fibre opticreadout for susceptibility field-strength levelling

Free-field antennas such as:

Long wire an tennaTuned dipole setsBroadband biconic antennaLogarithmic conical spiral antenna' (circularlypolarised) and log-periodic antenna (linearlypolarised)Horn antennas of various sizesDish reflector type an tennas for microwaves


10Hz - 40GHz

o 000

SIGNAL IN

TUNED DIPOLES

RF LEVELLING AMPLIFIER t-------CONNECTED TO

OFIELD SENSING PROBEvia FIBRE OPTIC CABLE

LOOP ANTENNA

(H FIELD) RF POWER OUTPUT TOSUSCEPTIBIUTY ANTENNAS ....- ...

MONOPOLE ANTENNA

~IGH GAIN REFLECTOR MICROWAVE ANTENNA

~ ~ORNANT~ LOG. CONICAL0/ LjSPIRALANTENNA

SMALL LOOP "H FIELD" ANTENNA _ ~m-~;30fn SPECTRUM ANALYSER OR EMI RECEIVER.----:-;;:;C ~m,3m,

_ --;to\S\~NCS.~RO\SS

PARALLEL PLATE LINES

TEM CELLS ETC (not to scale)

I I IBATTERY POWERED "E FIELD" SENSORWITH FIBRE OPTIC CONNECTION TOFIELD LEVELLING AMPLIFIER TO CONTROLFIELD LEVEL DURING A SUSCEPTIBILITYTEST

p"'igure 3.4 Antennas and sensors used in radiated emission (RE) and radiated susceptibility (RS) testing

Figure 3.5 Examplesof antenna frequencycoverage .for typicalEMC antennas

ANTENNA or SENSOR DEVICE FREQUENCY COVERAGE

SMALL LOOPSLARGE DIAMETER LOOPS -- ---- -----1m MONOPOLELONG WIRETUNED DIPOLES --

---

BICONIC DIPOLE - -LARGE LOG. CONICAL SPIRAL --LARGE LOG. PERIODICLARGE HORN --SMALL HORNS - -

- -,.....

SMALL LOG. CONICAL SPIRAL -DISH REFLECTOR + FEEDS -

- .......FIBRE OPTICLY COUPLED

E FIELD SENSORSPARALLEL PLATE LINEST.E.M. CELLSG.T.E.M. CELLS

10 100 1k 10k 100k 1M 10M 100MFREQUENCY Hz

1G 10G 1000

Bounded-wave antennas usually used for susceptibility testing:

Parallel-plate lines"rEM (transverse electromagnetic) cellserawford cellsGTEM cells (gigahertz TEM).

3.4 References

WHITE, D.R.J.: 'A handbook series on EMI', vol.3, section 6.3, pp. 6.20-6.21 (Formulas for

capacitive crosstalk between parallel wires) DWCInc., PO Box D,Gainesville, Virginia 22065, USA

2 PITT, A.D.: 'EMC guidelines'. BAe ST23357, Oct1979, pp. 10.2-10.6. British Aerospace Dynamics,Six Hills Way, Stevenage, Herts, UK

3 RICKETTS, L.W., BRIDGES, J.E. andMILETTA, J.: 'EMP radiation and protectivetechniques' (Wiley) p. 147

4 BURKE, G.J. and POGGIO, A.J.: 'Numerical electromagnetic code (NEC) - method of moments'.Technical document 116, Naval Ocean SystemsCenter, 1981

5 PRICE, W.O. and CANAGA, K.W.: 'Accuracyestimation in anechoic chamber testing'.Proceedings of IEEE conference on EMC, 1985,pp. 161-169

6 KING, R.W.P.: 'The theory of linear antennas'(Harvard University Press, 1956)

7 'ESA GEOS spacecraft development control plan'.BAe Dynamics, Filton, Bristol, UK, 1974

8 DUFF, W.G.: 'Fundamentals of electromagneticcompatibility'. Interference Control TechnologiesInc., Gainesville, Virginia, USA, p. 9.22

9 DEMARINIS, J.: 'The antenna cable as a source oferror in EMI measurements'. Proceedings of IEEEsymposium on EMC, 1988, pp. 9-14

10 AG.ARWAL, N.K.,JOSEPH, P.C. and KESAVANNAIR, P.N.: 'Evaluation of shielded anechoicchamber for EMC measurement'. Proceedings ofIEEE symposium on EMC, 1985, pp. 134-138

11 TSALIOVICH, A.: 'Absorber-lined open area testsite - a new type of EMC test facility'. Proceedingsof IEEE symposium on EMC, 1988, pp. 106-111

12 DVORAK, TJ.: 'Fields at a radiation measuringsite'. Proceedings of IEEE symposium on EMC,1988, pp. 87-93

13 COONCE, H.E.: 'On premises emission testing ofdigital switching systems'. Proceedings of IEEEsymposium on EMC, 1988, pp. 15-18

14 JOFFE, E.B.: 'Are RS03 limits in the HF bandrealistic?'. Proceedings of IEEE symposium onEMC, 1990, pp. 196-201

15 DASH, G.: 'A reference antenna method for


performing site attenuation tests'. Proceedings ofIEEE symposium on EMC, 1985, pp. 607-611

16 BERRY, J., PATE, B. and KNIGHT, A.:'Variations in mutual coupling correction factorsfor resonant dipoles used in site attenuationmeasurements'. Proceedings of IEEE symposium onEMC, 1990, pp. 444-450

17 BRENCH, C.E.: 'Antenna differences and theirinfluence on radiated emission measurement'.Proceedings of IEEE symposium on EMC, 1990,pp. 440-443

18 MISHRA, S.R., KASHYAP, S. andBALABERDA, R.: 'Input impedance of antennasinside enclosures'. Proceedings of IEEE symposiumonEMC, 1985, pp. 534-538

19 McCONNELL, R.A.: 'An impedance networkmodel for openfield range site attenuation'.Proceedings of IEEE symposium on EMC, 1990,pp. 435-439

20 MISHRA, S.R.: 'Effect of ground plane andchamber walls on antenna input impedance'.Proceedings of IEEE symposium on EMC, 1988,pp. 395-399

21 HUBlNG, T.H.: 'Bundled cable parameters andtheir impact on EMI measurement repeatability'.Proceedings of IEEE symposium on EMC, 1990,pp. 576-580

22 'A two-part five-day comprehensive training coursein EMC, Part 2'. VG A15393, Don WhiteConsultants;' PO Box D, Gainesville, Virginia22065, USA

Chapter 4

Measuretnent devices for conducted EMI

4.1 Introduction

The approach taken in this book with regard toordering and presenting information about thedozens of sensors, couplers, probes and antennasused in EMC measurement is to discuss them ingroups that are defined by the manner in whichthey couple to the signals being measured. Thedifferent physical processes related to each type ofcoupling can thereby be appreciated and apractical insight gained in to their operationwhich is reflected in the structure of the varioustest methods employing these devices.

Although this particular chapter concentrateson coupling devices used in conducted EMCtesting, all the groups of sensors discussed in thisbook, including radiated emission and susceptibility antennas, are listed here to show thenumber of groups and to illustrate the range ofsensors considered.

Sensors are grouped as follows:

Conducted emission and susceptibility tests (CE,CS)

• Direct connection devicesLine impedance stabilisation network

(LISN)10 flF feedthrough coupling capacitorInjection transformersHigh-impedance voltage probes

• Inductively coupled devicesCable current probesCable current clampsSurface current probesStraight and spiral wire inductive couplingInductive loops around boxesFerri te wands or probes

• Direct electrostatic dischargeHandheld spark generators

Devices for radiated emission (RE) tests

• Radiated emission antennasLong wire antennaMonopoles (passive and active)Tuned dipolesBiconic dipolesLog. conical spiralLog. periodicStandard horns

48

Ridged hornsHorn-fed dishesLF magnetic loopsFerri te-cored loopsLeakage probes

Devices for radiated susceptibility (RS) testing

• Free-field antennasShort transmission linesLong wire antennaBiconic dipolesI...Jog. spirals and periodicsStandard and ridged hornsDish-type reflectors

• Bounded-wave antennasParallel-plate strip linesCrawford cellsTEM and GTEM cells

• E-field sensorsBattery powered monopole sensorsElectrically short dipolesDiode detection dipolesFibre-optic coupled sensorsMonopolesMonoconesRadhaz monitors

• Indirect ESDVarious metal plates onto which the sparkis discharged

4.2 MeasureD1ent by directconnection

Devices that are directly connected to the EUTare of two types:

(i) Networks that produce a defined loadimpedance as a function of frequency to thecircuit being tested. The most commondevice of this type is the line impedancestabilisation network (LISN).

(ii) Probes that can be connected to the circuit tobe measured withou t significantly alteringthe RF voltages, currents and circuitimpedance. High-impedance RF voltageprobes commonly used with oscilloscopes,transient recorders or spectrum analysers arean example of this second type of directconnection device.

MEASUREMENT DEVICES FOR CONDUCTED EMI 49

Zs0.03uF600V

15A

50A

TO POWER SOURCE8

C2

25 k ohm 20W R4

L (APPROX. 56uH)

O.1uF 600V

TO EQUIPMENTUNDER TEST

A

NC

Z1

50 OHM COAXIALTERMINATION IN PLACE OF RFI METER

(aj

(J) 50:;EI0

UJ40

()Z<t:0UJ 30a..~

LL0

UJ 20:::::>....J<t:>UJ 10r-:::::>....J0(J)co 0<t:

0.014 0.1 1.0 10

FREQUENCY MHz

(bj

Figure 4.1 56p,H line impedance stabilisation network(LISNj(a) Circuit diagram(b) US MIL-STD 462

is given for use in MIL STD 462 N3 which issuitable for measurements over a higher frequencyrange up to 50 MHz, see Figures 4.2a and 4.2b.

Figure 4.3 shows the basic circuits for three LISNsrequired to carry out testing to CISPR 16, FCC(MP4) and VDE 0876. Specifications BS3GI00,NWS3 and DEF STAN 59-41 require the use of a5 pH LISN for tests up to 150 MHz which incorporates special design elements to suppress unwantedhigh-frequency impedance variations owing tocomponent self resonance. See Figure 4.4a for thecircuit details and Figure 4.4b for the limits ofacceptable impedance with frequency.

DEF STAN 59-41 specifies the addition of a10 pF feed through capacitor connected across thepower input to the LISN for AC lines and a30,000 pF capacitor for DC supplies to furtherstabilise the power supply impedance. Figure 4.5shows the limits of acceptable impedance for theLISN with the power supply terminal connectedto the case.

Filter incoming RF noise from the maInssupplyProvide a known impedance as a function offrequency to the equipment under test(EDT), andProvide a matched 50 ohm RF connection tothe EMI meter.

One of the most commonly used LISN designs isthat required for meeting tests CE02, CE04, CS02in notice 3 of MIL STD 462 [3]. The circuit ofthe LISN is shown in Figure 4.1a with itsimpedance profile from 14 kHz to 10 MHz inFigure 4.1b when the measurement port is loadedwith 50 ohms. Note that the EDT line impedanceis 50 ohm down to about 1 MHz. A second circuit

4.2.1 Line impedance stabilisationnetwork

The development of the LISN goes back to the1950s when early EMC engineers needed todevelop a means of providing a standard powersupply impedance network for aircraft, in anattempt to correlate equipment EMC bench testresults with those carried out using actual aircraftsupplies. In the civil sector, the parameters of themains network were determined by analysingmany measurements of the RF impedance ofdomestic, industrial and other supply systems [1].The mean values were found to be wellrepresented by an equivalent circuit with 50 ohrnsin parallel with an inductance of 50 pH. Goodagreement was possible between several countriesand so this network was adopted by CISPR inpublication 16 [2J and has since been incorporated into a number of standards includingBS800jEN55014.

When an LISN jartificial mains network with adefined impedance is used, it is possible to simplyand quickly measure the RF voltage as a functionof frequency appearing on the conductor undertest. The specified levels for the test are written interms of a measured RF voltage as a function offrequency. Thus the voltage measurements madeusing an LISN can be compared directly withspecified limits and many commercial standards,such as BS800jEN55014, BS6527jEN55022,VDE0877 and VDE0871, etc., specify conductedEMI only in terms of RF voltage across astandard LISN.

However, knowing the RF impedance of thenetwork also permits the calculation of the RFcurren t flowing along the conductor into theLISN terminal. This can then be compared withmeasurements made using alternative directtechniques such as inductive current probes asused in MIL STD 462 tests CEOI to CE05.

Practical LISNs fulfil three functions:

(aj

L150uH

FCC MP-4

VDE 0876 Teil1

CISPR PUBL. 16

TO EUT > TO MAINS SUPPLY

TO EUT TO MAINS SUPPLY

TO 50 ohmRECEIVER

TO EUT TO MAINS SUPPLY

TESTSAMPLE

L

TO 50 ohm

RECEIVER

TO 50 ohmRECEIVERor 50 ohm

TERMINATION

COAXIAL

~g~~~~~~R ...__.....MEASURINGSET

Figure 4.3 Various LISNs for use in testing commercialelectronic equipment

Figure 4.2 US MIL-STD 462 HF 5 IlH LISN( aj Circuit diagram (b j LISN impedance

50 A HANDBOOK FOR EMC TESTING AND MEASUREMENT,

A 50 ohm 50llH design (9 kHz-30 MHz) is givenin BS727 which allows the connection of the EMImeter to both the live and neutral terminals of thepower lead of the EDT by operation of a switch.See Figure 4.6a. Note the pi-section input filtersformed from L 1, C1/R1, C2/R2. Figure 4.6b showsthe acceptable impedance limits.

The impedance profile of the higher frequency5 IlH 50 ohm LISN (150 kHz-l 00 MHz) specifiedin BS 727 is that given in BS3G 100. BS 727 alsodefines the component values for a 150 ohmresistive LISN, see Figure 4.6a. A more sophisticated 150 ohm LISN that allows measurement ofboth the symmetrical and asymmetrical interference voltages from an EDT is specified in BS905pt I/EN550 13, see Figure 4.7 .

LISNs are usually mounted on a specifiedmetallic ground plane alongside the EDT as inFigure 4.8 and the EMI meter is connected via ahigh performance low-loss coaxial cable to themeasurement port. LISN measurement ports onlines which are not being measured areterminated in 50 ohms. Typical narrowbandvoltage limits (BS3G 100) for conducted interference measured in this way can be seen in Figure4.9. Limits applicable to interference frominformation technology equipment as specified byFCC part 15 and the now superseded VDE 0871(measured using a quasipeak detector in the EMImeter) are given in Figure 4.10.

LISNs can be usedjust to provide a stable knownterminal impedance for EDT power lines and theactual measurement of conducted current is madeusing inductive current probes. This is the case fortest DCEOI in DEF STAN 59-41, see Figure4.11a. The conducted current limits in this case arespecified in dB IlA not dB 11V. See Figure 4.11 b.

The LISN is a widely used coupling andstandard impedance device which is easy to useand has been incorporated into most commercialand military EMC standards. It represents realworld power line impedances well and is aninexpensive device, but is limited in respect of itsfreq uency span and its upper frequency limit,which is usually determined by spuriouscomponent resonances.

The LISN can be used to inject an RF signaldirectly onto a power line that feeds theequipment under test. Consider the circuit of thesimple network shown in Figure 4.2a. The signalis injected onto the line under test via the highquality low-inductance/low-loss capacitor C1. Theseries inductor L 1 provides a high impedance tothe injected RF being shorted out by the mainscircuit outside the test area. The capacitor CY

2dumps any RF signal which passes the inductorto ground and prevents it from contaminating theexternal mains system.

50

Iii'40

E..c.2-w 30()z«0wa.. 20~

z(J)

::J10

00.1M 1·0M 10M 100M

FREQUENCY Hz

(bj

MEASURElVIENT DEVICES FOR CONDUCTED EMI 51

For details of inductor see table

"'TOSOohm I rlMEASURING C1 C2RECEIVER O.OS,UF TO.OSpF

,.....,................ ,.......~~r~"r-,.

TO

EQUIPMENT 10ohm

TO MAINSSUPPLY

}-'igure 4.4 LISNsuitabIe for UKBS3G100 Pt.4 Sec.2(a) Circuit details( b) Limits ofacceptable impedance

DETAILS OF INDUCTORCURRENT INDUCTANCE INSIDE DIA. LENGTH No. OF TURNS CONDUCTORRATING CROSS SECTION

A ~H mm mm mm10 5 25.4 32 20 1.6dia100 5 50 115 18 6dla500 5 90 178 11 12.5 x 12.5 sq.

(aJ

100M 200M

----------"".,.-',""

1.0M 10MFREQUENCY Hz

O~ "-- ...L.__ ___IL..._ ...I___ ____'

0.1M

80----------r-----------r---------.,.------.

(j) 60E.c~UJ

~ 40<!oUJa..~ 20

(b) Reproduced by permission of BSI

50 __---r---...,.---r------r----,----,------,

_ 40

~"§ 30(II:g.w 20oz(§ 10wc..~ 0

The use of an LISN in this way is required forsome conducted susceptibility tests such as MILSTD 462 Notice 3 test CS02 (method B 150 kHzto 65 MHz). Signal voltages of less than 1 V RMSare normally injected but RF power of up to afew tens of watts can be injected by this means fora short time.

-1 0 L------'......__..,.;.__"--_---L.__--I--__~_----J

10k 100k 1M 10M 10

FREQUENCY Hz

*ACTUAL L1SN IMPEDANCE MUST LIE WITHIN THESE LIMIT LINES

Figure 4.5 LISN,for use with UK DEF STAN59-41 limits for impedance against frequencywith supply or load terminal connected tocase

Reproduced by permission of HMSO

4.2.2 10 JlF feedthrough capacitor

MIL STD 461/2 calls for the use of a good quality10 flF RF feed through capacitor which can beused to provide a known low RF terminatingimpedance on power lines in tests such as CEO 1,CE03 and CS06. See Figure 4.12. In the spikeimmunity test CS06, the RF short circuit


E MEASURINGRECEIVER

R3C3L

R2 ~ -- I

: EQUIPMENT:0-_..--.....----1.._---;,-0 UNDER I

I TEST I

R2~ ~

N

Figure 4.6 Dual lineLISN suitable foruse with BS727:Radio interferencemeasuring apparatus(a) -Circuit of 50 JlHdesign (b) Acceptableimpedance limits

COMPONENT VALUES FOR 50 ohm I 5O,..H NETWORK COMPONENT VALUES FOR 150 ohm NETWORK

COMPONENT VALUE COMPONENT VALUE

R1 10 ohms C1 1.2pFR2 5 ohms C2 8JJFR3 1kohm C3 O.25J,1FR4 50 ohm L1 250,uHR5 (short) l2 50}lHR6 50 ohms

COMPONENT VALUE COMPONENT VALUE

R1 open C1 openR2 short C2 0.251JFR3 open C3 O.1I-'FR4 150 ohm l1 shortR5 100 ohm l2 shortR6 50 ohm

(a)

100 r-------.,..--------.------,..--..,

50

enE

..c:,£.

~10z~o~ 5~

'~~------------N--------;",' , __ fllllJ-----, ---------; "

~' ",' TOLERANCE +/- 20%

;, ";; ;",; ;;

; ;;

--;'

10M100k 1M

FREQUENCY Hz

1'--- ...1.- 1-- --1._--1

10k

Reprod uced by permission of BSI (b)

provided by the two capacitors ensures that thefull high-frequency spike voltage is applied to theEDT power input. Because these capacitors areintended for use with power lines they mustoperate up to 600 V DC and be designed tohandle mains currents of more than 100 Awithout significant loss. Such capacitors are quitelarge, 85 X 85 X 70 mm, and are designed to bebolted to the RF ground-plane or to the screenedroom wall when being used as a power line filter.The RF attenuation characteristic of acommercial 10 JlF capaci tor (Solar 6512-1 06R)[4] is shown in Figure 4.13.

4.2.3 RF coupling capacitors

High-quality low-inductance RF couplingcapacitors are widely used in EMC testing to

inject RF voltages onto the power lines connectedto EUTs. The principle is to isolate the mainslow-frequency AC, or DC line voltage from thesignal injection RF power amplifier whileefficiently coupling the injected RF energy intothe power line circuit. Typical circuits may havean RF impedance from tens of ohms up to a fewhundred ohms. The coupling capacitor musttherefore have negligible series reactance at thelowest frequency of the test. The capacitor shouldnot exhibit self resonance at high frequencieswhere the self inductance due to foil constructionor lead lengths becomes significant.

In MIL STD 461, a capacitor with a reactanceXc < 5 ohms is required for the CS02 test over afrequency range of 50 kHz to 400 MHz. Thisrequires a capacitor with a value of at least0.1 fiF. See Figure 4.14. Practical coupling


Figure 4.7 150 ohmLISNfor measuringconducted interference(EN55013[BS905 pt.1])

Measuringapparatus

Resistor values for ISO ohm LISN

Value

r----------------------------------- ---~

5 : S R12 I~ I0:: L::>ou::zoo~....JUJo

I::>a.. r~ ,(j) Iz I 2~ I OUTPUT TO EUT I

::?E l_~monaifiiter il;'~;;:-ed- ------ --- - - - -----.1:---- JSWITCH S - Position 1 - symmetrical interference

Position 2 - Asymmetrical interference

ohms120150390270

2211050


capacitors are constructed from a number ofdifferent capacitors in a network designed tominimise self resonance in the VHF and UHFranges.

Coupling capacitors are also used for exan1ple inIEC 801 part 4 (BS6667) where high-frequencytransient bursts are injected into industrial processmeasurement and control equipment. See Figure4.15. The capacitors have a value of 33 nF whichresults in a negligible impedance to the frequencycomponents of the 50 ns (50% width) voltagespikes. Note also the use of ferrite dampedinductive and capacitive decoupling of theinjectors from the mains input.

The draft for part 5 of BS6667 of 1990 [5Jconsiders the immunity of industrial processcontrol equipment to voltage surges. In this

standard, capacitors with values of 9 or 18 JlF arespecified for injecting surges onto power lines andsmaller capacitors with a value of 0.5 JlF are usedfor injection onto input/output (I/O) or controllines. See Figure 4.16a. For line-to-ground testinga series resistor of 10 ohms is used with the 9 JlFcapacitor for power lines and a 40 ohm resistor inseries with the 0.5 JlF capacitor for I/O lines. SeeFigure 4.16b and 4.16c.

Boresero et al. [6J describe a current injectionmethod for evaluating the immunity of broadcastreceivers to RF interference based on an injectioncapacitor. See Figure 4.17. The reactance of thecapacitor must be small compared with the150 ohm resistive component of the totalgenerator and coupler impedance. Once more aninductive/capacitive filter forms an integral part

LINE IMPEDANCE STABILISATION NETWORKS(ONE FOR EACH POWER LINE).-------

POWER INPUTS -----AC or DC

50 ohm resistive terminationon line not being tested

Figure 4.8 Conducted emission measurement LISNs mounted on ground plane bench


BROADBAND CONDUCTED INTERFERENCE LIMITS POWER SUPPLY MEASURING SET

SCREENED ROOM WALL

30,OOOJJF CAPACITOR FOR DC SUPPLIES

L1SNs USED TO PROVIDEA DEFINED RF IMPEDANCEFOR POWER LINES

GROUND PLANE

EUT

CURRENT PROBESARE USED TO MAKE

THE MEASUREMENTS

80WC)

~ 70....l0> N 60WI()~z> 50W=t0::,...w(1)u.. > 400::0W.c.-0~en 300"0Z<!: 20en0<!:0 100::en

010k 100k 1M 10M 100M

FREQUENCY Hz

NARROWBAND CONDUCTED INTERFERENCE LIMITS

FREQUENCY Hz

150M10M1M

TEST METHOD DCE01(POWER LINE CONDUCTED

INTERFERENCE 20Hz - 150MHz)

100k10k1k

(a)

100Hz

10 L-__J..-..__J..-.. '-- '--_---L.__---L.__--'

10Hz

NORMALRFMEASUREMENTPORTSTERMINATED IN 50 ohms

(b)

(j).0o~ 130

~

~ 110

~ 90"0

~ 70:J

~ 50zwffi 30Ll..0::WI-~

FREQUENCY Hz

Figure 4.9 Typical conducted emission limits usingLISN (BS3G100 pt.4 sec.2)

w 90<:><!:.-..J 800>w()z 70wa::w>~=l 60W,...,-(1)Z >_ 0

0"8 50Zm(i5-o~

400a::a::<t:z 30

10k 100k 1M 10M 100M

100 ,.----,.----,----,....----r---.,---.,------,

FREQUENCY Hz

Reprod uced by permission of HMSO

(i) For AC power circuits

Figure 4.11 UK DEF STAN59-41 test DCE01using LISNs (a) Use oj current probes( b) Limits for aircraft use

resonant components are used. The EMC testengineer must take care however to ensure thatthe connection of the coupling capacitor does notlead to the following problems.

High-power line voltage can be coupled backthrough the capacitor into the signal source anddamage the source output circuit. The power linevoltage fed back through the coupling capacitoralso must not damage the input circuit of any RFvoltage measuring meter which is connected tomonitor the injected RF susceptibility signal.Some proprietary coupling capacitors (e.g. Solar7415-1) have built in high-pass filters to thepower line frequency in the monitor output toprevent this.

10M 30M3M

CLASS B VDE & FCC

1M

--- CLASS A FCC-

r-------I CLASS A & C VDE

300k30k 100k

RECEIVER BANDWIDTH--I_---RECEIVER BANDWIDTH ... !200Hz -19kHz

20 r.....-_--'"__--lo-__--I,..-__-i-__-'--__-'--_---'

10k

80

Figure 4.10 FCC and VDE interference limits forconducted emission tests on IT equipmentusing LISN


of the coupler and isolates the wanted signal sourcefrom the injected interference.

The use of coupIing capacitors for injecting RFinterference into power and signal circuits is acommon technique. It is simple and inexpensive.I t can be trouble free if good quality non self-

CI)...JW>W...J

W()ZW

~ $ 600::"'-~ ~~~OOJ~"O 40():JoZo()


(ii) For signal I/O and control lines

4.2.4 Distributed capacitance couplers

The use of lumped injection coupling capacitorshas a number of technical drawbacks which havebeen mentioned earlier. An additional problem isthe time, trouble and cost involved in making thedirect connection to the conductor under test bysplitting the cables or using special break-outboxes. It may not be possible to make such aconnection at all in the case of overall screenedmulticonductor cables or coaxial signal cables.

For some circuits to be tested, many of theseproblems of direct connection can be overcome bythe use of distributed capacitance clamps if thetesting standards permit. Using such a device thecable under test can simply be laid in the clampwith no direct physical connection being made.

A device specified in IEC80 1 part 4 is shown inFigure 4.18a. I t is used for testing as shown inFigure 4.18b. Such a clamp however only has acapacitance of between 50 and 200 pF for cablesof 4 to 40 mm dia. This low coupling capacitancerestricts efficient injection to high-frequency interference signals or pulses with fast nanosecondrisetimes.

In some cases it will not be possible to physicallyplace the signal or I/O cable under test in thecapacitance clamp owing to the inflexibility of thecable in question or the size of the clamp. In suchcircumstances IEC80 1 pt 4 permits the use of asimple wire or conductive metal tape to be usedby placing it alongside the cable under test toform a distributed coupling capacitor. See Figure4.19. 1'he capacitance between the cable undertest and the injection wire should be adjusted tobe in the same range as for the purpose designedclamp (50-200 pF).

Distributed capacitance probes can also be usedin conducted emission measurements. White [7]illustrates a spaced wire technique on a groundplane for measurifig common-mode cable

'The connection of a coupling capacitor can loadthe signal circuit thus reducing the wanted signallevel below that specified for correct circuitoperation. When the interference voltage is appliedvia the capacitor the ratio of the normal functionalsignal to injected susceptibility signal will be artificially low and the EUT will tend to fail the test ata lower level of injected interference than it should.

Direct capacitive coupling via lumpedcomponents is not particularly suitable for circuitsoperating at RF or carrying data with fast-risingand falling (nanosecond) edges, as the highfrequencies will see the injection capacitor as alow-impedance short.

50

TRANSIENT WAVESHAPE

(c)

(b)

(a)

TRANSIENTGENERATOR

~ 10IJF CAPACITOR

::r: 10IJF CAPACITOR

-=- SERIES INJECTION ON AN AC LINE

Figure 4.12 MIL STD 461 test using 10 J-lF capacitors(a) Spike injection test (b) Test CEOI(c) 10 J-lF Jeedthrough capacitors


70 r-------.,-------~-------r_------~------__,.------___.

TYPICAL PERFORMANCE [SOLAR 6512-106R]

1G100M10M

SAE MINIMUM ARP-936 REQUIREMENT

100k10k

20

50

10

60

1M

FREQUENCY Hz

INSERTION LOSS MEASURED IN a 50 ohm CIRCUIT as per MIL-STD-220A

Figure 4.13 Attenuation oj RF conducted current through 10 J-lFJeedthrough capacitor Reproduced by permission of Solar Electronics

co"0 40CJ)(/)

9 30

~5cm~

BLOCKING INDUCTOR

..--r-'lOWER SOURCE

TESTSAMPLE

Xc< 5 ohms

WAVEFORM MONITOR

POWERAMPLIFIER 500hms

OSCILLOSCOPEOR

TUNEABLE RFVOLTMETER

Figure 4.14 Conducted susceptibility test using RF coupling capacitor (MIL Syn 461 50 kHz-400 MHz)

33nF COUPLING CAPACITORS

TRANSIENT BURSTGENERATOR

Mains filter section

100,uH DECOUPLING INDUCTORS(With lossy ferrite damping)

L

EN

REFERENCE GROUND PLANE

EUT

INTERNALPOWERSUPPLY

Insulating supportReproduced by permission of BSI

Figure 4.15 Use ofcoupling capacitors inIEC801-4)· transientinjection onto power lines

emissions. AIm-long coupling wire is spaced 5 cmabove the cable under test, which is itself 5 cmabove the ground plane. See Figure 4.20.

4.2.5 High-impedance RF voltage probes

It is often required to make a measurement of RFvoltage on a conductor or component without

loading the circuit to any appreciable extent sothat a true representation of the signal characteristics can be observed. This implies that any directconnection device should have a high inputresistance and a low shunt capacitance to presenta high impedance across the frequency band ofinterest. For EMC measurements, such probesneed to cover at least DC to 1 GHz if possible.


EUT

Decoupllng network

I-o I---~L....L-L-J-L---------_+_-- ----__(

I

T-0--------------------------------

~

AC(DC)

POWER

SUPPlY

(a)

EUT

R=10ohmsDecoupling network

IO ~--('(Ynl---------_+_---_+_ --___<

Io ~___i!_____,._-'

IO---IIt---------------tlI----------___

I±

AC(DC)

POWER

SUPPLY

(b)

Decoupllng network

1.5 tnH INDUCTORS

R = 400hms

C=O.5pF

AUX.EQUIP. EUT

(c)

Figure 4.16 Various coupling capacitors used in IEC801-5 for transient burst injection(a) Test set up for capacitive coupling on ACIDC lines)· line-to-line coupling) generator output floating(b) Test set up for capacitive coupling on ACIDC lines)· line-to-ground coupling) generator output floating

or earthed(c) Test set up for unshielded 110 lines)· line to ground coupling



/ // / / / / /

J L 1--~

~C1-- D100 ohms

other cables for mains, loud speakersetc each terminated with a couplingunit (150 ohms)

L = Isolation inductance

C1 &C2 capacitors with lowRF impedance

-----_--..-----,,----....

\

+

//

EQUIPMENT UNDER TEST

../

Coupling unit I t

Coaxial cable, twisted pair ormulti-lead cable - screenedor unscreened

R int + R1 = 150 ohms

I = INTERFERENCE CURRENT

Wanted signal generatoror auxilary equipment

INTERFERENCE RF SIGNAL GENERATOR

Figure 4.17 Coupling capacitor used for signal injection (EN55020 [BS905}) Reproduced by permission of BSI

Probes are available in two types: passive andactive. Passive probes are usually built as voltagedividers with ratios of 10: 1 or 100: 1 and can presentinput resistances of greater than 1megohm. Thephysical construction of the probe and the selfcapacitance of the input resistor determine the shuntcapacitance. Probes of this type are commonly usedwith oscilloscopes for voltage measurements within alimited bandwidth of around 100 MHz.

If the signal to be measured is large enough towithstand the probe attenuation and thebandwidth of the probe is sufficiently high, theseoscilloscope probes can be adequate for EMC

measurements. When such probes are used tomeasure susceptibility signals impressed on powerlines care must be taken to ensure that, althoughthe amplitude of the RF signal to be measured isacceptable, the maximum probe input voltage isnot exceeded by the power line voltage.

Input parameters for typical passive probes canbe found in Table 4.1. Care must be taken toestablish the upper frequency limit of such probeswhen used for EMC testing as they will typicallynot perform satisfactorily much above 50 or100 MHz. Special high-frequency probes(HP 85024A for example) can be obtained with

Table 4.1 Examples ofprobes suitable for use in EMC measurements

Active! Probe Division ratio Input R Shunt C Compensates Max. voltpaSSIve for input

(ohms) (pF)P 10430A 10: 1 1M 6.5 1 M, 6-9pF 450P 10435A 10: 1 1M 7.5 1 M, 10-16pF 450P 10436A 10: 1 10M 11 1 M, 18-22 pF 450P 10440A 100: 1 10M 2.5 1M,6-14pF 450P 10437A 1:1 50

Freq. rangeA 85024A 1: 1 1M 0.7 0.3 M-3 G 1.5A 10: 1 15A 41802A 1:1 1M 12 5 Hz-100 M 50A 1124A 10: 1 10M 10 DC-100M 10A 100: 1 100A 1141A 10: 1 1M 7 DC-200M 200A 100: 1 Diff. mode

(All equipment - Hewlett Packard)


.-..---------- 1000 mm ----------~I

~UND PLANE SHAll HAVE A MINIMUM AREA OFf ~=~metre EXTENDING BEYOND THE DEVICE ON ALL

SIDES BY MORE THAN O.1m

INSULATING SUPPORTS

70mm

ooooo

COUPLING PLATES

ooI

.-- 1050 mm ------_....

(a)

EUT(1) EUT(2)

CAPACITIVE COUPLING CLAMP

AC MAINS SUPPLY

a.1m

Insulating support

GROUNDING CONNECTIONTO MANUFACTURERSSPECIFICATION

i1 i2

TO INTERFERENCE SIGNAL GENERATOR

(b)

AC MAINS SUPPLY

/

GROUNDING CONNECTIONTO MANUFACTURERSSPECIFICATION

Figure 4.18 Distributed capacitance clamp for use in IEC801-4 susceptibility testing(a) Construction of capacitance coupling clamp(b) Use of the clamp for injection into control and signal cables Reprod uced by permission of BSI

Figure 4.19 Tape or wiredistributed capacitor injection(for HF transients IEC801-4)

THIS CONNECTION SHALL BE ASSHORT AS POSSIBLE

AC MAINS SUPPLY

COMMUNICATIONS LINES110 CRICUITS

The coupling device shall be a conductive tapeor a metallic foil in parallel or wrapped aroundas closely as possible to the cables to be tested

The coupling capacitance shall beequivalent to that of the CLAMP

PROTECTIVE EARTH

/ 1Reproduced by permission oCBSI


5mm FOAM SUPPORT FOR CABLE

RF GROUND PLANE

10 x 15 em FOAM CHANNEL 1m long

//CONNECTOR FOR SHORT / OPEN CIRCUIT

Figure 4.20 Capacitive coupling wire used for cable emission measurements Reproduced by permission or leT Inc.

an input resistance of 1 megohm and a shuntcapacitance of only 0.7 pF.

Active probes can be used to make highimpedance measurements of small m V -level RFsignals over a wider frequency range. Care mustbe taken in the probe preamplifier design tominimise distortion products (important for EMCmeasurements) and to maintain a wide dynamicrange. Probes such as HP 41800A cover thefrequency range 5 Hz to 500 MHz with a unitarytransfer function and have an input impedance of100 kohm with 3 pF shunt capacitance.

As a further example, the Anritsu MA44B activeprobe has a frequency range from 10kHz to30 MHz, a unitary transfer function and an inputimpedance of 300 kohm with 20 pF. Its maximumRF input voltage is around 0.25 V. Activewideband probes are used mainly in low signallevel development or diagnostic testing applications where circuit boards are being examined forsources of interference.

Some passive direct connection probes can beused to measure the relatively large signals up to300 V pk. from exported transients as in the ·DEFsrfAN 59-41 DCE03 test for example, see Figure4.21a and b. They can be used to monitor theimpressed RF interference signal voltage of a fewvolts RMS on power lines as in MIL STD 462(N3) CS02. See Figure 4.22.

4.2.6 Directly connected transformers

Direct connection transformers are used in EMItesting for:

Series injection of audio- and low-frequencyinterference signals onto power linesMeasurement of low-frequency conductedemissions on power linesSeries injection of transients into power linesGenerating several kV pulses for use withspark gaps in discharging transients intoscreened cables or structures.

Audio-frequency signals may be injected intocircuits for EMC tests such as MIL STD 462CSO 1 using directly connected iron cored transformers. Such transformers have primary windingswith less than a 5 ohm impedance, a turns ratio of2: 1 and a heavy-duty secondary winding capableof carrying 50 A AC or DC. They are employedas shown in Figure 4.23 to series inject the audiointerference signal of up to about 100 W into themains circuit of the EDT. In some cases anadditional secondary winding is provided which isused to connect a meter/oscilloscope to monitor theinjected signal level.

Audio-frequency transformers can also be usedto couple low frequency conducted emissions frompower cables to an isolated measurement meter asin Figure 4.24. For AC lines it may be necessaryto introduce a mains-freq uency notch filter toprevent input overload of the EMI meter.

Spikes or transients with voltages up to around600 V pk., such as that shown in Figure 4.25a, canbe series injected into power circuits as in Figure4.25b using high-frequency ferrite cored' pulse transformers. Higher voltages of up to 1200 V can beinjected into 50 ohm circuits by using transformerswith a high turns ratio. Typical output winding


DC orAC LINE

~ ?'- ~TEST_~ ~

::c-=-10MF

+ --..J

LINE

TRANSFORMEri ------~ NOT USED

V TESTSAMPLE

AUDIO ISOLATION TRANSFORMER

PRIMARY WINDING P < 5 ohms SECONDARY WINDING S - 10hmFREQUENCY 30Hz 250kHz POWER CAPACITY 100WSECONDARY POWER LINE

CURRENT 50A AC/DC TURNS RATIO 2:1

10 MF

]?igure 4.24 Audio transformer used as a pick-up devicefor conducted interference at audio flowfrequencies

Some EMC specifications such as MIL STD1541 (for spacecraft hardware) require theinjection of a high voltage transient onto thestructure of the system under test. An EHT pulsetransformer such as Solar 7115-1 can be used togenerate up to 15 kV which can then bedischarged to the structure or equipment casebeing tested via a spark gap as in Figure 4.26.

Figure 4.23 Current injection via heavy currentiron-cored transformer

I AF. W A.F. ~OSCILLATOR AMPLIFIER P

NOTCH OR HIGH PASSTO SUPPRESSAC POWER LINEFREQUENCIES

I::UT

SWITCH

INSTRUMENTATION SUPPLY

(b)

(a)

SCREENED ROOM WALL

30,OOOuF CAPACITOR FOR DC SUPPLIES

POWER SUPPLY

EQUIPMENTUNDERTEST

BALANCED OSCILLOSCOPE PROBES

UI IIDIFFERENTIAL~ OSCILLOSCOPE

PROBES CONNECTED 50mm from EUT CONNECTOR

Figure 4.21 Direct voltage probe measurement used inUK DEp-' STAN 59-41 DCE03(a) Test configuration for AC supply( b) Test method DCE03 exported spikes

on power lines


{MIL·STD.461 CS02} EMI METER(N4-50kHz·400MHz) MEASURES RF CURRENT

}zgure 4.22 Passive and active probes used to measureamplitude of injected RF signal

inductance is around 300,uH. I t is possible for thetransient wave shape to be slightly degraded bythe transformer which has higher outputimpedance than the spike generator.

4.3 Inductively coupled devices

4.3.1 Cable current probes

The most widely used inductively coupled EMCmeasurement device is the cable current probe orclamp. I t is a conveniently small device, about5-10 cm across which is constructed in two halveshinged together. It can be clamped around asingle conductor or cable bundle with ease. Whenconnected to an EMI meter, sensitive measurements as low as a few microamps of RF currentcan be made. Figure 4.27 shows a typical probewith an aperture of about 4 cm in diameter.

A circumferential magnetic field exists roundany conductor carrying the RF current to be


TIME (microseconds)

(a)

H FIELD AROUND THE CONDUCTOR

H

CONDUCTOR CARRYINGA CURRENT

CURRENT FLOWING INCONDUCTOR UNDER TEST

BASIC RF CURRENT PROBE

SERIES INJECTION ON AC LINE

(b)

PARALLEL INJECTION ON DC LINE

SPLIT FERRITE TOROIDCONCENTRATES H FIELD AROUNDCABLE UNDER TEST THROUGHPICK-UP COIL

Figure 4.26 HF step-up transformer for spike injection

Figure 4.25 Transient injection from spike generatorswith transformer outputs (a) Transientshape with unterminated output as requiredby MIL STD 462 (b) Series injection onA Cline (left) and parallel injection on DCline (right)

:::::; 600 V OUTPUT

TRANSIENTGENERATOR

tFERRITE COREDHIGH FREQUENCYTRANSFORMER

TRANSFORMED UP TO :;:::,-10kv

EUT

SPIKE DISCHARGETO EUTCASE

Figure 4.28 Principle of cable current probe field aroundconductor

measured as. ,shown in Figure 4.28. The toroidalferrite core of the current probe concentrates thisflux within itself. The toroidal core is split toallow the probe to open. The mating surfaces ofthe two halves must be carefully machined andpositioned such that a low reluctance path ismaintained around the magnetic circuit of thetoroid. The permeability of the ferrite, and notthat of the air gap, must be the limiting factorminimising the overall toroid reluctance.

The concentrated flux in the toroid induces avol tage in the outpu t winding proportional to thepermeability, cross sectional area of the ferritetoroid and the number of turns in the outputwinding.

Typical EMGY current probe for measuringconducted RF emissions

Figure 4.27

\CABLE UNDER TEST

PROBE OUTPUT CABLETO EMI METER

Vout kflANf Icond . 4.1

where Vout output voltagek constantfl core permeabilityA CSA of coreN number of turnsf frequency

Icond current in conductor

The RF current probe is therefore a widebandRF transformer which uses the conductor inwhich the current to be measured is flowing asthe single-turn primary winding. See Figure4.29. Note that an electrostatic shield is includedIn the construction to prevent capacitive


50 100 2005 10.5.05 .1

.5

1.0

FREQUENCY IN MEGAHERTZ

2.0

.01 "--_--A._"--_---&_.a.--_---&_""'--__.L..--...I-o-....

.01

.05

(J)

~Io~w()z«owCl.

~a:I:l:(J)z«a:f-

OUTPUT TO COAXIAL CABLE,50 OHMS IMPEDANCE

Figure 4.29 RF cable current probe is transformer

V(:) ~ SECONDARY

POWER LINE /J ELECTROSTATIC

~~~~~~;ECAN ==== / SHIELD (CASE)

FERRITE CORE /O---~---~---:--i(f)--I: 0

TRANSFORMED DOV' / \ ~OWER LINE AC OR DC

50 OHM LOAD PRIMARY RF CURRENT TO BE ~C~~iN~U~:~~~GH(TEST SAMPLE MEASURED CONDUCTOR UNDERLEAD) TEST

coupling between the windings and theconductor under test. The performance of thecurrent probe may be expressed in terms of asensor transfer impedance:

50 100 200

500 1000

10.5

10 50 100


.05

-10 L.-__--A._--'-- -'-----& -"

1

(a)

-15 1--__..L.----'--__--L...----I"--__.L..--&-__--I...---lL.---I

.01

::2:IoWzo~ +309wcoa:~ +20>~«fg +10~w()z«owCl.

~a:I:l:(J)z«a:f-

+10 ....----....---__---.,-----.,.---...-__-- r-----,

(b)

Figure 4.30 Transfer Junctions oj typical current probe( a) Transfer impedance in ohms( b) Tran~fer impedance in dB ohms


Figure 4.31 Transfer Junctions oj HF current probe

-10

Reproduced by permission or Camel Labs Corp.

Reproduced by permission of' Camel Labs Corp.

-5

+5

currents of the order of 200-400 amps, dependingon power line frequency, can be accommodatedbefore saturation results.

Because the current probe forms a transformerwith the conductor under test as the primarywinding, a small series resistance of a fraction ofan ohm is introduced in to the primary circuitowing to transformation of the 50 ohm (plus any

4.2

The transfer function of a more sensitive(Z == 0 to 15 dBohm) and higher frequency(1 MHz-1 GHz) current probe (Carnel Labs.Corporation 94111-1) [8] can be seen in Figure4.31.

Current probes are sensitive well-behavedsensors which are easy to use and give repeatableresults. RF currents from microamps up to 20 Aor more can be measured. However, care must betaken with regard to the total current (RF andpower line components) flowing in the conductorbeing measured. At high combined current levelsit is possible to saturate the ferrite toroidal coreand thus change its small-signal permeabili ty.This results in incorrect measurement of the RFcomponent of the total current. Power line

where Vout is the voltage developed across a 50 ohmtermination on the output and I cond is the currentflowing in the conductor being measured. Typicaltransfer impedances are about 1 ohm, giving1 volt output per amp. Practical current probesare optimised with internal loading to yield flattransfer impedances over most of their operatingfrequency range. The transfer impedance of atypical general performance current probe(Carnel Labs. Corporation 91550-2) [8] can beseen in Figure 4.30a. The frequency dependenceindicated in eqn. 4.1 can be seen below 100kHz.The upper frequency limit of a current probedesign may be determined by factors such as selfresonance and ferrite losses.

The probe transfer impedance is often expressedin terms of dB above or below 1 ohm as in Figure4.30b. This convenient notation allows the EMCengineer to calculate the measured current from:


internal probe resistance) secondary load by theeffective turns ratio of the probe. In mostmeasurement situations the additionaltransformed resistance has no effect on theoperation of the circuit under test.

Care should be exercised when current probesare used to monitor circuits which may besensitive to additional resistive loading. Freerunning RF oscillators and circuits exhibi tingspurious oscillation are examples where frequencypulling may result from the additional loading.The spectrum observed with the probe in suchcases may not be truly representative of thatunder unloaded conditions.

Current probes are used extensively in EMCconducted emission testing to military specifications such as MIL STD 462 and DEF STAN 5941. Their use is less evident in EMC standardsrelating to testing of commercial electronicequipment where the LISN method currentlypredominates.

A current probe will measure the sum of all

currents flowing in a cable bundle, or the screencurrent in the case of a screened cable, see Figure4.32a. If the probe is placed around oneconductor in a bundle with another conductorclose by, then although the probe constructionoffers considerable rejection of unwanted signals,it may not measure just the current flowing in thecable under test, see Figure 4.32b. In such casesthe conduc tors should be separated a fewcentimetres if possible and any changes in themeasured current noted.

During development testing it may be advantageous to determine not only the magnitude butalso the nature of the interference current flowingin a set of conductors to learn something aboutthe probable source of the interference. The differential mode current can best be measured asshown in Figure 4.32c and the common-modecurrent as shown in Figure 4.32d.

The RF current in a cable will usually exhibitstanding wave patterns along its length. Inconducted emission EMC tests this phenomenon

iSCREENCURRENT

(a)

DIFFERENTIAL

MODE

(b)

COMMON MODE

..

i DIFFERENTIALMODECURRENT

i COMMON MODECURRENT

(c) (d)

F'igure 4.32 Making cable current measurements with a current probe( a) Measuring shielded cable screen current( b) Measuring of commonIdifferential mode( c) Maximising differential mode pickup( d) Maximising common mode pickup

Reproduced by permission of' leT Inc.


4.3.2 Current injection probes

Specially designed low-loss current probes canbe used to inject RF current into cables.Formal EMC tests using this technique are arecen t introduction and have been pioneered inthe UK primarily for testing militaryequipment. However, the technique can also beused for the developmen t testing of commercialelectronics.

The transformer action of a toroidal currentprobe is exploited to produce a current injectiontest for cables which does not require directconnection to the circuits being tested. Thismakes the tests quicker and potentially morerepresentative of the functioning system with anundisturbed electromagnetic topology. Anexample of the use of injection probes is thatcalled for in the clearance 'of military aircraftunder RAE technical memorandum FS510 [9J.The current probe injection test is now includedin DEF STAN 59-41 as DCS02 for power,control and signal leads over the frequency range50 kHz-400 MHz.

The precise test method is complex and thereader should refer to the appropriate referencefor a full understanding. The construction andperformance of the injection probe will beconsidered here as representative of this type ofEMC test device. A low-loss high-power currentinjection probe design capable of handling 200 Winput RF power was produced initially by ERATechnology in the UK and designated ERA36Aand ERA37A. The Carnel Labs. Corporationcompany in the USA has produced an equivalentinjection probe designated model 95242-1. Thecharacteristics of this probe are given in Table 4.2.

The probe has been designed to handle highinput RF power and has more core material andsmaller core gaps than the general emissionmeasurement probes. The low insertion loss ofabout 5 dB (see Figure 4.35a) over most of thefreq uency range also ~akes the device anextremely sensitive emission measurement probewith a transfer impedance as high as +40 dBohm. See Figure 4.35b.

In the injection mode the probe is used with a200 W amplifier and a directional coupler tomeasure the forward power to the probe. This isnecessary because when fixed around a bunch ofconductors with unknown RF impedance, theprobe can present a poor VSWR to the amplifier.

compared against the limits. The conductedemission limits for these two tests (for military landsystems EUTs) are shown in Figure 4.34a asexamples of the levels of conducted RF currentwhich can be measured with toroidal current probes.

EUT

SCREENED ROOM WALL

EUT

SCREENED ROOM WALL

(b)

MEASURING SET

TEST DCE02

POWER SUPPLY

50Q

GROUND PLANE


(a)

GROUND PLANE

EUT

30,OOOjJF CAPACITOR FOR DC SUPPLIES

POWER SUPPLY MEASURING SET

TEST DCE01

Fzgure 4.33 Positioning ofRF current probes on powerand signal cables in UK DEF STAN 59-41tests

IS only important when measuring VHFfrequencies on cable longer than a few metres.The position and amplitude of RF currentmaxima and minima can be explored by movingthe probe along the cable. In some EMCstandards that call for the use of current probes,their position along the cable is specified withregard to the distance from the cable connectorson the EUT. In the DEF STAN 59-41 DCE01test this is 5 cm from the LISN connection on thepower lead. See Figure 4.33a. For the DCE02 test(see Figure 4.33b) with leads longer than 1 m andfor frequencies above 30 MHz the probe ispositioned 5 cm from the connectors at both ends ofthe cable and the greatest emission levels are


100 MHz, 25

100 MHz, 40

100 MHz, 10~--------...... CLASSC

LIMITS FOR LAND SYSTEMS

~-------... CLASS B

2 MHz, 0,..----10-0-M-H-z,-0-t CLASS A and E

~---------...... CLASS 0

TEST METHOD DCE01/2

140y-----.,..-----...,.--------------,..-----...,.------

o

~ 1202-co~ 100zwa:gs 80Uz@ 60(J)

~wo 40wI-U:::>o 20zou

Figure 4.34 Typical testlimits for conductedcurrent as measured withcurrent probe

100MHz10MHz1MHz100kHz10kHz1kHz

-20 ""-- """'- """-- __

100Reproduced by permission or HMSO

30 min.

1: 10.8,uH, ±200/0350 MHz, ±10 0

/0

As a check on the RF current being induced intothe cable a measurement current probe is placed5 cm from the connector of the EDT and theinjection probe as in Figure 4.36.

4.3.3 Close magnetic field probes

RF pickup devices that could be considered assmall magnetic field an tennas have beenincluded in the section on inductively coupled

Table 4.2 High power current injection probe

Specifications model 95242-1Frequency range 2-400 MHzWindow diameter 40 mmOutside dimensions 102 x 130 x 60 mmWeight 1.60 kgInput connector Type NMaximum input power 200 WMaximum core temperature 80°CRecommended maximum 35°C

temperature riseMaximum time for

continuous rating at fullpower

Turns ratioInductanceSelf-resonan t frequencyImpedance at resonance:

unloaded 76 + jO ohms95241-1 calibrations jig 50 + jO ohms, ±5 %

Frequency 2 5 10 20 50 100 200 300 350 400(MHz)Insertion 17 9.5 6.5 6.5 5 5 5 5 5.5 6loss (dB)

devices as they operate at frequencies up to1 GHz and at a distance of only a few mm fromthe curren t carrying conductor. Thus theyoperate in the near field, and with the probeelements being small loops, the predominantcoupling is inductive.

Small probes or wands as they are sometimesreferred to, are an important tool for diagnosticand development testing at board and box level.I t is possible to make a set of probes coveringvarious frequency ranges and to calibrate themsufficient for nonconformance test purposes.Proprietory equipment employing dual pickuploops and a balun can be obtained such asHPl1940A/11941A. These probes have calibratedtransfer functions to yi~ld the absolute magneticfield st~ength in dB,uA/m from the EMI meterreadings in dB,uV. For example, from 9 kHz to30 MHz with probe HPl1941A and 30 MHz to1 GHz with HPl1940A [10].

Close-in probes can be used for tracking downsources of RF emission from components, circuitboards, apertures in equipment boxes and leakagefrom connectors and screened cables. Whenconnected to a spectrum analyser that can rapidlysweep over wide bandwidths to reveal theamplitude of components of complex signals, theEMC engineer has a powerful diagnostic tool.

4.3.4 Surface current probes

These small inductively coupled sensors aredesigned to measure the RF current flowing in aparticular direction on a conducting surface. Theunit is a proximity RF current transformer wherethe surface on which the insulated device sits formsthe primary circuit. The probe is placed with its


Figure 4.35 Performancecharacteristic of typicalcurrent injection probe

200 300 4005001005020

FREQUENCY IN MHz

1052

\\

\ \\ \\ ~

\ :\\ :\

\ ~

\ ''- "'- ..................."....-.---- .",/, ----------

" ,~ ,"""'---- .,;--------------_......

20

18

16

CO 14"0

~en 12en0

10-J

Z0i= 8a:w

6en~

4

2

01

(a)

Injection Probe Model 95242-1

50

en~ 20:r:0~w() 10z«0wa.~ 5a:wLLC/)Z«a:.-

2

2 5 10 20 50 100 200 500

(b) FREQUENCY IN MHz Reproduced by permission o[ Camel Labs Corp.

insulated base against the conducting surface inwhich the RF current is flowing, as in Figure 4.37,and is orientated to obtain the maximum responseon the EMI meter to which it is connected.

The RF current flowing in the test surfacebeneath the effective area of the probe is obtainedby dividing the measured EMI voltage by thetransfer impedance of the probe in ohms, in thesame manner as for a cable current probe.' SeeFigure 4.38 for the transfer impedance of typicaldevices such as Fischer probes [11] modelnumbers F-91 and F-92.

The total RF current flowing in a surface can be

measured by dividing the surface into probefootprint segments along a line normal to thedirection of current flow and summing themeasurement made at each segment.

These probes operate over a frequency rangefrom around a hundred kHz up to a few hundredMHz and can be used to measure the skin currentswhich are set up on system structures such as cabletrunking, equipment racks or mainframe computercabinets when illuminated by electromagneticradiation. They are also useful in determining thedistribution of interference ground currents withinsystems such as military vehicles and spacecraft.



MEASUREMENTCURRENT PROBE Injection currents controlled

to give known/wantedmeasured current levels inthe cable under test

RFPOWERMETER

Produce records ofpower meter readingsfor given measuredcable current values

Cable into which current is beinginjected presents variableimpedance to injection probeleading to VSWR variations

SIGNALGENERATOR

EMIMETER

AMPLIFIER

DIRECTIONAL COUPLER

FORWARD POWER/TO PROBE

___ INJECTION PROBE

TESTSAMPLE

Figure 4.36 Current injectionand measurement probesused together to stimulatecable to a known level

Reprod uced by permission of' ICT Ine.

model no. F·91FISCHER PROBE

-10 ---------.--------..--------,.----.

absorbing element to be adjusted along the lengthof the mains input cable to an EDT. Theimpedance discontinui ty represented by theabsorbing clamp on the single-wire transmission

~I0T""

UJ>0 -20

/CD«.0"0UJ0Z«0UJa.. -30~0:UJlL(f)Z«a:I-

-40

1MHz 10MHz 100MHz

FREQUENCY

FISCHER PROBE model no. F-92

+10~I0T""

UJ>0CD«.c"0UJ0Z«0UJa..~ -100:UJlL(f)Z«a:I-

1MHz 10MHz 100MHz 1000MHz

FREQUENCY

Figure 4.38 Transfer impedance oj surface currentprobes

Reproduced by permission of Fischer Custom Coms.Inc.

B FIELDSurface current probe is orientated formaximum pick-up

Figure 4.37 Using surface current probe

4.3.5 Cable RF current clamps

In his survey of EMC measurement techniquesJackson [1] gives a brief history of thedevelopment of the cable current clamp frominitial experiments in Sweden. Experiments werecarried out to enable radiated field measurementsto be made in a laboratory by using a movablecoaxial filter on the mains lead of an EDT tomaximise the radiation from the equipment.Further work by the Swiss PTT laboratories ledto the use of ferrite rings, and this evolved intothe ferrite cable clamp.

For a full history of the development of thisprobe consult the reference given in CISPRpublication 16, Appendix S. The probe specifiedin CISPR 16 was conceived in 1966 for use inradiated emission testing in an attempt to obtainbetter agreement between measurement resultsand operational experience.

The use of the clamp requires the position of the


Figure 4.39 Use ojabsorbing clamp Jormeasurement ofinterference power30 MHz-300 MHz

Typical exanlple

A

MOVEABLE CLAMP

d~ \..--f=E::::::::t--_H_---l F

~1-B---t-C-+--F=D=f----EtG-J-----B-II-_~~~~~~-E,t=:==:::1...;;.;;.;;;..;\~-~Q:

1----\--When L~4.2m, F min~ 40MHzFIXED

CABLE UNDER TEST ABSORBER

DETAILS OF ABSORBING CLAMP

CABLEUNDER TEST

EUT

d

SINGLE TURN MEASUREMENT CABLESHIELDED CURRENT PROBE ABSORBER RINGS

/---,..........,.-------~-----E~~~~~~~}~-~i-+-H--,.~1~-:

60rl~~~_____ ~~5.f8~ METERl=1~~r=:H:~t:=+~~I:::::::l------===RRRR_=Jf

--~12-18==m~::---t=:rF=:rt==:t-,.---r:==n:==n:==n==nF=rl::=:r1F=r_==--:.=-=-:.= 0 EfEjEj~ TO

3 r~gs 56 r?ngs \ BU

MAINS

ABSORBINGFERRITE CLAMP Reproduced by permission of BSI

line, causes the line impedance presented to thein terference source to change in such a way thatthe maximum RF power is transferred to theassociated measurement probe. In this way, ameasurement can be made at each interferencefrequency which represents the maximum possiblepower which can be radiated by the equipmentand its cable.

The construction of the clamp is shown inFigure 4.39. The key measurement component ofthe clamp is the Faraday loop of coaxial cablewhich is wound around three ferrite rings,labelled C in the diagram. This acts as a toroidalcurrent probe and produces an output voltageproportional to the RF current flowing on thecable under test. Further along the cable from theEUT which generates the interference emission,are anumber of lossy ferrite rings which act toabsorb the interference signal once it is past thecurrent probe. The assembly is moved up anddown the cable under test to obtain maximumreadings at each frequency of interest. Otherferrite rings are placed around the measurementcoaxial cable from the current probe to absorbany spurious signal pickup on the shield.

The measurement of RF power available to beradiated from the cable under test at eachfrequency of interest is made by a substitutionmethod where the signal reading from the EMI

meter is compared with that obtained during acalibration of the clamp in a special jig andrecorded on a calibration chart. The chart is arecord of the clamp insertion loss as a function offrequency from 30 to 300 MHz. Details of thecurrent clamp and the calibration method can befound in EN55014/BS800 and VDE0875.

The probe- may ,give readings which correlatewell with everyday experience of radiated interference from typical commercial equipment, but it istime consuming to have to adjust the position ofthe probe along the line to obtain a luaximumsignal reading at every frequency of interest in anemission spectrum. The need to physically movethe probe means that it is very difficult toautomate the process in order to speed up themeasurement.

The absorbing clamp method, although widelyused for testing commercial electronicequipment, is not without its critics. Kwan [12]presents a keen analysis of the drawbacks of themethod and suggests how the device may beimproved by separating the current probe andthe mismatch/absorption unit so that they can bemoved apart as necessary, to ensure that theabsolute maximum energy transfer to the probeis obtained.

An idea of the interference RF power levels thatcan be measured with the absorbing clamp


Table 4.3 Typical interference levels on cables that can be measured using absorbing clamp device (EN55014)

Frequency Household and Portable tools: rated power of motorrange similar appliances

~700W 700-1000 W 1000-2000W

MHz dB (pW) dB (pW) dB (pW) dB (pW) dB (pW) dB (pW) dB (pW) dB (pW)quasipeak average* quasipeak average* quasipeak average* quasipeak average*

30 to 300 Increasing linearly Increasing linearly Increasing linearly Increasing linearlywith the frequency with the frequency with the frequency with the frequencyfrom from from from45 to 55 35 to 45 45 to 55 35 to 45 49 to 59 39 to 49 55 to 65 45 to 55

* If the average limit is met when using a quasipeak detector receiver, the test unit shall be deemed to meet both limitsand measurements with the average detector receiver need not be carried out.

TRANSIENT SOURCE POWER LINE FREQUENCY SOURCE

BOX INDUCTION WINDING CABLE INDUCTION WINDING

JACKSON, G.A.: 'Survey of EMC measurementtechniques', Electr. Commun. Eng. ]., March/April1989

2 'Radio interference measuring apparatus andmeasurement methods'. CISPR 16

3 MIL STD 462 Notice 3, 1970, pp. 21-234 'Instruments, components and accessories for the RFI/

EMC engineer'. Solar Electronics Co., 901 NorthHighland Avenue, Hollywood, CA 90038, USA

4.4 References

the induction cable. A current of 20 amps ispassed through the wire at the power linefrequency appropriate to the unit under test(usually 50, 60 or 400 Hz). The field producedaround the induction cable is closely coupled tothe cable under test and the coupled power lineinterference signal enters the EDT via the cableconnectors. EDT power line cables are exemptfrom this test.

Signal cables are to be tested in bundles orgroups. However, White [13] questions thevalidity of spiralling the test wire around thecable to be tested. He suggests laying the wirealong side and parallel to, the cable under test tomaximise the induced voltage in any cable pairin the harness being subjected to the inductionfield.

A very similar power line induction field test iscarried out on the EDT itself by wrapping thewire around the equipment case at a pitchseparation of 30 cm over the heigh t of the boxand applying the 20 A current. See Figure 4.40.

Spikes or transients which are produced fromthe generator detailed in test CS06 of MIL STD462 are also fed into the same induction cablewrapped around both the box and interfacecables, to perform transient pulse tests. Thecorrect applied stimulus level is obtained bysetting the spike generator outpu t to 100 Vmeasured across a 5 ohm load.

EUTor

LOAD

CABLE UNDER TEST(not power cables)

\

>oo

MIL STD 462CS 06

method can be seen in Table 4.3. This shows thelimits set out in EN55014 for the interferencewhich is tolerable from household appliances andportable power tools. Levels of around 35 to50 dB PWare specified when measured using anaverage and quasipeak detector.

AC POWER MAINSVARIAC

Figure 4.40 Magnetic induction tests using simple wirecoils on boxes and cables

4.3.6 Magnetic induction tests

The susceptibility of boxes of electronics andattached cables to magnetic induction fields atvery low frequencies can be determined bygenerating fields from nearby wires or coilscarrying heavy current at the power linefrequencies. Such tests are specified in MILSTD 462 (RS02) and call for heavy-duty wires tobe wound around the interface signal or controlcable under test in a spiral with a pitch of twoturns per metre. See Figure 4.40 for the layout of


5 Draft BS6667, 1990: Electromagnetic compatibilityfor industrial process measurement and controlequipment, Part 5: surge immunity requirements

6 BORESERO, M., VIZIO, G. and NANO, E.: 'Acritical analysis of the immunity methods forsound and television broadcast receivers'.Proceedings of lEE symposium on EMC, UK,1990, pp. 219-226

7 'A two-part five-day comprehensive training coursein EMC' part 2. Don White Consultants, PO BoxD, Gainesville, Virginia 22065, USA, page 2.1.28

8 'Calibration & operation manuals for Carnel Labs.Corporation current probes 91550-2 and 941 f 1-1'.Carnel Labs. Corporation, 21434 Osborne St.,Canoga Park, California, 91304-1520, USA

9 'Recommended test specification for the electromagnetic compatibility of aircraft equipment'. Technicalmemorandum FS(F)510, RAE Farnborough,Hants, UK

10 1990 product guide. Hewlett Packard, p. 12811 'Fischer surface current probes'. Fischer Custom

Communications, Box 581, Manhattan Beach, CA90266, USA

12 KWAN, H.K.: 'A theory of operation of the CISPRabsorbing' clamp'. Proceedings of lEE symposiumon EMC, 1988, pp. 141-143

13 'A two-part five-day comprehensive traInIngcourse in EMC' part 2. Don White Consultants,PO Box D, Gainesville, Virginia 22065, USA,page 2.2.23

Chapter 5

Introduction to antennas

\

diameter d

(bj

(aj

BALANCED MATCHED\ TRANSMISSION LINE

DIPOLE CONDUCTING ELEMENTS

IfSURFACE CURRENTS INDUCED BYINCIDENT WAVE

RF POWER EXTRACTION(is V TECHNIQUE

....-----1] LOAD Z

VOLTAGE DEVELOPED =VPOWER DELIVERED P

Holm

Holm

Small gap

Ev/m

S (x, y, z) \

POWER IN WAVEFIELD ARBITRARY 3D CONDUCTING SURFACEPw Ex H W/M2 ( IN REAL ANTENNAS THE STRUCTURE IS

CHOSEN TO HAVE SPECIFIC PROPERTIES)

5.2 EMC antenna basics

5.1 Arbitrary antenna and simple dipole( aj Arbitrary antenna concept( bj Simple nonarbitrary antenna - dipole

5.2.1 Arbitrary antennas

Consider the impact of an electromagnetic waveshown in Figure 5.1, which is defined by E) l!(wavefield impedance), wavefront phasecurvature and the direction of the wave normal k,which is incident on an arbitrary conductingsurface S from which power can be extracted to aload Z. A linear transfer factor can be establishedfor the voltage delivered to the load in terms ofthe incident wavefield. This factor is a function ofall the specific conditions under which it wasobtained. It depends on the detailed nature of the

E vim

Wave impE~dorlceIZw

In the following chapters, antennas that arecommonly used in EMC radiated emission andsusceptibility testing are examined individually indetail. Particular characteristics are explored andreference made to tests that employ the varioustypes of antenna.

INCIDENTWAVEFIELD

\

EMC antennas

tests on large items of equipmentmay have to be made on an open range in thepresence of ambient electromagnetic noise, anuncertain ground plane and possible reflectionsfrom objects. All these effects can increasemeasurement uncertainty. Standards coveringradiated emission testing of commercial electronic

and particularly informationusually specify testing on a

type of open test site that has to becalibrated before use. Repeatability of

test results measured on an open range is howeveralso open to question and continues to stimulatedebate in the EMC community. See Chapter 3References 11, 12, 15-17 and 19.

Whether the radiated emission testing is carriedout in screened rooms or on an open field test site

antennas used are the components in themeasurement chain. What these various antennas

measure, and the influence that surroundhave on these measurements, is often not

and can lead to considerable"YV'l11C'11Y\rl .c>Y'C'i-"l' Y\r-41Y\r<' and error.

......... A.''-''--'".L 'J'L.L.LL"-.':;".L.L'--'L.L'--' antenna theory is complex andmathematical and much has been written

on the basic of antennas [1-4] and theof many antennas for communica-

tions use from HI" to millimetric wavelengths7]. It is neither possible nor appropriate to coversuch in detail in this text on EMCmeasurement, therefore emphasis is placed on thet-JA. '--""",',.L'--'L-l,.L aspects of antenna characteristics which

engineers and test technicians can use withview to minimising potential measurement errors.An overview of the important basic characteris

tics of antennas is presented to establish aframework of equations within which

antenna types may be discussed further.

EMC measurements can be complex andinherently unrepeatable. This is particularly truewhen Hlaking radiated emission or susceptibilitymeasurements in anything other than ideal freespace test conditions. Military EMC standards

require measurement to be made inchambers which mayor may not have

radio absorbent material (RAM) to damp outwave reflections; if uncontrolledan increase in measurement

72

INTRODUCI'ION TO ANTENNAS 73

electromagnetic wavefield mentioned earlier andon the exact form of the conducting surface, theway the power is extracted from it and thecomplex impedance of the load. The transferfactor is meaningful only for this specific situationand is not applicable to any other generalsituation where the wavefield is different, theconducting surface is different, or the geometricalrelationship between the two has changed.

A particular conducting surface or antenna (S)with its specific transfer factor may have beendesigned to be exactly what is required to meet aparticular need, say for a communications link.But it may be of little use as a generalmeasurement antenna for EMC work which mustbe wideband, inexpensive, portable and can yieldan absolute measurement of field strength via acalibration curve of its transfer factor which hasbeen derived under controlled conditions.

5.2.2 EMC antennas

Table 5.1 Radiation fields from Hertzian dipole

If the current in the Hertzian dipole is1 10 cos OJt, the radiated electric field intensi tyE and magnetic field intensity H are given inSI units by

E i e [60n 10 dl sin e sin (OJt - 2nrjA)JjAr

H == i¢ [10 dl sin e sin (OJt - 2nr jA) Jj2Ar

where r is the distance from the centre of the dipoleto the field point (r, e, ¢) and i e and i¢ are unitvectors, respectively, in the directions of increasingeand ¢. The electric vector E is proportional tosin eand the field radiation pattern of the dipoleconsists simply of a plot of sin eagainst e.

OJ == angular frequency of driving current2nf

f == frequencyA wavelength of radiation

dl == elemental length of Hertzian dipole

The nature of antennas used In EMC measurements is quite different from those required forspecific functions such as communications. Theoptimum design of EMC test antennas hasreceived relatively little academic attention todate. The design aim for an EMC antenna is toproduce a simple, cheap, rugged, widebandconducting structure that is coupled to a givenload impedance such that the output voltage canbe related simply to the E (electric) or H(magnetic) field strength in a known mannerunder general conditions of use.

Ideally, the transfer factor, or antenna factor,should be calculable directly from a knowledge ofthe geometry of the conducting surfaces, as in thecase of special D-dot antennas [8J so that anabs9lute measurement of field strength can bemade. If this is not possible, the antenna factorshould be derivable from a simple set of measurements using known wavefields to yield a reliablecalibration curve.

There are some simple antenna configurationsfor which solutions of the induced current andvoltage may be calculated for a specifiedexcitation [9J. Equally, the E and H fieldsproduced by the antenna for a given currentexcitation can also be calculated. The best knownof these is that for the electrically short Hertziandipole and an oscillatory excitation current. Forexample Benumof [1 OJ gives the formulas inTable 5.1 for the E and H fields from a dipolegiven an excitation of the form

5.3.1 Gain

5.3 Basic antenna paratneters

5.2Pin (W) X gain

4nR2

Antenna gain

I t is possible therefore to understand how asimple nonarbitrary conducting structure such asa short dipole behaves, and to use this knowledgeto interpret the wavefield it will generate from agiven driving current. I t is also possible tocalculate the reciprocal behaviour of the dipoleoutput voltage in terms of the incident electromagnetic wavefield which is to be measured.

where W == power in watts. Note that for anisotropic source,

Gain is a measure of the ability of an antenna byvirtue of its specific conducting structure toconcen trate a transmitted (radiated) signal in onedirection, or to receive a signal from only onedirection, as opposed to all other directions [11 J.Gain is usually expressed in dB relative to aperfect isotropic radiator.

Consider Figure 5.2 where two antennas, oneisotropic and one with gain, both radiate into asurface at a point P at distance R to deliver a unitpower density Pd (watts/square metre):

input power to isotropicantenna for unit power density at P

input power to directiveantenna for unit power density at P

Thus

5.11 == 10 cos OJt

where 1 is the time-varying driving current, 10 is thepeak driving current, and OJ is the angular frequency.

5.3


DIRECTIVE ANTENNA GAIN = P in (ISOTROPIC) (for a given P d )P in (DIRECTIVE)

Figure 5.2 Concept oj antenna gain

5.9

5.10

gain G

where A is the wavelength. Using eqns. 5.8 and 5.9the received power P can now be expressed as

A2C x PdP == ---4n

of the antenna. For example the physical frontalarea of a horn or parabolic dish antenna is relatedto the energy capture aperture. The two are notequal owing to the way in which the electromagnetic fields are distributed nonuniformly over thephysical area.

The gain of an antenna with a given physicalaperture is dependent on the wavelength of theradiation being transmitted or received. The relationship between the aperture as defined by eqn.5.8 and gain is also dependent on wavelength:

4n X aperture

A2

5.4

E H

Lk

POWER DENSITY ONSURFACE = P d W/M 2

ANTENNA SYSTEMCASE 1 - ISOTROPIC DEVICECASE 2 - DIRECTIVE DEVICE

and

-T~-+----t/

CASE 1 - P in W (ISOTROPIC)CASE 2 - Pin W (DIRECTIVE)

P in (DIRECTIVE) < P in (ISOTROPIC)

4n<0

These relationships can be used to derive both thetransmitting and receiving antenna factors.

where <0 is impedance offree space 377 ohms, andE is the magnitude of the wave electric field at adistance R. To produce a 1 Vim field at 1 m froman isotropic antenna an input power of 33 m W isrequired. For 100V/m a power of 330W isneeded. Also

or 5.11

5.3.3 Transmitting antenna factor

Consider a feeder cable delivering a power Pt to anantenna. The voltage Von the line of impedance <is given by

5.5

where H is the magnitude of the wave magneticfield. Thus to produce 1 A/m at 1 m a power of4800 W is required.

For a directive antenna,

4nR2 E2

Pin<0 X gain

5.6

and Pin4nR2 H 2 <0

5.7gaIn

V2

PI = ZFrom eqns. 5.6 and 5.12

V2 4nR2 E 2

< <0 X gain

5.12

The ratio E / V is called the transmISSIon antennafactor (TAF) and relates the field strength E(V/m) produced by the antenna at a distance Rmetres with an input signal of voltage V across aline of impedance < ohms. This relationship isuseful in calculating the field strength incident onan equipment at a distance from a transmittingantenna in a radiated susceptibility test.

The Lorenz reciprocity theorem [1 OJ shows thatthe gain figure for a transmitting antenna is thesame as that for the identical receiving antenna.However, to better understand the concept ofreceive gain it is necessary to introduce the idea ofantenna aperture.

5.3.2 Aperture

Consider an antenna in an incident field with apower density of Pd watts/square metre whichdelivers a power P watts to a matched load. Then

therefore 5.13

and the aperture has the dimensions of area. Theaperture is not identical with the physicalantenna area, nor its projected area onto a planenormal to the incident wave direction. In somecases it can be identified with the physical shape

P == Pd X aperture 5.8 5.3.4 Receiving antenna factor

Bennett [12J states that 'antenna factors are widelyused, yet the literature does not appear to includeany detailed mathematical characterisations ofsuch factors', and proceeds to develop usefulformulas for some standard dipole antennas.

INTRODUCTION TO ANTENNAS 75

V outloutZout

RECEIVER

ANTENNA ILOAD

(a)

lout (Zout + Za)

(b)

Zm Zmlout 19

MID - PLANEI

II

I

II I

I Irdr

GENERATOR

I

Figure 5.3 Antenna mutual impedance coupling( a) Identical antennas separated by d( b) Equivalent circuitZg == generator impedanceZa == antenna impedanceZm == mutual coupling impedanceZout == load impedanceIg == generator currentlout == load current

characteristics of the antenna and the relationships involving gain and distance which describethe coupling between the antennas will no longerrigidly apply. The antenna radiation pattern,antenna factor and feed point impedance willdepart from calibrated values and any measurements then made will be subject to an increaseduncertainty.

Consider two identical antennas separated by adistance d in Figure 5.3a. There will be a mutualimpedance term Zm introduced into theequivalent circuits of the two antennas as shownin Figure 5.3b. The mutual impedance of simpleantennas can be calculated from the antennadimensions and spacing and may be found instandard texts. The circuit equations relating toFigure 5.3b are

5.3.5 Antenna phase centre

5.3.6 Mutual ante11na coupling

When operating an antenna close to anothersimilar antenna, ground plane, earth or screenedroom wall, undesirable mutual coupling will takeplace between the antennas, or between theantenna and its electrical image in any reflection.Such an effect will alter the normally calibrated

When an antenna is radiating into free space, thewavefront at great distance (in the far field) isusually assumed to be plane. This is howeveralvvays an approximation as the radiation isemitted from a structure with a finite size and sothe wavefront will have a slight curvature at afinite distance. The apparent centre of thecurvature is usually situated on the antennastructure and is called the phase centre.

When making calculations or measurementswhich involve the distance from an antenna to anobject, the distance from the phase centre shouldalways be used. This can be important when suchdistances are comparable with the antennadimensions, as in the case of some EMC radiatedemission measurements where the EDT toantenna separation is only 1 m and the antenna isabout half-a-metre long. The position of the phasecentre on most antennas is obvious, for exampleat the feed point of a dipole, but for somefrequency independent antennas such as logperiodics and log spirals the phase centre willmove along the axis of the antenna withfreq uency. This can lead to measurement errorsand even calibration errors at VHF jDHF whenthe two-antenna method is used for such antennaswith a short 1 m separation.

In general, the receIvIng antenna factor isdefined as EjV where E is the magnitude of theelectric field of the incident wave and V is thevoltage delivered from the antenna to the load ofimpedance Z. It can be shown that

~ = ~ [z :~oainr 5.14

Knowledge of the derivation of these two antennafactors is most important to the EMC engineer asit permits the setting up of experiments tocalibrate antennas and derive the factors withconfidence. Otherwise the engineer has to rely onthe manufacturer's data supplied with proprietaryequipment which may contain artifacts of possiblyimperfect calibration procedures. Manufacturer'santenna factors are usually reliable and availablein graphical form as a function of frequency.Examples for specific antennas are shown later.


which leads to 10,000...-------------------.....,

5.3.7 Wavefield impedance

As the separation d becomes large the value of <mtends to zero and the current flowing in thereceiving antenna lout as a result of mutualimpedance coupling tends to zero. The couplingbetween the two antennas then reverts to onlyradiative coupling which is determined by thenormal gain equations given earlier.

The problem of mutual coupling betweenantennas and their images in the earth and otherground planes bedevils EMC measurements andgives rise to much comment [13-18]. This subjectis discussed again later in the text concerned withopen range and screened room radiative testing.

--Induction field-- ---Far field----I

Zo = 120 1t = 377 n

ZH = Zo dJd2 + 1

Zo = 1201t = 377n

ZE = Zo v'd2+1--d--

IEa and H<j>U+

I

30

100

(J)

E'§ 300

3=N

3000

1000

5.15lout = Zm- (Zout + ZaJ (Zg + Za)

<m

101.0104----------+------------'

0.1

DISTANCE FROM SOURCE d (increments of r = _A_)21t

Figure 5.5 Variation of waveJield impedance withdistance from source5.16<ohms

A simple representation of a sinusoidally varyingelectromagnetic wave is shown in Figure 5.4. Theelectric field E V 1m and the magnetic fieldcomponents H Aim are orthogonal and the wavepropagation velocity is v m/s. In a vacuumv == c == 2.9979 X 108

, normally approximated to3 X 108 m/s. The wave impedance

EV/m

HA/m

The solution of Maxwell's equations for thisantenna element [9J leads to the components forthe electric and magnetic fields shown in Table 5.2.

5.3.8 Near-field/far-field boundary

An understanding of the origin of the Inverselinear, quadratic and cubic dependence of fieldstrength with distance can be obtained byconsidering the elemental dipole in Figure 5.6,which is of length h and carries a uniform sinusoidally oscillating current of the form

and not as an absolute distance in metres. Agraph of wavefield impedance as a function ofdistance from the source in terms of wavelength,for both an electric and magnetic oscillator, canbe seen in Figure 5.5. Note that close to thesource, that is at distances of less than )"/2n, boththe E- and H-fields vary rapidly as square andcubic terms with distance. Beyond this distancethe wavefield impedance from either type ofsource tends to the same constant value of 377ohms or 120n. This is known as the free-spacewave impedance which occurs in what is calledthe 'far field' of the source.

5.17I == 10 exp (jwt)

v

The wavefield impedance close to a source of electromagnetic radiation is determined by the natureof the source. For example, for an electric-fieldsource such as a small dipole oscillating charge,the wave impedance close to the source will behigh at several thousand ohms, as E is muchgreater than H. For a small electrostaticallyscreened oscilIa ting current loop (with minimurnexposed voltages) the magnetic field componentpredominates and the ratio of EIH is low,therefore the wavefield impedance is low, perhapsonly a few tens of ohms close to the source.

I t is useful to define the distance from an EMsource in terms of the wavelength being radiated

Figure 5.4 Plane-polarised wave

z

OSCILLATINGDIPOLECHARGE ~

H$

Efl Er

p


Table 5.2 Solution ,[or field components for short dipole

10 h CP 1). B 5.18H¢ ==- -+- SIn4n r r2

Er ) casB 5.19

10 h CWIl 1 Zo). 5.20E() == -+-.-+- smB4n r Jwcr3 r2

Balanis [4] states that the reactive near-fieldregion is defined as that region of field immediatelysurrounding the antenna where the reactive fielddominates, and is taken to exist out to a distance of

3

R < 0.62 [~ ] 5.27

He defines the radiating near-field (Fresnel) regionas that region between the reactive near field andthe radiating far field wherein radiation fieldspredominate and where the angular field distribution is dependent on the distance from theantenna. 'The inner boundary is taken to be thatin eqn 5.27 and the outer boundary is at

2D2

R == T 5.28

where

Er == radial electric fieldE() elevation electric fieldH¢ azimuthal magnetic field

r distance from current element10 == current on elementh == length of element

Zo == impedance of surrounding mediumc == permittivity of surrounding medium

J1 == permeability of surrounding mediumf3 == propagation constant of mediumw == angular frequency of current 10

To help clarify equations 5.18, 5.19 and 5.20 notethe following relationships

w == 2nf 5.21

wherefis the frequency in hertz

f3 == 2nlA 5.22

Af == c 5.23

where A == wavelengthc == speed of electromagnetic wave

propagation

c 1/Vii8 5.24

...---------t----y, I'" I

" I' ....... , I

" I'" I......

Figure 5.6 Oscillating dipole element

Equation 5.19 for the radial electric field E r hasonly two terms in 1/r2 and 1/r3

. This means thatEr decays rapidly with distance from theoscillating element and is therefore onlyimportant close to it. The 1/r3 term can beidentified with the field calculated for an electrostatic dipole.

Equation 5.18 for the azimuthal magnetic fieldshows a l/r and l/r2 dependence. Close to thecurrent element the 1/r 2 term dominates and it isin phase with the excitation current 10 • It can beidentified as the usual magnetic induction fieldobtained from Ampere's law.

Equation 5.20 for the elevation axis E-fieldcontains terms in l/r, l/r2

, and l/r3. The higherorder terms dominate close to the source and areidentified as the dipole and induction fields, withthe 11r term being the radiation field which isdominant at large distances from the source.

These equations show that the characteristics ofthe field close to the source are different fromthose for the field at a position far from theradiating element. The radiation field E and Hcomponents are in phase, spatially orthogonaland the ratio of E to H is the free-space waveimpedance Zoo

The region in which the higher order termsdominate is known as the near field and thatwhere the l/r terms dominate is called the farfield. Examination of eqns. 5.18 to 5.20 will showthat the l/r2 terms are of the same magnitude asthe l/r terms at a distance of A/2n and this issometimes referred to as the near-field/far-fieldboundary. A more comprehensive analysis ofnear-field transition to the far field is given byYaghjian [19] and he classifies the boundarydistances as shown in Figure 5.7.

Zo

f3

5.25

5.26

78 A HANDB'OOK FOR EMC TESTING AND MEASUREMENT

10

41t

VARIATION OF PATTERNWITH DISTANCE IN THEFAR FIELD AROUND THEANTENNA PATTERNFIRST NULL

1t 21t 31tU =( DOd sine

-40~__....a.-_""""'_-""'o...&-_--a......... ___

o

o

-30

co"0

018TANCE FROM ANTENNA (m)

..

z -10a:w

~a:w~o~ -20>

~wa:

REGIONS OF STABLE ANGULAR PATTERNAT FAR FIELD DISTANCES

>()zw::> 100awa:::u..

NI~

Figure 5.9 Relationship of near-fieldlfar-Jield boundarywith frequency d == A/2n

Reproduced by permission of Scientific-Atlanta

Figure 5.8 Variation of antenna pattern with far-jielddistance. Calculated radiation patterns ofparaboloid antenna for different distancesfrom antenna. Source: HOLLIS) ].S.)LYON) T.]. and CLAYTON) 1. (eds):,Microwave antenna measurements).Scientific-Atlanta) Inc.) July 1970

RANGE r

FAR FIELDF(<1>,0) e ikr

r

",\

RADIATING \NEAR \FIELD ~

_1---...... REACTIVE INEAH IFIELD I

\ IFRESNEL! FRAUNHOFER

I I I

I : I :I I I I

I I I :~ Io (O.6211"537T) 2 0 2 /A

FIELDDISTRIBUTION

Figure 5.7 Classification offield regions aroundantennas

for D » A. The far-field region can be defined asthat region of the field of an antenna where theangular field dis tribu tion is essentiallyindependent of distance from the antenna.Outside the far-field boundary defined by eqn.5.28 the angular distribution is not entirelyindependent of range, but the differences aresignificant only at angles corresponding to thefirst null in the pattern. See Figure 5.8 for thepatterns at different distances from a paraboloidantenna [4].

Knowledge of the near-field/far-field boundaryfor antennas used in EMC measurement is veryimportant. Most radiated measurements at box orsub system level are made close to the equipmentunder test (EDT). Often the antenna being usedeither to receive or transmit with respect to theEDT is not clearly in the far field.

In such a situation the transmitted or incidentwave cannot be accurately defined by reference tocalibrated far field behaviour. The antennabecomes arbitrary as discussed in Section 5.2.1and the measured signal is only meaningful in thecontext of that particular situation, so that noabsolute measurements of E or H for the wavecan be made.

Figure 5.9 shows a graph taken from BS800(now EN550 14) of the near-field/far-fieldboundary distance in metres against frequencybased on the simple A/2n rule. For those predominantly military tests which call for radiatedemission and susceptibility measurements to bemade at 1 ill from the EDT it can be seen thatthis is only valid down to a frequency of 50 MHz.In the case of standards for measuringcommercial electronic equipment, such as FCCpart 15 or EN55022 where radiated emissionmeasurements are allowable on an open site at a


Usually the additional A is left out of thecalculation, but is included here to cover thesituation where the maximum aperture dimensionD in Figure 5.7 is less than a wavelength. TheRayleigh distance to the far field should properlybe measured from the outer boundary of thereactive fields around the antenna [19].

Using the eqn. 5.28, for the example of a 50 cmdiameter dish antenna operating at 10 GHz, thefar field is at 17m from the dish and so all usualEMC measurements at this frequency would bevery much in the near field giving rise to resultswhich at the least would be difficult to interpretand may be suspect.

range of 3, 10 or 30m from the EUT, it can beseen that the measurement is in the far field forfrequencies down to 15 MHz, which is below thelowest frequency to be measured in the test(30MHz).

The simple A/2rc formula yields values for thefar-field distance of only a few centimetres atfrequencies above 1 GHz. It may seem that allmicrowave measurement can automatically becarried out in the far field with a test distance ofonly 1 m. In practice, this is not the casebecause the antennas used in the microwaveregIme are usually of a large aperture, possiblyemploying parabolic reflectors. For suchantennas the near-field/far-field boundaryshould be defined by

5.30

3 dB beamwidth == 90°3 dB beamwidth == 87°3 dB beamwidth == 78°3 dB beamwidth == 64°3 dB beamwidth == 47.8°

Beamwidth

Table 5.3 Calculated beamwidths for varzous dipolelengths

Dipole length

l « Al A/4l A/2l 3A/4l A

The beamwidth is shown in Figure 5.10 as beingmeasured from the halfpower (-3 dB) points. Ifone calculates the total power radiated in themain beam between the first zeros or nulls anddivide this by the mean radiated power levelcalculated over all angles the figure obtained iscalled the directivity of the antenna.

Beamwidths can be calculated for real antennaswith more physically meaningful current distributions but the mathematics are nontrivial. InFigure 5.11 a one can see the polar diagram fordipoles of various lengths. These are calculated[4] using the current distributions in Figure 5.11 band the 3 dB beamwidths are given in Table 5.3.Note that the difference between a halfwave andelectrically short dipole is small.

from d > - D/2 to d < D/2, I == constantotherwise I == 0 the radiated power plot will takethe form

5.29distance

5.3.9 Beamwidth

The angular width of the main beam between thehalfpower points is termed the halfpower or-3 dB beamwidth and is a measure of the degreeto which the antenna can confine and concentratethe radiation towards a single point. Thebeamwidth may be specified in the plane of theelectric field or the magnetic field produced bythe antenna and the values are not necessarily thesame. Simplistically, knowledge of the beamwidth and a separation distance from the antennato a point of interest will allow the spot size whichcan be illuminated by the antenna to becalculated. By reciprocity, this is also the size ofthe area on an extended EUT from whichemission signals will be received most efficientlyby such an antenna.

Consider for example, an antenna aperturewhich has a constant illumination function as inFigure 5.10, producing by Fourier transform thefar-field radiation pattern shown above it. If theillumination function is a true 'boxcar', where

Although it is possible to calculate the beampatterns and hence beamwidth for an antennastructure, this is usually only carried out forspecific communications antennas. Even in thesecases the patterns are always measured in an RFanechoic chamber or on an open test range forconfirmation. Antennas for use in EMC testingare measured individually and calibration curvesof antenna factor and a beamwidth figure areusually supplied by the manufacturer.

Specific data are available for the 3 dBbeamwidth of certain common types of antennawhich may be used in EMC radiated emissionand susceptibility testing. For example, forreasonably high-gain pyramidal horns the RSGB[20] gives a graph of gain (for a 50% efficiencyhorn) against 3 dB beamwidth as in Figure 5.12.

A polar diagram is given for a typical high-gain~ntenna [21] in Figure 5.13a and a graph of 3and 10 dB beamwidths as a function of antennagain in Figure 5.13b. The approximate formulawhich may be used as a rule of thumb relatinggain to beamwidth is [21]


5.39

5.38

5.37

5.40

f)E60

BIA (deg)

f)H68AlA (deg)

and gain G42,000

f)E X f)H

Other useful approximate formulas for horns [23]in general use are

where G is the gain not expressed In dB, and f) E

and f)H are in degrees.The exact formulas for the gain and beamwidth

of antennas in the far field depend on the detailedcurrent distributions on the conducting surfaces ofthe particular antenna. Thus the shape of a hornor other antenna (length, aperture size, E and Hplane dimensions) willaffect the radiationpattern. This accounts for the small differencesbetween some of the approximate formulaspresented, each of which is appropriate for thatparticular type of antenna. Such formulas if usedcarefully can indicate the gain and beamwidth forother antennas, butare limited to those withsignificant gain at the wavelength of interest [21].

Thus the EMC engineer is caught in somethingof trap: a clear idea of the far-field distance andspot size for the antennas to be used is needed butusually the time and resources cannot be devotedto calculating these antenna parameters from firstprinciples. T'he simple formulas for high-gainantennas are all slightly different; care must betaken not to use these for antennas for which theywere not derived. There appears to be no clearguidance available as to the limits of applicabilityof some of the rules of thumb.

In reality, the best approach for the EMCengineer who wishes to understand theperformance and limitations of the wide range ofantennas that may be used, is to make carefulmeasurements, either on an open range or in ananechoic chamber, of the antenna patterns andgain at various distances of interest. This is not asimple nor a quick task, however, and requiresspecial test equipment, facilities and time to makereliable measurements at the range of frequenciescovered by each antenna.

The correct understanding and use of antennasis one of the most difficult aspects of EMCtesting. As such testing becomes more importantwith increasing business and legal implicationsthe technical uncertainties which surround the

5.35

5.34

5.32

5.31

5.36

PEAK SIDE LOBE- LEVEL

+7tPATTERN ANGLE RADS-

I

164.3jG2 (deg)

f) _ 51AE - B

G == 4nABjA2

G == 10 ABjA2

RADIATED POWER dB

Po I---j----OdB

3dB

I

1

27 000123 dB beamwidth == T

I APERTURE IN '" s ---

C"'-l WAVELENGTHATWHICH

ANTENNA IS OPERATING

~---D- . I

I

3 dB beamwidth == 2.86jG2 (rad) 5.33

2 nd 1 sfNULL NULL

3 dB beamwidth

Z0:

~0..UJenzo0..en-UJ---

IiI~

Horns are one of the general antenna types usedextensively in EMC testing and usefulapproximate expressions for the gain andbeamwidth are [22]:

The 3 dB beamwidths in the E- and H-planes are

or

where G is the gain, A is the wavelength, and Aand B are the H- and E-plane aperturedimensions as in Figure 5.14. Note that thetheoretical gain [20] of a long tapering hornwhere a plane wave emerges at the front apertureIS

Figure 5.10 Relationship between antenna physicalaperture: illumination function andradiated far-jield pattern

where G is the numerical gain (not expressed indB) and beamwidth is in degrees. This is identicalto


00 ELEVATION ANGLE e

302520

GAIN OF HORN ( dB )

Beamwidth against gain for pyramidal horn

10080

60

40(J)Q) 30Q)

enQ) 20"0

I 15......0

~ 10:2: 8L5 6coco 5"0 4C")

3

2

115

Figure 5.12(a)

1800

300

;-----t---+-~I---_t_-_+_-~~___tI 2700900

Reproduced by permission of' Wiley

Figure 5.11 Dipole antenna patterns and currentexcitations (a) Dipole pattern responses forvarious lengths (b) Current excitationpatterns for various dipole lengths

I_

\ /DIPOLE ELEMENTS

£/2 "I-

(b)

/12----1

Reproduced by permission of' Prentice Hall Computer Publishing

halfwave dipole beamwidth is 78° which yields aspot size of 1.6 m at a distance of 1 m. Estimatesfor other dipoles can also be made using Figure5.11a. However for a low-gain, dipole-likeantenna operating below about 50 MHz andworking at 1 m from the EUT, calculating even acrude spot size in this way is undesirable as1m < A/2n: the E UT is in the near field of theantenna.

For a large-aperture dish reflector an tennaworking at microwave frequencies, the near fieldmay extend well beyond the antenna-to-EUTdistance as 1m < 2D IA and again (assuming ameasurement at 1m) the EUT is in the near fieldof the antenna. In this case the crude spot sizemay be taken to be the dish diameter D, with thepower density falling off at a rate of

where Pd is the power density at a distance x fromthe edge of the beam cylinder, Pmax is the maximum power density in the cylinder, P is theinput power to the antenna in watts, A is the areaof the reflector of diameter D, and x is thedistance normal to the beam with x == 0 at thebeam edge [24]. See Figure 5.15.

Using the criterion of D« A/2 use far-fielddistance of A~2n, and if D > A/2 use far-fielddistance of 2D 1A.

Taking into account these comments, the farfield boundary distances have been estimated fortypical EMC antennas [25] and are shown inFigure 5.16. The calculated spot sizes (covered by

use of antennas for radiated emiSSion andsusceptibility measurements will attract greaterinterest.

5.3.10 Spot size

Using the calculated or measured far-field 3 dBbeamwidth and with an antenna to EUT distanceof 1 m (as required by MIL STD 461 forexample) it is possible to estimate a rough spotsize which is approximately equally illuminatedby the antenna. Eqn. 5.33 is useful in this resp~ct

but should only be applied to high-gain antennas.For low-gain devices such as a dipole it is possibleto use some calculated results to indicate thebeamwidth and spot size. Using Figure 5.11a the

and

24xlD (dB)

4PIA


Reproduced by permission of RSGB

HORN GAIN dB

:r:I- 100

tlrl0 8 @]Tzw =KJ....J 6 CD BwI -l> ~l---1« 4 ~A---1

~u.. L = axial length to apex

0 A = width of aperture in H planeCf)

28 = width of aperture in E plane

~c::WI-Z 10..i 8c::0 6

a::i'<i 4

u..0z0 2Ci5ZW~

C5 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Reproduced by permission of Prentice Hall Computer Publishing

Figure 5.14 Gain ofpyramidal horn in terms of itsdimensions measured in wavelengths

5.3.12 PolarisationBalanis [4J defines the polarisa tion of an antenna

5.3.11 Effective length

Connor [26J defines the effective length of anantenna in terms of the nonuniform current distribution on its surface. Such a distribution is shownin Figure 5.11 b for a dipole. The relationshipbetween the effective length Le and the physical\length L is

Le area under nonuniform current distribution

L area under uniform peak distribution

The effective length of a long wire antenna forexample will be a fraction of its physical lengthowing to the nonuniform current induced by thewave along its length. The shorter effective lengthleads to a lower output voltage for a givenincident field strength than would be predictedsimply using the physical length of the antenna.

applicable in the far field. The spot sizes given inFigure 5.1 7 are shown as a broken line over thefrequencies for which the EDT is in the near field(Figure 5.16) at 1 m and it is unsafe to rely on farfield formulas such as eqn. 5.33 to derive the spotSIze.

(a)

30

20

108

CI>6Q)

~C) 4Q)"0

IJ- 20

~~ 1« gain ~ 27,000w 0.8co (83dS)2

0.6

0.4 e10 dB ~ 1.83 8 3dB

0.210 20 30 40 50 60

ANTENNA GAIN dB

(b)

Figure 5.13 Relationship between beamwidth and gainfor typical high-gain antennas (a) Typicalpolar diagram of high-gain antenna(b) Approximate relationship betweenantenna gain and beamwidth

the 3 dB beamwidth) using the information given inthis section for typical EMC antennas [25J aregiven in Figure 5.17. The concept of a definablebeamwidth, and therefore spot size, is only really


EUT is not in the far field

ANTENNA FAR FIELDf------------ IS AT 2 0 2 / A ----------

Spot size is less than EUT size

/

Pd EUT

Power density falls as 24 x / 0 dB for x > 0/2

T!

o

1

Figure 5.15 EUT innear field of high-gainantenna D == reflectordiameter A == reflectorarea

Reprod uced by permission or ArLechHouse Inc.

in a given direction as 'the polarisation of theradiated wave when the antenna is excited'. Thepolarisation of the wave is defined as thatproperty of the radiated electromagnetic wavedescribing the time-varying direction and relativemagni tude of the electric field vector; specifically,the figure traced as a function of time by theextremity of the E-field vector at a fixed locationin space, and the sense in which it is traced, asobserved along the direction of the propagation.

Thus the wave shown in Figure 5.4 is said to bevertically polarised. Polarisation may be classifiedas linear, circular, or elliptical. Linear and circularpolarisations are special cases of elliptical. Thesense of circular and elliptical polarisation can beright hand (clockwise) or left hand (anticlockwise) .

In EMC testing many of the antennas such asdipoles, biconics, log periodics and horns arelinearly polarised and require measurements to bemade in two orthogonal planes, usually verticaland horizon tal. Antennas such as conical log

spirals are circularly polarised and only onemeasurement needs to be made.

5.3.13 Bandwidth

Bandwidth may be considered as the range offrequencies either side of some central frequencywhere characteristics such as polar pattern, gaininput impedance, sidelobe levels, 3 dB beamwidthor polarisation are within an acceptable value ofthose at the centre frequency.

The bandwidth of an antennna is therefore notdefined by a single absolute figure. In the case ofantennas used by EMC engineers the bandwidth isusually limited by variation in input impedancewhich is specified in terms of VSWR (voltagestanding wave ratio) with respect to 50 ohms.Engineers should be aware that all the other characteristics of an antenna (such as beamwidth, spotsize and far-field boundary) may also change withfrequency, and while the VSWR may be the

FREQUENCYpermission or BAeReprod uced by

Dynamics Ltd.

Figure 5.16 l~r-field

distances for selection oftypical EMC testantennas

USUALTESTDISTANCE1m

SMALL RIDGED GUIDE HORN2-18GHz

LARGE HORN ANTENNA400 MHz - 1 GHz

1 GHz

o... ~

100 MHz

5.0 --------...-------------------....18" dia. PARABOLIC REFLECTOR i i

AND FEED HORNS4-12GHz 6m@4GHz

7 m ® 12 GHz

1.0

EW

~ 3.0«I-(J)

oo.....IWIT: 2.00:

Lt.

o.....IWIT:0:«WZZOzi=-::>W«00

~~~~ZlW«2WWO:0:1::>(J)«W2

Wo«20Cf).....II-WZU::Wo:2«WLL

§§~Cf)«W2


2.5 ...---------.....----------.....----------....

TYPICAL STANDARDGAIN HORNS

- - -. dashed line indico1es thatEUT a1 1 m would be in the near fieldand no reliable spot size can bedetennined

lOOPffi~~~~~~~~~:'"LAROE HORN \

\\ PARABOLIC REFLECTOR "SMALL HORNS \ & HORN FEEDS \

\ --_.._-~~ \

tt._._ "'",\,

BICONIC------.- ANTENNA

0.5

2.0

en~ 1.5

Q)EwNCi5

b 1.00-W

Figure 5.17 Estimate ojspot sizes Jor typicalantennas used in EMCmeasure- ments atdistances oj 1 m

Reproduced by permission or RAeDynamics Ltd.

100 MHz 10Hz

FREQUENCY

100Hz 1000Hz

obvious and limiting parameter the effect of theseother changes on the measurement should not beignored.

lie between 50 and 300 ohms with a dipole at firstresonance having a value of72 ohms.

5.4 References

where ZA is the antenna input impedance, RA isthe antenna resistance, XA IS the an tennareactance, and

where Rr is the radiation resistance and RL is theresistive loss of antenna components. An antenna isusually operated at or around resonant frequencieswhere the input impedance is nearly resistive andhas a value which is convenient for coupling to anexternal load. Typical useful antenna impedances

5.3.14 Input impedance

The electrical complex impedance presented bythe antenna at its input terminals will contain acomponent known as the radiation resistancewhich is related to the power loss radiated awayfrom the antenna. I t is a fictitious resistance [26Jwhich represents the radiative power loss in theantenna equivalent circuit. The radiationresistance should be large compared with any realresistance in the antenna (e.g. element resistance)such that at resonance the efficiency of the deviceis high and most of the power is lost to theradiation resistance.

The antenna feed-point impedance is

KING, R.W.P.: 'Theory of linear antennas'(Harvard University Press, 1956)

2 SCHELKUNOFF, S.A. and FRIIS, H.T.:'Antennas theory & practice' (Wiley, 1952)

3 JOHNSON, R.C. and jASIK, H.: 'Antennaengineering handbook' (McGraw-Hill, NY, 1961)

4 BALANIS, C.A.: 'Antenna theory analysis &design' (Wiley, NY, 1982)

5 'Radio communications handbook (RSGB, 1982,5th edn.) Chaps. 12 and 13

6 IEEE Transactions on Antennas7 'Reference data for radio engineers' (Howard W.

Sams, 1977) Chaps. 278 D-dot sensor, model HSD-2 & HSD-4. EG & G

Electromagnetics, 2450 Alamo Avenue, SE, POBox 9100, Albuquerque, NM 87199, USA

9 MARVIN, A.C.: 'Practical notes on antennas forEMC engineers'. Study note 2/10/79, BritishAerospace Dynamics EMC Group, Filton, Bristol, UK

10 BENUMOF, R.: 'The receiving antenna', Am. ].Phys. 1984,52(6), pp. 535-538

11 SARGENT, j.: 'Choosing EMC antennas'.Interference Technology Engineer's Master, 1989,pp. 182-304

12 BENNETT, W.S.: 'Properly applied antennafactors'. IEEE Trans. EMC-28 (1), 1986

13 BENNETT, W.S.: 'Antenna to ground planemutual coupling measurements on open field sites'.Proceedings of IEEE symposium on EMC) 1988,pp. 277-283

5.41

5.42RA == Rr + RL

INTRODUCTION T'O ANTENNAS 85

on E'MC') 1990,of IEEEpp.

19 YACHJIAN, A.D.: 'An overview of near-fieldantenna measurements', IEEE Trans. AP-34

20 EVANS, D.S. and JESSOP, C.R.:manual' (RSCB, 3rd edn.) p. 8.69

21 EVANS, D.S. and JESSOP, C.R.:manual' (RSCB, 3rd edn.) p. 8.46

22 'Reference data for radio W.Sams, 1977) pp. 27-37

23 Microwave Journal tJu.,,-,~~vU,'_~\._Hh.J

24 KEISER, B.: of compat-ibility' (ArtechHouse, 1987) 335

25 EMC facility brochure BT BritishDynamics, Filton, Bristol, UK, 1982, p. 8.7 and1990 issue

26 CONNOR, F.R.: 'Antennas' Arnold,London) p. 2

14" KENDALL, C.: '30m-site attenuation improvementby increasing the transmit antenna height'.Proceedings of IEEE symposium on EMC) 1985,pp. 346-350

15. BRENCR, C.E.: 'Antenna differences and theirinfluence on radiated emission measurements'.Proceedings of IEEE symposium on EMC) 1990,pp. 440-443

16 MISHRA, S.R. and KASHYAP, S.: 'Effect ofground plane and charnber walls on antenna inputimpedance'. Proceedings of IEEE symposium onEMC) 1988,pp. 395-399

17 MISHRA, S.R., KASHYAP, S. andBALABERDA, R.: 'Input impedance of antennasinside enclosures'. Proceedings of IEEE symposiumon EMC) 1985, pp. 534-538

18 McCONNELL, R.A.: 'An impedance networkmodel for open field range site attenuation'.

Chapter 6

Antennas for radiated emission testing

Figure 6.1 Simple monopole antenna and base loadingcoil

6.1 Passive Illonopoles

6.1.1 Construction

TAPPEDBASE LOADINGCOIL

~50QUNBALANCEDOUTPUT

TRANSFORMERCOUPLED OUTPUTFROM LOADINGCOIL

~50nIII I UNBALANCED: I OUTPUT.1

Band 1: 10 to 250 kHzBand 2: 250 to 500 kHzBand 3: 500 kHz to 1 MHzBand 4: 1 to 2 MHzBand 5: 2 to 4 :NIHzBand 6: 4 to 8 l\t1HzBand 7: 8 to 16 MHzBand 8: 16 to 3211Hz

Table 6.1 Typical monopole antenna frequency bands

10 kHz to 32 MHz in eight bands

Figure 6.2 Simple impedance matching circuits formonopole antennas

transformer coupling or a tapped loadinginductor as shown in Figure 6.2.

The design of an EMC antenna is perhaps morecomplicated than that of a communicationsantenna by the need to achieve good sensitivity(good effective height) over a wide band offrequencies. This requires that the total frequencyrange of interest (usually 10 kHz to 30 MHz) foran EMC monopole be segmented into bands witha different impedance matching circuit for eachband.

MonDpole antennas are used mainly for testingmilitary equipment inside screened rooms where agood antenna ground plane reference-is available.I t is appropriate therefore to demonstrate its usewith reference to a n1ilitary standard.

MIL-STD 462 calls for the use of a monopoleand a typical passive antenna is 41 inches longwith an effective height of 0.5 m. Such a devicemay have up to eight frequency bands as shownin Table 6.1.

RF GROUND PLANEREFERENCE FOR MONOPOLE

___ Monopole effective height f e

~Capacitance to "ground" Cm

~~ em and~ate at centrefrequency of F MHz

\ \ \UNBALANCEDOUTPUT

lIoWI..-lct:o~ EI~0-W..-lo0-oZo~

These are amongst the simplest receiving antennasused for EMC measurements. They consist of aconducting rod of defined length connected to animpedance matching circuit which usually feeds a50 ohm cable to a matched 50 ohm input of anEMI receiver. The impedance-matching circuitconsists essentially of a base loading coil withsufficient inductance to resonate with thecapacitive reactance of the monopole elenlent atfrequencies of interest, see Figure 6.1. A moresophisticated matching circuit may use

Antennas for radiation emission measurement aretreated separately from antennas for radiatedsusceptibility testing to show the importance ofdifferent antenna parameters in the two cases.With antennas for emission measurements the keyparameters are bandwidth, sensitivity, dynamicrange and absence of cross or intermodulationproducts in the case of active antennas with builtin amplifiers. Important parameters for antennasused in susceptibility measurements includebandwidth, gain/power requirement, beamwidth/spot size, power dissipation, size and mass. Suchtopics are discussed in the following chapter. Thisone looks in detail at the types of receivingantennas that are widely used for radiatedemission testing. They are discussed in sequence,beginning with the passive monopole which hasthe least complicated construction.

86

AN1'ENNAS FOR RADIATED EMISSION rrESTING 87


6.1.2 Performance

FREQUENCY IN MHz

Figure 6.3 Antenna factors for typical passive 41" rodantenna suitable for use with MIL STD 461

ZoU5C/)

~wow~oc:t: E0::>0::1.Zeoc:t:-oCO z~o0::0::c:t:Z

FREQUENCY

MONOPOLE ANTENNAFREQUENCY RANGE -

Zo 100

~ 95

~ 90o N 85~ ~ 80

~ ~ 75c:t:>0:: :::i. 70

~ ~ 65~ ~ 60

~ 55~ 50 L--_---L__--L.__.....L-:~_....J..___.L___~

co 10kHz 100 1MHz 10 100 1GHz

Counterpoise bonded to

ground Plane~ I

Figure 6.5 FrequenC)J coverage and example of sensitivityachievable with monopole antenna

}'igure 6.4 Test set-up for measurement of radiatedemissions (10 kHz-30 MHz) u)ithmonopole antenna

poise for the monopole is at the base of theelement and must be large enough to adequatelyterminate the electric field lines from the rod asillustrated in Figure 6.1 such that the rod toground plane capacitance does not change significant!y if the ground plane is enlarged further.1'he rod element uses the ground plane as itsvoltage reference and the field strengthmeasurement will be in error if too small aground plane is used. The size of the 41-inchmonopole antenna ground plane is specified inMIL STD 461A and is 60 cm square. In MILSTD 462 note 3 the monopole ground plane isbonded to the E UT ground plane bench by asolid copper extension of the counterpoise. SeeFigure 6.4.

Monopole antenna measurements can be inerror if the rod element is too close to theconductive screened room walls or ceiling. Thisarises because too many of the electric field linesassociated with the rod capacitance areterminated other than on the reference ground

201816

:F :4.0 4.5

SAND 6

*Note decreasing sensitivityat low frequencies

BAND 1 :b : : : ].01 .05 0.1 0.15 0.2 0.25

BAND 2 :b: : ; ~.25 .30 .35 .40 .45 .50

BAND 3 :E: : : : ~.5 .6 .7 .8 .9 1.0

BAND 4 :~ : : : : j1.0 1.2 1.4 1.6 1.8 2.0

BANDS :E: : ; ~2.0 2.5 3.0 3.5 4.0

: :;:: j5.0 5.5 6.0 6.5 7.0 7.5 8.0

BAND? :E: : : I : : 18 9 10 11 12 13 14 15 16

BAND 8 :k: : : =:322 24 26 28 30 32

co"'0C/)0::oIoi1Zoi=oW0::0::ooc:t:ZZWIZc:t:

The receiving antenna correction factors for atypical commercially available passive 41-inchmonopole [IJ are given in Figure 6.3. An antennafactor of between 24 and 50 dB must be added tothe output voltage to obtain the electric fieldstrength in V jm.

An example of the use of a passive 41-inchmonopole to measure the radiated emissionsfrom an EDT on a groundplane is MIL STD462 note 3 test RE02-1, see Figure 6.4. The areaof maximum emission on the EDT is located byprobing its surfaces with close-in field probes andthen orientating the EDT so that this area isfacing the monopole. The antenna to EDTseparation is 1 m and the frequency range of thetest is from 14 kHz to 30 MHz. When connectedto a low-noise EMI measurement receiver theantenna is sensitive enough to measure fieldstrengths of a few microvolts per metre at10 MHz. The performance is sufficient tomeasure signals in the range 14 kHz to 30 MHzat below the specification limits for the RE02 testshown in Figure 6.5.

A monopole measures the E-field of the incidentwave with a polarisation along the axis of themonopole element. If the rod is vertical then theantenna is most sensitive to a vertically polarisedE-field. The ground plane or electrical counter-


receivers owing to the number of narrow resonantbands which the antenna has to maintain itssensitivity. The monopole element itself oilers arelatively high reactive (capacitive) impedance atits base over most of the operating freq uency rangebelow its self-resonance, which occurs at low VHF.If an active amplifying device with a high input,yv\'---"D.rtr"Y\f'D were used to buffer the rod to the low50 ohm output, it would be possible to produce therequired impedance transformation without loss of

Moreover, a frequency shapingnetwork could be included in the circuitry toproduce a flat antenna transfer function. Incommercially available active monopole antennassuch as the Carnel Labs. Corporation model95010-1 [2J, the first active element is a field-effecttransistor with a very high input impedance. Thisparticular active monopole has a specificationshown in Table 6.2.

I t has a single frequency compensated band overthree decades wide with a 0 dB flat transferfunction when extended to 50 inches and used withtop loading. rrhe addition of the top loading plateincreases the capacitance of the rod to the groundplane and increases its over the standard41-inch configuration. This highly sensitive devicecan detect narrowband emissions down to belowthe levels shown in Figure 6.7a and broadband

down to those in Figure 6.7b.

6.2.2 Disadvantages

Power supP0J: A battery pack is required, usuallyhoused within the base box of the antenna whichalso contains the active circuit. The ElVIe testengineer must make pack checks bothbefore and after each test to be sure that themeasurements are made with a properlyactive circuit.Front end damage: Care must be taken when

the rod or moving the antenna thatstatic does not accumulate on the test

m.onopoles

--..,..,.............r--'~

MONOPOLECOUNTERPOISE

SCREENED ROOM CEILING

6.6 ejfect oj j)lacing monof)ole antennatoo close to conducting surfaces in screenedroom

SIDE WALL

Reprod uced by permission of ICT Ine.

in 6.6. l'his leads to a detuning of the~J..J.."'-,A.'cJ..J..~A.> with a consequent in the antenna

rprllllrprr.p.n1- ",---,..o"",+,£:>r1 in MIL STD 462

the rod nlust greater than 30 crn fromand 1 m from the walls of the

room. Where these distancesshould be increased to the maXilTIUm available by

in a chamber which islined with radio absorbent Inaterial (RAM).

the effectiveness of RAM at theTrp'r1"'£:>Yl,r""::>(' for which are used is ratherlimited and the criteria for an unlinedroom should be LLIJIJ.!.i'",--,"--l.

amjJlifier)

loaded rod: 0 dB8 dB

.LJ'-/IJ''-/LI.'-lvil.L on receiver bandwidth and frequency

Carnel Labs. 950101-110kHz to 40 111HzOne±1 dB from 10 to 25 kHz±0.5 dB from 25 kHz to 40 MHz50 inch top-loaded rod: 1 m41 inch rod: 0.5 m10 Mohm shunted by 8 pF50 ohms50 inch41 inch

/lV/m)

active monofJole antenna

ANTENNAS FOR RADIAl'ED EMISSION TESTING 89

6.3.1 Introduction

NARROWBAND SENSITIVITY+10r-----..,------r----~--

600

FREQUENCY MHz

200 400

A. DIPOLE2"I

co"C

W...J«() -20CJ)

>-0:«0:I-000: -40«wCJ)z0a..CJ)w -600:w0:::>I-:.Ja..

-80:2«

connecting a balanced feeder to the elementsacross the gap. A dipole can be of any length withrespect to the wavelength of an incident wave,from a hundredth of a wavelength to a fewwavelengths. Practical dipoles for communications purposes are built to resonate with thewavefield to maximise receiving sensitivity at afixed frequency. The most common dipole is ahalf-wavelength long, making each element aquarter-wave long.

For EMC measurements, dipoles must be ableto receive signals over a very wide band offrequencies from about 30 MHz to 1 GHz. If asingle dipole is used which is for example 4 mlong, tuned to 38 MHz, t:he frequency response isof the form shown in Figure 6.8. Thus the dipolewill only respond well to certain harmonicfrequencies in the band of interest and is not veryuseful for broadband EMC measurements.

This problem can be overcome by tuning thedipole to a new resonant length at each frequencyof interest in the band being measured. Theantenna characteristics such as correction factorand beamwidth at the first resonance can beestablished and a useful measurement of thewavefield can be made. This is the basis of thetuned-dipole radiated emISSIon test methodswhich are required by commercial specificationssuch as FCC part 15j and EN55022/BS6527/VDE0871. However, tests carried out in this wayare time consuming and the method is not wellsuited to automated scanning emissionmeasurement with swept frequency EMI receiversor spectrum analysers.

10 MHz 40 MHz

10 MHz 40 MHz1 MHz

50" TOP-LOADED CONFIGURATIONReceiver random noise BW = 1 kHz

100 kHz

100 kHz 1 MHz

FREQUENCY

One-metre rod: 0 dB ACFNarrowband: 107 dBIlV/mBroadband: 73 dBIlV/m/MHz

41 inch rod: 8 dB A.C.F.Narrowband: 115 dBIlV/mBroadband: 81 dBIlV/m/MHz

-20L ...L__-==r:~~=~~!!!!!!!!!!~

10 kHz

~>~ -10"0

BROADBAND SENSITIVITY+70.-----,------r--------r-------,,...--

i +60

> +50~"0 +40 -..I..- -'- ---L__-L--1

10 kHz

Table 6.3 Overload jield levels jor active monopole

(b) FREQUENCY

Figure 6.7 Active monopole antenna sensitivity

6.3 Tuned dipoles

engineer which can be discharged to the rodelement and therefore into the gate of the FET. Itis relatively easy to damage some active antennasand they should not be moved or touched withoutgrounding the rod to the box.Intermodulation distortion: Any active component onlyhas a limited range of operation where its transferfunction is linear. If large signals are present andthe device is working on a nonlinear part of itsoperating curve then spurious distortion and intermodulation signals can be created. With thedynamic range of EMC measurements often beingin excess of 60 dB it is possible to measure andrecord small spurious signals at harmonic and intermodulation frequencies, if large signals are presentout of band. An active antenna with a bandwidth of40 MHz is also prone to overload and signaldistortion when measuring impulsive broadbandnoise. This will result in observing an incorrectspectrum of the pulsed wavefield being measured.Typical maximum field strengths that must not beexceeded for the 1 m and 0.5 m effective heightmonopoles [2J are given in Table 6.3.

Maximumsignal input(overload)

(a)

A dipole antenna is constructed from two long thinco-linear conducting elements with a small gap inthe middle. The electrical outpu t is luade by

rzgure 6.8 Typical response oj tuned dipole (4 m longat ),/2). Dipole parameters: length400.0 cm; diameter 1.0 em; load 100 n

Table 6.4 Tuned dipole parameters

90 A HANDBOOK FOR EMC TESTING AND MEASUREMENT-

6.3.2 Practical tuned dipoles

n616670

Dipole R in

--L/A. = 1.0

L / D RATIO = 2000

0.470.480.49

L/A at resonance

L/ A. = 0.8

\

~ (0.025) (F in MHz)

CJ)

:2:r:o:><:w()z«

~w -1000a::

50500

50000

L/D ratio

30al"'Ca:0 20t-U«LL« 10zzWt-Z 0«

-1020

Reproduced hy permission of BS

DIPOLE COMPLEX IMPEDANCE Z c R + j X

Figure 6.11 Complex impedance oj thin dipole asJunction oj element length/wave length

-2000---------------

40~---~---T"""-----r-----r----""

2000r------------------..,

Reproduced by permission of NBS

50 100 200 500 1000

FREQUENCY MHz

Figure 6.10 Theoretical antenna Jactor Jor thin resonantdipole

The theoretical antenna factor for a thinresonant dipole with a lossless balun and cableconnected to a 50 ohm receiver is given by Maand Kanda [3J as shown in Figure 6.10, where <ais the dipole impedance (taken to be 70 ohms).

L dipole elelnent lengthD dipole element diameterRin dipole feed point impedanceA wavelength at resonance

FIRSTRESONANCEA. DIPOLE2"R ~ 70n 1000

6.3

6.2

6.1

50n INPUT

Leff == Lire

50n COAXIAL CABLE

\ ~~~f~s [8J

L!::====~~========:'::::1 EMI

~ . METER

\ SWR "LOSSES"

OUTPUT CONNECTORV out

ANTENNA FACTOR = EVout

EXTENSIONARM

BALUN

BALUN LOSSES

\

L tan(reL r I2L)Lejjff == ------

. re

TELESCOPIC ELEMENTS

Tuned and rrlatched halfwave dipole ojvariable length Jor EMC measurements

H

Z=72Q

T

Ic<1-q-

II

1Figure 6.9

To achieve self-resonance of the dipole (zeroreactance) experience shows that it is necessary tomake it slightly shorter than a half-wavelength.Schelkunoff [4J derived the required length interms of the length-to-diameter ratio of the elements

[1 - 0.2257 ]

L r = (L/2) In(L/D) - 1

If the dipole is shortened slightly to the requiredlength for resonance (eqn. 6.2), the theoreticalresistance R at resonance depends on the length-todiameter ratio. Typical values are given in Table 6.4.

A practical tunable dipole is shown in Figure 6.9with telescopic elements which can be extended tobe a quarter-wavelength long over the frequenciesof interest. At resonance the dipole presents abalanced impedance of 72 ohms across the feedpoint. This must' usually be transformed by thebalun (balanced to unbalanced transformer) to a50 ohm output for connection to a 50 ohm coaxialcable and thence to the matched input of theEMI meter.

The effective length of an infinitesimally thinhalfwave tuned dipole in free space is given byMa and Kanda [3J as

where D is the diameter of the dipole element, L istuned length, and L r is required length. Theeffective length of a dipole near to resonance isgiven in Reference 3 as

ANTENNAS FOR RADIATED EMISSION TESTING 91

RFOUTPUT

(aj

IIIIIII

._---J----..,/(

II () tj ()\:..>..)_'*l_"

~t--I--~--I

\I TELESCOPING ROD

Il SHIELDEDI CASE

: J __j

,---_ I'\ I

" "'" I I'J "ll I I

,_)(_)i-t( _I II

___~ JBALUN

r------IIIIIIIl _

A Ar--"4----1-4--j

\TELESCOPING ROD

6.3.3 Commercial EMC tuned dipoles

Consider the performance of some commerciallyavailable tuned dipoles for EMC testing to meetcivil and military standards. The Carnel Labs.Corporation models DM~105A (T1, T2 and T3)are a set of three dipoles [4] which can be tunedfrom 20 MHz to 1 GHz. They are supplied with afrequency calibrated 'tape measure' which can beused to set the element lengths q uickly andcorrectly without calculation. The antenna factorfor the far-field performance is shown in Figure6.12. The solid line A is the antenna factor for thedipole tuned to each individual frequency. It isuntuned below 27 MHz which is the frequency atwhich the telescopic elements are fully extended.At frequencies below 27 MHz the efficiency of thedipole starts to fall as its effective length decreases,it also becomes mismatched with the inclusion ofcapacitive reactance, see Figure 6.11.

The plot in Figure 6.11 shows the compleximpedance as a function of dipole length towavelength with the first useful resonance close to0.5 and the second close to 1.5.

RFOUTPUT

Reproduced by permission of' Camel Labs. Corp.

(b)

Figure 6.13 Examples of baluns (a) Wound balunforuse at HF/ VHF (b) Stub balun for use atUHF

r--'IIII----I

"'BALUN

r---~

IIIIL __

6.3.4 Radiated emission testing

Testing to meet EMC standards for commercialequipment accounts for the major use of tuneddipoles, but they are sometimes used to determine'antenna induced voltage' called up by militarystandards. The civil tests are commonly carried

losses in the commercial antenna over thetheoretical one are due mainly to balun losses.

A typical balun configuration used for the lowerfrequencies up to 200 MHz is shown in Figure6.13a. Similar choke designs can be found inReference 5. A suitable VHF/DHF balun fortuned dipoles is shown in Figure 6.13b andfurther details are given in Reference 6.

Figure 6.12 Antenna factors of tuned dipoles for usewith FCC Pt. 15j (tuned down to30 MHz) and EN55022 (tuned down to80 MHz)

~ FARFIELD ANTENNA~ 35 CORRECTION FACTORa: 30 Model OM -105 A - T1 antenna 3m. above ground

O~ ....----20 - 200 MHz ." (\\25 \\~ ~p(\o

~ ~\~Y;. 0 \0 ~e ~t\1-Z 20 s \u(\e \0 '2.1o Q\~\~s oo~(\t5 15 \e(\9J'~ 10 oo~(\ Model OM -105 A - T3~ 5 \u(\e~() ~t\1-~ ,~~ ~ 400 - 1000 MHzo - trCC "\Oc\O« 0 'o~ ~\~"Y;. ':\0(\(\0 Model OM - 105 A ! T2~w -5 ~e~\cO~ S ~ 140 - 400 MHzt- ;(,."(\f?P \ ~

~ -102'-!-O-......3....0--S0---1......00---20I-O----5......0.....0--10--'00

FREQUENCY IN MHz

For FCC part 15j testing of informationtechnology equipment the dipole can be tuneddown to the lower test frequency of 30 MHz.When testing to the European close equivalents ofEN55022/BS6527/VDE0871/CISPR22 the dipoleis tuned at all frequencies down to 80 MHz, but isthen fixed at the resonant length for thatfrequency while testing is continued down to30 MHz. In such a case the antenna correctionfactor for the dipole would be that shown by thebroken line in Figure 6.12. The lower solid line Bis the theoretical antenna factor derived from theNBS calculations in Figure 6.10. The increased


~-------MAJORDIAMETER;::: 2 d------~I

----1------ ---~~ I~~'/" ...........

/ ",/ MINOR DIAMETER ""-/ ;:::~ ~

/ \/ \

\\\ DIPOLE OR -- II'" OTHER ANTENNA /

"'~ I ~///........... -------------..;...-

Figure 6.14 Plan view of open-field test site. Boundaryoj area defined by ellipse. Area to be jree ojreflecting objects e.g. buildings)fences)trees) poles) etc.

Reproduc~d by permission of BSI

out on an open-si te tes t range wi th the tuned dipoleantenna at a distance d from the EDT. Dependingon the specification and the equipment class thisdistance can be 3, 10 or 30 m. The site must haveno reflecting objects within an ellipse with a majordiameter of 2d, see Figure 6.14. There must also bea conductive ground plane of a specified minimumsize to provide unvarying ground interactionproperties. The dipoles and EUT may be set up asin Figure 6.15.

There are anumber of potential disadvantageswhich the EMC engineer must be aware of whenconducting radiated emission testing with tuneddipoles. The tests can take a long time, as thedipole has to be adjusted at each frequency ofin terest in the emission spectrum of the E UT.Some specifications call for testing 'with bothhorizontal and vertical polarisation, and the

---1/:;;;;;~METAL GROUND PLANE

EUT HEIGHTTYPICALLY 1m.

ANTENNA HEIGHTVARIES BETWEEN 1 ·4m.

EQUIPMENT SITED ON AREFLECTION-FREE OPEN AREA TEST SITE

Figure 6.15 Use oj tuned dipoles to measure emissionsJrom computer equipment carried out onopen-area test site

height of the antenna must be changed at eachfrequency of interest to determine the maximumemission level from the EDT.

For the FCC tests at low frequencies the manipulation of the large dipoles becomes a problem andthe lower element (vertical polarisa tion) gets veryclose to the ground. This problem is not so severefor the EN55022 test \vhere the dipole length isrestricted to that for resonance at 80 MHz(elements about 1 m long).

The most serious problems arise as to what therather large antenna actually measures. There areconsiderable mutual impedance problems due toground reflections [7-9] and the extended nature ofthe FCC type antenna integrates the field variationsalong its length: this leads to a field strength estimatethat differs from those made with the EN andCISPR (80 MHz) antenna or a compact broadbandbiconic (where permitted) [10].

Experienced test engineers are aware of some ofthese measurement problems and are careful notto become over confident about the validity of atechnique simply as a result of its being specifiedin a national standard.

Tuned dipoles are easily sensitive enough tomake EMC emission measurements when usedwith a low-noise EMI receiver and quasipeakdetector. Signa~ .levels set for radiated interferencelimits in standards such as EN55022 are notdifficult to achieve. These are

30 dBflV/m (QP) from 30-230 MHz37 dBfl\T/m (QP) from 230-1000 MHz

Ambient radio noise experienced when makingmeasurements on an open test range can be aproblem and this can mask the signal from theEDT. There is little that can be done to minimisesuch a problem other than siting the test area in alow radio noise location.

6.4 Electrically short dipoles

6.4.1 Special short calibration dipoles

Calibration dipoles are not widely used for EMCtesting but a knowledge of their existence andsome idea of how they work is essential for theprofessional EMC engineer. There are two typesof calibration antennas which measure electricand magnetic fields. They are based on shortdipoles and small loops. This section deals onlywith these special dipoles used for standardE-field measurement and calibration.

6.4.2 Roberts dipoles

Wilmar Roberts was assistant chief of the FCClaboratory (1967) when he undertook the


""TAPERED RESISTIVEDIPOLE ELEMENTS

DIODE RECTI FI ER

Figure 6.17 Advanced lnicroscopic resistively tapereddipole with built-in diode detector. Themicroscopic dipole probe sensor shown nextto a sewing needle Jor comparison oj size

...........BALANCED HIGH IMPEDANCECARBON FILAMENTSCONDUCT DC FROM THEDIODE TO A MILLIVOLTMETER

6.4.4 Microscopic dipole probes

Berger, Kumara and Matloubi describe a specialhigh field strength probe manufactured byElectromechanics Co [16J which is based on theNBS small tapered resistive dipole work by Kanda[1 7J. The device uses diode detection and highlyresistive leads but makes use of resistive dipoleelements. It can operate fronl 1 MHz to 10GHzwithin 1 dB accuracy at field strengths from 1 V1mto 1 kV1m. The dipole elements, detector andresistive leads can be seen in Figure 6.1 7.

would expect froin a resonant dipole into astandard 50 ohm load. I ts effective height is smalland the device is insensitive compared with aresonant dipole.

The second is to connect a high impedancebalanced DC voltmeter across the short dipolewhich has a low turn-on-voltage Schottky diodeconnected across the feed point. The balancedconnection to the display meter is made using aresistive filter line constructed from carbonimpregnated plastic in a nylon jacket. Each linehas a resistance of about 6000hm/cm. Thus ashort dipole with a practical sensitivity is achievedby using the high-impedance detector/meter andthe wavefield is not disturbed in the vicinity ofthe dipole by the balanced connections due to itshigh resistance.

Although a short dipole is capable of making anaccurate field measurement almost at a singlepoint in the wavefield it has two drawbacks foruse other than for controlled calibration typetesting. I t is insensitive and requires a fieldstrength of a few hundreds of m V /m foroperation. I t is also frequency insensitive (allspectral information being lost in the diodeconversion to DC). I t cannot be used where morethan one signal is present at a time.

5

4a::~ 3CJ)

> ROBERTS2ANTENNA

120 30

z 4::::>-I

COMMERCIAL ANTENNA« 3co /'" ROBERTSCJ) 2CJ) ANTENNA0 /-Ico,..,

020 30 50 100 200 300 500 1000

FREQUENCY (MHz)

Figure 6.16 VSWR and losses Jor typical commercialbalun in tunable dipole Jor EMCmeasurements and Roberts balun Jorcalibration dipole

50 100 200 300 500 1000

FREQUENCY (MHz)

development of standard dipoles which haveantenna factors close to those for a theoreticaltuned dipole [11]. Their chief characteristics are alow VSWR and low balun losses over a wide rangeof frequencies [12, 13]. The balun loss and VSWRperformance of the Roberts dipole are shown inFigure 6.16 compared with commercially availabletuned dipoles such as in Section 6.3.3.

6.4.3 Small nonresonant dipoles

Electrically short dipoles can be used for some EMCradiated emission testing. BS727 in paraH3.3permits the use of a dipole shorter than half awavelength but longer than a tenth of a wavelengthto be used for testing commercial electronicequipment. Short-dipole antenna correction factors(50 ohm load) are provided in graphical form inFigure 7 ofAppendix H in that document.

Workers at NBS (National Bureau ofStandards) in the USA produced a number ofdesigns for calibration dipoles [14J and sphericaldipoles have been used at the National PhysicalLaboratory (NPL) in the UK. It is not intendedto deal in depth with this topic here, save to saythat the approach is based on two characteristicsof dipole behaviour.

The first is to reduce the dipole length to a fewcentimetres or less [15J which has anumber ofeffects. The device averages the field over only asmall region. The short dipole is almost purelycapacitive and presents a high reactive impedanceand is not capable of delivering the currents one

6.5.2 Commercial biconic antennas

Figure 6.18 Basis for theoretical calculation of radiationfields and feed-point impedance jorinfinitely long bicone elements

Typical commercial biconic antennas used inEMC testing are the Emco 3108 [21 J and theCarnel Labs. Corporation 94455-1 [22]. Theseantennas consist of a pair of open stiff wire framebicone elements and a balun as in Figure 6.21.For an example of the gain and antenna factor ofa commercial biconic antenna see the figures for

P

FIELDS PRODUCEDAT POINTPE, H (r, 8,$)

..............

..............

..............

'.I

z

x

CONICAL ELEMENTSOF APEX ANGLE <X

The biconic dipole is a broadband derivative ofthe long thin resonant dipole. Balanis [18J givesthe theoretical basis of operation of a biconicdipole in detail. The treatment considersconical di pole elements of infini te length andcalculates the radiation pattern and feed-pointimpedance as a function of cone angle. SeeFigure 6.18.

Schelkunoff [19J devised a transmISSIon lineequivalent method of calculating the feed-pointimpedance for finite length small angle biconicelements. The resistance and reactance of thefeed-point impedance calculated by Jasik [20J areplotted in Figure 6.19 and show how the antennabecomes more broadband as the cone halfangleIncreases.

Approximations to true biconic antennas can bemade in the form of triangular sheet 'bow tie', orbow-tie wire simulations as in Figure 6.20. A wirebicone with approximately eight intersectingwires has a performance almost equivalent to asolid sheet bicone. The intersecting wire bicone isthe one most often used in EMC testing as it islight, rugged and compact with elements about1 m long covering the frequency range from 20 to200 MHz.


6.5 Biconic dipoles

6.5.1 Introduction

CONE ANGLES

- a/2= 5°----. a 12 = 11°-- a /2 =1/100°

SEE Fig. 6.18en:EJ:o

SOURCE: JASIK

8 StCONE LENGTHIN WAVELENGTHS

4

1t II A.

~'\

I \ n'\I \ II \

\ II • . I \If ;\\ '.-, If\\ ~

,\ " I" W~\' , I '\ 0

U. \I.~\, ~" I II ~\ I I ~\ . .....I I ,~

~

100

1,000

THIN DIPOLE

I

'II ,I ,

12 7£:' A. 6 8 10 0 12

1st BICONE RESONANCE~

...--....----.----.--... 10,000

o

en~J:o.5a:w()

~~Ci5wa:.....::>a..~

Reproduced by permission of'McGravv-Hill

Figure 6.19 Calculatedfeed-point impedanceofjinite length biconerelating angle tobroadband response


(dB)12.813.512.58.89.58.19.07

10.014.813.513.614.115.016.418.318.718.519.8

Antenna factor*

0.050.070.150.500.591.061.061.050.500.931.191.341.331.160.900.971.171.00

Gain

";/,.. ...---........,, ............1m ANTENNA FACTOR,,;"; ....

,,;

.......---_ ...,,;

,,;\ ,,;

....., ...// FAR FIELD ANTENNA FACTOR

EMC03108BICONE

25

30r---r------,---~---.,---- ......

5

20

(MHz)30405060708090

100120140160180200220240260280300

zoi=&3 ~ 20

§~ 1500 T2 DIPOLE~ t; 10 /dJLE 5I- 0 DIPOLE -- CARNEL LABS~ TUNED DIPOLES

-5 L.--_.L-_--'-__---I ~I______l

20 30 50 100 200 400

FREQUENCY IN MHz

Antenna factor of wire biconic antennacompared with tuned dipoles

Frequency

zoi=cnf:d"O 15a::z~~() 0 10«I-z~zu..WI-Z«

Table 6.5 Gain and antenna jactor for wire biconeantenna

O~---L_--'-_---'-_--A-._......I-_.....L.-_...L-_~---'

20 40 60 80 100 120 140 160 180 200

FREQUENCY IN MHz

}'igure 6.23 Far-Jield and 1 m antenna factors fortypical wire biconic antenna used in EMCtesting. Biconic antenna: G'Iarnel Labs.94455-1

ANTENNA FACTOR OF WIRE BICONIC ANTENNACOMPARED WITH TUNED DIPOLES

Figure 6.22

Reproduced by permission of Carne! Labs Corp.

*Specification compliance testing -factor (1.0 m spacing)to be added to receiver meter reading in dbpV toconvert to field in tensi ty in dBpV Im (Emco 3108)

6.5.3 Use of biconic antennas

Some biconic antennas are designed specifically todrawings given in MIL-STD 461A and are usedextensively for test methods specified in MILSTD 462 from 20 or 30 MHz up to 200 or300 MHz depending on the capability of specific

Reproduced by permission of Emco

Figure 6.21 Typical wire bicone antenna used in EMCtesting. Biconical antenna Emco 3108

BOW-TIE WIRE APPROXIMATION

Figure 6.20 Antenna shapes derived from basic solidbicone

the Emco 3108 given in Table 6.5. Note that theantenna factor is for 1 m spacing betweencalibration antennas so that it may be used fornormal military standard emission testing, and isnot necessarily the far field antenna factor, whichshould be used for 10 and 30 m testing incommercial standards. In Figure 6.22 the '1 mclose-in' antenna factor for the biconic is plottedwith the '3 m close-in' antenna factor for the T1and T2 tuned dipoles. The shorter element lengthof the biconic results in a less efficient antennabelow 80 MHz. The far field and 1 m antennafactors for the Carnel Labs. Corporation 94455-1are given in Figure 6.23. When the antenna isused at 1 m from the EDT it appears to be predominantly a few dB less sensitive than for receptionof far-field signals.


bicone models. They can also be used for sometests specified by civil standards for commercialelectronic equipment such as informationtechnology. For example, EN55022 (Section 10.2)permits the use of aerials other than tuned dipolesproviding that the results can be correlated with abalanced tuned dipole with an acceptable degreeof accuracy. More explicit req uirements for theuse of complex broadband aerials is given inBS727 (para H4.2).

Biconic antennas can be used with a low-noise50 ohm EMI meter to make sensitive measurements over frequencies from 20 to 300 MHz todetect signals below the severe levels set in MILSTD 461 C (using a peak detector function) asshown in Figure 6.5.

The chief advantage of a broadband biconicantenna is that it provides good sensitivity over awide band and is easy to use, unlike tunabledipoles, and enables tests to be made quickly withautomated scanning receivers or spectrumanalysers.

6.6 Wideband antennas

6.6.1 Introduction

Although the biconic antenna or transmISSIonline is defined by an angle, its current distributiondoes not fall away to zero with distance and itcannot be successfully truncated to form afrequency independent antenna. Other antennashapes which are specified by angles do havecurrent distributions which decay with distanceand can exhibit frequency independent properties.

6.6.2 Log-periodic antenna

If an antenna is designed so that its physicalstructure is periodic with the logarithm offrequency, and the variation of structure is smallor negligible over a single period, then a practicalfreq uency independen t antenna can beconstructed [25]. For a more detailed explanationsee Balanis [23]. There are many interesting andbizarre shapes which fulfil these requirements anda variety of useful communications antennas havebeen built from planar and wire log-periodicstructures, planar and wire trapezoidal toothedlog periodics and log-periodic slots. The configurations most widely used for performing EMC testsare log-periodic dipole arrays and the conical logspiral. Usually the first is linearly polarised andthe second is always circularly polarised.

6.7 Log-periodic dipole antenna

Consider a set of dipoles with characteristics andspacings which change in a log-periodic manneras in Figure 6.24. Define

Ideally, the dipole gap sizes and the elementdiameters should also change in a sinlilar fashion.However, if they do not, and constant values areused, it has little impact on the practicalperformance of the antenna. If the elements areclosely spaced and all fed in phase from the narrowend then an endfire beam propagates backwards inthe direction of the larger elements. If eachsuccessive dipole is fed in antiphase as in Figure6.24b the endfire beam emerges forwards from theshort end of the array. In either case, the antennamust have a balanced feed which is usuallyimpractical. An unbalanced coaxial feed cable maybe used by staggering the elements and connectingthe coaxial inner conductor as shown in Figure6.24c. This is the configuration that is mostfamiliar to EMC test engineers as a number ofcommercially available antennas are of this form.

6.5

6.4

-1 InlJ. == tan -

Xn

and

Before the 1950s antennas with broadband inputimpedances and pattern characteristics coveredfrequency ratios of about 2: 1. The advanceddesigns which came later extended bandwidths to40: 1 or more [23]. These designs were referred to asfrequency independent and they had the propertythat their geometries were specified by angles.

Antenna characteristics such as impedance,beam pattern, polarisation etc. remain unchangedwhen the physical size of the antenna changes,provided that the operating frequency alsochanges by the same ratio. Thus the antenna characteristics are invariant if the electrical size of theantennna does not change. If the shape of anantenna is therefore completely specified byangles then its performance is independen t offrequency [24].

An infinite biconic dipole is one such antennawhich is only specified by the cone angle as inFigure 6.18. To make practical truncatedstructures it is necessary that the current on thestructure decreases with increasing distance fromthe feed point. If the current beyond a certaindistance is negligible the structure may betruncated and removed. Such an antenna has alow-end cut-off frequency above which theantenna characteristics are the same as those forthe infinite structure. The upper cut-off frequencyis determined by the size of the feed point whichmust be less than A/8, where A is the upper cut-offwavelength.


1.0

0.9

0.8

>- 0.7()z

Iw<3 0.6u::u..

ll R 0.5wLC)

z 1~ 0.4Ci« 0.30:: T = 0.89

0.2a =45°

+8 ......---------------...+7+6+5+4+3+2 -....-. _....-. _ .....

O_:::;;;;;"a.._...s..- .&-- _

o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

L/A

Figure 6.25 Radiating efficiency of log-periodic dipolearray. L width oj array) a halfangleoj apex

0.1

COAXIALLINEINPUT

BALANCED FEEDELEMENTS OFPHASE ... t

tBeam' producedat this end

(b)

(a)

BEAMWIDTH FREQUENCY MHz

Figure 6.26 Typical performance parameters Jor logperiodic antenna

for EMC measurement as scanning receivers can beused to make rapid measurements which satisfy awide range of military and civil standards. I t is wellcapable of measuring field strengths (in itsoperating band) below the severe limits specified forthe MIL STD 461C RE02 test given in Figure 6.5.

100 300 500 700 900 1100 1300 1500

GAIN FREQUENCY MHz

50

50

~~=--....:;;;a700 800 1100 1300 1500

FREQUENCY MHz

908070605040 ......-50~...............1...00-3..00-500.......-7...00-800.......-11...00-1...30-0-...150D

130

110

170 ......--------------........

J:

b~~«wCOCO"0(I)

(J)

@ 150

Cl

a:~(J)>

UNBALANCEDFEEDBeam formedat this end ( c)

Figure 6.24 Log-periodic dipole array antenna(a) Array dimensions (b) Balancedantiphase Jeed (c ) Unbalanced coaxialfeed

Reproduced by permission of Wiley

The radiating efficiency of a log-periodic dipolearray is given in Figure 6.25 showing how thischanges with the largest dipole dimension [26].The antenna achieves 85 010 efficiency with thelargest dipole element of 0.6 A. Therefore theseantennas may be compact, at least not significantly larger than a halfwave dipole at the lowestfreq uency of interest.

Typical gain, beamwidth and VSWR (50 ohms)for a log-periodic dipole array operating from50 MHz to 1 GHz are given by [27] and shown inFigure 6.26. A drawing of a typical antennawhich can be used for both EMC radiatedemission and susceptibility measurements is givenin Figure 6.27 and its performance is illustrated inTable 6.6. The gain, VSWR (50 ohm) andantenna factor as a function of frequency from150 MHz to 1.1 G Hz for this typical antenna aregiven in Figure 6.28.

This type of sensitive wideband antenna is ideal


0::otUit«zzWtZ«

25

15

10TYPICAL ANTENNA FACTOR

6.0

~O ~

8.0

a:~C/)

>

co"0 7.0

C/)

==::r:ooLO

2.5 5

2.0

1.5

1.0 .-..- .-.. _ .....a 100 200 300 400 500 600 700 800 900 1000 1100

FREQUENCY MHz

Figure 6.28 Gain, antennaJactor and VSWRJor typicalcommercial log-periodic antenna.

40"

Reproduced by permission or Amplfier Research

Reproduced by permission or Amplifier Research

Table 6.6 Performance qf commercial log periodic

Figure 6.29 Spiral plate antenna configuration.L determines lowest Jrequenc_y that isradiated efficiently . Feed point is at centre

L

Another example of a frequency independentantenna, the structure of which is defined byangles, is the conical spiral, a nonplanar extensionof a spiral-plate geometry as shown in Figure6.29. The length L determines the lowestfrequency which can be propagated, and atfrequencies above this the pattern and impedanceare frequency independent. 1~he ends of thespirals are tapered in thickness to provide a betterimpedance match. rrhis type of antenna iscircularly polarised.

When the conducting spiral lies on the surface ofa cone, the antenna has only a single pattern lobetowards the apex of the cone with a maximumalong the axis. Typical conical log-spiral antennas(Stoddart 93490-1 & 93491 used for EMCmeasurements are shown in Figure 6.30 withantenna factors in Figure 6.31.

6.8 Conical log.-spiral antenna

type C female102 x 13 x 91 cm

(40 x 5 x 36 in)

1.8: 11.5: 1

150 to 1000 MHz

2000 W1500 W

750 W6.5 dB min7.5 dB av±1.0 dB50 ohms nom

o 0 0 0 0 0 0 0 00000

o 0 0 0 0 0 0 0 00000

o 0

o 0

Gain flatnessImpedanceVSWR

AR model ATlOOO

maxlmull1average

Beamwidth (average)E planeH plane

Front to back ratio(minimum)

ConnectorSize(wxhxd)

Frequency rangePower input (maximum)

150-250 MHz250--500 MHz500-1000 MHz

Power gain (over isotropic)

Figure 6.27 Dimensions oj typical log-periodic antennaused in EMC measurements (150 MJ-Iz1GHz)



far-field distance

where D is the largest diameter of the an tenna,or 5A whichever is less. Thus for an antenna ofdiameter of 25 cm the far-field distance at200 MHz is only a few centimetres. Sometimesit is not made clear where this distance shouldbe measured froIn on the antenna. The phasecentre of the antenna (defined in Chapter 5)should be used as the assumed source ofradia tion. I t is then logical to use this as thepoint from· which distances should bemeasured.

However, the phase centre moves along thean tenna axis as a function of freq uency and themeasured an tenna factors correspond to thevarious positions of the phase centre and not to astandard distance of 1 m. Theoretical calculations and measurements by Marvin [28J haveresulted in an expression for the distance of thephase centre from the apex of the antenna as afunction of wavelength. This can then be used tocalculate the correction to the gain figuresobtained by the SAE ARP-958 calibrationmethod. The gain figures for the standard (ARP958) and corrected calibration method whichtakes accoun t of phase-centre movemen t, aregiven in Table 6.7 for an Emco model 3103conical log spiral antenna.

Measured antenna patterns are shown inFigure 6.32 for the vertically polarised E-field at200, 500 and 900 MHz. A typical beamwidth forthis type of antenna is 90 0 and the spot size at1 m in midband is about 2 m. The VSWR(50 ohms) for the model 3103 is typical of thistype of antenna and is usually less than 2: 1. SeeFigure 6.33.

Like all wideband frequency independentantennas conical log-spiral an tennas are easy touse and automated measurements may be madequickly. With these circularly polarised antennasthere is no need to evaluate horizontal andvertical polarisations separately. If the polarisation components must be measured separately alinearly polarised log-periodic dipole antennacould be used.

The antenna factors are determined by the MIllSTD 461 (para 5.2.7) which is based on SAEARP-958 two antenna method using shortseparation distances and may not produce anantenna factor calibration which is suitable forfar-field measurements. Often the manufacturersrecommend that the far-field distance iscalculated for these log conical spiral antennas byuSIng

10

140cm

1

98

Frequency coverage:200 MHz - 1 GHz

Frequency coverage:1 -100Hz

r-13cm~1·10GHz

ego CARNEL LABS MODEL 93491-2

Log spiral pattern conductorprinted onto conical surface

4 5 6 7

FREQUENCY GHz

3

Model no. 93490-1

Model no. 93491-2

2

200 300 400 500 600 700 800 900 1000

FREQUENCY MHz

COAXIALCONNECTOR

~36an ~I200 MHz· 1 GHz

ego CARNEL LABS MODEL 9349Q.1

52 r---oy---oy----r---.,--~- __- __- __-_

co 50~

~ 48ex: 46~o 44~ 42zo 40~~ 38

~ 36

8 34

~ 32z~ 30

~ 28·26 __.a..-_.a..-_~_...L.-_.....I.- """"_"'"

1

Figure 6.31 Typical log conical spiral antenna factors200 MHz-l GHz and 1-10 GHz

co 27

~ 26

ex: 25

~ 24o~ 23

5 22

~ 21

~ 20ex:o 19o« 18zm 17

~ 16«

Figure 6.30 Examples oj commercial log conical spiralantennas

EXAMPLES OF COMMERCIAL LOG CONICAL SPIRAL ANTENNAS


Table 6.7 Standard gain for conical log spiral antenna (measured at 1 m) and corrected gain taking into account phasecentre movement with frequency

Frequency(MHz)

Standard measured gain(linear)

Corrected gain(linear)

Antenna factor based onstandard gain (dB)

100200300400500600700800900

1000

0.211.672.232.112.352.241.852.111.892.10

2.913.262.792.922.672.122.372.072.26

17.014.016.319.020.522.324.525.126.627.0

Emco Model 3103

Reproduced by permission or EMCO

Reproduced by permission or EMCO

Figure 6.32 Example of variation with frequencyof conical log spiral antenna polardiagram (Emco 3103)

At microwave frequencies where waveguides are ofpractical dimensions, it is possible to radiateenergy from the open end _of the guide used as acrude antenna. This type of antenna is almostnever used in EMC testing owing to its lowdirectivity [29]. Open waveguides are howeversometimes used as feeds to illuminate parabolicreflector antennas [30J.

A horn antenna can be considered as a taperingextension to a waveguide which provides anincreased aperture size leading to improveddirectivity. The basic theory of horn antennas canbe found in almost any standard text on antennasincluding [29-33J. Various types of horn antennawhich are based on rectangular and circularwaveguides are shown in Figure 6.34. The typemost frequently used in EMC measurements isthe pyramidal horn. I t is cheap to construct andits design is well understood.

Consider the pyramidal horn in Figure 6.35defined by its principal dimensions as shown. Theradiation pattern of a horn is determined by theamplitude and phase distribution of E- and Hfields across the respective aperture planes. Thedifference in phase over the aperture denoted by rin Figure 6.35 is a result of the principle of equalityof path length (Fermat'-s principle). For adequateperformance most horns have this parameter r ofless than 0.25L in the E-plane and about 0.4L inthe H-plane [34J. Expressions for the gain and3 dB beamwidth of pyramidal horns have beengiven in Chapter 5 equations 5.34 to 5.40.

Sometimes horn an tennas are used in the nearfield when making EMC measurements and workby Ma and Kanda [35J has resulted in anaddition to the Shelkunoff gain eqn. 6.34 whichshould be used in the near field:

6.9 Horn antennas200 MHz

500 MHz

900 MHz

1800EMC03013

3.0

1.0

ex:~

~ 2.0

100 200 300 400 500 600 700 800 900 1000

FREQUENCY MHz

Figure 6.33 Typical VSWR for comm.ercial conical logspiral antenna (Emco 3103)


6.7

6.8

88A8E == 13 deg. for B/A < 2.5

79A31 +A deg. for A/A < 3

OJ 0,"'0

en0::0z ..... -1-()

<t:c:(Ou..zz0::00-:r:t;

:::>0wex: 2 3 4 5 6

~\ 0° /

, I

I

DISTANCE FROM HORN m.

Figure 6.36 Typical near-field gain reductionfactorsforpyramidal horn at 1 GHz (Ma and Kanda)

Reprocl ucecl by permission or NBS

The radiation pattern from a typical horn with

horn operating at 1 GHz over an antenna to EUTdistance of 1 to 6 m.

Silver [32] gives approximate formulas for the10 dB beamwidth of typical horns with averageflare angles of 20°

900~------",-~__--&-------+-270°r ,/

Sectoral E - plane

Exponentially taperedcone

Exponentially taperedpyramid

CIRCULAR HORNS

RECTANGULAR HORNS

Pyramidal

PYRAMIDAL HORN

Reproduced by permission 01' McGraw-Hili

Figure 6.34 Various possible types of horn antenna

H PLANE VIEW E PLANE VIEW

Figure 6.35 E- and H-plane views of rectangularpyramidal horn antenna

1500

1800

2100

Reproduced by permission or McGraw-Hill

32ABG ~x RHRE 6.6

where RH and RE are the gain reduction factorsdue to the E- and H-plane flares. These factorsare calculated in Reference 35 and expressedgraphically in Figure 6.36 for the example of a

Figure 6.37 Exalnple of E- and H-plane beamwidthsand polar diagrams for pyramidal hornantenna (Source: Balanis). E-planebeamwidth > H-plane beamwidth.11 == 12 == 6A, A == 12A, B == 6A,a == O.5A_, b == O.25A, - E-plane,- - - H-plane

Reproduced by permission or Wiley


CD 21"CZ 20

~ 19

2 3 4 5

FREQUENCY GHz I

; ;~~----r---.---~3 4 5 6 7 8 9 10

FREQUENCY GHz I; :~_r----r-\I'lG-\G""--'8l--""'O\

8 9 10 11 12 13 14 15 16 17 18

FREQUENCY GHz

Figure 6.38 Frequency coverage for horns with standard gain 20 dB) for various waveguide sizes. Typical beamwidthof horns is 18° with sidelobes at -13 dB. Data for commercial horns manufactured by [/lann Microwave

Reproduced by permission of Flan Microwave

Reproduced by permission of Amplifier Research

A single or double ridge can be introduced into arectangular waveguide as shown in Figure 6.40.Marcuvitz [38J gives tables of modified cut-offwavelengths for propagating modes and showsthat the equivalent transverse network for thedominant waveguide mode is a junctioncapacitance shunted by two H-mode transmissionlines with open- and short-circuit terminations.

The effect of the central ridge is to load thewaveguide and increase its useful bandwidth bylowering the cut-off frequency of the dominantmode [34, 39]. A thin ridge or fin is also effectivein producing the loading of a central ridge. Ifsuch a fin is extended from the waveguide sectioninto a pyramidal horn, as in Figure 6.41, the

dirnensions of those in Figure 6.35 is gIven InFigure 6.37 [31 J.

A wide range of waveguide horns are commercially available and typical gain against frequencydata [36J for various wave guide sizes are given inFigure 6.38:Note that the bandwidth of the horns(and waveguides) is rather small, at less than oneoctave.

At lower frequencies horns can still be used butare usually not fed by a long waveguide. Acoaxial cable is employed and connected to a stubin a short section of waveguide directly attachedto the horn. The specification of a commerciallyavailable horn [37J for use in EMC testingoperating in the frequency range 400 MHz to1 GHz is given in Table 6.8, with typical gain andVSWR curves in Figure 6.39.

6.10 Ridged guide horn antennas

TYPICAL UHFHORN ANTENNAFOR USE IN EMC TESTING

400-1000 MHz1000 W10 dB min typicallyincreasing to 15 dB at1000 MHz50 ohms nom2.5:1 max1.5:1 av9.1 kg (20 lb)56.4 x 79.3 x 73.7 cm(22.2 X 31.2 X 29.0 in)

Example ofgain and VSWRforcommercially available UHF horn antenna.A.R. model AT 4001

600 700 800 900 1000

FREQUENCY MHz

Manufactured by A.R. model no AT 4001

Frequency rangePower input (max)Power gain

Table 6.8 Example of UHF horn for EMC usespecification

400 500

~ 1.5

~

8

ImpedanceVSWR

Figure 6.39

Weight (max)Size (w x h x d)

;;; 2.5:2J:o 2o1.0

ANTENNAS FOR RADIATED EMISSION rrESTING 103

2

p--'igure 6.43 ~ypical VSWR againstjrequency .forcommercial ridged guide horn antenna.Electro-Metrics RGA-100

0:::

~ 1.5>

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18FREQUENCY GHz

Tb

L

Yo

A.C

EQUIVALENT CIRCUIT

E FIELD INWAVEGUIDE DOUBLE RIDGE GUIDE

T

0;01 -1

Ylo

SINGLE RIDGE GUIDE

CROSS SECTION OF WAVE GUIDE

CURVED (fronHed)CORNER

FEED

PLANE

Reproduced by permission or Electro-Metrics

laboratory with a large number of antennas oflimited bandwidths. The gain against frequencyperformance of a commercial wide band ridgedguide horn [41 J is given in Figure 6.42 togetherwith the receiving antenna factor. A typicalVSWR plot for this antenna is given in Figure6.43 and shows a rather uneven match to 50 ohmsat the lower frequencies.

Reflector antennas operate successfully when theiraperture dimensions are many wavelengthsacross. They can be constructed in a number ofdifferent ways as shown in Figure 6.44 and explanations of their design can be found in standardantenna texts. Of particular interest to the EMCengineer is the parabolic reflector antenna, as it iswidely used from about four to above 18 GHz forboth radiated emission and susceptibility testing.Geometrical optics based on ray tracing may beused to study certain aspects of these antennas,while a full analysis requires the use of electromagnetic field theory [42J.

A parabolic surface defined in Figure 6.45 hasthe property of being able to convert a divergentbeam into a near parallel one with the minimumof aberration. This is clearly useful for point-topoint communications where narrow beamwidths

6.11 Reflector antennas

15

14en-0

ANTENNA FACTOR 0:::13 40 0

I-en 12 0-0 <CZ LL

:;: 11 35 <C0 z

10 zWI-

9 30 ~

8

72 4 8 10 12 14 16 18

FREQUENCY GHz

qp ../

WIDEBAND

COAXIAL FEED ~I""'. """I

SHORT SECTION OF PYRAMIDAL HORNDOUBLE RIDGED CONTAINING TWO FINSWAVEGUIDE

Figure 6.41 Construction oj double ridged pyramidalwideband horn antenna

useful bandwidth of the antenna can be increasedmany times [34, 40].

This increase in bandwid th is very useful to theEMC engineer who wishes to cover the requiredmeasurement spectrum with as few changes ofantenna and connections to the EMI receiver aspossible. This will save on testing time and reducethe initial capital cost of equipping the test



Figure 6.42 Example ojgain and antenna Jactor jordouble ridged guide horn antenna. ElectroMetrics RGA-IOO

Figure 6.40 Single and double ridged wave guide

Reproduced by permission or Electro-Metrics f'igure 6.44 Geon1etry oj reflector antennas


Figure 6.45 Geometry ojparabolic r~flector

where A is the apparent aperture area and S is thephysical aperture area, and the factor 0.54 is dueto the nonuniform illumination of the reflector.The gain of this type of antenna is

8

20

- 3 dB BEAMWIDTH

3 4 5 6 7 8 910 122

18" diam.parabolic reflector,

7" focal length 91892 - 1

FEED HORN FEED HORN91'890-1 91891-2

4.6-7.3GHz 7.3-12 GHz0

:b~oq-

- ~ II

0 .....: II~LO

0) II ~ coII ~

to ~ -~

CD --1co JI ........

t- if co -~ -t co "0

f"'0 PJLO

CD 00 II"'0 C\Jco co 0"'0 N IIet) 110N

<:>II

0 ~

151

-8

24

23

22CO"'0a: 210I-ULt 20«zzw 19I-Z«

18

17

16

Reproduced by permission of Wiley

-6 -4 -2 0 2 4 6

OFF AXIS ANGLE (DEGREES)

Figure 6.46 Typical polar diagram offront-jedparabolic r~ector antenna. Parameters:JID == 0.82) D == 25cm)f== 20.5cm)Jrequency == 35 GHz

FREQUENCY GHz

Figure 6.47 Antenna Jactor) gain and beamwidth JorlJ)pical parabolic r~ector antenna used inEMG~ testing

Care must be taken when using these antennasin standard EMC measurements, as theequipment under test possibly at a distance of 1 mis almost always in the near field, with all thea ttendant problems of uncertainty in waveimpedance, gain, antenna factor, beamwidth andspot size. The manufacturers of commercialantennas for EMC measurement do not alwaysspecify the method used to calibrate the antenna

6.9

6.10

6.11

y2 = 4 fx

A == 0.54 x S

G == 0.54 (nDIA)2

o

7 X 104

83dB == ---JD

where 8 is the -3 dB beamwidth,Jis frequency inMHz, and D the physical diameter of parabolicin feet.

An example of a typical radiation patternproduced by a high-gain parabolic reflectorantenna which might be used in a communicationlink is given in Figure 6.46 [44]. Notice that the-3 dB beamwidth is of the order of 10.

1'ypical of conlmercial parabolic reflectorantennas produced for EMC testing is the CarnelLabs. Corporation 91892-1 18-inch diameterreflector with horn feeds 91890-1 (4.4-7.3 GHz)and 91891-2 (7.3-12 GHz). The antenna factors,gains and beamwidths (far field) are given inFigure 6.47. The beamwidths are much largerthan for communications antennas with values of4 to 9°.

are used. But in EMC testing parabolic antennasare used to increase directivity and gain over awide frequency range using a simple horn feed sothat small signal levels, such as those in Figure 6.5,can be measured with a good signal-to-noise ratio.

The apparent aperture of a parabolic reflector isgiven in Reference 43 as

where D is the physical aperture diameter. The3 dB beamwidth for a parabolic reflector wasgiven in eqn. 5.31 and can be used to drive thefollowing


6.12 Magnetic field antennas

6.12.1 Introduction

6.12.2 Passive loops

Loop antennas generate an output voltage bymagnetic induction and the relationship of opencircuit EMF (below loop resonance) andmagnetic field strength is

«-l00WI-0() 140a:en 130'U 120:r:I- 110<:>z 100w0:: 90I-00 800-l 70wIT: 60()

f= 50wz<:>«

10 100 1k::E 10k 100k

140

120

CD'U 1000::0I-

~ 80LL Factor for dB I J,JV I Mz0 60

50.Q input -(jj0::W> 40z00 Factor for dB I PT

20 50.Q input

010 100 1k 10k 100k

FREQUENCY Hz

Figure 6.49 M agnetie field emission lirnits forMIL STn RE 01 test in dB pieotesla.Limit,for RE01

FREQUENCY Hz

Figure 6.48 Antenna eonversionfaetorsfor MIL STnloop (13.3 em dia.)

Electrostastically shieldedDiameter: 13.3 cmArea: 139 cm2

Turns: 36Wire type: 7-41 Litz.

A typical comn1ercial loop antenna of this type isthe Solar Electronics 7334-1 [45] which has atransfer characteristic shown in Figure 6.48. Noticethat the lower curve on the graph relates EMImeter "indicated voltage to magnetic field strengthin dB picoteslas and the upper curve relates outputvoltage to E-field strength in dB flV 1m (assuming afree-space wave impedance). Great care should beexercised in using a loop antenna to derive E-fieldstrengths as most measurements at these lowfrequencies are made in the near field where thewave in1pedance is not likely to be that of free space.

As an example of using a small loop look at thetest method of MIL STD 462 N3 REO 1. Thisreq uires that the loop is placed 7 cm from the faceof the EUT with the plane of the loop parallel to

6.12e == flr flo N A (j) H

Loop antennas can be used to measure thestrength of the magnetic component of the electromagnetic wavefield produced by an interferencesource. The design of loops varies with theapplication and frequency range. In all cases theloops are electrostatically shielded by enclosingthe wire turns inside a tubular conducting shieldwhich is broken at some point around theperiphery to prevent the shield acting as ashorting turn.

Loops are used primarily at low frequenciesfrom a few Hz to tens or hundreds of kHz. Forexample the MIL STD 462 N3 RE01 (30 Hz30 kHz) and RE04 (20 Hz-50 kHz) tests specifythe use of a 13.3 cm-diameter loop over thesefrequencies. Larger loops, but with areas of lessthan 1 square metre, are used for example tomeasure field strengths from 100 kHz to 30 MHzin accordance with VDE 0877 pt2. Such measurements are called for in VDE 0871/EN550 11 inconnection with industrial, scientific and medical(ISM) equipment.

where e == open-circuit output voltageflr rela tive permeabili ty of coreflo permeability of free spaceN == number of turnsA == area of the loop(j) angular frequencyH == magnetic field strength

Taking as an example the small loop, which is wellspecified in the US MIL STD 461/2, the followingcharacteristics apply:

and derive the antenna factor and the EMCengineer is often left to wonder if the figuressupplied are related to the far-field performanceor tor aIm measurement.

Until manufacturers produce full traceableperformance data with their antennas, it is safestfor the EMC test specialist to calibrate antennasusing the methods approved by the appropriateEMC standards. In the UK, antennas can besubmitted to an independent calibration serVicesuch as the National Physical Laboratory.


6.12.4 Loop calibration

There are a number of ways of calibrating loopantennas [45, 49] but the most common approachis the coplanar two-loop method. This is shown inFigure 6.52 where the generating loop produces aknown field strength along the axis of the loop tobe calibrated by measuring the voltage developedacross a calibrated resistor to derive the drivingloop current. A similar calibration technique isspecified in IEEE Std 302 [50] where the loopsare situated 1 m apart.

Investigations by Millen [49] have shown thatthere are wide variations by up to 7 dB in themeasurements of magnetic fields made bydifferent commercial loop antennas in testjcalibration situations. The greatest problemsoccur near the loop resonances. He concludes thatloops do not produce the same output voltages inuniform and nonuniform fields and that it is not

6.12.3 Active loops

To ex tend the frequency range of a loop antennawithout either incurring a large number of bandswitches, or reducing the number of turns (andthe sensitivity) to avoid loop resonance, while atthe same time equalising the freq uency responsea t low freq uencies, it is necessary to develop anactive loop antenna. An active broadbandamplifier is included in the antenna in the sameway as for the active E-field monopole. Thus thesame comments apply with regard to batterypower and overload distortion (see Section6.2.2). An example of a commercially availablecalibrated active loop is the Electro-MetricsALR-30A [48]. Tl1is device has a diameter of43 cm (17 in) and is calibrated from 9 kHz to30MHz.

co 80CL

"'00:0 w.... 60 ()()« zLL «Z 0

W0 40 C-OO ~a::LU 0> «z 20 00 ...JU

~«z 0 sonzLU....z 600n« -20 100kOc-o0 0.01 0.1 1.0 10 100 250...J

FREQUENCY IN KILOHERTZ

}-'igure 6.51 Antenna conversion factors for large loop76.2 cm diameter

Electrostatically shieldedFrequency range: 20 Hz to 250 kHz (calibrated)

usable up to 30 MHz76.2 cm (30 in)0.456 m 2

1150, 600 or 100 kO

Diameter:Area:Turns:Load impedance:

Figure 6.50 Example of typical loop antenna emissionnleasurement (MIL SYD 461j2 RE04).Plane of loop should be 90° to plane of testsample face

The antenna conversion factor for this loop isgiven in Figure 6.51. It could be used in VDEtesting but would require additional calibrationup to 30 MHz. Another example of a largediameter loop antenna which can be used forVDE or TEMPEST (secure communications)testing is the Electro-Metrics ALP-70 [47] whichhas a diameter of 63.5 cm (25 in) and is calibratedfrom 10 kHz to 30 MHz.

the face. The EUT is then examined to find theposition of maximum radiation at each frequencyof interest and the levels are recorded andcompared with the permissible limit which isshown in Figure 6.49. Small loop antennas mayalso be used at greater distances from the EUT.Such a test configuration is called for MIL STD462 N3 RE04 and is shown in Figure 6.50 withloop positioned at 1 m from the EUT.

Larger passive loops are also used for measuringmagnetic field strengths in this way. An example isthe Carnel Labs. Corporation 94608-1 [46] whichhas the following specification:


Reprocl ucecl by permission of Solar Electronics

Figure 6.52 Co-planar two-loop calibration setup(a) Illustration oj'layout (b) Circuitschematic

usa.~

~en

1A 810-1 ~

10-2 g10-3 ff

:c10-4 ::

10-5 ~

10-£ ~oowa::5aw

100k a:

B is measured here

LIMIT FOR RS 01

1k 10k

FREQUENCY Hz

100

Loop specifiedfor magnetic field radiatedsusceptibility test ( MIL S T D 461/2RSO1). For this configuration oj loop:B == 5 x 10-5 T/ A at measurement face.Loop self-resonance occurs at above 100 kHz

:5enwb~160

~140I-:§ 120

~ 100I-

~ 80wr:: 60~ 40

~ 20oi=wz

"c(

:E 10

~12 em-----1I I

I I

I

0.3 em I LOOP (n = 10 tums) OF-L ....-------- 00.16 INSULATED"!T COPPER WIRE

5 em -"----- ~~~E~~~DUCTING

llo---_ _=___--

Figure 6.54 Limits of magnetic field strength JorMIL STD 461C pt. 4 RS 01susceptibility test

Figure 6.53

The test limits are given in MIL STD 462 N3directly in dB,uV across the calibrated resistor. Inlater versions (MIL STD 461 C pt.4 the fieldstrength limits which the EDT must withstandare given in Figure 6.54, together with theapproximate loop driving current required toproduce the field.

The use of various loop antennas in lowfrequency EMC field measurement illustrates thatgenerally, military EMC standards require awider range of tests, over a broader range offreq uencies and using a wider range of antennasthan do standards relating to commercialelectronic equipment. I t is inevitable thereforethat tests in military EMC standards have beenused to illustrate the use of the widest range ofantennas, sensors and measurement techniques.

LOOP BEING CALIBRATED(RE 01)

(b)

(a)

TRANSMITTING LOOP

\

( RS 01)

13.25"33.57 em.

~

L J

1.0nPRECISIONRESISTOR

1 Q PRECISIONRESISTORIN SERIES

WITH LOOP

POWER SIGNAL GENERATOR orSIGNAL GENERATOR and AMPLIFIER

6.12.5 Magnetic field susceptibility tests

A typical magnetic field susceptibility test is thatspecified in MIL STD 461/2 as RSOI (30Hz30 kHz) where a small 12 em-diameter loop iswound with ten turns of insulated copper wire asshown in Figure 6.53. In the test the face of theradiating loop is placed 5 em from the face of theEDT being investigated. The alternating voltageacross a calibrated resistor in series with the loopis measured to derive the current in the loop toproduce the required field strengths.

valid to calibrate loop antennas in a nonuniformfield, such as that which would be produced withtwo separated coplanar loops.

To achieve the most accurate measurements,loop antennas should be calibrated in a uniformfield which could be provided by a Helmholtzcoil. Currently this would require the testengineer to calibrate his own loops, which willtake time and involve some equipment costs.Whether this is done or not, engineers should beaware of these calibration problems, and shouldexpect an additional measurement uncertaintydue to calibration factors when making magneticfield measurements.


6.13 References

Passive Monopole Antenna 94592-1, Carnel Labs.Corporation, 21434 Osborne Street, Canoga Park,CA 91304, USA

2 Active Monopole Antenna 95010-1, Carnel Labs.Corporation, 21434 Osborne Street, Canoga Park,CA 91304, USA

3 MA, M.T. and KANDA, M.: 'Electromagneticcompatibility and interference metrology'. NBStechnical note 1099, NBS,. Boulder, CO 80303,USA, july 1986, p. 76

4 Carnel Labs. Corporation instruction manuals fortuned dipoles DM-I05A-Tl/T2 & T3, CarnelLabs. Corp., 21434 Osborne Street, Canoga Park,CA 91304, USA

5 'RSGB radio communications handbook'. PottersBar, Herts, UK, 1982, 5th edn., p. 12.42

6 'RSGB radio communications handbook'. PottersBar, Herts, UK, 1982, 5th edn., p. 13.6

7 McCONNELL, R.A.: 'An impedance networkmodel for open field range site attenuation'.Proceedings of IEEE symposium on EMC, 1990,pp. 435-439

8 KENDALL, C.: '30 m site attenuation improvementby increasing the transmit antenna's height'.Proceedings of IEEE symposium on EMC, 1985,pp. 346-350

9 BENNET, W.C.: 'Antenna to ground plane mutualcoupling measurements on open field sites'.Proceedings of IEEE symposium on EMC, 1988,pp. 277-283

10 BRENCH, C.E.: 'Antenna differences and theirinfluence on radiated emission measurements'.Proceedings of IEEE symposium on EMC, 1990,pp. 440-443

11 DASH, G.: 'A reference antenna method forperforming site attenuation tests'. Proceedings ofIEEE symposium on EMC, 1985, pp. 607-611

12 'A new wideband balun'. FCC project 3235-16dated 3/4/57

13 'Construction and testing of antenna balunassemblies for general purpose use by the FieldEngineering Bureau'. FCC project 3235-25, dated10/11/67

14 MA, M.T. and KANDA, M.: 'Electromagneticcompatibility and interference metrology'. NBStechnical note 1099, sections 6 & 7, NBS, Boulder,CO 80303, USA

15 CAMELL, D.G., LARSEN, E.B. and ANSON,W.J.: 'NBS calibration procedures for horizontaldipole antennas'. Proceedings of IEEE symposiumon EMC, 1988, pp. 390-394

16 BERGER, H.S., KUMARA, V. and MATLOUBI,K.: 'Considerations in the design of a broad bandE-field sensing system'. Proceedings of IEEEsymposium on EMC, 1988, pp. 383-389

17 KANDA, M.: 'An isotropic electric field probe withtapered resistive dipoles for broad band use,100kHz-18GHz'. Proceedings of IEEE symposiumon EMC, 1986

18 BALANIS, C.A.: 'Antenna theory, analysis &design' (Wiley, NY, USA) chap. 8, pp. 322-330

19 SCHELKUNOFF, S.A.: 'Electromagnetic waves'(Van Nostrand, NY, 1943) chap. 11

20 jASIK, H.: 'Antenna engineering handbook'(McGraw-Hill, NY, 1961) chap. 3

21 Emco Biconical Antenna 3108 instruction manual.Electro-Mechanics Company, PO Box 1546,Austin, TX 78767, USA

22 Carnel Labs. Corporation biconic antenna 94455-1instruction manual. Carnel Labs. Corp., 21434Osborne Street, Canoga Park, CA 91304, USA

23 BALANIS, C.A.: 'Antenna theory, analysis anddesign' (Wiley, NY, USA) chap. 10 pp. 413-435

24 RUMSEY, V.H.: 'Frequency independentantennas'. IRE National Convention record,part 1, 1957, pp. 114-118

25 'Reference data for radio engineers' (Howard WSams) chap. 27, p. 27-17

26 ISBELL, D.E.: 'Log periodic dipole arrays' IRE,1960, AP-8 (3), Fig. 7

27 'Antennas, antenna masts and mounting adaptors'.American Electronic Laboratories Inc, Lansdale,Montgomeryville, PA 18936, USA, catalogue7.5M-7-79

28 MARVIN, A.C.: 'Gain and phase centre evaluationof the EMCO conical log spiral antenna'. BAe EMCstudy note 005, 29/1/79, British AerospaceDynamics, Filton, Bristol, UK

29 COLLIN, R.E.: 'Antennas and radio wavepropagation' (McGraw-Hill) p. 182

30 RUDGE, A.W., MILNE, K., OLVER, A.D. andKNIGHT, P. (Eds.): Handbook of antennadesign, Volume 1. (Peter Peregrinus, London,1982) chap. 4

31 BALANIS, C.A.: 'Antenna theory, analysis anddesign' (Wiley, NY, USA) chap. 12, pp. 532-571

32 SILVER, S.: 'Microwave antenna theory anddesign' (Peter Peregrinus, London, 1984) section 10

33 SOUTHWORTH, G.C.: 'Principles and applications of waveguide transmission' (Van Nostrand,1950) section 10.1

34 KRAUS,j.D.: 'Antennas' (McGraw-Hill, 2nd edn.)p.647

35 MA, M.T. and KANDA, M.: 'Electromagneticcompatibility and interference metrology'. NBStechnical note 1099, NBS, Boulder, CO 80303,USA, chap. 5.3, pp. 67-72

36 'Standard gain horns' data sheet. Flann MicrowaveInstruments, Dunmere Road, Bodmin, Cornwall,UK

37 Antenna data sheet for AT 4001. Amplifier research(USA), EMV Ltd, 11 Drakes Mews, Crownhill,Milton Keynes, UK

38 MA.RCUVITZ, N.: 'Waveguide handbook'(McGraw-Hill) section 8.6, p. 399

39 COHN, S.B.: 'Properties of the ridge waveguide'.Proc. IRE, August 1947, 35, pp. 783-789

40 WALTON, K.L. and SUNDBERG, V.C.:'Broadband ridged horn design', Microwave ].,March 1964,7, pp. 96-101

41 Operating instructions for RGA-I00. ElectroMetrics, 100 Church St, Amsterdam, NY 12010,USA

42 CONNOR, F.R.: 'Antennas'. (Edward Arnold,London, 1984) section 5.3, pp. 53-57


43 'Reference data for radio engineers' (Howard WSams, 1977) chap. 28, p. 28-20

44 BALANIS, C.A.: 'Antenna theory, analysis anddesign' (Wiley, NY, USA) chap. 13, p. 616

45 'Calibration of loop antennas, RFljEMCInstruments, components and accessories for theRFI jEMC engineer'. Solar Electronics Co, 901North Highland Ave., Hollywood, CA 90038,USA

46 Carnel Labs. Corporation loop antenna 94608-1.Carnel Labs. Corp., 21434 Osborne Street, CanogaPark, California 91304, USA

47 Electro-Metrics loop antenna ALP-70, Electro-

Metrics Ltd, Ivel Rd, Shefford, Beds., SG 17 5JU,UK

48 Electro-Metrics loop antenna ALR-30A, ElectroMetrics Ltd, Ivel Rd, Shefford, Beds., SG17 5JU,UK

49 MILLEN, E.: 'A comparison of loop antennas'.Proceedings of IEEE symposium on EMC, 1990,pp. 451-455

50 IEEE Std 302-1969: Standard methods formeasuring electromagnetic field strength forfreg uencies below 1000 MHz In radio wavepropagation. IEEE, 345 East 47th St, NY,NY 10017, USA

Chapter 7

Use of antennas for radiatedsusceptibility testing

7.1 Introduction

The types of antenna commonly used for RFradiated susceptibility testing are treatedseparately from antennas used in emISSIonmeasurements as the an tenna parameters relatingto reception and transmission are differen t.

7.1.1 Types of antennas used insusceptibility testing

Antennas for EMC radiated susceptibility orimmunity testing. fall into two classes: freewavefield and. bounded wavefield, addressedseparately in this chapter. Generally, free-fieldantennas are used for tests on large systems orsubsystems, and on units at frequencies above30 MHz. Bounded-wave devices such as parallelplate lines are generally used for testing smallunits about 30 cm high at frequencies below300 MHz. There are of course exceptions to thesegeneralities and bounded-wave devices forexample can be used to test large vehicle systemsat kV 1m field strengths in NEMP (nuclear electromagnetic pulse) measurements. Small units orcomponents can be tested in bounded-wavedevices (with extended frequency coverage up to1 GHz) such as Crawford cells or GTEM(gigahertz transverse electromagnetic mode)cells.

Free-field antennas are not as efficient atproducing high field strengths (over a largevolume) as bounded-wave devices are over asmall volume. In many cases of conformancetesting, the use of free-field antennas orbounded-wave devices is specified over a givenfrequency range by the EMC standard whichapplies to the EDT. For those development testswhere the EMC engineer has some discretion inchoosing antennas, the more efficient boundedwave devices will probably be chosen if the sizeof the EDT and test frequency range permits.The bounded-wave radiators are less costly, andwi th some closed devices wi th no RF leakagesuch as a Crawford cell, the tests may be carriedout in an ordinary laboratory rather than ashielded room.

110

7.1.2 Standards requiring immunity tests

Radiated susceptibility testing is carried outregularly and extensively on military equipmentsbefore going into service. Each country, andsometimes each armed service in that country, hasa specification and set of test methods which willdirect the testing in a detailed way. ImportantEMC testing standards covering militaryequipment are listed in Chapter 2.

Many EMC standards relating to commercialelectronic equipment do not as yet requireradiated susceptibility or immunity testing to becarried out as part of product certification.Examples of exceptions being IEC 801-31BS666 7-3 for industrial process con trolequipment, EN55101-3 for informationtechnology equipment and NWML0320 forcertain items of metrology equipment. There isalso a European harmonised generic immunitystandard EN50082-1 relating to any domestic,commercial or light industrial equipment notcovered by a product specific immuni tystandard.

Radiated susceptibility testing is one of the mostexpensive aspects of EMC assessment. This ismainly due to the capital cost of the range of highpower broadband RF amplifiers needed to drivethe variety of an tennas used to cover frequenciesup to 1 GHz (civil) and 18 or 40 GHz (military).

Free-field tests must be conducted in a metalscreened room of sufficient size to house the EDTand test antenna as shown in Figure 7.1.Reflections should be suppressed wheneverpossible by using radio absorbent material on thewalls and ceiling inside the room as shown inFigure 7.2. The screened room and radioabsorbent material are also costly items andincrease the capital investment which must bemade by the test laboratory if radiated susceptibility testing is to be performed.

In what follows, antennas commonly used infree-field EMC radiated susceptibility testing aredescribed individually and typical field strengthsproduced for various input RF powers indicated.This is followed by a similar treatment forbounded-wave devices.

USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING III

7.4

W0.0333.3333333332

Lossless DM-I05A TljT3

Power input

W0.022.0420420449

This type of broadband antenna is described inChapter 6. It is often used for both radiatedemission and susceptibility testing in thefrequency range 20 to 300 MHz because of itswide bandwidth which is achieved without anylength adjustment. This makes it convenient touse during swept frequency testing which reducestest times and therefore keeps costs to a minimum.

Table 7.1 Input power requiredfor various field strengths

Reasonable field strengths of around 10 V jm at1 m can be obtained for a few watts input RFpower. Thus the dipole antenna is quite efficientwhen used at resonance. It would be impracticalto use a dipole with some loss (such as the DM105A T IjT3 designed primarily for reception) togenerate fields above about 100 V jm as the balunand other lossy components would begin to heatand sustain damage.

The problem with using tuned dipoles for EMCradiated susceptibility testing is that relativelyefficient performance with a low VSWR isrestricted to the frequencies close to the dipoleresonance. The antenna therefore has to be tunedat each frequency of interest when testing acrossthe required band and this would be very timeconsuming. I t is for this reason that tuned halfwave dipoles are rarely used In practicalimmunity testing.

7.3 Biconic dipoles

Fieldstrengthat 1 m

where Pd is the power density in Wjm 2 and Pin isthe input power to the antenna in watts. Relatingthe E-field strength to the wavefield powerdensity by the impedance of free space, the inputpower required for various E-field strengths inTable 7.1. is calculated from

Vjm1101001000

In the case of a practical antenna with a slightlylossy balun, such as the Carnel Labs DM-I05ATljT2jT3 which were discussed in Chapter 6,where the gain is given as 0 dB from 27 to1000 MHz, eqn. 7.4 becomes

E 2 == 30 X Pin 7.5

POWERSUPPLY

7.3

ANECHOIC CHAMBER

I.

TRANSMITIING ANTENNA 1 i

:_~

A 0.13A2[IJ 7.1

GA2

and also A -- [2J 7.24n

so gaIn G 0.13 x 4

G 1.63 or 2.1 dB

Lossless tuned halfwave dipoles have anapproximate effective area A given by

Using eqn. 5.2 the approximate power density onan EUT at 1 m from the lossless tuned halfwavedipole is calculated as


FREE-FIELD ANTENNAS

7.2 Tuned halfwave dipoles

Figure 7.2 Radio absorbent material lining inside ojscreened room used for radiated susceptibilitytesting


Figure 7.1 Standard test set up in screened room forradiated susceptibility testing

SIGNAL SOURCE &POWER AMPLIFIER

112 A HANDBOOK FOR EMC TESTING- AND MEASUREMENT

Reproduced by permission of' Amplifier Research

50---....-----r----.----~-----.

where Pd is the power density at 1 m and in asimilar manner to eqn. 7.4 for the log-periodicantenna:

7.6

7.7

0.398Pin5Pin

4n

E2 377 X 0.398 Pin

E2 150 Pin

J:- 40I-w00::zl-

30WWo::~1- .........(J)(J)

2001-...J...JwOu:~ 10

00 2.0 4.0 6.0 8.0 10.0

POWER (WATTS)

120

J: - 100I-W

~ ~ 80~~~ 00 60a~

rf! ~ 40U-_

20 30 40 50 60POWER ( WATTS)

LOG PERIODIC ANTENNA AT 1000 (150 MHz - 1 GHz)

Figure 7.3 Power against field strength at l,m fortypical log-periodic dipole antenna

I t is now possible to calculate the approximateinput power required to produce a given fieldstrength at 1 m from this log-periodic antenna.Figure 7.3 shows the relationship in graphicalform for field strengths from 10 to 100 V 1m. Thislow loss and robust commercial antenna forexample, can produce field strengths at 1 m of upto 500 V 1m and handle input powers of over1.5kW.

frequency is also given and reference is made to acommercial log-periodic dipole antenna producedby Amplifier Research, Model AT 1000. Theaverage gain (calibrated at 1 m) for this typicalantenna is 7.5 dB ± 1dB and if a small loss isassumed of 0.5 dB for connectors etc., it isreasonable to use an average gain figure of 7 dBor a factor of five.

In a similar manner to deriving eqn. 7.3, forthis log-periodic antenna the relationshipbetween input power and wavefield powerdensity at 1 m is

Fre- Typical Field strengthquency gaIn

1Vim 5V/m 10V/m 20V/m

MHz W W W W30 0.04 0.83 21 83 33240 0.08 0.42 11 42 16850 0.13 0.26 6.5 26 10460 0.38 0.088 2.2 8.8 3570 0.59 0.056 1.4 5.6 2280 0.94 0.035 0.88 3.5 1490 1.00 0.033 0.83 3.3 13.2

100 0.98 0.034 0.85 3.4 13.6120 0.69 0.048 1.2 4.8 19.2140 0.76 0.044 1.1 4.4 17.6160 0.88 0.038 0.95 3.8 15.2180 1.31 0.025 0.63 2.5 10200 1.62 0.021 0.53 2.1 8.4220 1.42 0.023 0.58 2.3 9.2240 1.04 0.032 0.80 3.2 12.8260 0.90 0.037 0.93 3.7 14.8280 0.80 0.042 1.1 4.2 16.8300 1.02 0.033 0.83 3.3 13.2

At 200 MHz this antenna is almost as efficient as alossless tuned dipole, but is less so at the edges ofthe band over which it operates. At 30 MHzwhere it is electrically short and inefficient (beingphysically only 1.3 m long) it requires 40 times asmuch power for the same E-field as a losslesstuned dipole.

7.4 Log-periodic dipoles

The simplified theory of operation and typicalconstruction of this type of antenna is discussed inChapter 6. Typical gain as a function of

Emco 3108 high power biconical antenna

Table 7.2 Approximate power requirements againstfrequency for field strengths at 1 m spacing

Care must be taken to ensure that biconicantennas that are suitable for radiated emissiontesting are also capable of handling the RFpower for radiated susceptibility testing. Somereceiving antennas may have baluns which aretoo lossy or have built-in resistive networks toaid matching or the production of a flatfrequency response. In such cases the powerdissipated in these lossy elements could lead tooverheating and damage.

High-power biconic antennas such as the Emco3108 (discussed in Chapter 6) are speciallydeveloped for radiated susceptibility testing. Asan example of the performance of such anantenna, the power requirements for given fieldstrengths at 1 m are shown in Table 7.2.

USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING 113

7.5 Conical log-spiral antennas

Table 7.3 Power against field strength for typical logconical spiral antenna at 1 m spacing

7.9

7.10

Thus for a field strength of 10 V1m at 1 m only0.33 W is required. Care must be taken when.using simple far-field gain formulas such as eqn.5.2 on which these calculations are based, whenconsidering high gain antennas which may haveextended near fields out to beyond the usualantenna to EDT distance of 1 m.

produce a truly frequency-independent value forthe constant which relates electric field strength toantenna input power as in eqn. 7.8 for example.However, as a guide, take a single gain value of10 dB at a frequency of 580 MHz and use it toderive the value ofE at 1 m as

This yields an input power requirement of 83 Wfor a field strength of 200 V1m at 1 m and at afrequency of 580 MHz. It is evident from thissimple calculation that the use of a horn antennaof this type is an efficient way of producing highfield strengths at 1 m suitable for EMC radiatedsusceptibility testing. Graphs of field strengthswhich can be produced at 1 m by the AT4001horn antenna for a number of input powers canbe seen in Figure 7.4.

For standard .gain waveguide horns such asthose referred to in Figure 6.38 the average gainfor a given waveguide size is about 20 dB ± 1 dB.Deriving the approximate relationship betweenfield strength and input power as in Section 7.2,

7.8

Field strength

1Vim 5V/m 10V/m 20V/m

W W W W0.256 6.409 25.6 102.40.019 0.484 1.9 7.60.016 0.403 1.6 6.40.014 0.343 1.4 5.60.014 0.355 1.4 5.60.014 0.350 1.4 5.60.016 0.401 1.6 6.40.015 0.384 1.5 6.00.018 0.441 1.8 7.20.023 0.571 2.3 9.2

Fre- Gainquency

MHz100 0.13200 1. 72300 2.07400 2.43500 2.35600 2.38700 2.08800 2.17900 1.89

1000 1.46

Typical input powers for various field strengths aregiven for this conical log-spiral antenna inTable 7.3.

This type of antenna is also discussed in Chapter 6and typical gain against frequency figures aregiven. The typical midband gain for the commercially available Emco 3103 (calibrated' for 1 m) isabout a factor 2.3 or 3.6 dB. Deriving the relationship between E-field at 1 m and antenna inputpower,

Emco 3103 conical log-spiral antenna

Although the manufacturer shows the antennabeing usable down to 100 MHz the VSWR asshown in Figure 6.33 rises to almost 4: 1 at thisfrequency. If it were used to create a significantfield strength of say 20 Vim at 1 m (requiring aninput power of around 100 W) care would beneeded to avoid reflected power causing damageto the RF power amplifier output stage. For thisreason many modern commercial amplifiers foruse in EMC testing have output stage protectionin addition to current trips to prevent reflectedpower damage.

7.6 Horn antennas

1000800

600Wex:

400r-w~

(f)200r-

....J0>::c 100r- 800Z 60Wex:r- 40(f)

0....JW 20u:

The theory and design of horn antennas is discussedin Chapter 6 and typical gain against frequencyfigures are stated for pyramidal horns. An exampleof a large low-frequency horn used for EMCradiated susceptibility testing is the AR AT400 1.1'he gain varies from 10 dB at 400 MHz to 15 dB at1 GHz (Figure 6.39). It is therefore not possible to

10 ~---"'_~_..I--_.L--~_~_-'300 400 500 600 700 800 900 1000

FREQUENCY MHz

Figure 7.4 Field strength at 1 m produced as function offrequency for various input power levels frompyramidal horn antenna

Reproduced by permission or Amplier Research


7.7 Parabolic reflector antennas

Octave-bandwidth travelling wave tube poweramplifiers are available which can produce up to200 W output at these frequencies. If this powerlevel were applied to the parabolic reflector

The characteristics of the parabolic reflectoran tenna are also discussed in Chapter 6 andtypical gain and beamwidth figures are given forcommercial antennas which are widely used forEMC measurements at microwave frequencies.As an example, the Carnel Labs 91892-1 18-inchdiameter reflector and two horn feeds (91890-1,4.4-7.3 GHz and 91891-2, 7.3-12 GHz) areshown in Figure 7.5. Table 7.4 lists the antennagain values, gives the field strength/powerconstants and indicates the approximate relationship between field strength at 1 m and requiredinpu t power to the antenna feed horns at theupper and lower freq uencies of the bandscovered.

7.8.2 Requirements for civil aircraft

Concurrent with the introduction of digital cockpitinstrumentation and fly-by-wire technology, theproposed field strengths which civil aircraft mustwithstand increased dramatically at the end of the1980s [3]. The new draft certification requirements are being proposed for aircraft todemonstrate that they can operate safely in ahigh-intensity radiated field (HIRF). This hasimplications for both equipment (EMC standardDO 160 ch.20) and system level testing. Anexample of the draft RF environment (worldwide) is given in Figure 7.6 and shows worst-casepeak field strengths of over 10kV /m.

7.8 Radiated im.m.unity fieldstrength requirem.ents

7.8.1 Requirements for commercialproducts

The approximate relationships derived for theelectric field strength (at 1 m) as a function ofantenna input power and the tables of typicalvalues produced from them, have been set ou t fora number of antennas commonly used in EMCradiated susceptibility testing: The tables arecalculated for field strengths in the range 1 to100 V /m as this covers the values commonly usedin immunity testing to meet most civil andmilitary standards.

The field strength specified in the civil standardIEC80 1-3/BS6667-3 (Susceptibility to radiatedelectromagnetic energy of industrial processmeasurement and control equipment) over thefrequency range 27-500 MHz is normally restrictedto 10 V /m or less. This is typical of currentstandards applicable to commercially producedelectronic equipment. These levels may increase inthe future as the commercial and domestic EMenvironment becomes more severe, as has been thecase in the military field over the last ten years.

antenna at, say, 7.4 GHz, it would result in a veryhigh field strength at 1 m from the dish of approximately 1.9 kV/m.

The near field/far field boundary for the 18-inch(46 cm) parabolic reflector antenna at a frequencyof 7.3 GHz calculated from eqn. 5.28 is approximately 10m. An EDT at 1 m from the antenna isclearly in the near field and care must beexercised in applying simple formulas to derivethe field strength for a given input power to theantenna. The experienced EMC test engineer willwhere possible, calibrate his own antennas at thevarious ranges of interest and derive a moreprecise set of power tables which can be used withconfidence.

REFLECTOR DISH

HORN FEED(7.3 - 12 GHz)

i 4cm-1

/~HORN FEED COAXIAL INPUT

( 4.4 - 7.3 GHz ) CONNECTOR

r-18" diam.

l_Figure 7.5 Parabolic reflector antenna andfeed horns for

use in radiated susceptibility testing

Table 7.4 Power against field strength table for an 18-inch diameter parabolic reflector antenna

Frequency

4.4GHz 7.3 GHz 12GHz

Gain dB 23 28 32Gain (x) 200 600 1600£2

fiOOO 18000 480007C Const.

Field Required input powerstrengthat 1 m 4.4GHz 7.3 GHz 12GHz

Vim mW mW mW1 0.16 0.05 0.0210 16.6 5.5 2.08100 1660 555 208


limits in the HF band have been addressed [4]and nonradiative methods of susceptibility testinghave been devised using high power Bel currentprobe injection techniques [5-8] and directinjection of RF into the structures of items fortest. The recently introduced BCI (bulk currentinjection) techniques will be dealt with InChapter 9.

In the following section the design and use ofsusceptibility antennas for use at HF and beloware discussed and the problems of generatingother than modest field strengths are explored.

------AVERAGE

-"'1.-"'"1.'LII r L_J

1 10 100 1,000 10,000 100,000FREQUENCY MHz

Draft worldwide severe RF environJnent(116190)

--PEAK

E100,000

> 10,000I...." 1,000zwex:

100....000

10-'wu::

1.01 .1

Figure 7.6

7.8.3 Mili tary requirements

In the military EMC world, MIL STD 461 alsoshows this trend towards higher field strengthsspecified for radiated susceptibility tests. Figure7.7 gives the levels required by MIL STD 461A/Bfor the RS03 test when applied to equipmentsfrom all three services. In general the values arebelow 100 V 1m but in special cases (some aircrafton carrier decks) the limit is raised to 200 V 1m.

I t can be seen by extrapolating from Table 7.2(biconic antenna) that to test even smallequipments to this level would require poweramplifiers with outputs of 1.3 kW at 100 MHz and16.8 kW at 40 MHz. Such powerful testequipment would be very expensive and specialultra-Iow-Ioss antennas would be needed tosurvive this power input. In general the situationbecomes worse at lower frequencies whereantennas have a lower power gain, and becomescritical in the HF band where inefficient 'fringefield transmission line' antennas (discussed subsequently) are commonly used.

It is at present impossible to carry out for areasonable cost a 200 V1m radiated susceptibilitytest through the HF band on a complete systemwhich has dimensions of more than a few metres.The problems of susceptibility testing to the RS03

7.9 E-field generators

7.9.1 Construction

An E-field generator, as its name implies, is notexactly an antenna. It is a compact radiatingdevice which produces a localised high electricfield in its vicinity. It does not have a well-definedradiation pattern and does not produce anintentional beam of any _kind. These devicessometimes exploit the fringe radiation from ashort open transmission line which is energised toa high voltage from a powerful amplifier via abroadband RF step-up transformer.

A sketch of a transmission line antenna is givenin Figure 7.8 and shows the transmission line

HIGH LOCALliE" FIELD

1 1 Vim (Army)0.3 ,.", I I I I

10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz 100 GHz

FREQUENCY

Figure 7.7 Radiated susceptibility limits for us servicesas given in MIL STD 461 A and B(Test RS03)

10 VIm (Navy & USAF)

300 \

POSITIONING OFIIEII FIELD GENERATOR AND EUT

Figure 7.8 Typical construction oj high E-Jieldgenerator

200 V1m for non-metallic aircraft andabove-decklf~selage equipment

/ I

-

(all services) 40 Vim (Navy) ---L......

(all services)I pi

Jo----ool _

(Army & USAF)3-

10

100~ \30-


~2kW

7.9.2 Practical devices

FREQUENCY MHz

Figure 7.11 Example of variation in field strength forconstant input power of unlevelled E-jieldgenerator inside shielded chamber

0.01 .1 1 10 100 200

A. R. Model AT 3000

150

Reproduced by permission or Amplfier Research

Reprod uced by permission or BAe Dynamics

E; 100

o...JWu:w 50

Figure 7.10 Compact highfield strength/high powerlocal E-field generator

they become self resonant. The field strength canvary considerably with frequency for a constantinput power and it is usually impractical toexpect to produce a given field strength at aparticular point close to the generator simply bymeasuring the device input power and extrapolating from a calibration curve. An example ofthe variability of field strength with frequencyfor a constant 1 kW input power is given for amoqel EFG3 when used in a reflecting screenedroom in Figure 7.11.

E-field generators are therefore normally used inconjunction with a small broadband (10 kHz300 MHz) E-field monopole detector which may

E FIELD~ 1 kV 1m

1d = 1 m

I500nHIGHPOWERLOAD

ISHORT TRANSMISSION LINE "RAILSII

~1 kV

Figure 7.9 Typical construction of high-jield strengthlocal E-field generator

Several commercial designs of wide band E-fieldgenerators are available. Typical of these are theIFI EFG3 [9J (10 kHz-220 MHz) and theAR AT3000 [10J (10kHz-30MHz). Thegenerators for laboratory use are about1 m X 1 m X 8 cm and are mounted on a nonconductive stand or tripod. They may haveextending arms or rods which enhances the fieldbetween them. A diagram of the AR AT3000 isgiven in Figure 7.10.

Although E-field generators can operate over awide frequency band they do not always have awell behaved frequency response at VHF where

3 : 1 RF TRANSFORMER(WIDE BAND)

rails, the impedance transformer/balun and thehigh power low inductance loads. In some designsthe load is external to theE-field generator. TheEDT is usually placed 1 m to the side of thedevice in the fringe field of the short transmissionline. The live rails, or plates, can often beextended to the side of the main an tenna toprovide a higher field region between them.

A reasonable impedance match is made to thepower amplifier by the use of the high power lowinductance RF absorbing resistors across the railsand VSWRs of around 2.5 to 3: 1 can be achievedprior to the EDT being pu t in place or theantenna being operated in a small screened room.Generators constructed in this way are notefficient but have a wide bandwidth, from 10 kHzto 30 MHz, as the loading on the amplifier isessentially resistive.

A wide-bandwidth transformer is used to stepup the RF voltage from the 50 ohm input by upto a factor ten to increase the voltage across thetransrnission line and thereby increase the E-fieldproduced between the rails and around thedevice. A circuit diagram and electricalparameters for a typical generator are shown inFigure 7.9.

lISE OF ANTENNAS FOR RADIATED SUSCEPT'IBILITY TESTING 117

L== ==1

RF POWERAMPLIFIER

Figure 7.13 Field strength Jor given power as functionojfrequency, Jor large commerciallyavailable E-field generator (EUT size2x3x2m)

Power input, cw maximum 3000 wattsFrequency range 10kHz· 20 MHzImpedance 50 ohms, VSWR 2.5:1 maximum, 1.5:1 averageElectric field intensity ( at 2500 watt input) 200 vim minimum between elementsRF irlput IconnlBctor ' 'yp" v female

GENERATOR SPECIFICATION (A.R. AT 3001 )

so0'----.0L...1

-.01-2 -.....J.o.....s--L.1-..J..2--.S1---1L--~-~--'7":::----=-=-----=-!.50

FREQUENCY MHz

FIELD STRENGTH BETWEEN ELEMENTS


not so large as to significantly alter the intrinsicperformance of the E-field generator mutualimpedance coupling to the active elements. Theexact manner in which these tests are conductedis still to some extent a matter for the EMC testengineer.

E-field generators produce fields with differentimpedances at different frequencies as the EUT isalmost always in the reactive near field of thedevice. The wave impedance is usually greaterthan that of free space, particularly at lowfrequencies from 10 kHz to a few tens of MHz.Thus the susceptibility of the ED'T may not bethat which it would be if subjected to a free-fieldplane wave with the same field strength. Indeed,using these generators it is often difficult to knowexactly what wavefield the EDT has beensubjected to, apart from knowing the oneparameter which is measured (E-field).

I t is possible to produce reasonably uniformfields over a large volume by constructing devicesup to 3 m high and 2 m wide. An example of sucha large E-field generator is the AR AT300 1(10 kHz-20 MHz) which can be used to subjectsystems up to the size of small vehicles to high Efields. This commercial device has a frequencyresponse which is shown in Figure 7.13.

E-field ge~<;rators are very inefIicient devices andrequire a few kW ofRF input power to generate afew hundred V 1m fields over a relatively small testvolume of around a cubic metre. This makes thetest method extremely expensive as it requires abroadband kW RF power amplifier. More efficientbounded-wave devices for conducting EMCradiated susceptibility tests on small equipmentsare considered subsequently.

sao r__--.---..------,--,---r---,--Ir----.---r---r-,-----,

W "S 400~ .~300

~ :g 200-- 3':(J)o1-0c5 ~ 100>"5

SMALL BATIERY POWERED_________E FIELD MONOPOLE SENSOR

FIBRE OPTIC LINK

~/

HIGH POWER E FIELD GENERATOR10 - 100 V / m constant field strength

1====;:::::====::1

100 W - 2 kW VARIABLE OUTPUT POWER

J?igure 7.12 E-field generator used to produce knownlevelled E-field againstJrequency.

be coupled via a fibre optic link to a broadbandRF levelling preamplifier as in Figure 7.12. Theinput power from the main amplifier is then automatically controlled to produce a constant E-fieldat the position of the sensor. If the maximumoutput power required from the main amplifier isgreater than that which can be supplied, thelevelling will not take place and the E-field willfall below that demanded. The VSWR of the Efield generator varies with frequency and mayexceed the limits into which the amplifier candrive the power. Under these circumstances theprotection devices in the amplifier will trip outand shut off the power. This is more likely tooccur when a large EDT is being tested, or whenthe E-field generator is working in a smallscreened room with insufficient spacing to thewalls, floor or ceiling.

Even with the limitations described, at first sightthe levelling technique overcomes the problem ofthe highly variable frequency response of the Efield generator. However, when the EDT isplaced in the field and the E-field sensor is placedon, or close to it, this will still result in a differentlevelled field. That is, the levelling now takesplace taking into account the diffraction fieldfrom the EDT. The diffraction fields of the EDThave thus been compensated for by the levellingloop and the field strength to which the EDT issubjected is not the same as that which would bederived by using the levelling loop without theEDT present.

To overcome this problem it is possible to usecomputer-based instrumentation to record theinput power needed as a function of frequency toproduce the levelled field without the EDTpresent and to replay these power settings whenthe EUT is in place. This will then subject theEDT to a known calibrated field, providing it is


7.12

7.11

COAXIALFEEDER

INSULATORS

~

R3

DRIVING VOLTAGE EL

Field strength atEUTis EV/m

J

where EL is the driving voltage, E the E-field onthe centre line of the EDT and kd the attenuationconstant (TAF),

between two insulators mounted on the oppositewalls of the screened room, below the ceiling level,at between 1/4 and 1/3 of the height of the room.A coaxial feed is constructed from a 1 in. diametercopper pipe with a 16 AWG bare wire centreconductor. The outer of the coaxial line isgrounded to the wall of the room at its base andthe signal generator or power amplifier connectionis also made to the coaxial feeder at this point. I t ispreferable to have the signal generator or poweramplifier outside the room (for convenience andsafe operation) but the length of the connection tothe feeder should be kept as short as possible andcertainly less than one tenth of a wavelength at30 MHz (1 m). If the amplifier or signal generatoris inside the screened enclosure and the coaxialfeeder centre conductor is extended to connect tothe amplifier output, its length must be kept to lessthan 1 foot (30 cm) [11 J.

There is a careful procedure which must befollowed in MIL STD 462 N3 to derive thevalues for the terminating resistors (R2 and R3 inFigure 7.14) for any given long wire installation.These resistors correctly load the feeder and longwire line so that the generator can feed RF powerto a well matched line to radiate in the room atfrequencies up to 30 MHz. Once the noninductiveloading resistors have been determined the samevalues maybe used each time the line is erectedto conduct a test. The final configuration is shownin Figure 7.15 and the equation relating the Efield produced at the test sample to the drivingvoltage at the base of the feeder (which is thetransmitting antenna factor, T AF) is

7.1 0.2 Use in testing military equipment

The MIL STD 462 notice 3 [12J is the bestexample of an EMC standard which calls for aradiated susceptibility test using a long wire line asshown in Figure 7.14. The line is suspended tautly

7.10 Long wire lines

7.10.1 Advantages

Another form ofE-field generator or antenna that iscapable of producing high field strengths is the longwire line. This technique has the advantage overshort transmission line E-field generators in thatthey operate in the dominant TEM (transverse electromagnetic mode) using the conductive enclosurein which they are erected as the outer sheath of acoaxial line [11 J. This allows some understanding ofthe field pattern and the wavefield impedance atpoints around the wire and in the vicinity of theEDT. The equipment being tested is placed underthe wire in the screened room and it should be smallcompared with the size of the room (less than aquarter of the linear dimensions of the room). Longwire lines can therefore be used for generating highfield strengths on relatively large objects (0.5-1 m 3

)

if used in a good sized screened enclosure. Testobjects of this volume would normally be too big tobe tested in a parallel plate line or other boundedwave device (discussed later in this chapter).

The long wire line can only be used below about30 MHz [11 J in the average sized screened roomused for example for EMC susceptibility testing toMIL STD 461 (5 x 4 x 3 n1), as higher order modeswill be set up at frequencies above this and theordered nature of the TEM field will be destroyed.

Figure 7.14 Typical screened room layout for using longwire antenna (MIL STD 461)

Figure 7.15 Geometry for long wire line relating fieldstrength at E UT to driving voltage


where d) d1 and d2 are the distances shown inFigure 7.15, and Z is the characteristic impedanceof the long wire line (R2 ) . The a ttenuationconstant (TAF) is independent of frequencyresulting in the straightforward performance ofthe radiated susceptibility test when using a longwire line. Although the T AF will be a good guideto the relationship between the E-field generatedat the EUT and the driving voltage or inputpower (unlike the case of E-field genera tors) it isadvisable to have a small fibre optically coupledE-field sensor [13J at the EUT location tomeasure the actual E-field generated.

The long wire line maybe considered as a freefield radiating antenna, within the confines of thescreened room. I t is also equally tenable toconsider it as a simple TEM bounded-wavedevice with the screened room itself forming theouter conductor of a coaxial line.

BOUNDED-WAVE DEVICES

contribute to the upper frequency of simplelines for use in an EMC laboratory, beinglimited to about 30 MHz.

The larger the EUT the larger must be the line toaccommodate it, and thus the lower must be theupper frequency limit of the line. Very largeparallel-plate lines have been built which approximately simulate predominantly low-frequency(less than 10 MHz) nuclear electromagnetic pulse.Complete vehicles can be irradiated in such linesat high field strengths of around 50 kV 1m.

7.11.2 Line impedance

The characteristic impedance of a parallel plate isthe key design parameter. I t determines the voltageacross the plates for a given input power andtherefore determines (crudely) the E-field as this isthe plate voltage divided by plate separation. Thecharacteristic impedance itself is determined by thephysical configuration of the line:

7.13

Figure 7.16 Parallel stripline jor radiated susceptibilitytest RS03 US MIL STn 46112

The construction of a simple parallel-plate linewhich is suitable for testing to MIL STD 462RS03 is shown in Figure 7.16. I t can be seen thatthe line is a sizable laboratory device which can

7.14

7.11.3 Construction

Z == Zo X hlw

if h « w (i.e. h < O.lw), where

h == plate separationw == plate widthZ characteristic line impedance

Zo == impedance of free space (377 ohms).

POWER INPUTEND

where Z is characteristic impedance of theparallel-plate line, L the inductance per unitlength, and C the capacitance per unit length. Itcan be shown [15, 16J that for an air-filled linethe approximate impedance is

Bounded-wave devices is the name given to a classof EM-field generating devices where the radiationis largely confined by the radiating structure. Thistends to make these devices efficient but only ingenerating fields over a small volume.

(i) The height of the EUT is restricted to aboutone third of the line spacing. 'This meansthat for practical laboratory lines of lessthan 1 m separation [14J the EUT musthave dimensions of less than 30 cm. For theparallel-plate line specified in MIL STD 462(notices 1 and 3) with a plate separation of18 inches (46cm) the EUT can have nodimension larger than 6 inches (15 cm) if itis to be tested in all three axes.

(ii) The useful frequency range of the susceptibility test is limited by the cut-off frequencyfor the line where higher order modes beginto propagate in addition to the simpleTEM. The overall VSWR of the line isdetermined by the InJection and loadrnatching sections at either end of theparallel section and by the variation of lineilTIpedance with frequency. All these factors

7.11 Parallel-plate line

7.11.1 Properties

The parallel-plate line is the simplest of thebounded-wave devices and a line of about 0.5 mseparation is capable of producing high fieldstrengths of up to a 100 V 1m for moderate inputRF powers of less than 100 W. There are twomain limitations in its use for EMC susceptibilitytesting:


SOURCE

50nSIGNAL

SOURCE 520

(0)Insertion loss

=4.2dB

PARALLEL PLATE LINE

(b)Parallel plate

line

LOAD

800 46.4Q

(c)Insertion loss

=30dB

TERMINAL STRIP(9 tags)

TERMINAL STRIP(see fig. 7.21 )

WOODEN PLANK

METAL BOX WITH FILTERS

FRAMEWORK OF WOODEN BEAMS( section 50 x 50 mm )

HF MEASURINGPROBE

EARTH

BLOCKS OF WOOD(400 x 200 x 125 mm)

SUPPORT OFFOAM PLASTIC


Figure 7.20 Parallel-plate line with tapered source andload sections IEC/ 801-3 for testingindustrial electronics

Figure 7.19 Example ofparallel-plate line usedfortesting commercial equipment (DIN 45305)

son 122 n

Q: :Q61~: Z=150QSOURCE SOURCE PAD LINE

reduce the effects of sharp impedance discontinuities which adversely affect the VSWR.~ more sophistica ted line [1 7] is required for

testIng commercial equipment to IEe801-3jBS6667-3 in the frequency range 27-500 MHz andis shown in Figure 7.20. The test volume of the lineis defined by a cube of 0.8 m side. The maximumrecommended EDT size is a cube with a 25 cmsi?e. 1~he lin~ has two tapers each 0.8 m long butstIll uses reSIstor pads for the source and loadmatching networks. The condition associated witheqn. 7.. 14 clearly does not hold (as the plateseparatlon and wid th are identical) and the characteristic impedance cannot be calculated using this

TERMINAL STRIP

SUPPLY t INPUT &OUTPUT CABLES( 3 + 2 + 2 wires)

All cables ~are twisted

I

calibration chart for RS03 1WIL461 parallel stripline

7.18

7.17 Matching networks at input and load endsof RS03 parallel stripline

24 r---,--,---r--,---,----,--r---.---.---~ 22

~ 20~ 18~ 16QEuI 00 14iI ~ 12

~~ 10t- 8&1 6..JW 4

20o~_~--=----:~~-~--'--...1.--L--l--J

1 2 3 4 5 6 7 8 9 10

V - VOLTAGE ACROSS LOAD ( VOLTS)

test a small item of equipment withdimensions less than 6 inches (15 cm) . Thedimensions of the line lead to a characteristic

of 83 ohms and to drive it froln aconventional 50 ohm signal source an impedance

network is required. This -is -shown in7.17 a and has an insertion loss of 4.2 dB.

. line termination is shown in Figure 7.1 7c andIS a 83 ohm load with a voltage dividerattenuator with a 50 ohm output for connectionto an EMI meter which enables the RF voltageacross the to be measured. A suitableattenuator in the line to the EMI Ineter has a loss

30dB.A calibration factor for the stripline is

shown 7.18. A field strength of 100 V jmcan be with 25 W dissipated in the loadwhich 65 W from the 50 ohm signal source.

A 150 ohm impedance parallel-plate linewith a 0.8 In separation for use incommercial to the DIN45 305 [14] standard is shown in Figure 7.19

with the and load circuits. Thisline tapered sections from the

to the source and load resistive networks to

USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TEST'ING 121

TERMINATIONcomprising 3 separate layers of

377 n /0 conductive plastic film,joined at ends and connected

to plates

for 10 V / m

W = E2 = 100 = 0.833 WZc 120

100 1k 10k 100 kiM 10M 100 MFREQUENCY Hz AFSC DH 1 - 4 LINE

BNC orN TYPECONNECTOR

6m~

~1m

~

INPUTCONNECTOR

7.11.5 Field uniformity and VSWR

All parallel-plate lines suffer a degradation in fielduniformity and VSWR when an EDT isintroduced into the line. A study made by Ankeand Busch [15] has produced an estimate of theVSWR seen at the input of the parallel plate linespecified in MIL STD 462 when EDT objects ofvarious sizes are introduced into it.

Let the wid th of the line be Wand the height ofthe line be H. Now let the height of the object be hand its wid th be equal to that of the line, anddefine R == W/H and r == W/(H h). Thenhili == 1 - R/r and the VSWR at the input to theline is given in Figure 7.23. The effective value ofR for the MIL STD line is about 1.5 and thestandard req uires that 'in no case shall the testsample be closer than 10 cm to the top plate'.Thus the ratio of the maximum height of theobject to the height of the line (h/H) is 0.78 andr == 6.8.

Reproduced by permission of' US DoD

WAVE LAUNCHER;'-611 copper on Perspex

WAVE LAUNCHER DETAIL

Reproduced by permission of' US DoD

F'igure 7.22 Power inputfor 10 Vim field in efficientparallel-plate line. No source or loadresistive pads

Figure 7.21 Sophisticated parallel stripline "Loith wavelauncher US AFSG'V DH 1-4

i 1000

~ 100

0:: 10w~ 1 ... -'

a. .1I-~ .01~ .001 L...--_---'-__..I-_-...L__-L-__L--_--'-_-----I

10

7.11.4 Complex lines

I t is possible to design more sophisticated parallelplate lines using long and carefully tapered transmission lines or wave launchers of complexgeometry. A 6 m long tapered line with aImseparation and multiple strip conductors insteadof plates (to reduce transverse currents) has beenbuilt [19J to operate at frequencies above 30 MHz.

A large 1 m plate separation design using a wavelauncher is specified in AFSC DH 1-4 [20J asshown in Figure 7.21. I t uses a distribu ted loadmade from conductive plastic sheets with377 ohm/square. Because it uses no lumpedresistor matching pads it is extremely efficient andcan produce a consistently high field strength as afunction of freq uency. A field strength of 10 V /mcan be produced for just 0.83 W from DC to20MHz. See Figure 7.22.

simple formula. The value of the matched load is135 ohms which is close to the line impedance.

rrhis particular line is specified for use up to500 MHz, which is high considering the largeplate separation. I t is fed in such a way that thelive plate is the lower one which sees the groundplate above it and the actual ground below it isseparated by 0.4 m. This is a crude coaxial configuration which can help to extend the upperfrequency of the line. The design does not includedistributed RF absorbing materials to provide RFdamping which are sometimes used in highperformance lines. Care would be needed whencarrying out tests in this line and the test engineershould monitor the actual E-field components inthe line, preferably at points along its length, asthe test frequency is changed.

I t is interesting that this IEC80 1-3 striplinemakes provision to extract the EDT cables via aset of filters placed above the top (grounded)plate, and then run them down the tapered inputsection to the input RF connector. They are thentwisted around the RF coaxial cable for somedistance away from the line before beingconnected to the EDT support equipment. Thisconfiguration helps to minimise the interactionbetween the EDT cables and the fields in the lineso that the test will predominantly reveal thesusceptibility of the box of electronics rather thanits connecting cable.

The EDT box is tested in three axes to determinethe worst-case susceptibility. Schemes have beensuggested [18J to reduce the testing time bymounting the EDT at 45° to the plates in bothlongitudinal and transverse axes and then rotatingit through 360° to expose all facets of the EDT tothe maximum field. Such novel approaches havenot yet been included in test standards.


120

100

w 80ex:.....w

60~

(j)..... 40...J0>

20

01 10 100 500

FREQUENCY MHz

Figure 7.25 Parallel-plate line calibration: fieldstrength Jor constant input voltage, withoutabsorbent material Jor damping

7.11.6 Use in screened room

A parallel-plate line will radiate a significantamount of energy away from itself and must beused inside a shielded enclosure for all but thelowest power tests. Porter [21] has conductedexperiments to determine the effect of a screenedroom on the performance of a parallel-plate linederived from that specified for testing con1mercialelectronic equipment in IEC80 1-3, as discussedearlier. A particular a.pplica tion of this line hasbeen incorporated in a report [22] and is used bythe DTI to satisfy the requirements of ECEReg.13 (annex 13) [23] and 85/647 EEC annex x[24] with regard to the immunity of antilockbraking devices.

The screened enclosure in which a parallel-plateline is operating behaves as a cavity resonator [25]with frequencies given by:

J == _1_ V(a/x)2 + (b/y)2 + (c/::l 7.152VJi8

where a, b aQd c are integers (one of which may bezero) and x, y and z are the dimensions of thecavity, with f1 the permeability of free space and 8

the permittivity of free space. The screened roomin which the experiments were conducted was6.7 x 2.6 x 2.2 m and had a fundamentalresonance at 62 MIlz. A 10 V 1m field calibrationof the line was therefore subject to progressivechanges above this value as the modes set up inthe screened room interact with the line, seeFigure 7.25. By placing RF absorbing material inand around the line, together with a stack of

line performance! However, a strong VSWReffect would be noticed at about 50 MHz withthis EDT in the line where the impedance reachesover 200 ohms compared with the ideal value of83 ohms.

83 n line

W 150z::JawN~ 25.....I c:~.g 100

~~.....I E~~<co.O-.S 50

OL-- --'- .....Io-__~_ ____I

0.1 1 10 30 100

FREQUENCY MHz

Figure 7.24 Variation oj line input impedance withJrequency, with EUT present and absent.- - - - with EUT--- without EUT

a:~ 2.5>

5r-----~-----,...---::I.-----,------.

Tracing these values onto the R == 1.5 curve (forthe MIL STD line) the worst-case VSWR for anEDT which is only 10 cm less than the 46 cmpIate height will be close to 3: 1. Anke and Busch[15J measured the input impedance of the MILSTD line as a function of frequency up to100 MHz and revealed an increase above thenominal 83 ohms which starts at 8 MHz andpeaks with a value of 154 ohms at 20 MHz. Seethe heavy line trace marked 2 in Figure 7.24.This shows that the relatively simplistic design ofthe MIL STD line is rather inadequate at thehigh end of its specified operating frequencyrange of 30 MHz. When an EDT is placed in theline which is 50 cm long X 50 cm wide and 30 cmhigh, the measured input impedance of the linechanges to that shown as the light trace 1 inFigure 7.24. Within the 30 MHz frequency rangethe insertion of the EDT actually improves the

200..-------,-------y---r--...".-----.,

o 0.5 (R~~~5) ~---.

LIMITATION OF MIL STD 46f

Figure 7.23 VSWR at input to parallel-plate line asJunction oj relative height oj E UT to line.Let W == width ojplate line, H == heightojplate line, h == height oj EUT.Define R == WIH, r == WI (H-h) ,hlH == l-Rlr


1-----------6.7m---------

7. 12.2 erawford cell performance

7.12.1 Basic construction

7.12 TEM cells

where Z == characteristic impedance of line,C == capacitance/unit length (pF /cm), £1' == relativepermittivity, and dimensions w) band t relate tothose of the cell componen ts shown in Figure 7.29.

Keiser [29J also gives figures for typical cut-offfrequencies above which more complex modeswill propagate in the cell. These are given inTable 7.5 as an indication of the upper limit ofsimple TEM operation for a typical line (withC == 0.087 pF/m.). Other investigations intoTEM-cell cut-off frequencies have also beenreported [30, 31].

A TEM cell is constructed by gradually expandingthe size of a coaxial transmission line to dimensionswhich are large enough for a small EDT to beplaced between the inner and outer conductorswithout significantly altering the properties of theline.

The Crawford cell [26-28J was designed by M.L.Crawford at the National Bureau of Standards inthe USA as a means of establishing standarduniform electromagnetic fields in a shieldedenvironment [29]. Compared with a parallel-plateline the Crawford or TEM cell has twoadvantages: operation at higher field strengths forthe same input power, and operation up to higherfrequencies. TEM cells have the disadvantage thatthey can only accommodate small objects for testwithout being scaled up to sizes where the SWRbecomes a problem. 'Typical sizes for test objectsare 15 x 5 x 10 cm and are much smaller thanthose which can be tested in open parallel-platelines.

are called TEM cells. They are however usuallyquite small and can only be used for susceptibilitytesting at component or board level.

Because the cell is based on a coaxial line with anexpanding cen tre conductor surrounded by acarefully designed tapered rectangular conductingbox it has an almost constant impedance forfrequencies up to around 1 GHz. Figure 7.28shows the construction of a Crawford Cell andindicates the small volume into which the EUTcan be placed. Keiser [29J gives the characteristicimpedance of a square section Crawford cell as

94.15Z == ohms 7.16ft;[ we]b(1 - t/b) + 0.088 £1'

I \ABSORBERSTACK

1.3m

SCREENED ROOM"

___ RF ABSORBING BLOCKS

1-2.23 mI

i

~I~DOOR ··~i,~

120

LU100z

:::i~ 80J:..... Q)o '-~Q5 60a: E..............(f)~ 40o 0..J >LUiI 20

01 10 100 500

FREQUENCY MHz

PLATE LINEII

I

11---1aii..:af-.·1.9 m-:----+-J~-2.6m

III

Figure 7.27 Controlled field strength obtained inparallel-plate line Jor 50 W input powerwith absorber material in place around lineand in screened room

Figure 7.26 Disposition oj RF absorbing blocks aroundparallel-plate line to control standing wavesin screened room

absorber at a specific location in this particularroom, shown in Figure 7.26, it was possible tocontrol the interaction between the room and theline to produce the field strength calibration for aconstant 50W input power shown in Figure 7.27.While there are still variations in the plot, theyare much reduced over the original disturbancesin Figure 7.25 and show the line being useful upto almost 500 MHz.

These examples of the use of parallel-plate transmission lines indicate the type of effects which theEMC test engineer must be aware of when usingthem to perform radiated susceptibility testing. Ingeneral, these efficient and economical devices arebest suited to testing small objects at high fieldstrengths, with simple lines being limited tofrequencies below about 30 MHz. There are moresophisticated transmission-line devices which aretotally enclosed, operate up to higher frequenciesand can be used outside a screened room. These


TEST VOLUME At frequencies well below the cell cut-off frequencySpiegel et al. [32J have calculated (usingquasistatic approximations) the electric fieldstrength as a function of position above and belowthe centre conductor or septum. These calculations were made for the 30 cm-square TEM cell inuse at the National Bureau of Standards, andthen compared with measurements which hadpreviously been made. The plots in Figure 7.30show how the field varies across the workingspace above and below the septum.

Figure 7.28 Ijpical Crawford cells

OUTPUT CONNECTOR

Side view

~6~~~~~L - :j~TVOLUMESEPTUM PLATE INSULATING LOW LOSS SPACERS

----------.,. .-----OUTER BOX

INNER PLATE

Top view


b +I~---I

iTI I

1---w~·1

1--1--- b ------.tal

\

OUTER BOX

SEPTUM PLATE

7.12.3 Wave impedance in TEM cell

Because the structure of a TEM cell is so simpleand the wavefield is well behaved below thecu t-off freq uency, few engineers have investigated both the electric and magnetic fieldspresent in the working space [32J. This has ledto the assumption that the E-field can bedetermined from a consideration of the cell as aparallel-plate capacitor and the H-field is equalto the E-field value divided by 377 ohms. Thusthe H-field determination relies on the relationship between E and H for free-spacepropagation. For most applications this roughtreatment is sufficient, however at lowfreq uencies this approach is progressively unsatisfactory [32].

The issue of wave impedance in either a TEMcell or a parallel-plate line is an important one forthe EMC engineer, as the value of the H-fieldcomponents can have a significant effect on theinduced currents in the EDT, and thus affect itssusceptibility. If the same object were to be testedin apIate line and in a free field at the samefrequency and E-field strength, there is noguarantee that the immunity of the object to thetwo wavefields would be identical, as the H-fieldin the line may not be related to the E-field valueby the impedance of free space.

7.12.4 Field distortions in TEM cell

}'igure 7.29 Cross section dimensions of square sectionCrawford cell

Table 7.5 Dimensions and cut-offfrequencies of aCrawford cell

b Cut-offw

frequency

cm cm cm MHz150 124 0.157 10050 41 0.157 30030 25 0.157 500

When an EDT is placed into the TEM cell itdistorts the field in the line and loads it at somepoint along its length. Because the field strengthchanges from the unloaded condition it is thendifficult to know what value to ascribe to asusceptibility which might be observed for theobject under test. Foo et al. [33J have performed2D finite element EM computations for TEMcells with various dielectric and conductiveobjects inside them. As a guide to the percentagechange in E-field which will occur in a TEM cellthe calculations are performed around a semicircular cylindrical EDT, and result in the datashown in Figure 7.31.


SEPTUM PLATE---- -------- 15em --C::=::::=:::J

o _-r---.--...,..-.,...-r---r--r--r-......--r--I"'"""""T--'--""-"'"

:c 0.1I- 0.2

~ 0.3

~ 0.4

~ § 0.5Cl a. 0.6

u:I ~ 0.7u: ~ 0.8

~.8 0.9i= 0 10« .rrJ 1.1a:: 1.2

1.3

1.4

CROSS SECTION OF ASQUARE CRAWFORD CELL

30em-....----

26.25 cm

22.5 cm

18.25 cm

Figure 7.30 Variation offieldstrength with position insidesquare cross section Crawford cell

1.4

~ 1.3<:) 1.2

rIi 1.1

~ E 1.0CfJ.2 0.9g g- 0.8W C/)

iI ~ 0.7UJ 03 0.62::.0 0.5

~ 0.4

rrJ 0.3a:: 0.2

0.1I I I

Oem

11.25 cm

7.5cm

3.75cm

30 em wide---~~I

o 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

DISTANCE FROM SIDE WALL

The plots show the percentage field distortionat the centre of the cell (above the septum) forcylinders of various radii and dielectricconstants. The upper heavy curve is for a

1-----0 I

conducting cylinder and this result is mostappropriate for the type of metal box objectwhich is usually subjected to EMC susceptibility testing. I t is clear that significantdistortions of about 50% of the assumed fieldvalue are encountered by a conductive objectwhich is 1/4 the height of the working part ofthe cell.

Figure 7.31 Effect oj EUT in TEM cell.Inset: upper half oj TEM cell

%

41

7.12.5 Other uses of TEM cells

In addition to the conventional use of TEM cellsfor high field strength wide bandwidth EMCsusceptibility testing of small items, these devicesare used for other purposes. The measurement ofthe shielding properties of materials includinggaskets has been undertaken using TEM cells[34, 35]. Special TEM cells have also been usedto test the susceptibility. of circuits on printedcircuit boards to fields with deliberately highwave impedances to simula te the conditionspertaining to near field coupling at circuit boardlevel [36]. TEM cells are especially useful in thefield of investigating the biological effects ofintense EM radiation [37, 38]. These cells cangenerate accurately known fields at very highlevels using simple apparatus and for areasonable cost. A graph of field strength againstinput power for a cornmercial TEM cell [52J isgiven in Figure 7.32.

NORMALISED RADIUS OF CYLINDER r

conductor

SEPTUM PLATE

2015108

0.15 b .2 b .25 b .3 b .35 b .4 b .45 b

25

33

58

78

100

r;::)w

~~ 124o0-rz(50-

~o....JWu::w~zoi=0:o~CS#.


7.13 GTEM cells

A TAPERED 50 n -----TRANSMISSION LINE

IN/PUT CONNECTOR \ \

\ \ \ \\ ~ \

CLOSED METAL

RESISTIVE MATCH FOR END WALLRETURN CURRENT ON SEPTUM

OFFSET SEPTUM

~ 15% SECTOR OFSPHERICAL WAVE FRONT RADIO ABSORBENT MATERIAL( ie. almost a plane wave I - acts as a high frequency load for the wave

7.13.2 Typical construction

The concept has been the subject of a patent [41 Jand has been variously reported by theoriginators [42-44J. A vertical cross section of aGTEM cell is shown in Figure 7.34. It isconstructed as a tapered section of a rectangular50 ohm transmission line. At the apex of the cell isa precision made transition from coaxial cable tothe transmission line.

The travel time for any signal path from thesource to the load at the opposite end of the cell isthe same, as the RAM-loaded end wall is curved.Also in a GTEM cell, there are no shape discontinuities, such as exist in Crawford cells, which canact as sources of diffracted non-TEM radiation.As th-e radiation travels along the line, thestrength of the E-field varies as l/distance in asimilar manner to that for a spherical plane wave.

The offset septum plate is terminated by adistributed noninductive resistive load across thelarge end of the cell and provides a return pathfor the current via the conducting outer case.

7.13.1 Description

The GTEM or gigahertz transverse electromagnetic mode cell is a high-frequency variant of theTEM cells discussed. It is a single-taperdevelopment of an asymmetrical TEM cell with anoffset septum plate for increased working volume.I t also has both a current load connected to theseptum and distributed wave termination in theform of a RAM wall at the end of the enclosingtaper. I t may be viewed as a careful combinationof aspects of a TEM cell and an anechoicchamber. l'hese features endow the design with theuseful properties of a large working volume andhigh freg uency performance in addition to thenormal features of Crawford-type TEM cells.

T720

~t--1350-----j

300

---+-7504 rI

w 10000-U5 E 900z- 800->J:- 700..... w 600C)~z::> 500w-J0: 0 400~> 300ot-...Jw 200W W

100u::t-. 1 5 10 50 100 500

OFFSET SEPTUM PLATE

ITI I ~r60°-+-9°°+6001

all dimensions in mm

7.12.6 Asymmetric TEM cells

Asymmetric TEM cells have been designed [40J toincrease the working space where the EUT can beaccommodated for a given overall cell size. In anormal sguare cell the EUT height is limited toabout one sixth of the cell overall height. Theseptum is offset in the asymmetric design asshown in Figure 7.33 leaving a greater spacebeneath it. The field distortion that is producedas a result can be mitigated by making the cellwider, but these changes usually result in compromising the cell cutoff freg uency.

The construction and operational parametersassociated with TEM cells have been discussed, andexamples offactors which EMC engineers should beaware of, such as bandwidth and field distortion,have been presented. These low cost, simple tooperate devices are widely used for EMC susceptibility testing and for other related purposes. Theycan produce high field strengths over wide bandwidths but are usually limited to small test objects.


INPUT POWER ( WATTS)

Figure 7.32 Field strength against input RF power forcommercially available Crawford cell.Specification AR TC0500: frequencyrange) de to 500 MHz)· power input)maximum) 500 W)· impedance of cell)50 ohms)· VSWR) maximum) 1.2:1 to250 MHz) 1.4:1 to 500 MHz)· cell totalwidth) 1 m)· cell total height) 30 em)· celltotal depth) 50 em)· septum depth) 37 em)·maximum dimensions of device under test(w x h x d)) 15 x 5 x 10 em

Figure 7.33 Example of asymmetric TEM cell Figure 7.34 Schematic diagram of GTEM cell

USE OF ANI'ENNAS FOR RADIATED SUSCEPTIBILITY TESTING 127

1.20....---------,.------------.

SEPTUM LOAD

PYRAMIDS MADE FROMRADIO ABSORBENT MATERIAL

• act as a good wideband load to wave field

WAVE LAUNCHING SECTIONSfor both CW &pulsed testing\

INPUT CONNECTOR

Offset septum leaves a largevolume for testing

RF TRANSPARENTEND WALL _____

RF SEALED OUTER STRUCTUREeliminates leakage

7.13.3 Power requirements

The field strength which can be generated in aGTEM cell as a function of input power and cellsize has been determined [43] and is shown inFigure 7.37. For aIm final septum height itrequired just less than 1kW to generate 200 V/n1over the test object volume with a uniformity of±l dB. Very large GTEM cells with final septumheights of 5 m have been constructed forautomobile EMC testing [43] but it can be seenby extrapolating from Figure 7.37 that RFamplifiers capable of delivering more than 10kWare needed to produce high field strengths(200 V 1m) in such a large volume.

A three-dimensional view of a GTEM cellsuitable for immunity testing of medium sizedelectronic units with sides up to 50 cm long, suchas PCs (personal computers) and peripherals, isshown in Figure 7.38.

frequency response of ±3 dB is outstandingly goodover the frequency range up to 1GHz. The ± 1dBE-field contour for aIm GTEM cell is shown inFigure 7.36 with an indication of the size of anEDT which has dimensions of 1/3 of the septumheight and cell width. It is clear that an EDT ofthis size is immersed in an almost uniform field.

+-1 dBenvelope

SEPTUM

"

E FIELD1-000.. - --- - -i

H FIELD - -- -~ ....- - ..-..-.

110dB

T

0.00 1-- _

-1

Il-e:>zW0::--11---1(J)W

aO--1~~WL1.1-we:>>z~---IW0::

1.00

0.20

~ 0.80e:>jjjI 0.60--J--JW() 0.40

oCELL WIDTH

Figure 7.36 Field uniformity contours in GTEM cell.Inset: An EUT with dimensions i oj cellworking size

10 100 1000FREQUENCY MHz

SEPTUM HEIGHT - 1 m

Figure 7.35 Flat response oj E- and H-Jields inGTEM cell

The performance of a GTEM cell with aImmaximum floor to septum height produced withinAsea Brown Boveri by Hansen et al. [43] is shownin Figure 7.35. The flatness of the E- and H-field

1,000'

o::Ew .......6>i!: ~ 100,o=>ffi50::>1-0ClJz9 S2 10~g§u.~

1......_ ......._ ........_--a._---It...- _

o 0.5 1 1.5 2 2.5 3 3.5

INNER CONDUCTOR HEIGHT m

Figure 7.37 Field strength as Junction oj workingvolume height in GTEM cell Jor variousinput RF power levels

Figure 7.38 General viezRJ of GTEM cell with E UT inplace.[or testing (G T EM is a trademark ofEmco)

7.13.4 G1-'£M cells for emission tes ting

It has been suggested [43, 44J that GTEM cellscan also be successfully used for radiated emissiontesting of suitably sized objects. The EDT ispositioned as for a susceptibility test but the inputconnector to the cell now becomes the outpu twhich is conl)ected to an EMI meter. Thesensitivity of the cell used in this way is reportedto be high [43J and successful measurements havebeen made and compared with VD~ and FCCmeasurements made on OATS (open area testsites) .


TEST CHAMBER HEIGHT m

Figure 7.39 Pulsed E-Jield strength against test volumeheight Jor pulse generator amplitude ojlOOkV

'Reference data for radio engineers'. (Howard W.Sams, 1977) p. 27-8

2 'Reference data for radio engineers'. (Howard W.Sams) p. 28-20

3 BULL, D.A. and CARTER, N.J.: 'Testing civilaircraft and equipment to the new external RFenvironmental conditions'. Proceedings of IEEEsymposium on EMC, 1990, pp. 194-203

4 JOFFE, E.B.: 'Are RS03 limits in the HF bandrealistic?' Proceedings of IEEE symposium onEMC, 1990, pp. 196-201

5 DEF STAN 59-41, DCS02, Ministry of Defence, UK6 CARTER, N.J., REDMAN, M. and WILLIS, P.E.:

'Validation of new aircraft clearance procedures'.Proceedings of IEEE symposium on EMC, 1988,pp.117-124

7 KERSHAW, D.P. and WEBSTER, M.J.:'Evaluation of the bulk current injectiontechnique'. Presented at IEEE symposium onEMC, 1990, 14 unnumbered pages not bound intoproceedings

8 BURBIDGE, R.F., EDWARDS, D.J., RAILTON,C.J. and WILLIAMS, D.J.: 'Aspects of the bulkcurrent immunity test'. Proceedings of IEEEsymposium on EMC, 1990, pp. 162-168

9 Model EFG3 E-field generator operating instructions. Instruments for Industry, Inc, Nj, USA

10 Model AT 3000 E-field generator, AmplifierResearch, 160 School House Rd, Souderton,PA 18964-9990, USA

11 WHITE, D.R.J.: 'Handbook series on electromagnetic interference and compatibility, volume 2:EMI test methods and procedures'. Don WhiteConsultants, Germantown, Maryland, USA

12 MIL ST'D 462 notice 3 (EL). Army Department,Washington, DC 20360, USA 9 Feb 1971, pp.118-124

13 E-field sensors EFS 1/2/3. Instruments for IndustryInc, Nj, USA

14 DIN 45 305 part 302: Methods of measurement onradio receivers for various classes of emission; Methodsof checking the immunity fronl interference fields ofradio receivers. Beuth-Verlag, Berlin 30, Germany

15 ANKE, D. and BUSCH, D.: 'Parallel-plateantennas: field distortion caused by test objects'.Institution of Electronic & Radio engineers, 1985,55, (6) pp. 210-216

16 'Reference data for radio engineers'. (Howard W.Sams) p. 24-22

17 '0.8 m parallel-plate line'. ERA report 80-135, ERA,Leatherhead, Surrey, UK

18 BRONAUGH, E.L.: 'Simplifying EMI immunity(susceptibility) testing in TEM cells'. Proceedingsof IEEE symposium on EMC, 1990, pp. 488--491

19 MARVIN, A.C. and THURLOW, M.: 'A 1 mseparation strip line using tapered impedancetransformer sections'. British Aerospace Dynamics,Filton, Bristol, UK

20 'Design handbook, electromagnetic cOlTIpatibility'.Department of Defense, Washington DC, USA,AFSC DH 1-4

7.14 References

3 4 52

w::>-'~~ 100wD-C-'wu:oa:I-~ 10 --11-__----''__ _

W 0.5

->~

E1000

7.13.5 Pulse testing

Because the GTEM cell has such a flat frequencyresponse over a wide band it is ideally suited tomaking distortion-free fast pulse measurements.These cells can be used to propagate simulatedNEMP waveforms over test objects at full threatfield levels as required by MIL STD 461 C testRS05 (50 kV 1m). The peak field strengthobtainable as a function of final septum height for alOOkV input pulse amplitude is given in Figure 7.39[44J.

The GTEM cell is the result of progressivedevelopments in design from simple parallel-platelines, through TEM Crawford cells andasymmetric TEM cells combined with someproperties of RF anechoic chambers to yield amultipurpose reliable piece of test equipment formodern EMC testing. Its advocates point out thatit is less costly and more useful than anechoicchambers for small test items. I t appears to be avery versatile equipment which may becomewidely used in future years.

The great advantage of making emiSSionmeasurements in this way is the absence ofambient signals which so often confuse open-sitemeasurements and slow down the testing. Usinga GTEM cell also obviates the need for thelarge number of measurement antennasdescribed in Chapter 6, and as no antennachanges are necessary the testing is accomplishedmore quickly. Further work will need to be doneto gain general acceptance for this GTEMmethod of emission measurement, but it wouldappear to be a very promising cost-effectivetechnique.


21 PORTER, R.S.: 'A high field strength low costcomponent susceptibility test facility'. Proceedingsof IEEE symposium on EMC, 1990, pp. 227-231

22 MADDOCKS, A.J.: 'Draft specification for themeasurement of immunity of road vehicle anti-lockbraking system to electromagnetic radiation'.Project report 5043/4R6/4, 1983, ERA,Leatherhead, Surrey, UK

23 'Uniform provisions concerning the approval ofvehicles with regard to braking'. ECE Reg. 13/05,UN Economic Commission for Europe, 1988

24 'On the approximation of the laws of member statesrelating to the braking devices of certain categoriesof motor vehicles and of the trailers'. EC Directive71/320 amended by 85/647EEC, Commission of theEuropean Communities, 1985

25 HARRINGTON, R.F.: 'Introduction to electromagnetic engineering'. (McGraw-Hill, New York,1958)

26 CRAWFORD, M.L.: 'The generation of standardEM fields using TEM transmission cells'. IEEETrans., 1974, EMC-16, p. 189

27 CRAWFORD, M.L.: 'Measurement of EMradiation from electronic equipment using TEMtransmission cells, NBS international report 73-303,1973

28 CRAWFORD, M.L. and WORKMAN, j.L.:'Using a TEM cell for measurement of electronicequipment'. NBS technical note 1013, 1979

29 KEISER, B.: 'Principles of electromagnetic compatibility'. (Artech House, 1987, 3rd edn.) p. 346

30 HILL, D.A.: 'Bandwidth limitations of TEM cellsdue to resonances'. ]. Microwave Power, 1983, 18,pp. 182-195

31 WElL, C.M., JOINES, W.T. and KINN, j.B.:'Frequency range of large scale TEM moderectangular strip lines'. Microwave ]., 1981, 24,pp. 93-100

32 SPIEGEL, R.J., JOINES, W.T., BLACKMAN,C.F. and WOOD, A.W.: 'A method for calculatingthe electric and magnetic fields in TEM cells atELF'. IEEE Trans., 1987, EMC-29 (4)

33 FOO, S.L., COSTACHE, G.l. and STUCHLY,S.S.: 'Analysis of electromagnetic fields in loadedTEM cells by finite element method'. Proceedingsof IEEE symposium on EMC, 1988, pp. 6-8

34 CATRYSSE, j.: 'A new test cell for the characterisation of shielding materials In the far field'.Proceedings of IEEE symposium on EMC, 1990,pp. 62-67

35 BROWN, j.T.: 'Using TEM cells for shieldingperformance evaluation'. Proceedings of IEEEsymposium on EMC, 1990, pp. 495-499

36 DAS, S.K., VENKATESAN, V. and SINHA, B.K.:'A technique of electromagnetic interferencemeasurement with high impedance electric and lowimpedance magnetic fields inside a TEM cell'.Proceedings of IEEE symposium on EMC, 1990,pp. 367-369

37 BLACKMAN, C.F. et at.: 'Induction of calciumion effiux from brain tissue by radio freq uencyradiation: Effects of modulation frequency andfield strength'. Radio Sci., 1979, 14, (6S), pp. 9398

38 HILL, D.A.: 'Human whole body radio frequencyabsorption studies using TEM transmission cellexposure system'. IEEE Trans. 1982, MTT-30,pp. 1847-1854

39 TC 0500 TEM cell. Amplifier Research, 160 SchoolHouse Rd, Souderton, PA 18964-9990, USA

40 VERHAGEN, V.H.A.E.: 'Analysis of anasymmetric TEM cell for immunity testing'.Proceedings of IEEE symposium on EMC, 1990,pp. 157-161

41 HANSEN, D. and KOENIGSTEIN, D.: PatentCH 670 174 A5: Vorrichtung zur EMI-Prufungelectronischer Gerate, 1989

42 KOENIG·STEIN, D. and HANSEN, D.: 'A newfamily of TEM cells with enlarged bandwidth andoptimized working volume'. Proceedings of 7thinternational symposium on EMC, March 1987,pp. 127-132

43 HANSEN, D., WILSON, P., KOENIGSTEIN, D.and SCHAER, H.: 'A broadband alternative EMCtest chamber based on a TEM cell and anechoicchamber hybrid'. Proceedings of IEEE symposiumon EMC, 1989, vol. 1, pp. 133-137

44 GARBE, H. and HANSEN, D.: 'The GTEM cellconcept; Applications of this new EMC testenvironment to radiated emission and susceptibilitymeasurements'. Proceedings of IEEE symposium onEMC, 1990, pp. 152-156

Chapter 8

Receivers, analysers and•nneasurennentequlpnnent

8.1 Introduction

This chapter discusses the types of electronic testequipment commonly used in EMC emission andimmunity testing over the frequency range from afew hertz to tens of gigahertz.

8.1.1 Outline of equipment

EMC emission testing is carried ou t using a sensoror pickup device connected to a receiver,spectrum analyser or other item of measurementequipment which gives a voltage reading that canthen be converted to the quantity being measuredvia the sensor calibration or transfer function.1'he receivers, or EMI meters as they aresometimes called, are complex items of RFtechnology which in some cases have beenespecially designed for EMC test work.

Susceptibility testing requires the use of a rangeof CW, modulated CW and pulsed signal sources.High output powers are often needed and areproduced using broadband high-power RFamplifiers. Other equipment which is commonlyused directly in RF susceptibility testing includesdirectional couplers, power circulators, highpower RF broadband loads, diode detectors, RFpower meters and frequency meters.

The practising EMC test engineer must befamiliar with the performance capabilities andlimitations of all this laboratory electronic instrumentation, and be able to use it correctly to carryout testing to a wide variety of EMC standards.

The managers of companies engaged in testwork should appreciate the sophisticated natureof EMC test equipment and be able to evaluateits technical merits, the capital outlay required,probable life, calibration and maintenance costsand reliability to make cost-effective investmentdecisions. The customers of EMC test facilitiesshould be familiar with the type of instrumentsused to make measurements on their testspecimens and able to appreciate how the highcost of this equipment contributes to the facilitytariff.

8.1.2 Groups of equipment

EMC test equipment, including that mentioned in

130

Section 8.1.1 is discussed in two groups: that usedfor emission testing and that used for susceptibility testing

INSTRUMENTATION FOR EMISSIONTESTING

8.2 EMI receivers

8.2.1 Design requirements

EMI receivers are frequency tunable audio, RF andmicrowave variable bandwidth voltmeters whichcan measure and display the absolute amplitude ofa complex unknown input signal. The receivers areusually superheterodyne equipments which havebeen designed carefully to measure the amplitude ofCW, broadband noise and impulsive noise signalsaccurately using a wide range of intermediatefrequency and post-detector bandwidths. The keydesign features of a typical midfrequency range(10 kHz-30 MHz) EMI receiver are

Wide tunable frequency range, up to three orfour decadesHigh sensi tivi tyflow noise figure, < 10 dB(0.01 flV in 100 Hz)Good input VSWR < 1.5: 1Good gain flatness across band « ± 2 dB)Good absolute measurement accuracy(uncertainty < ± 2 dB)Built-in switchable calibration sources for CWand impulse signals to enable substitutionmeasurements to be madeGood out-of-band signal rejection > 100 dB(using inpu t bandpass and tracking filters)Careful mixer design giving overload signalwarning and low harmonic distortion, intermodulation, LO leakage, image responses andspurious signal responses of > 70 dB downWide-range input and IF attenuators withcoupled actionLO output with usable power> - 20 dBmLow parameter drift with time/temperature(e.g. frequency and amplitude)Good dynamic range of 0-60 dB in a singlerange, and 0-120 dB with attenuatorsGood dynamic range with impulsive signals 060 dB with preselection

AUDIOOUTPUT

VIDEOOUTPUT

BUFFEREDOUTPUTS

VOLTMETER

L--

FREQUENCY

Figure 8.1 Simplified EMI receiver block diagram

designs will be different for these equipments froD1the midrange HF jVHFjUHF instruments. Forexample, it has been extremely difficult to providegood front end preselection filters in a smalllightweight unit for the low frequency receivers.The more complex input filtering and multipleheterodyning designs used in microwave receiverscan affect the sensitivity, input VSWR and gainflatness which may not be as good as the midfrequency range receivers.

The design of EMI receivers with all thefeatures listed above is a considerable task andmanufacturers must inevitably make compromisesbetween some of the design parameters. Aninsight into the design process and some appreciation of the engineering which justifies the highcost of these sophisticated receivers may be gainedby reference to Coney and Erickson [1 J.

A simple block diagram of a basic EMI receiver isshown in Figure 8.1. A more detailed diagram of atypical receiver is given in Figure 8.2 and details ofan RF. front-end based on a commercial receiver [2Jwhich makes use of a number of separate octavewide receiver modules is shown in Figure 8.3. Thesemodules are switched into the signal path to coverthe appropriate frequency being analysed. Therestricted bandwidth of the individual modulespern1its a design with good out-of-band signalrejection and intermodulation suppression.

RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT 131

vVide selection of measurement IF bandwidths1 kHz-10 MHz with known 3 and 6 dBbandwidths and shape factorsWide selection of post-detector bandwidths,1 Hz-100kHzSelection of detector functions

PeakSlideback peakQuasipeakAverageBFOFM

Manual and sweep frequency tuningAutomatic tuning via a digital data busAutomatic frequency control (AFC),switchableSelection of output ports (amplitude)

Linear IFLog IF (0-70 dB)Linear video (0-5 MHz)Log video (0-5 MHz, 0-70 dB)Panel meterAudio outputPlotter ou tpu tData bus output

Selection of output ports (frequency)Panel meterPlotter x-drivePanoramic display x-drive

(spectrum analyser display)Remote controlAll main functions and outpu ts should beaccessible via a common standard data bus(e.g. IEEE 488)The equipment should be rugged, lightweight(man portable), battery powered (as anoption) and should have a good EMI-shieldedcase (better than 100 dB) .

EMI meters which cover the low audio-frequencyrange below 10 kHz and microwave receiversabove 1 GHz may not be able to satisfy all theparameter requirements listed, as the circuit

INPUTPORT

ep

PLOTTEROUTPUTx DRIVE(FREQUENCY)

OUTPUT LEVELMETER

LINEARIF OUTPUT

LOG IFOUTPUT

Figure 8.2 Block diagram oftypical EMI receiver

Reproduced by permission or Camel LabsCorp.


Figure 8.3 Example offrontend of commercial EMfrecezver

Reproduced by permission of Camel LabsCorp.

'-------« AFC

IFOUTPUT

where N == noise power in WK == Boltzmann's constant,

1.38 x 10-23 W IK/HzT == temperature of receiver front end

(typ. 21° C == 293K)B == receiver bandwidth in HzF == receiver noise factor (a multiple),

FdB is the receiver noise figure

If the receiver sensItIvIty is defined as that signalpower which is equal to the referred input noise

8.2.2 Selectivity and sensitivity

High signal rejection to frequencies other than theone being measured is the key performance featureof an EMI receiver or meter. The spectrumpresented to the input port is likely to be completelyunknown, and may contain a time-varying mixtureof high- and low-level narrow-band signals,together with bursts of random and impulsivenoise. Without narrow bandpass or tracking filtersat the input to the receiver, the first RF amplifier ormixer stage will be overloaded [1 J and a forest ofspurious signals will be generated, leading to aninaccurate assessment of the spectrum.

A good review of the circuitry and operation ofEMI meters can be found in White (volume 2) [3Jand is not repeated here. I t is necessary however tohave some idea of the sensitivity which EMImeters can achieve. Receiver sensitivity is definedwith reference to the equivalent noise powertranslated to the receiver input port, i.e. the outputor indicated noise power divided by the receivergain for a given set of parameters such asfrequency, attenuation, IF and post-detectorbandwidtho The noise power is defined as

Reproduced by permisssion of ICT Inc.

-87

>--97 ~

i=-107~

~E-117 ffi !g

>-127 0

w-137 a::-30

>- 201--

~s!:: 0. 10(/)Z

rna(/)0 0a::ll')UJ-» -10W::1,OeD~ '0 -20

power, then

-40 ..I.-__--I-__---L__-L.-.--..J-1471 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz

RECEIVER BANDWIDTH

Figure 8.4 Receiver sensity as function of bandwidth fornarrowband signals and various noisefigures. F == receiver noise figure

SmindBm == -114+FdB + 10log1o B 8.2

For a 50 ohm input impedance,

SdB,uV == SdBm + 107 8.3

Examples of receiver sensitivity against bandwidthfor various receiver noise figures are given inFigure 8.4.

The narrowband sensItIvIty for a receiver isdefined with regard to the random or thermal noisepower referred to its input port as given in eqn. 8.1.Such noise is incoherent and does not have apredictable phase relationship between incrementalfrequency components of the noise waveform. Thecomparison of intrinsic receiver noise power andrandom noise or CW input signals results in thedefinitions of S, the receiver sensitivity.

If the input signal is broadband coherent

8.1N == KTB x F

133

10 100 1000 10,000

IMPULSE REPETITION RATE (Hz)

0:oIo

I- LU 0 .....-----AM~:===-_.-1!J1IJIIIIt~tu I1- 0

5~ -10

a:~~«UOLUI-

tu~o ~ -40

....JW0::

Reproduced by permission of ICT Inc.

Figure 8.6 Output Jrom quasipeak (OJ?) detectors asjunction oj impulse repetition rate.-6dB bandwidth == 9 kHz

F =35 dB

F =30dB

RECEIVERS, ANALYSERS AND MEASUREMENT'

8.2.3.2 Quasipeak detector (various

rrhe function of this detector is to an outputreading which correlates well with the oJL-<.'J I'u"'JLLV ... ~..L

assessed annoyance when listening to discontinuousimpulsive noise which may be heard from abroadcast radio receiver. There are a number ofdifferent specifications for quasipeak thetwo most widely used are the ANSI andCISPR (16) standards. 1'he rise and fall times aredifferent and given in Table 2.5 in Chapter 2.Graphs of indicated output amplitude as a functionof pulse repetition frequency for peak, CISPR,ANSI and RMS detectors are given in Figure 8.6for a 6dB bandwidth of 9kHz (CISPR 16) [5J.'The equivalent graphs for the CISPR 120 kHzbandwid th are given in Figure 8.7

Quasipeak detectors are almost used forEMC emission measurements of commercialelectronic equipment being tested to standardssuch as US FCC, German VDE, British BS or theharmonised European EN series of standards.

-7+F lOlogloB 8.4

o

30

40

50,-~-----~---,.----r-----

-10

F =25dB

F=20dB

F =15 dB

F =10 dB

F=5dBF=3dB

-20 L..-__-'-__--'- L..-__-'-__--I F = 0 dB

100 Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz

RECEIVER BANDWIDTH

r::>i=(jjZLUC/)

O:N

~ ~ 20LUu>~~ 100'0Z«OJo«o0:OJ

Reproduced by permission of leT Inc.

Figure 8.5 Receiver sensitivity as junction of bandwidthJor broadband signals and various noisefigures. F == Receiver noise figure

This assumes that the coherent signal power In agiven bandwid th increases as the square ofbandwid tho Examples of receiver sensitivity tocoherent broadband signals as a function ofbandwidth and noise figure are given in Figure 8.5.

8.2.3 Detectors

impulsive noise, then there is a defined phase relationship between adj acen t incrementalfreq uencies. In this case, the receiver will measurea different rate of increase of impulsive signalpower to receiver noise power as the bandwidth isincreased. This leads to an increasing sensitivi tyto the coherent signal as the receiver bandwidth isincreased, provided that the signal bandwid th isalways greater than the receiver bandwidth.White [3J gives the following definition of receiversensitivity to impulsive noise:

Reproduced by permission or ICT Inc.

PEAKoI----------------~_-_t-6

-12

-18

-24

-30

-36

-42

-48

-54

-60~_____L. _I__-----I-----L----.....

0.1

0:oIU

I-W::::>1o..LU1-0::::>~0«o:W00..1-«°0~I-WW0>o..i=0:)

W0:

1 10 100 1000IMPULSE REPETITION RATE ( Hz )

Figure 8.7 Output Jrom quasipeak (OJ?) detectors asjunction oj impulse repetition rate.-6dB bandwidth == 120 kHz

This function allows the measurement of the peakvalue of a time varying signal to be measuredduring a preset window or gate tilue anddisplayed as the RMS value of an equivalentsinewave. Typical gate times are in the region of3 ms to 3 seconds. A brief discussion of the factorswhich determine optimum gate times for variousmeasureluents may be found in Reference 4. MostMIL STD 462 emission measurements forexample are made using a peak detector.

In addition to the high sensItIvIty, high dynamicrange and good unwanted signal rejection, EMIreceivers have a number of special detectorswhich meet the requirements of various EMCstandards and test specifications. The main typesof measurement detectors are as follows.

8.2.3.1 Peak detector


8.2.3.3 Slideback peak detector

This function is available on some receivers andenables the operator to manually set a thresholdlevel which causes the amplitude meter to readwhen a corresponding signal level is encountered.If the peak value of an incoming time varyingsignal reaches this threshold an audible tone maybe sounded to attract the operator's attention. Bysuccessively reducing the threshold level theoperator can determine the amplitude of the peaksignal level. By tuning the receiver he can ascertainthe frequency dependence of the peak signal. l'hetime-varying triggering of the slideback peakdetector by a complex signal can help the EMIengineer to identify the source of some interferencesignals to particular functions and circuits.

8.2.3.4 Average detector

This detector produces an output which is proportional to the average of the modulus of a sinusoidalsignal. I t is mostly used for determining the levelsof CW carrier signals and is sometimes called thefield intensi ty detector.

8.2.3.5 AMjFM detectors

Also included in most EMI receivers are a simpleaudio AM detector and an FM detector with anoutput bandwidth which may be up to 10 MHzwide to assist the operator in identifyingmodulation types and probable EMI signal sources.

8.2.~ Commercially available EMIreceIvers

There are a number of commercial EMI receiversavailable and examples of frequency coverageagainst model type are given in Figure 8.8. EMImeters are complex receivers specially designed toaccurately measure a wide range of unknownsignal types. 'T'hey are expensive and several will

FREQUENCY1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz 100 GHz

\1M 7

NMJ/27 I INM 37 /57

NM67

EATON

IESH3 ESJp

ROHDE & SCHWARZ

EMC 11

EMC30

EMC60

ELECTROMETRICSIFE~

Figure 8.8 Examples qffrequency coverage bycommercial EMI receivers

be req uired to cover the frequency range 10kHzto 18 or 40 GHz. Many can be swept internally toproduce a spectrum display and almost all have astandard digital interface bus through which theycan be remotely operated. They are however mostuseful, not as a spectrum analyser, but for makingaccurate and reliable measurements of specificsignals within a complex spectrum. This leads toa level of measurement confidence which isnecessary to meet the EMC test standards laiddown by procurement or licensing authorities.

8.3 SpectrulTI analysers

8.3.1 IntroductionSpectrum analysers are somewhat complementarytoEMI receivers and many EMC laboratories willuse both equipments during development andconformance EMC testing. The spectrum analyseris an extremely versatile piece of equipment whichenables the EMC engineer to quickly gain anoverview of the nature of a signal or completeemission spectrum. In contrast to EMI receiversmost scanning superheterodyne spectrumanalysers have wide-bandwid th front ends with apreamplifier or mixer as the first stage and withonly minimal lowpass filtering. This can makethem more prone to signal overload in thepresence of broadband impulse noise signals thanthe more protected EMI receiver front ends.

The noise figure for a superheterodyne spectrun1analyser at around 20-30 dB is usually higher thanthat for an EMI receiver which may be in theregion of 6-12 dB. However, the advantagesoffered by the much wider frequency span (andlower cost) of the spectrum analyser must be setagainst this lower sensitivity.

Unlike EMI receivers, spectrum analysers areused in a wide variety of applications in electronicand communications engineering: sales specificallyfor EMC work form only a small fraction of thetotal market. As interest in EMC has grown and theneed for cost-effective multipurpose RFmeasurement instrumentation has increased, manufacturers have become more aware of the need tosupply spectrum analysers with preselectors and arange of detector functions to enable theseupgraded equipments to be used for EMCconformance testing to some regulations.

8.3.2 Analyser typesThere are three types of spectrum analyser:

(i) Frequency scannIng superheterodynereceIvers

(ii) Contiguous bandpass filter banks(iii) Digital signal capture and software Fourier

transform systems


The frequency-scanning receiver is by far the mostwidely used spectrum analyser. One of the firstcommercial equipments was produced by HewlettPackard in the early 1960s followed by the morecompact and capable 141 T series which came intowide use after its introduction in the late 1960s.Since that time the spectrum analyser has benefitedfrom the revolution in measurement instrumentation brought about by the introduction ofmicroprocessors during the 1980s. There are a wide range ofexcellent processor based synthesised LO analysersavailable covering frequency ranges from 10kHz to2 GHz and beyond with bandwidths as low as10Hz. Such performance is available from a singleequipment and at a reasonable cost.

Any scanning receiver cannot detect more thanone frequency (bandwidth) at any instant andthis can result in problems when measuringpulsed signals. I t is sometimes difficult to resolvethe difference between a multiple line spectrumand a low repetition rate broadband impulsivesignal without extensive manipulation of scan rateand bandwidth. In certain circumstances it maybe necessary to produce a spectrum display froma single pulse (e.g~ NEMP) and this cannot beachieved using a scanning-type analyser.

For crude single pulse spectrum analysis a seriesof contiguous bandpass filters can be used whichproduce a histogram-type spectrum display. Withthe advent of very fast analogue-to-digitalconverters and fast Fourier transform software ithas now become possible to capture a single pulsedwaveform and then compute its spectrum. Thereare a number of commercial systems available withtime resolutions of about 1 ns [6]. Such analysersare not commonly used in EMC testing at presentbut with the increasing interest in interference

produced by switching and other low repetitionrate pulsed interference events these software basedanalysers will find increasing use.

8.3.3 Analyser operation

A simplified schematic diagram of a typicalscanning heterodyne spectrum analyser is given inFigure 8.9. An example of an analyser input andfirst mixer sections which largely determine thenoise figure, dynamic range, intermodulation andspurious responses is shown in Figure 8.10. Ingeneral, spectrum analysers do not have such agood noise figure as a purpose-designed EMIreceiver, with typical values being 20-30 dB forHF jVHF equipments and higher than this formicrowave analysers. It can be seen in Figure 8.10that with the input attenuator set at 0 dB forgreatest sensitivity, the first mixer is divorced fromthe input port only by a small fixed attenuator anda lowpass filter. When a broadband coherentimpulse signal is fed to the analyser input it ispossible to overload the mixer with only modestsignal amplitude levels. White [3J suggests that inthe limiting case, when the input signal isproduced by a very wideband fast-edged impulsegenerator, a spectrum analyser may have littleusable dynamic range without a preselector.

In my experience, spectrum analysers when usedwith care, noting their broadband dynamic rangelimitations, are a most useful piece of equipment.They are the real-time eyes of the EMC testengineer, rapidly conveying the full spectrum ofinformation about the unknown signal beingmeasured. By tuning in to a single frequency ofinterest and putting the analyser into a video signaldisplay mode it is possible to see the modulation

B lock diagram ofmultiple-stagesuperheterodynespectrumanalyser

Reproduced by permissiolJ of" HewlettPackard

INPUT LOW PASS 3dB FIRST LOW PASS SECOND NOTCHATTENUATOR FILTER AITENUATOR MIXER FILTER MIXER FILTER

t

RF n 2ndlFINPUT ..1. .L OUTPUT

r 1 n 1 W~t i

'" /1sfLO. BAND PASS FILTER 2ndL.O.

Figure 8.10 Spectrumanalyser wideband front end

Reproduced by permission of HewlettPackard

136 HANDBOOK FOR EMC TESTING AND MEASUREMENT

Direct With Frequencyinput preselector range

dB dB GHz14 20 18-26.514 23 26.5-40

Table 8.1 Microwave receiver noise figure

A bank of fixed bandpass filters can be switchedmanually or automatically into the signal pathprior to the first preamplifier or mixer circuit. Amore advanced automatic tracking bandpassfilters based preselector is available with somespectrum analysers, turning them into wide bandEMI receivers over the frequency range of 20 Hz2 GHz, which includes the frequency bands ofinterest for FCC and CISPRjEN specifications.

One such system is offered by Hewlett Packard[13J with the model 85685A. It contains bothtracking bandpass filters and preamplifiers coveringthe frequency range 20 Hz to 2 GHz and endowsthe spectrum analyser with 30 dB improvement inamplitude measuring range [14]. Belding [15Jdiscusses the theoretical basis for spectrum analyserpreselection and explores the measurernentsituations where its use is advantageous withparticular reference to FCC type open range testing.

Preselectors are also available for microwavemeasurement receivers such as the EMC-60 withan FE-60 [ 16J frequency extender allowingcoverage froIYl 18 to 40 GHz. The improveddynamic range and reduced spurious responsesobtainable with the preselector are accompaniedhowever by an increase in the receiver noisefigure, owing to additional front-end componentsand some increased signal attenuation in thepassband. Typical noise figures with and withoutpreselection are shown in Table 8.1.

8.4.2 Bandlimiting filters

Individual filter units are used in EMC testing for avariety of purposes. They are often built to be easilyconnected into the coaxial line from a sensor orantenna to a receiver or RF voltmeter and may belowpass, highpass, bandpass or bandstop. Thesefilters can be used to bandlimit spectra containingsmall signals of interest in the presence of largesignals, to achieve the best receiver sensitivity andlinearity. Tunable narrowband rejection filters[17J are sometimes used to reduce the dynamicrange of signals containing high-level carriers orother narrowband emissions so that lower levelsignals can be investigated.

Lowpass filters are used for example in test configurations for l11ethods CS-Ol, CS-03, CS-04, CS-05,CS-08 and RS-04 of MIL STD 462. Commerciallyavailable filters of this type [18J can have stopband

1Hz 10Hz 100Hz 1k 10k 1001< 1M 10M 100M 1G 100 100G

3585 B ~590 ~ I8592 B

Hewlett 8566 B

Packard

I TR 4172

Advan1est

M~ 611A

MS620

MS68

MS 2802 A

MS710

Anritsu

I FSA

Rohde & FSBSchwarz

Modern spectrum can be equipped withfilters to reduce the problemswide bandwidth out-of-band

Preselectors and filters

Figure 8.11 Examples qffrequency coverage bycommercial spectrum analysers

the display screen as andisplay. This often enables the test

and customer's technical representative tothe source of an interference signal and

IJ_LLJllJ'J'.LLll- the culprit circuit. This leads to spectrumused during development

testing.of spectrum analysers lies with their

range, functional displays andcompact nature. The cost per octave covered is muchless than that of an EMI receiver and a goodspectrum component ofawell Modernsystems are controlled for ease of'---'!J"v.L ....., ...... '-/.L.L and data recording. Many are tailor-made

EMC test work with the inclusion of bandpass orl- ... ......,v....... JL ...... .:;;.. YYr·""'c.'plpr ....nrc and a range of IF bandwidthsand detector functions MIL STD 461,FCC and CISPR. The covered by

'An 'A hTcp'rc are shown Figure 8.11.has been in this chapter to

an overview of the properties of spectrumand EMI meters of interest to thosewith EMC The design of

receivers is an extensive subject in itself and isdealt with in a number of texts, for exampleReferences 7 and 8, which El\1C test personnelmay find useful. Some manufacturers offer a

of notes relating to receiverswhich are also valuable in

'J't-'l- ....lAA ... U ....... F-, EMI measurements [9-12J.


floor levels of better than 100 dB and can beobtained with typical cut-off frequencies and-100 dB frequencies as shown in Table 8.2.

Table 8.2. EMI measurement filters (lowpass)

Cut-offfrequencyf -100dB frequency 3f

MHz MHz0.1 0.30.2 0.60.5 1.51.0 3.02.0 6.0

sequence repeats up to a cut-offfrequency of50 MHz

Highpass filters are needed for methods CE-O 1and CE-02 of MIL STD 462 to eliminate thefundamental power line frequencies andharmonics which otherwise would saturate ordestroy EMI receiver front ends. Typical filterresponse curves are given in Figure 8.12 where itcan be seen that the two lowest frequency filtersprovide 100 dB of attenuation to 50/60 Hz and400 Hz mains power frequencies, respectively. Allfilters used for bandlimiting signals before theinput to an EMI receiver are matched 50 ohmdevices and usually have coaxial connectors.

Bandpass filters are often used for isolating aknown frequency of interest from a spectrum withhigh-level adjacent noise bands so that thereceiver or spectrum analyser can be adjusted formaximum sensitivity, linearity and suppression ofspurious responses. It is important for the EMCengineer to have access to a range of high q uali tyfilters for the purposes described to avoid thepitfalls of making measurements on signals thatcause the measurement instrumentation to displayfalse amplitude data about the spectrum being

investigated. Instrumentation can be overloaded,or operate in a nonlinear manner whenconfronted with an unknown and complexspectrum containing a mixture of high and lowlevel CW and impulse signals. The ability toinsert bandlimiting filters and adjust the receiverinput attenuation provides an effective way ofensuring that these errors are minimised.

8.5 Itnpulse generators

8.5.1 Description

An impulse generator is a device which produces aseries of very short subnanosecond-duration pulseswith a variable pulse repetition frequency oftypically 50 Hz-10kHz to yield a flat widebandpulsed RF spectrum of known spectral densitymeasured in dBIlV/MHz. Impulse generators areused in EMC testing for a variety of purposes [19]:

Broadband calibration of EMI receivers orspectrum analysersShielded structures attenuation measurement'Transient testing of devices such asinformation technology equipmentDetermination of filter frequency responsesCircuit coupling or crosstalk measurement.

The main use of impulse generators with acalibrated output is as a secondary calibrationstandard for EMI receivers. The frequencycoverage of an impulse generator can extend froma few hundred Hz to 1 GHz with a spectralflatness of ± 1 dB. The output level is usuallyadjustable in 1 dB steps up to a value of above100 dBIlV/MHz.

8.5.2 Design

Reproduced by permission or Solar Electronics

Rc » <:'0with Rc as the charging resistor and <:'0 as the lineimpedance. The capacitance of the short length ofline and the charging resistor determine thecharging time constan t and the line charges to avalue Vc in that time scale. When the closingswitch, which may be a mercury-wetted reedrelay, is operated, the stored energy in t~1e transmission line is allowed to escape into the loadresistor (50 ohm) and a short voltage pulse is

The generator is normally designed to produce ashort pulse at a high-voltage level by alternatelycharging and discharging a short length of coaxialtransmission line. Figure 8.13 shows a coaxial lineof about 10 cm length being charged from a stablehigh-voltage source through a high-value resistor,where

100k100

0

CD -20"0Cf)Cf) -400--IZ0 -60t=a:w -80Cf)

~

-100

1k 10kFREQUENCY Hz

Figure 8.12 Example of the insertion loss of commercialhighpass filters (50 n coaxial) for use inEMC testing. Solar type -7205


Figure 8.13 rast dischargecoaxial line impulsegenerator (a) Impulsegenerator diagram( b) Waviform andspectrum from impulsegenerator

STABLEHIGH VpLTAGESUPPLY

~ 100V

50n

OUTPUT SPECTRAL DENSITYUP TO 100 dB Jl V / MHz

"FLATil PORTION OF OUTPUT SPECTRUM

__11dB--T

4 • FREQUENCY

Impulse generatorused to calibrate LINE SPACINGreceivers over AT PRFthis frequencyrange.

>-.....U5zwo..J«Cl: N.... J:frl~0..-'en>w::tC)«~

~

FOURIERTRANSFORM

ov""'"-----.......;;::::::11111.....TIME

-t=:::.5 ns---

..... w::::>00.«..... ~::::>00>

PRF ~ 100 Hz100 V,..-----·--....

formed at the ou tpu t terminals. The pulse wid th isdetermined largely by the length of the coaxialline. White [3J gives the voltage spectral densityof the ou tpu t pulse as

pulse. Using this technique it is possible toproduce reliable and cheap instruments withspectral flatness of ± 1dB to above 1GHz.

The first null value in the spectrum occurs at]~ IltHz.

If the pulsewidth is typically 0.5 ns long the firstzero is at 2 GHz. When] « lit the (sin n ]t) In]texpression tends to 1 and

1-'his shows that at freq uencies well below the firstnull the signal spectral density is flat with anamplitude of 2 V:. t. Thus these impulse generatorsare useful as calibration devices as the spectraloutput can be accurately determined from simplemeasurable quantities. It is possible to improvethe useful upper frequency of the flat part of thespectrum by designing a mismatch peaking circuitwhich increases the higher frequencies in the

S ]2 V:. t sin (n] t)

( ) ~ V/Hznit

where S(]) ~ signal spectral densityVc ~ charging voltage in volts

t ~ pulse wid th in secondsf ~ frequency

8.5

8.6

8.5.3 Use of impulse generators

Commercially produced impulse generators areavailable as stand-alone equipments [20J or arebuilt-in to some EMI receivers [21 J. Asmen tioned earlier the au tpu t swi tch from thecoaxial line maybe a relay. I t can be triggered toclose either linked to the power line freq uency, orto a variable frequency source. If pulse-repetitionfrequencies above a few hundred Hz are requiredthe switch is usually replaced by a solid-statedevice such as a fast FET.

A good reliable impulse generator is essential forthe EMC test laboratory. I t can be used tocalibrate and check EMI meters and spectrumanalysers for amplitude accuracy, spurious signalgeneration and dynamic range compression; or itcan be used to give a direct reading of equivalentbroadband signal strength by adjusting theoutput to give the same reading on the EMImeter as the unknown interference signal beingmeasured. l'his is generally known asmeasurement by substitution and is a veryaccurate but slow method of measurement.

EUT

producingnoise burst

when switchedon or off

EXPORTED TRANSIENTNOISE BURST

/

VOLTAGE PROBE

DIGITAL TRANSIENTRECORDER OROSCILLOSCOPE


8.6 Digital storage oscilloscopes conductor switching and regulating circuits inmains power supplies. In such cases the noise bursts

8.6.1 Advantages of digital oscilloscopes may be locked to the power line waveform; in other

Some of the more comprehensive EMC immunity applications the noise bursts may not be synchro-tests such as those in MIL STD 461/2 (CS06, CS10/ nised or may even occur randomly. The detailed11, RS05) and IEC801/BS6667 call for continuous vvaveform within a burst is often complex and mayand transient stimuli signals to be imposed on the be rather variable but if examined closely it usuallyequipment under test and monitored with a fast oscil- has some distinguishing feature such as an obviousloscope using a high-impedance probe or other oscillatory frequency generated by unintentionalsensor. There are also conducted emission tests such inductance/capacitance, which may indicate theas MIL STD 461/2 CE05, CE07-1, SP-P-90203 [22J source of the interference. Using conventional oscil-and EN550 14/BS800 in which transient or very low loscopes it can sometimes be difficult to examinerepetition rate fast burst type signals must be individual time-expanded bursts in detail owing tomeasured. For most of these tests details of the peak triggering and screen writing speed limitations.amplitude, waveshape and repetition frequency Some conducted emission tests require themust be measured. exported spikes from an EUT to be measured

Traditionally, oscilloscopes have been used to when the device is turned on or off or functionedmeasure the signal ofinterest which is usually super- in some other way. Figure 8.15 shows a typicalimposed on a power line waveform which may itself measurement setup where a single transient musthave an amplitude of ± a few hundred' volts. be measured on top of the power line voltage. InTriggering a conventional oscilloscope on the some cases, it is possible to use lowpass filters towanted interference signal can present problems reduce the amplitude of the power line waveformespecially ifits amplitude and position with respect and obtain a better trace on a conventional oscillo-to the mains frequency are variable. Setting up an scope. A digitising transient capture instrument oroscilloscope to record a single transient event is also digital oscilloscope is ideal to record this singleusually difficult and requires considerable experi- interference event. Figure 8.16 shows the productmentation and takes up valuable time in the EMCfacility. It is difficult to produce a record with thetransient in the centre of the scan even with delayedtrigger circuits on standard oscilloscopes.

Permanent records of transients have normallybeen made by photographing the screen, either with .an open shutter and a single scan, or by writing thetrace on to a storage screen and then using a timedexposure. Many of these problems are eliminatedwhen digital storage oscilloscopes are used and themeasurement and recording of transient waveformscan be achieved quickly and accurately.

MAINS POWER WAVEFORM

Figure 8.14 Transient capture oj noise burst on mainspouJer Line

8.6.2 Typical waveforms to be measuredFigure 8.14 shows typical recurrent noise burstswhich may be produced by poorly designed semi-

INDUCTOR

Figure 8.15 Measuring exported voltage spikes withdigital transient recorder

1O~s 100ms 1mS 10mS 100mS

DURATION OF TRANSIENT ENVELOPE

-600

600----r----------.....-----------

Figure 8.16 Example of exported spike limits fromSP-P-90203

> 200w

~ 0~o> -200

~0.. -400

TRANSIENT NOISE BURST( with fine structure )

III-I~"",,,,"'--#---~-""~--I"TIME ms

wo~..Jo>wz::J


0.1

1+----0.9

....zw~ 0.5+--+-+---------....:::J()


Figure 8.19 Typical output waveform oj ESD generator(BS6667 pt2)

200 ms----...,IIw IC)

~I

...J J

0 I>.... I timezw IU5 Iz ,«a: Il- I

> 200 ms ...1I

}'igure 8.17 Example of exported transients defined inBS800. Individual impulses shorter than200 ms spaced closer than 200 ms continuingfor more than 200 ms


Digital transient recorders can be extremely usefulin EMC measurement. The advent of fast, reliableADC integrated circuits and inexpensive memorieshas led to a number of instruments which for thefirst time make the capturing of low repetition orsingle-shot fast transients a simple procedure.

There are three types of waveform digi tiser:

(i) transient digi tiser with circular addressing(ii) random interleaved sampling RIS digitisers

and(iii) sampling digitisers.

When armed the first type continually digitises anincoming waveform and constantly overwrites ablock of memory from start to finish and back tothe start, in a circular fashion. When a triggerpulse is produced the digi tisa tion continues until auser-specified post trigger time has elapsed andthen the digi tisa tion ceases and the memory islocked. The captured waveform complete withpre- trigger information is read ou t from thememory and displayed on a screen.

Instruments with a digital output bus candownload the data to a PC or direct to a plotter.A typical instrument will have a sampling rate ofaround 100-200 megasamples per second, a

8.6.4 Digital transient recorderarchitecture

EDT. An example of the signal which must beinjected in accordance with MIL STD 461B CS06is given in Figure 8.18. The risetime of this spike isaround 2/ls which is relatively slow. Oscilloscopesand digital transient capture instruments need tobe capable of recording risetimes much faster thanthis. For example the risetime of the simulatedNEMP waveform used in MIL STD 461 C RS05 isIOns and that for the ESD pulse specified inIEC80 1/BS6667 is 5 ns. As an example, the ESDwave shape is given in Figure 8.19.

252010 15

TIME lIs

5

Example of injected spike waveformMIL STn 461B CS06. Use left or rightordinate whichever is less in particularapplication

Figure 8.18

8.6.3 Recording injected pulses forimmunity testing

Fast-transient recorders are also used to monitor theamplitude and waveshape of transients or spikesbeing injected onto the power lines connected to an

specific SP-P-90203 specification limits fortransients emitted from an avionics EDT in theform of amplitude-time envelopes for AC and DCsupplies. The exported transients must lie withinthese envelopes.

Confirmation of this requires that the worst-casetransient can be recorded over a time scale of microseconds to many hundreds of milliseconds. Otherexamples of the type of lo"v repetition-rate noiseburst signals of interest in EN550 1/BS800 as emittedfrom household electrical appliances can be seen inFigure 8.17. This specification permits the use of adisturbance analyser [23] which can measure andrecord transient amplitude and occurrence data forstatistical analysis. Such instruments are complementary to detailed measurements of individual eventswith digital oscilloscopes.

400CJ)

w~I-

300 ....J

g~ 0>t::e 200 wO:X 0

:2Q) ;:)

«g> 100 I-w= ::i~~ a..

0 :2a:::Q) «CJ)~ w

~

-100 a:::CJ)


repetitive signal effective sample rate of 4 GS/s, asingle-shot bandwidth of 100-300 MHz, an 8 to12 bit ampli tude resolution and two or moreinput channels. l'ypical instruments are LeCroy9410, HP 54112D and Tektronix RTD710A.Among the fastest single-shot digitisers currentlyavailable is the LeCroy 7200 series with fourchannels operating at 1 GS/s and an analoguebandwidth of 400 MHz.

The RIS digitiser can yield improved waveformresolu tion in some cases by sampling anumber ofidentical input transients at slightly differentstages in the waveform and then interleaving allthe data.

A sampling digitiser or sampling scope can onlywork with a long train of identical pulses or acontinuous waveform. Under these conditions adigitised waveform can be produced with aneffective bandwidth of beyond 30 GHz [24].

In addition to the types of instruments alreadydiscussed there are some transient captureequipments using special very fast waveformstorage techniques which can also provide adigital output. The Tektronix 7250 can captureand display a single transient waveform with aSOps risetime and the Tektronix model SCD1000/5000 has a time resolution of 5 psjpoint.

Some digitisers have built-in signal processingcapability and can quickly calculate the Fouriertransform of a captured waveform and displaythem together on a single screen. Instruments suchas HP 5180T/U, Tektronix TD2301 and LeCroy7200 series have this ability to convert from time tofrequency domain. These powerful instrumentshave a great deal to offer the practising EMCengineer or equipment designer who needs todevelop electronic products which can meet EMCstandards, as they can give a rapid insight into thefull nature of a wide range of interference signals.

An excellen t technical tu torial concerning thefundamentals of digital transient captureinstruments IS produced by LeCroy [25J.Inspection will demonstrate the capabilitiesavailable and need for careful specification of therequirements for instruments of this type beforepurchase by EMC engineers, if they are to obtainfull advan tage from this technology.

8.7 AFjRF volttneters

When performing low-frequency conductedsusceptibility tests such as MIL STD 461/2 CSO 1and CS02 it is necessary to measure the injectedsignal voltage on the lines to the EUT withou tloading them with a low impedance (e.g. 50 ohmsinput of an EMI meter). I t is also inappropriateand unnecessary to use expensive and sensitive

EMI meters or spectrum analysers fitted withhigh-impedance probes to measure these signalswhich often have an amplitude of a few voltsRMS. The preferred economical method is to usea high impedance RF voltmeter which canmeasure the required signal at a fraction of thecost of using an EMI meter. There are severalfactors which determine which type of ACvoltmeter is appropriate:

For conducted susceptibility tests on DC lines abroadband AF/RF voltmeter can be used. Thereare a number of designs using diode bridges,crystal detectors and sometimes preamplifiers toprovide measurement from a few kHz to above2 GHz and amplitudes over the range illV to 10volts. AF /RF voltmeters with both balanced andunbalanced inputs are available. A usefuldescription of AF/RF voltmeter design techniques,including explanation of average, RMS and peakreading meters, crest factor and form factor hasbeen produced by Rohde and Schwarz [26J.

For measuring signals injected into AC powerlines a tuned AC voltmeter is req uired which canfilter out the power frequency and its harmonicsproducing a measurement of only the injectedsignal amplitude. These equipments often use acombination of fixed frequency highpass filtersand a tunable bandpass filter with bandwidths aslow as 10Hz in the case of AF voltmeters.

The frequency range of _the test, e.g. audiofrequencies up to 50 kHz for MIL STD 461/2CS01 and RF frequencies up to 400 MHz forCS02, will determine the requiremen t for differen tAF and RF selective voltmeters. The use of a highinput impedance (> 100 k ohm/3-5 pF) frequencyselective RF voltmeter is shown in Figure 8.20 ina MIL STD 461/2 CS02 test setup. When makingmeasurements on AC lines, care must be taken toensure that adequate power-frequency rejection isembodied in the AF/RF voltmeter to enable thecorrect amplitude of the injected susceptibilitysignal to be measured. A range of selective AF/RFvoltmeters is commercially available from anumber of electronics instrument manufacturers.

8.8 RF power tneters

RF power meters fall into two types: ou tpu t powermeters and directional power meters. Outputpower meters measure the RF power delivered tothe sensing head and display it either on ananalogue meter or on a digi tal display or via adigital interface bus. The power meter sensorhead should have a very low SWR « 1.3: 1 [27J)over a wide frequency range (up to 26.5 GHz) ifmeasurement uncertainties owing to reflectedpower are to be minimised.


CURRENT PROBE

Figure 8.20 Use of highimpedance RF widebandvoltmeter in conductedsusceptibility testing.Susceptibility limits are interms of both voltage andcurrent

ACorDPOWERINPUT

MIL STD 461 CS 02TEST

measuressusceptibility-voltage

measures susceptibillitycurrent

EUT

HIGH INPUTIMPEDANCERF VOLTMETER- wideband/ tunable

Most RF power meter sensor heads areconstructed using one of two basic mechanisms[28J: conversion of RF power to thermal energyand measurement by thermocouple, and dioderectification and measurement by a sensitivevoltmeter. In general, the first is used formoderate to high power meters and the secondfor detecting powers as low as 100 PW (- 70 dBm) .

In addition to their general use as measurementmeters, for checking the output power of RFamplifiers for instance, these instruments are veryuseful in performing semiautomatic EMC RF andmicrowave radiated susceptibility measurements. Insuch a test configuration the radiated field strengthat the EDT has been previously measured anddefined in terms of the forward RF power deliveredto the radiating antenna, see Figure 8.2l.

A power meter and a suitable directionalcoupler, or a directional power meter [29J can beused to measure the power to the antenna andconvey the level to a computer controller whichcompares the measured value with the valuerequired to produce the nominated field strengthat that frequency. The controller then adjusts theoutpu t of the signal source to reach the requiredpower to be delivered to the antenna.

SOFTWARE& DATA

Figure 8.21 Automatic control offield strength inradiated susceptibility test using RF powermeter

An automatic power measurement loop such asdescribed, can rapidly generate the correct fieldstrengths on the EDT as the frequency in the bandis being swept, and so increase the throughput andproductivity of the test house. Software can bedevised to ensure that at no time during the powersetting process does the resultant E-field on theEDT exceed the required value at each frequency.Users of EMC test facilities should look for theability to perform semiautomatic tests of this typewhen choosing where to have their equipmentstested, as they are likely to obtain faster tests andbetter value for' money.

8.9 Frequency m.eters

These instruments are not now used as widely asthey once were following the introduction of lowcost, reliable and accurate synthesised signalsources that can be used in EMC testing. Suchsources are accurate to a few Hz in hundreds ofMHz. A frequency meter is however useful forcarrying out spot calibrations on signal sourcestogether with a spectrum analyser to checkharmonic purity. A modern spectrum analyser witha synthesised LO can itself be used to measurefrequency to within a few Hz in a GHz and cantherefore be substituted for a frequency meter.

INSTRUMENTATION FORSUSCEPTIBILITY TESTING

8.10 Signal sources

8. 10. 1 Signal synthesisers

In the past, EMC testing has required a widerange of signal sources to meet the frequency,amplitude and waveshapes needed for susceptibility testing. Frequencies covered extended froma few Hz to 40 GHz and a large number ofsources were required. Most early signal


generators did not benefit from synthesisertechnology and thus were subject to time andtemperature drifts and were also manually driven.White [30] summarises the situation with regardto nonsynthesised sweepers and power oscillators.

Modern synthesised signal sources can ~over verywide frequency ranges and only a few instrumentsare needed in a modern well equipped EMC testfacility. These highly capable equipments tend tobe expensive, and relying on so few sources tocover the required frequency range can present asignificant problem when they are not availableowing to periodic calibration or malfunction, asthe ability to conduct a whole range of tests maythen be affected.

In specifying the requirement for suitablesynthesised signal sources for use in EMC testingit is necessary to consider

Frequency rangeFrequency resolutionFrequency stabilityMaximum output power and attenuator rangeAbsolute level accuracySpectral puritySettling timeModulation capability AM, FM, pulse andpossibly phase modulationEase of programming and type of data busCase shielding against RFISize, weight, reliability and cost.

It is possible to cover the frequency range up to1 GHz specified for immunity testing ofcommercial electronics equipment with only twosignal sources. For example;

• R&S APN 1 Hz-260 kHz, resolution 0.1 Hz,[ < 20 kHz and 1 Hz f > 20 kHz, output50 f.1V to 20 V

• HP 8657A 100 kHz-I040 MHz, resolution1 Hz, output -143 to 13 dBm.

Coverage from 1 to 18 GHz can also be obtainedin a single equipment such as the HP 8672S(100kHz-18GHz), resolution 1 Hz and 3kHz,output 120 to +13 dBm (2 dBmf > 2 GHz).

For a full understanding of the variety ofsynthesised signal sources available and theirsuitability for EMC testing the reader shouldconsult a range of specialist instrument manufacturers. Figure 8.22 is a guide to the range oftypical synthesised signal sources which areavailable.

8.10.2 Signal sweepers

Sweepers do not usually have the frequencyaccuracy of synthesisers bu t can produce alevelled output (within about 1 dB) as the

10Hz 100Hz 1 kHz 10 kHz 100kHz 1 MHz 10MH~ 100MHz 1GHz 10GHz 100GHz

1 MHz AFG/U 20

APN 260 )3SPNSMG 1

SMGU/SMHU 2.16 4.32

SWM 02/05 18

Rohde & SchwarzMG 440 E 20

.1MG 545 E 0.5

MG 649 A 2

Anntsu

8904 A 6001133258

8656 B 0.99

8644 A 2.06

86600 2.6

~2 26

Hewlett Packard 8360 series 50 110

Figure 8.22 Examples of synthesised signal generators

frequency is rapidly swept between the ends of agiven band. The oscillators are usually voltagetuned (varicap diode or YIG) depending onfreq uency, and the fast outpu t sweeps can be usedfor spectrum/network analysis with couplers,detectors and a suitable display. When manuallyoperated or used with long scan times (10-100 s) ,sweepers can be used as the signal source forsusceptibility testing. Most are programmable andthey are usually less expensive than synthesisers.

8.10.3 Tracking generators

These devices are variable frequency oscillatorswith maximum output powers in the range of 0 to10 dBm which can be locked to the centrefrequency of the measurement bandwidth of aspectrum analyser and track its scan accuratelyover the band of interest. This ability results in acheap scalar network analyser which can be usedby EMC engineers and equipment designers tomeasure filter responses, cable crosstalk, shieldingefficiency, cable losses and antenna passbands andcorrection factors.

When used with two current probes as shown inFigure 8.23 this simple network analyser can yielda great deal of information about the RFcoupling behaviour of a complex systenl of interconnected equipments, such as a distributedmainframe computer installation, and can helpuncover frequencies at which radiated emissionsor susceptibilities are likely to occur. I t has beenshown that the RF common-mode cable responsein such a system is often closely correlated withthe radiated emission from them [31], as inFigure 8.24. I t also indicates frequencies ofprobable susceptibility.


Both scantogether

FREQUENCYTRACKINGGENERATOR

SIGNALPASS BAND OFSPECTRUMANALYSER

cable # 2

cable # 1

WQ:::>f::J0..::E«

R X PROBE

SIGNALLOOP

IlJ SPECTRUMJJL. ANALYSER

~t.===========f=2~

TRACKING GENERATORSIGNAL SOURCE

----CABLEUNDER TEST

CABLE UNDER TESTTRANSFER FUNCTION

(raw data)

Figure 8.23 Trackinggenerator and spectrumanalyser used to measurecable transfer functions inmultibox E UT

SIGNAL TRACKS WITH ANALYSER• to measure probe I probe coupling

Reproduced by permission or BAe Dynamics Ltd.

transducer to perform the EMC test in question.Thus amplifiers will usually have a stable inputsignal from a well defined source impedance butmust drive current probes, capacitive couplingblocks and a wide variety of antennas which areoften situated in reflective screened rooms. Suchoutpu t loads will not in general present a goodVSWR to the amplifier output. In fact, EMCtesting is one of the most demanding uses towhich high power broadband RF amplifiers canbe put in terms of output VSWR and final stageamplifier protection.

The frequency range of EMC susceptibility testsis large (up to 18 or 40 GHz) and a number ofamplifiers are required to cover this range in upto ten contiguous bands. As they also need toprovide at least 100 W output power in bandsbelow 1 G Hz and 10-100 W above this frequencythey tend to be one of the most expensive capitalassets in the test laboratory. I t is thereforeimportant that EMC test engineers and managersin companies developing products which mustmeet EMC standards, and who plan to invest inEMC test equipment, should understand theperformance limi tations and design compromisespertinent to RF amplifiers.

One of the most difficult requirements to satisfy isthe production of an amplifier with broadbandcoverage and high power in the same package. Partof the problem lies in the bandwidth limitation ofthe power devices themselves where the internalcapacitance is the limiting factor. If this capacitanceis reduced by making smaller semiconductordevices, their power handling is also reduced. The

10080

RADIATED SIGNAL STRENGTH1m from the cable

FUNDAMENTAL LINE SELF RESONANCE

\

20

CURRENT TRANSFERCHARACTERISTIC

as measured in'\ 8.23

-8001-------1------1-----'-----...1.----"'o 40 60

FREQUENCY MHz

Figure 8.24 Current transfer characteristic and measuredradiated signal strength from simple testtransmission line

Power amplifiers are required to raise the signalpower outpu t from a synthesiser or sweeper signalsource to a level required by the outpu t

8.11 RF power all1.plifiers

8.11.1 Introduction

Many spectrum analysers, including those withsynthesised local oscillators have builtin trackinggenerators and this extends the capability of thespectrum analyser in carrying out diagnosticEMC measurements as described.

-70

O..-------r------.-----r------.-----__,

·10

_ ·20CO"0

W...J«u(/)

wCl::>I:J0-~«


with the widest bandwidth and the highest powerpossible within the budget constraints.

EMC immunity levels are likely to increase inthe coming years and more RF power will berequired to carry out the tests. A factor of threeincrease in field strength in a radiated susceptibili ty specification will req uire a factor 10increase in power amplifier capability. Amplifiercost increases very rapidly with output power andit is therefore preferable to purchase expansionpotential at the start rather than pay for increasingly capable new equipment.

A chart of high-performance amplifiercapability [32J (power against frequency) is givenin Figure 8.25. The trend represented in thisfigure is a movement fron1 very broadband at lowpower (1 GHz at 5 W) to a smaller bandwidth atvery high power (100 MHz at 10kW).

Figure~8.25 Examples oj RF amplifier power outputand bandwidth. Data Jor examples fromamplifier research


1000800400 600

FREQU~NCYMHz

10 kW ( 10kHz - 100 MHz)

I I2.5 kW ( 10k - 220 MHz)

I

1 kW (220 - 400 MHz)1kW (10 k -220 MHz) I750 W ( 10k - 220 MHz) 750 W ( 400 MHz - 1 GHz)

I I

300 W ( 400 MHz - 1 GHz)250W(10 kHz- 200 W ( 220 - 400 MH~ ) I220 MHz)

100 W (100 -1000 MHz)

50i(1 -1000

I

MHZ)

10 W ( 1 - 1000 MHz)

I I5 W ( 500 kHz - 1 GHz)

I I

10

10,000

1.01 1 100 2CfO

1000

I-~ 100I-:::>oex:w

~a.

amplifier designer faces limits in device gainbandwidth product and power handling which canonly be minimised by careful circuit design.

Unsophisticated broadband high-gain (1 MHz1 GHz, 30 dB) amplifiers have a tendency tobreak into oscillation and much design effort isdirected towards ensuring stable operation, particularly when driving poorly matched loads whichis the norm in EMC testing.

8.11.2 Specifying an amplifier

Specifying RF amplifiers for a range of EMC testsis a demanding task and mistakes can lead togaps in the testing offered, an unacceptable levelof amplifier malfunction or an increase in testtimes, all of which can have an adverse economicimpact on the facility operation. The followinglist suggests the performance parameters whichshould be addressed when evaluating thesuitability of an amplifier for use in EMC testing.

8.11.2.1 Frequency range

This should be as broad as possible consistent withcost. The fewer band switches or amplifier changesthe faster the test can be conducted. Typicalpractical amplifier frequency ranges are

Solid-state amplifiers (100 W-1 kW)10Hz -100 kHz10 kHz -200 MHz

200 MHz-400 MHz(for power output of > 100 W)200 MHz-1 G Hz (for ou tpu t powers < 100 W)

TWT amplifiers (10W-100W)1 GHz - 2GHz2GHz 4GHz4GHz - 8GHz8 GHz -12.4 GHz

12.4 GHz-18 GHz

TWT amplifiers (1 W-I0W)18 GHz -26.5 GHz26.5 GHz-40 GHz.

8.11.2.2 Power output

Typical power output required for 1-3 m EMCsusceptibility testing at up to 10 V jm is 10 to 100 W.The amplifier manufacturers do not always make itclear whether they are specifying maximum outputpower, mean power (according to some definition)or minimum output power. Whenever possible,amplifiers for use in EMC testing should be specifiedagainst minimum power output. This will enSlJrethat the field strengths or induced current levels canbe produced and will give in some cases up to 3 dBheadroom in performance [32]. Experience suggeststhat the best purchase strategy is to specify units

8.11.2.3 Amplifier gain

The amplifier gain required will depend on theoutpu t level of the signal source being used andthe outpu t power calculated as being required toproduce the required field strength or inducedcurrent specified in the tests. Usually the signalsource will have an outpu t in the range 0-+13 dBm and a gain of 40-50 dB will be requiredto reach full outpu t in excess of 100 W. Themaximum output power of an amplifier will varyacross its frequency range typically by ± 1.5 dB,and the rated output should be specified at the


]± 1.5 dB

_ .........± 1.5dB

--.......... =& 1.5dB

( 1.5 dB = x 1.4) _

min gain = 54 dB----- - -------------_________-----50L

250L

___~~ !!,i~::.~ _

Saturated output power, CW

20,000 r----r:':~r:--__,-~--.--.......---r-......-~....-......----.- ......-.--.......-

10,000

5000

2000

200

ffi 100

~ 50oa. 20LLa: 10

5

Figure 8.26 Amplifier outputpower exceeds rated powerthrough frequency band for ~

high quality equipment. w( Selection from Amplifier ~Research products) ~

:::J0....:::Joow

~


2 5 10 20 50 100 200 500

FREQUENCY MHz

~cc"00:W~o0-I::>0I::>o

lower bound of this range to ensure the specifiedoutput power is always available. See Figure 8.26.

8.11.2.4 Gain compression

An amplifier will respond nonlinearly when theinput signal is sufficient to drive the output near tosaturation, when no more power can be generated.As the output approaches saturation the amplifiergain is compressed and starts to fall off asillustrated in Figure 8.27. Knowing the outputpower at which the gain compression is 1 dB forexample, is important if the equipment is to beused for amplifying signals with superimposed AM.If AM signals are passed through a nonlinearamplifier, spurious distortion signals will begenerated. This is not so important if FM orpulsed signals are being amplified, and amplifiersare often run at the saturated level for these signals.

GAIN THEORETICALCOMPRESSION LINEAR I

( 1.0 dB ) OPERATION ~~#

T7-----~-71r ~

/ ~ IOUTPUT POWER ACTUALat 1.0 dB gain AMPLIFIERcompression I OUTPUT

IIII

(G = slope) IMaximum drive level for -I

linear operation IINPUT POWER dBm

Figure 8.27 Gain and output power behaviour nearamplifier maximum output


8.11.2.5 Harmonic distortion

This is caused by nonlinearities In the amplifierresponse and creates spurious signals atharmonics of the fund amental at the expense ofpower in the fundamental signal frequency. Thechoice of amplifier design is importanthere;Class A operation results in the greatest linearityand the lowest distortion figures. Unfortunatelythis mode of operation is not suitable for highpower systems owing to energy dissipation.Typical worst-case harmonic distortioncomponents are 15-20 dB down on thefundamental signal at the rated power output ofa commercial amplifier.

The control ofharmonic distortion is important inEMC susceptibility testing as it is possible to ascribethe failure of an EDT to the susceptibility signal atthe fundamental frequency when it may have beencaused by a harmonic signal. In the worst casewhere the transfer function of the antenna or probeis such that it preferentially selects the harmonic atthe expense of the fundamental, a ghost susceptibility can be produced. Where possible, amplifiersshould be operated well below gain compression toreduce the harmonic distortion to a minimum. Thisis a further reason for purchasing amplifiers with agreater power capability than is strictly necessaryfor a particular test.

8.11.2.6 Intermodulation distortion

This effect is generated when two or more signalsare fed into the amplifier and nonlineari ties act tomix the signals together producing sum anddifference frequencies. This effect is not assignificant for EMC testing as harmonic distortion


because usually only one signal is amplified at atime and injected into or radiated onto the EUT.

8.11.2.7 Output protection

Traditionally, high-power RF amplifiers haveused directional couplers on the output to detecta poor match with the load. The signal from thereverse power port of the directional coupler isused to trip the power supply to the final stages ofthe amplifier and thus protect them againstdamage caused by the absorption of the reflectedpower. This protection feature is necessary butcan cause problems in EMC susceptibility testingwhere serious mismatches occur frequently as thesignal is swept through the frequency band andthe amplifier repeatedly trips out, extending thetest time considerably.

Modern high-power semiconductor RFamplifiers [31 J can be designed conservatively sothat the reflected power from an open or shortcircuit can be absorbed in the output stageswithout causing damage. This results in the abilityto conduct the EMC susceptibility test in a singlesweep without repeated stops to reset the amplifier.

8.11.2.8 TWT microwave amplifiers

Travelling wave tube amplifiers are currently usedto provide high output powers of 100 W or moreup to around 18 GHz. They are based on athermionic valve or tube the general constructionof which [33J is shown in Figure 8.28. Typicalamplifiers have an octave bandwidth and gains inthe region of 35--40 dB. Sometimes TWTs arenoisy amplifiers and are often combined withsolid-state microwave preamplifiers in a singleequipment to improve the system noise figure.

l'he protection of travelling wave tube highpower microwave amplifiers [34J relies on VSWR

r--------------- -----,

II CONTROL ELECTRON BEAM I

IANODE FOCUSING MAGNET ATIENUATOR TUBE BODY

\ / / COLLECTOR

!H~ 1 II~~~~ I:.: >;:::==::tl1fJWm~ tmm~~~I a T6I CATHODE ELECTRON II HELIX BEAM I

/l JTRAVELLINGWAVE TUBE

Figure 8.28 High-power microwave travelling wave tubeand power supplies

Reproduced by permission or Macmillan

detection, tube element current monitors, andpower supply shut-down techniques. MakingEMC susceptibility measurements at microwavefrequencies with TWT amplifiers is much moretime consuming than at below 1 G Hz using solidstate equipment. The amplifiers only have anoctave bandwidth, the waveguides and antennasneed to be 're-plumbed' with almost every changeof band and the TWT amplifier is designed totrip ou t in the presence of high levels of reflectedpower due to mismatches.

8.11.3 RF amplifiers - conclusions'The selection, purchase and use of high-powerbroadband amplifiers for use in EMC susceptibility testing calls for careful consideration if thebest outcome is to be obtained within a limitedbudget. Only large test houses and aerospace!military and automotive companies will have aneed to purchase the largest I-10kW amplifiersfor large system evaluation. More modestamplifiers in the region of 100 W outputaresufficient for testing most military and commercialelectronic equipments and small units. Amplifierswith outputs of a few watts are extremely usefulin susceptibility development testing of small unitsor circuit boards when used in conjunction withbounded-wave devices such as GTEM cells.

8.12 Signal modulators

8.12.1 Modulation requirementsThe requirements for the modulation characteristics of EMC susceptibility test signals vary widelywith the specification. Often a minimum set ofconditions is laid down with the proviso thatother modulation types, levels and frequenciesshould be added if the EUT is designed tooperate in an environment that contains thesespecific signals. For example, an equipment thatis to be sited near a radar transmi tter may haveto pass a susceptibility test with the specificmodulation characteristics of that radar.Generally, the specification of modulation signalsis more detailed for tests on military equipmentthan for those on commercial electronics.

A typical set of modulation rules for tests onmilitary equipment is given in MIL STD 462(N3, p117) and is listed in Appendix 2 TableA2.1. A more comprehensive list of modulationrules for susceptibility tests on military equipmentis to be found in DEF STAN 59-41 (part 3 page12); and is given in Appendix 2 Table A2.2.

8.12.2 Built-in modulators

From inspection of these tables it can be seen thatthe modulation types required for EMC tests fall


into four basic types: CW, AM, FM and pulse. Itis possible that other more sophisticated types ofmodulation such as phase modulation, may occasionally be required, but in most cases modulationsources which can deliver the four basic types areadequate for EMC testing. Modern s)'nthesisedsignal sources usually have built-in modulatorswith a reasonable capability for AM, FM andpulse operation. Typical performance figures aregiven in Table 8.3.

Table 8.3 ~ypical built-in modulator performance

AM - Modulation frequency to 50 or 100 kHzModulation depth to 99 or 100 %

FM Modulation rate typically 50 or 100 kHzbut up to 10 MHzDeviation typically 100 kHz-20 MHz

Pulse - Risetime from 5-400 nsPRF typically 0.1-1 MHzbut up to 10 MHz

Not all synthesised signal sources will have the fullrange of modulators and care must be exercised inmatching the performance of the modulator withthe modulation requirements for the range of testswhich must be performed. A reasonable costconscious compromise is to use internal modulatorsfor AM and FM and slow (> 0.5 f.1s) jlow repetitionrate « 1 MHz) pulse modulation, together withan external fast pulse modulator [35J.

External fast « 100 ns) pulse modulators maybe based on PIN diodes and are usually onlyneeded for simulating radar modulation atmicrowave frequencies. The requirement is likelyto be specified in the test plan or contractual documentation in addition to the more general range ofmodulation parameters set out in the relevantEMC standards.

A wide range of pulse generators is availablefrom a number of electronics instrument manufacturers which are suitable signal sources to drivemodulators. Most have 5 V rrTL compatible50 ohm outputs [36J, and some are capable ofhigher outpu t voltages [3 7J .

8.12.3 Arbitrary waveform generators

A synthesised signal generator or sweeper with asuitably capable internal modulator section willrequire an input from a mod ulation signal source.This can be a simple analogue function generator[38J or a sophisticated synthesised function [39J orarbitrary waveform generator [40]. Typicalfunction generators will produce sine, square,triangle, trapezoidal AM and pulsed waveformswith a variable modulation depth at frequencies upto about 20 MHz. They may also include FM and

sweep-frequency capability. An arbitrary functiongenerator can produce any programmed waveformrequired with frequencies up to about 300 MHz.Some arbitrary waveform generators can takedirect digital information from a transient digitiserused to record a signal and then reproduce theexact waveform which has been captured.

LeCroy [41 J produced a useful tu torial referenceconcerning arbitrary waveform or arbitraryfunction generators.

The ability to synthesise any type of complexwaveform is invaluable in conductingdevelopment research into RF system susceptibility. Measured real-world interference currentson system cables can be reproduced accurately ata range of increasing power levels to determine thesusceptibility threshold for the system. Arbitraryfunction generators are also extremely useful inproducing specific waveforms, such as those forNEMP or ESD tests, as the complex waveformcharacteristics can usually be programmed fromsimple mathematical functions and can bechanged at will. This obviates the need for a rangeof waveform specific signal generators andalthough arbitrary waveform generators areexpensive, they may prove to be a cost-effectivechoice for a modulation signal source.

8.13 Directional couplers, circulatorsand isolators

8.13.1 Amplifier protection devices

High-power EMC susceptibility testing calls forthe use of high-power amplifiers covering anextended frequency range including microwavebands. The widely varying load VSWR forces theneed for protective devices to be installed on theamplifier output. Directional couplers, powercirculators and waveguide isola tors are all used inthis application. The choice of which device touse will depend on the specific amplifier outputcircuit, the load and whether the output line iswaveguide or coaxial cable.

8.13.2 Directional couplers

A directional coupler is an RF transmission linedevice which bleeds off a known portion of powerflowing in the forward or reverse direction in thewaveguide or coaxial cable. This power can bemeasured and the value of the main power flowcan be calculated. The device can therefore beused to sample the forward power flow, tomeasure the reverse power and perhaps use it toshut down an amplifier which is driving a badmismatch. With appropriate detectors and metersthe reflection coefficient or VSWR can be continuously displayed.


apart where n is any positive integer and Ag is thewavelength in the guide, the forward powerflowing from port 1 through the two holes and inthe direction of port 4, will add and produce anoutput. The backward waves in the secondaryguide propagating toward port 3 are out of phasewith each other and cancel to produce no outputat that port. Thus the coupler acts in adirectional manner.

8.13.2.1 Waveguide couplers

Consider the two parallel waveguides shown inFigure 8.29. Liao [42J gives an explanation of thefunctioning of this type of directional coupler. If asmall hole is made between the two waveguides, afraction of the power flowing in the primary guidewill propagate into the secondary guide throughthe slot antenna and then flow outwards in bothdirections. If two identical holes are made Ag /4apart, or more strictly at a distance

1.51.0

0.8---~---...-----------.wC>«~o>~!:(~~5!zu.. wf2 ~ 0.4wo::C>::>«(J)~~>:iE

~t;c0::w>w0::

FORWARD POWERMEASUREMENT PORT------------C 1 2 3 D--....--- -- -- ---

- A B----lIoo-

POWER ---~-----~--INPUT ~'A. 9 / 4-1-'A. 9 /4-1 \ POWER OUTPUT

PRIMARY WAVEGUIDE

ACTUAL FREQUENCYDESIGN FREQUENCY

}-'igure 8.30 Improvement in useful bandwidth withthree-hole waveguide coupler( a) Two-hole coupler( b) Three-hole coupler

8.7L

L=(2n+1>\

POWER IN . I. 4" .1 POWER OUT\.. Primary waveguide. /

PO~ - - - ----""' - - --1Iloo--'- - -"' - - - - - _1 PORT 2i I I I------Ii' Irl ;

PORT 3 cancelled=:='=.(=~-=~-.:~=---=:==added PORT 4i

Secondary waveguide Output used to measure

forward power inprimary waveguide

Figure 8.29 Operation of simple two-hole waveguidedirectional coupler. Ag == Wavelength in thewaveguide

Reproduced by permission or HMSO

Typical values for coupling factor given by Dunlopand Smith [44J are -3, -10, -12 and -20dB.

Directivity: The existence of the coupling holesthemselves will produce reflections in both theprimary and secondary guides and there will notbe total isolation between ports 3 and 4. Thedirectivity is a measure of the ratio of power atthese ports for a matched load at port 2:

directivity == 1010g (P4/P3) 8.9

The useful bandwidth of the coupler is clearlylimited by the fixed hole separation and phasecancellation which can be achieved. Thebandwidth can be increased by moving to athree-hole coupler as shown in Figure 8.30 wherethe amplitude of the leakage wave through themiddle hole 2 is twice that of the other two holes1 and 3 [43]. The reverse voltage divided by theforward voltage in the secondary guide is ameasure of the useful performance of thedirectional coupler. I t is plotted against frequencydivided by design frequency in Figure 8.30 andthe increase in useful bandwidth of the three-holecoupIer can be seen in Figure 8. 30b.

There are two main criteria which define theperformance of a directional coupler:

Coupling factor: With matched loads on ports 2 and3 in Figure 8.29 the coupling factor is defined as

For a good coupler design this value is in the regionof30 to 40 dB. Most waveguide couplers are limitedto octave bands and a commercial example of ahigh performance coupler is the HP 752A series[45]. It is possible to put two waveguide couplerstogether as shown in Figure 8.31 and install diode

Diode detector produces DC voltageproportional to reverse power

"C ~ MATCHED LOAD

4 FORWARDVr POWER-TO-LOAD

___~I I ~

:~:::;-~I_~B~./'1~ --1 Vf REFLECTED'A.g POWER

"4 FROM LOAD

MATCHED 0

LOAD Diode detector produces DC voltageproportional to forward power

Figure 8.31 Dual directional coupler

8.8 Reproduced by permission or HMSO


3

31------11

Za = impedance of waveguideA g =wavelength in wavegu ide

Coaxial and waveguide hybrid rings

E - PLANE WAVEGUIDE HYBRID RING3A.g

4

t 1E ---~-

SHUNT COAXIAL HYBRID RING

I2Zc

Forward power to hom is measured andfed back to signal source as a levelling loop.

DC signal is proportionalto reflected power DIODE DETECTOR

Reproduced by permision of Prentice Hall Computer Publishing

Figure 8.34

Figure 8.33 Dual directional coupler used to levelforward power and provide reflected poweramplifier shut down signal when conductingradiated susceptibility test

8.11

8.10

\OUTPUT

CONNECTOR

INNER OUTPUTCONDUCTOR CONNECTOR

AUXILIARYCONDUCTORS

foLOAD

VSWR == 1 +P1-p

and

8.13.2.2 Coaxial couplers

The need for ease of connection and maximumbandwidth in transmission lines and componentsfor use in susceptibility testing, drives the EMCengineer to the widespread use of high-grade lowloss coaxial cable where possible. Thus directionalcouplers based on coaxial rather than waveguidedesigns are useful.

Kraus [46] gives an explanation of thefunctioning of a coaxial directional coupler whichuses small lengths of wire parallel to the centreconductor within the coaxial system andterminated at opposite ends by a resistance equalto the characteristic impedance of the wire. Theother ends of .the wires are brought through theouter sleeve as shown in Figure 8.32 to form theforward and reverse voltage ports. A commercialexample of a broadband 2-18 GHz directionalcoupIer with precision type N coaxial connectorsis the HP 7725 [47] which has a coupling factorof 20 dB and a directivity of around 30 dB with apower handling capacity of 50 W. Such a coupleris ideal for use with broadband network analysersor as a component in levelling broadband poweramplifiers for EMC susceptibility measurementsas shown in Figure 8.33.

detectors in the forward- and reverse-coupled portsto enable the forward and reverse power to bededuced from the voltages Vi and Vr • Then thereflection coefficient p is

OUTPUT FOR WAVE TO THE LEFT( REFLECTED POWER)

Reproduced by permission of McGraw-Hill

Figure 8.32 Coaxial directional coupler

8.13.3 Hybrid rings, circulators andisolators8.13.3.1 Hybrid ring

A four-port hybrid ring can be constructed Inei ther waveguide or coaxial transmission lines as

AUXILIARYCONDUCTORS

OUTPUT FOR WAVE TO THE RIGHT( FORWARD POWER)

in Figure 8.34. I t has the property that there is nodirect coupling between arms 1 and 4 or between2 and 3 [48]. Power flows from port 1 to port 4only by virtue of reflections generated bymismatches at ports 2 and 3. This device can beused as automatic amplifier output protection as ahigh-power load can be placed at port 4 and theamplifier power entering port 1 will be automatically diverted safely to this load should amismatch occur at the normal load port.


8.13.3.2 Circulator

A circulator is a nonreciprocal transmission devicethat uses the property of Faraday rotation of thewavefield in a ferrite material [42]. I t is amultiport waveguide junction in which the powercan flow only from the nth port to (n + 1) th portin one direction, see Figure 8.35. The design usesa combination of side-hole directional couplersand nonreciprocal phase ferrite shifters. Thisdevice can also be used to protect the ou tpu t ofexpensive microwave amplifiers when conductingEMC radiated susceptibility tests.

microwave amplifier tubes from dalnaging theinput signal generator. The isolation is gainedhowever at a price, namely the slight forwardpower insertion loss, which might typically be upto 1 dB at frequencies below 26 GHz. Isolatorsare a useful component in constructing robusthigh-power microwave susceptibility test systemswhich can withstand the daily 'abuse' frommismatches caused by wavefield reflections insideshielded rooms, and the occasional cable fa ul t orbroken waveguide flange.

8.13.4 Protection devices - conclusion

PORT 1

PORT 4

Figure 8.35

PORT 2

Four-port circulator

PORT 3

All the devices described above can be used toprotect the expensive investment of high powerbroadband amplifiers and signal sources whichare used in EMC testing. They are inexpensive bycomparison, and their specification should beconsidered carefully together with the amplifiersand antennas which make up the test system. Aworking knowledge of their function and capabilities is essential for the EMC engineer who isentrusted with carrying out susceptibility testingwhile protecting the amplifiers and keeping therepair time for this equipment to an absolutemInImum.

8.13.3.3 Isolators

Waveguide isolators also use Faraday rotationand incorporate a 45° waveguide twist to isolatethe forward and reverse power in the guide [41].Dunlop and Smith [44] explain the principle ofoperation and the usual waveguide configurationis shown in Figure 8.36. The resistive vane isoriented in the plane of the reflected wave E-fieldacting as the built-in reflected power load. Theseisolators are often used to prevent the reversepower flow from the inputs of high-power

REFLECTEDPOWER

450 rotatedclockwise

E fields of reversedpower absorbed inresistive vane

,.>

POWER h:,. -,. "'IIN l' ~ 45

0rotated

E,."-~ clockwise

1~ 450 rotated

~ anti-clockwise

Figure 8.36 E-field rotation in waveguide isolator

Reproduced by permission or Van Nostrand Reinhold

8.14 Automatic EMC testing

8.14.1 Introduction

When the number of tests being performed in afacility to a single or restricted range of specifications increases, EMC engineers consider theoptio.n of automating the process. A companywhose products are all tested to EN 55022 mightfind it economically worthwhile investing in themicroprocessor-based measurement equipmen tand desktop or minicomputer controllersrequired. They could then purchase or write thesoftware needed to carry ou t these particular testsand ensure tha t all tests were carried ou tidentically. In addition, automation of EMCtesting has a number of potential advantages.

• I t increases the work through put by speedingup measurement.

• I t can be used to deskill the testing job,although this is probably a shortsightedeconomy because EMC testing is bestundertaken by staff who will be alert tounusual phenomena and be capable of theircorrect interpretation.

• It allows reports to be produced quickly andcheaply to a standard format.

In most automated EMC test facilities the testingcost can be kept low and the job satisfaction for the


test engineer improved as the equipment takes careof the routine aspects of measurement, leaving theengineer free to concentrate on quality issues andexperimental or diagnostic procedures which needthe flexibility of the human mind.

8.14.2 Automated emission testing

Fully automatic, blind EMC testing is notpossible, nor perhaps desirable. Computer assistedor semiautomatic testing which cuts down testtime and drudgery, most certainly is. Thedevelopment of suitable software that can copewith all the decisions that need to be made duringan emission test is not trivial. Consideration mustbe given to

Frequency stepping incrementScan speed and the assembly of multiple scansto fully cover the bandwidth of a pulsed signalspectrumSelection of bandwidth according to test specification, measurement frequency, or signalconditionsAlteration of detector function and classification of signal typesReal-time manipulation of input and IFattenuation depending on signal levelDetermination or recognition of backgroundambient signals and appropriate marking(when making open-range measurements)Switching antennas or sensors with frequency,or location on or around an EUTAdjusting the height of an antenna to findmaximum signal strength from an EDT on anopen siteDriving the turntable in steps during an opensite testPeriodic calibration routinesData storage and manipulationStorage and retrieval of dozens of sensortransfer functionsStorage, retrieval and comparison of specification limits with measured levelsCalculation of out of limits signals, datapresentation and storagePlotting routines and printing of test instrumentparameters to meet QA requirementsFiling of archive material in the form ofremovable discs, tapes or cassettes.

Many of these factors are considered in a paper bySikora [49J dealing with the production of EMCtest automation algorithms and methods ofperforming computer controlled EMI compliancetests are examined by Derewiany et at. [50J. Abrief description of some hazards to be avoidedwith automated EMI testing IS given byArchambeault [4J.

Most manufacturers of EMI receivers andspectrum analysers for use in E~1C emissiontesting supply computer assisted test suites withsoftware tailored to military and commercialequipment test standards. The verification anddocumentation of these software packages aremost important as they form part of the traceablequality chain which underpins statements ofmeasurement accuracy and repeatabili ty.

8.14.3 Automated susceptibility testing

I t is more difficult to automate radiated andconducted susceptibility testing because thecontrol computer must be able to monitor theperformance of the EDT directly, to stopfrequency scans and carry out investigativeprocedures at susceptible points. In addition, thenumber of connections between signal sources,modulation sources, power amplifiers andantennas or injection probes mean that the testingwill inevitably be time consuming.

While computers can be useful in setting up testconfigurations and in controlling some signalsources and feedback levelling loops, susceptibilitytesting is labour intensive as it is of a primarilyinvestigative nature.

8.14.4 In the future?

There can be no doubt that computer technologywill become increasingly powerful, cheap andavailable. Software will also improve and excitingprospects lie ahead in the field of EMC design,modelling and testing. The bringing together oflarge complex design models and responsive EMCtest algorithms within the framework of aknowledge-based architecture may result in muchmore careful and discriminating tests beingcarried out on particular types of equipment, orthe execution of specific tests designed to revealthe true EMI characteristics of individual itemsbeing tested.

8.15 References

CONNEY, M. and ERICKSON, S.A.:'Considerations in the design of a multi-bandwidth,sensitive, wide dynamic range, wide frequencyrange EMI receiver'. Proceedings of IEEEsymposium on EMC, 1990, pp. 634-637

2 EMI meter NM17-27 instruction manual. CarnelLabs. Corp, 21434 Osborne Street, Canoga Park.CA 91304, USA, p. 5.15

3 WHITE, DJ.: 'A handbook series on electromagneticinterference and compatibility, volume 2 test andmeasurement procedures'. Don White Consultants,Germantown, Maryland, USA, pp. 3.66-3.86


4 ARCHAMBEAULT, B.R.: 'Hazards to avoid withautomated EMI testing'. Interference technologyengineer's master 1985, pp. 118-122

5 DUFF, W.G.: 'Fundamentals of electromagneticcompatibility'. Interference Control TechnologiesInc, Gainsville, Virginia, USA, pp. 319-320

6 'TD2301/TD1301 High performan~e digitizersystems with Fourier analysis capability'. Tektronixcatalogue, 1990, pp. 162-163

7 STURLEY, K.R.: 'Radio receIver design'.(Chapman & Hall, London, 1949)

8 LANGFORD-SMITH, F.: 'Radio designershandbook' (Iliffe Books, London, UK)

9 'AN 246-1/5952-8815 Optimising the dynamicrange of the HP 3585A spectrum analyser'. HP,Winnersh, Wokingham, Berks, UK

10 'AN 150/5952-1213 Spectrum analyser basics'. HP,Winnersh, Wokingham, Berks, UK

11 'AN 150-1/5954-9130 Spectrum analysis AM andFM'. HP, Winnersh, Wokingham, Berks, UK

12 'Signal strength & interference measurements'.News special 1, Rohde & Schwarz, Ancells BusinessPark, Fleet, Hampshire, GU13 8UZ, UK

13 CISPR EMI receIvers 8573A/8574A. HewlettPackard 1990 product catalogue, p. 125

14 85685A preselector, Hewlett-Packard 1990 productcatalogue, p. 126

15 BELDING, R.L.: 'RF preselection requirements forspectrum analysers'. Proceedings of IEEEsymposium on EMC, 1985, pp. 94-97

16 Microwave EMI receiver EMC-60/FE-60. ElectroMetrics Ltd, Ivel Rd, Shefford, Beds, SG 17 5JU,UK

17 Band rejection filters TRF11-TRF15. Electro-MetricsLtd, Ivel Road, Shefford, Beds, SG17 5JU, UK

18 'High and low-pass RFI filters, instruments,components and accessones for the RFI/EMCengineer'. Data sheet 6625A, Solar Electronics Co,901 North Highland Ave., Hollywood, CA 90038,USA

19 KEISER, B.: 'Principles of electromagnetic compatibility' (Artech House, Norwood, MA, 3rd edn.)p. 347

20 Carnel Labs. Corporation (formally Eaton) 91263-1stand-alone 1 GHz impulse generator

21 EMI meter NM17-27 with built-in impulsegenerator instruction manual. Carnel Labs. Corp,21434 Osborne Street, Canoga Park, CA 91304,USA, p. 2.1

22 SP-P-90203 Product-specific EMC standard for theTornado military aircraft, Panavia, Europe

23 Disturbance analyser Series 606, Dranetz company,USA

24 Digitising oscilloscopes HP 54120 series, HewlettPackard 1990 test and measurement catalogue. HP,VVinnersh, Wokingham, Berks, RG11 5AR, UK, p. 60

25 'Fundamentals of digital oscilloscopes and waveformdigitising, A technical tutorial'. 1990 productcatalogue, section IV-I. LeCroy, 700 ChestnutRidge Road, Chestnut Ridge, NY 10977-6499, USA

26 1990/1 measurement equipment catalogue. Rohdeand Schwarz UK Ltd, Ancells Business Park, Fleet,Hampshire GU13 8BR, UK, pp. 384-387

27 'Power sensors' in Test and measurement catalogue

1990. HP Winnersh, Wokingham, Berks,RG 11 5AR, UK, p. 204

28 1990/1 measurement equipment catalogue. Rohdeand Schwarz UK Ltd, Ancells Business Park, Fleet,Hampshire GU13 8BR, UK, p. 428

29 Directional power meter RFM 100 in Rohde andSchwarz measuring equipment catalogue 1990/91,p.429

30 WHITE, DJ.: 'A handbook series on electromagneticinterference and compatibility, Volume 2 test andmeasurement procedures'. Don White Consultants,Germanton, Maryland, USA, p. 3.92-3.103

31 PITT, A.D.: EMC guidelines, ST23357. BritishAerospace Dynamics Ltd, Filton, Bristol, UK, 1979

32 'Guide to broadband power amplifiers'. AmplifierResearch, Rev. 0190, 160 School Hse Rd,Souderton, PA 18964-9990, USA

33 SCHEVCHIK, V.N.: 'Fundamentals of microwaveelectronics'. (Macmillan, NY, USA, 1963) pp. 186-203

34 TWT amplifiers. Keltec Florida Ltd, PO Box 862,Shalimar, FL 32579, USA

35 Fast pulse modulator, HPl1720A. HP 1990catalogue, p. 392

36 Pulse generator, PM 5771, 1 Hz-100 MHz. PhilipsInstruments UK

37 Pulse generator PG 73N, 0.ls-10ns. LyonsInstruments, Hoddesdon, Herts, UK

38 Analogue function generator HP 3212A. HP 1990catalogue. p. 428

39 Function generator AFG. Rohde and Schwarzmeasuring equipment catalogue 1990/91, p. 90

40 Arbitrary function generator 9100, 1990 productcatalogue. LeCroy, 700 Chestnut Ridge Road,Chestnut Ridge, NY 10977-6499, USA

4J 'The hows and whys of arbitrary functiongenerators, A technical tutorial'. 1990 productcatalogue, section IV-48, LeCroy, 700 ChestnutRidge Road, Chestnut Ridge, NY 10977-6499, USA

42 LIAO, S.Y.: 'Microwave devices and circuits'.(Prentice Hall, Englewood Cliffs, NJ, USA)pp. 158-169

43 GLAZIER, E.V.D. and LAMONT, H.R.L.: 'Theservices textbook of radio, volume 5, transmissionand propagation'. (HMSO, London, UK, 1958)p. 225

44 DUNLOP, J. and SMITH, D.G.: 'Telecom-munications engineering'. (Van NostrandReinhold, UK) p. 288

45 'Waveguide directional couplers HP752 series. HP1990 catalogue, p. 345

46 KRAUS, J.D.: 'Electromagnetics'. (McGraw-Hill,3rd edn.) p. 420

47 'Coaxial directional coupler' HP 772D, HP 1990catalogue, p. 344

48 'Hybrid ring, reference data for radio engineers' in 'ITThandbook' (Howard W. Sams) pp. 25/18 and 25/19

49 SIKORA, P.A.: 'Writing a useful EMI testautomation algorithm'. Proceedings of IEEEsymposium on EMC, 1988, pp. 377-382

50 DEREWIANY, C.F. and KOWALCZYK, K.R.:'Methods of performing computer controlled EMIcompliance tests'. Submarine electromagneticsystems dept. Naval Underwater Systems Center,New London, CT 06320, USA

Chapter 9

EMC test regitrles and facilities

9.1 Introduction

9.1.1 Main test regimes

This chapter examines the three principal testregimes and facilities in which these devices andequipments are used to conduct EMC tests:testing in screened chambers, open-range testing,and 'low-level swept' and bulk current injectiontesting. The majority of standard EMC test workcarried ou t on commercial and military electronicequipment falls into one of these three regimes.

9.1.2 Special testing

There are of course other specialised types of electromagnetic testing which are either related to thetype of electromagnetic threat employed orsignature being measured, or to the scale of thetesting on large systems. The special testingtechniques related to threat or signature type are

• Special electromagnetic threatsHIRF (high intensity RF) testsLightning strike testsNEMP (nuclear electromagnetic pulse)testingEMP (electromagnetic pulse) and HPM(high power microwave) testing

• Special signature testingTempest (emission security) testsSpacecraft EM 'cleanliness' (from DCmagnetic fields to millimetric waves)

• Special test techniques related to the scale of atest include

whole-ship testingwhole-aircraft clearancetelephone switching centreslarge distributed computer facilitieslarge communication centrestransportation signalling centres.

In some cases, it is possible that these large systemsneed to be tested for EMC, NEMP/EMP,lightning and Tempest.

The range of testing requiren1ents that facilitiesmust sometimes meet leads to increasing facilitycost, and the procurement or licensing authoritiesmay not impose the full range of specifications forall these electromagnetic effects on a given systemunless absolutely necessary. In a book such as thisdevoted to generalised EMC testing it is not

154

possible to cover all these interesting topics, butinformation is widely available in the proceedingsof relevant specialist groups sponsored by the lEE,IEEE and others. This chapter concentrates on thetesting regimes and test facilities most generally inuse for electromagnetic compatibility testing.

9.2 EMC testing in screenedcham.bers

9.2.1 Enclosed test chambers

In conducting EMC tests to measure bothequipment RF emissions and susceptibilities, it isdesirable to isolate the test space from the outsideelectromagnetic environment. If arbitrarily varyingambient RF signals (up to field strengths of 10 V /min industrial areas) are allowed to mix with thesignals of interest ofa mV/m or less from the EUT,it will be time consuming and almost impossible toseparate out the signals which need to be measuredto the levels required by the specifications.

Equally, it is undesirable (and illegal) to radiatehigh field strengths across whole bands offrequencies when conducting radiated susceptibilitytesting. rrhus the use of screened chambers becamewidespread as a result of military procurementEMC requirements which came to the fore duringthe late 1960s and early 1970s. Much of the customand practice with regard to the use of screenedchambers has been built up on the basis ofstandards imposed on the aerospace/militaryequipment industries such as MIL STD 461/2/3.Many of the tests currently being carried out areeither the same, or directly related to those in earlyspecifications such as this. Of course, increasedknowledge and test experience gathered over 20years have been incorporated into modifications totest methods, or to new tests such as those relatingto BCI (bulk current injection) contained in theUK DEF STAN 59-41 DCS02.

The advent of widespread governmental or selfregulation of EMI in the early 1980s connectedwith commercial electronic products such as digitalcomputers, brought about the widespread use ofEMC tests based in some cases on earlier CISPRtype methods. These new tests were largelyconcerned with the control of radiated andconducted emissions (FCC part 15, VCCI, etc) andmade use of open sites rather than screened roomsas suggested for testing military equipment.

EMC TEST REGIMES AND FACILITIES 155

R == 168 + 10logb/fll (dB) 9.1

S == R + A + B (all in dB)

where R == reflection lossA == absorption lossB == internal loss

The internal loss factor B is usually neglected if theabsorption loss is significan t [2]. The reflection lossIS

9.2A == 3.34tjfbll (dB)

where t == material thickness in mils (,thou')f == frequency in MHzb == material conductivityIl == material permeability

White [3J, quoted by Farsi [2J, gives a table forthe absorption loss per thousandth of an inchthickness for materials commonly used toconstruct shielded rooms, see Table 9.1.

The configuration of a typical small screenedroom complex used for EMC testing is shown inFigure 9.1. I t has the EMI receivers, poweramplifier 'transmitters' and the EUT shielded eachfrom the other, and all from the outsideenvironment. I t is possible to have a shielded roomonly for the EUT in a single cell facility, but theisolation scheme shown in Figure 9.1 is useful formulticell operation in a large test facility complexwhere emission and susceptibility testing ondifferent EUTs may be running simultaneously.

The practical performance of screened rooms isunlikely to be determined by the plane wavepropagation through the shield material butrather by leakage through panel seams, cornerjoints, penetration panels and access doorsurrounds. Typical shielding effectiveness of amoderate sized modular room of 12 x 7 x 5 mwith large 4 x 4 m doors, air vents and multiplepenetration panels is shown in Figure 9.2.

where f == frequency in MHzb == material conductivityIl == material permeability

The reflection loss for a plane wave is a function ofthe material conductivity and is high at lowfrequencies and reduces at higher frequencies.Expressions for the reflection losses of the electricand magnetic field components of a wave aregiven in Reference 2 which allow the loss to becalculated for nonplane waves such as occur inthe near field of a source.

The absorption loss is dependent on the type ofrnaterial and its thickness and is

Farsi [2] gives a good introduction to shieldingtheory and shows that for plane waves theshielding effectiveness is

Standard screened rooms are mostly constructedfrom thin galvanised sheet steel and woodsandwich modular panels that are clampedtogether with special pressed metal strips toproduce an RF-tight joint. This permits thebuilding of structures of variable size which canbe dismantled and re-sited if required. Othertypes of construction include self supporting allwelded sheet steel, copper sheet, metal foil, and'chicken wire', perforated sheet or expanded steelmesh. Each construction technique provides acertain degree of shielding for a given enclosedvolume at a particular cost. Careful analysis ofthe shielding requirement to carry out a givenEMC test must be undertaken if the correctperformance of room is to be achieved for theminimum cost.

9.2.2 Standard shielded enclosures

There are therefore almost two parallel streamsof EMC testing with each general approachhaving its own advantages and penalties optimisedfor the type and cost of testing which needs to becarried out in the civil and military fields. I twillbe interesting to see if the sharing of testinginformation between these two communities leadsto a consensus oh the best and most cost-effectivetest methods for particular types of equipment.

The practice of testing in screened rooms hasthen largely been established by the need to testequipment to military specifications where theEUT performance is critical, the unit cost is highand the project budgets are large. In these circumstances it is possible to build elaborate high cost(> £5 M) test facilities on the basis of large,sometimes very large (1 OOx 80 x 20m), shieldedenclosures in which to carry out the work. White[1] gives examples of large facilities constructedfor ballistic missile and aircraft testing. The onlyother product areas which have invested in suchlarge facilities are the automotive and spaceindustries and to a lesser degree the civil aircraftsector.

Screened test chambers used for EMC testingcan be divided into four types: standard shieldedchambers, shielded and anechoic chambers,mode-stirred chambers, and novel facilities.Screened chambers made from metal sheets areusually constructed in the form of a rectangularbox with parallel sides, but other configurationssuch as cylinders and tapers have been built forspecial requirements of items to be tested or tominimise reflectivity. Screened chan1bers are anexpensive item of equipment and costs typicallyrange from £20,000 for an 8 x 6 x 4 m room tomore than £20 M for one in which a large systemcould be tested.


Table 9.1 Electromagnetic characteristics of metals and absorption loss with thickness

Metal Absorption loss, dB per 0.001 in Relative Relative100 Hz 10 kHz 1 MHz conductivity permeability

100 kHz

Silver 0.03 0.34 3.40 1.05 1Copper 0.03 0.03 3.00 1.00 1Gold 0.03 0.28 2.78 0.70 1Aluminium 0.03 0.26 2.60 0.61 1Zinc 0.02 0.17 1.70 0.29 1Brass 0.02 0.17 1.70 0.26 1Nickel 0.01 0.15 1.49 0.20 1Bronze 0.01 0.14 1.42 0.18 1Tin 0.01 0.13 1.29 0.15 1Iron 0.44 4.36 43.6 0.17 1000Steel (SAE 1045) 0.33 3.32 33.2 0.10 1000Stainless steel 0.15 1.47 14.7 0.02 1000

EMI RECEIVER SHIELDED ROOM

Figure 9.1 Compact multiscreened room EMC test cell

EUT POWERLINE FILTERS

\

I II I

L:N:AJ__--\....\---.......lAIR VENTS TEST AREA SHIELDED ROOM

\POWER

LINEFILTERS

POWERAMPS &SIGNAL

____ SOURCES

~POWERAMPLIFIERSHIELDEDROOM

together. Figure 9.3 shows the attenuationafforded by a 6 mm-thick mild steel all-weldedconstruction with two moderate sized doors (2.5and 3.5 m sq.) and the usual complement of airven ts and penetration panels [4J.

The shielding performance of screened rooms tobe used for EMC testing is almost always measuredaccording to the method specified in MIL STD 285[5]. The original standard was introduced in June1956 and is still largely unchanged. Cardenas [6Jhas conducted experiments to define the mostpractical measurement methods based on MILSTD 285, which requires attenuation measurements to be made using a range of identicalantenna sets similar to those shown in Figure 9.4.

- 6 mm welded steel plate- - - typical modular

screened room

140

120

100co"CZ 800

~::J 60zw~.

« 40

20

01

Figure 9.3

10 100 1k 10k 100k 1M 10M 100M 10 100FREQUENCY Hz

Good low-Jrequency performance oj weldedsteel room. Welded room type: cylindrical14m dia X 12m high. -- 6rnm weldedsteel plate; - - - typical modular screenedroom

co"'0z 120o~~ 100w

~~ 80ooa:c 60wZw~ 40 i--_"""--_..J-_--'-_.....&_----I......._""'-_~_ ___'o 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz 1GHz 100Hz 1000Hz00 FREQUENCY

Figure 9.2 Typical attenuation oj modular steel-screenedroom. Dimensions: 12 X 7 X 5 m;Access: Doors 4 X 4 m and 1 x 2m,'Attenuation: measured at apertures(doors, attenuvents, etc.)

Improved low-frequency shielding can beobtained by constructing a chamber from largeheavy gauge steel plates which are all welded

Reprod uced by permission of BAe Dynamics Reprod uced by permission of BAe Dynamics

(a)

Electro-metrics ,RVR -25M

(c)

Shielded enclosure

Shielded enclosure

Receiver


Shielded enclosure

72" where practicable

(b)

Shielded enclosure

II

min. 2" ! 72" where practicablei! ~ ~1(1.·s..~A.12-8-2

I li-~( d)

Figure 9.4 Typical equipment corifigurations Jor measurement oj screened room attenuation. (a) LF magnetic Jield testequipment, (b) Planewave « 1 GHz) test equipment ( c) Electric Jield test equipment (d) Planewave(10 GH;~) test equipment

Strictly, the MIL STD 285 requires the Tx antennato be outside the room in all cases to minimise theswamping effect of ambients, but at VHF andabove, any signal which has leaked into the roomsets up standing waves which makes it difficult tolocate the source of a leak. At these frequencies theRx antennas have some discrimination againstambients and thus it may be better to put thereceive antenna outside the shield [6J.

The National Security Agency in the USAissued a specification NSA65-6 for the performance of screened chambers constructed frommetal foil which is shown in Figure 9.5. Althoughthe cheaper foil-shielded rooms covered by thisspecification were not intended for EMC testingthey are in fact quite adequate for this purposewith attenuation of 100 dB from 1 MHz to 10 GHz.

Well-constructed shielded rooms are very efficient

100co"0Z 80 Planewave0i= Electric Field specification« 60:::> sPecificationzw

~40

20

0.001 0.01 0.1 1.0 10.0 100 1,000 10,000

FREQUENCY MHz

Figure 9.5 Example oj screened room performancerequirement. Specifications NSA 65-6100 dB

at isolating the test volume from the outside electromagnetic world. Both radiated susceptibility andemission EMC tests can be conducted withoutcausing RF jamming problems to outside communications or being confused by the penetration ofambient electromagnetic noise. There is, however,a major drawback in the use of shielded enclosures.Because the walls, floor and ceiling are highlyconductive and usually orthogonal or parallel, theroom becomes a high-Q, multifrequency resonatorwhere con1plex standing wave patterns can be setup along all three principal axes.

The exact standing-wave patterns will dependon the shape of the box, the positions of thesource antenna, the measurement antenna (orEUT) and the frequency. Typical of the variationin field strength which occurs inside these rooms isthat shown in Figure 9.6 [7]. The test configuration is that specified by MIL STD 462 RS03 andthe lower panel of the diagram shows that thevariability in the field strength at the E UT is atleast a factor of ± 1O. This order of uncertaintywill be present in all radiated susceptibility andemission ITleaSUrements which are made inside asimple undamped shielded enclosure. A study ofthe effect of standing waves at frequencies below100 MHz in a shielded enclosure againstequivalent open-range measurements has beenmade by Stuckey et al. [8J.

Other investigators [9J have made confirmatorymeasurements of predictions generated by a 2Dcomputer model of reflection proce,sses inchambers. This model is based on amplitude andphase reconstructions from distributed Hugens


SIDE VIEW

f---10 ft.-----.,

r- .-

. 111

4 r-12 ft. 181l

t

-I

PLAN VIEW

POWER INPUT

(a)

BENCH MONITORPROBE

SCREENED ROOM( no absorber)

10 a: 00~ ~~3 ~~1

L..-..:....;L..;...,;;..:....::... --'----:...--:....~ ~~_--1 0

_---~~------------:-~ :r:50 1-_40 g~ E

rIJnl44+-I+I-1~'tWIA.1I~~....M~W.~..W4WlJll.j 30 ww-I " ---if8 u:g:::,- 0 (f)

1------------------y-----i100 :r:90 I-80 "70 z-~g ~ ~40 ~ >30 Q-20 ..J10 W

~_-_-...,..:....:..__\"""-+_=_~~_+___1~~.L.I....,;",~-O u:100 200 300 400 500 600 700 800 900 1000

FREQUENCY MHz

sources on the chamber walls. A visual impressionof the complexity of a typical standing-wavepattern at a single frequency can be gauged bystudying the model graphical output given inFigure 9.7. This shows the calculated standingwave pattern at 180 MHz with a centrallypositioned source in a 5 X 5 m (floor area)reflective shielded chamber.

I t is possible to exploit the standing waveproperties of a simple shielded room to generatehigh field strengths for limited input power [1 OJby using a special commercially available susceptibility antenna, known as the Cavitenna [11 J.The device, when fixed to the inside wall of thescreened room, excites the entire room using thestructure as a ground plane to increase theradiator's effective electrical size. Thus a 1.2 mlong antenna can produce equivalent fieldstrengths to a 5 m log periodic at a frequency of30 MHz. The antenna is set up above the groundplane bench as shown in Figure 9.8a; with the aidof a levelling loop amplifier it can efficientlyproduce the field strength from 30 MHz to 1 GHzshown in Figure 9.8b with an average value of25 V 1m for an input power of between 1 and 7 W.

Other techniques have been tried which aim to

1

4

2

40

20

10 ......&-..............-I-~........-I&o-l;.u...iY-~~ .....--Il........~

8

6

100......----~------ _

80

60

IGROUND PLANE I EUT I

I1 metre FIELD GENERATING

a..------ ~/ ANTENNA

;. 1 metre \'''-'''l~ ;. 1 metre

1 metreFIELD MEASURING t

ANTENNA~~

SHIELDED ENCLOSURE T~CONTROL FIELD

WA\ ~ 1fetre =10 VIm

STANDING WAVE PATIERN

E3>~ozwc::IenC..JWiI:cw~UCi~ 11...-.........................L.......L...-&--&-................--L.--A..--&.--I---I.........L:.....I.

40 50 60 70 80 90 100 110 120 130 140 150 160 170 180190200

FREQUENCY MHz

Figure 9.6 Variation offield strength at EUTfor constant10 Vim control field. Location of aerials forradiated susceptibility testing shown forMIL STD 462 test method RS03

Figure 9.7 G'Ialculated E-jield standing-wave pattern insquare cross section screened room. 5 m-squarechamber with central source at 180 MHz

Reproduced by permission or NPL/HMSO

(b)

Figure 9.8 ~]Jicient field strength generation inundamped screened room using wall-mounted,cavitenna)


Figure 9.9 Hooded test area configuration in screenedchamber

50 200FREQUENCY MHz

Figure 9.10 Frequency response of shielded room excitedwith small magnetic loop

ABSORBER WALL

REFLECTEDWAVES

SCREENED ROOM

>w 130 ........-co(J)"O~o

:c:'I-~°0moa:: a::1-0(J)woz...JwwwU:a::

()(J)

produced by the standing waves to manageableproportions. When used in conjunction with anautomatic field levelling loop, field variability ofaround ±2 dB is claimed from 50 to 100 MHz andbetter than ± 1 dB up to 550 MHz in a4 X 4 X 2.5 m chamber. This test configuration isuseful for testing an E UT up to a half-rack size.

Another approach [16J is to damp out theresonances by extracting energy with RAM atstanding wave sites of maximum field strengthwithin the room. The undamped frequencyresponse is shown in Figure 9.10 for a2.5 X 2.5 x 5 m room with a first resonance at75 MHz. Figure 9.11 shows how the first resonanceis reduced as the loading provided by the absorberis increased with increasing conductivity.

Finally, this briefsurvey oflow-cost techniques to·reduce standing wave problems in screened roomsby using only small amounts of RAM, includes asuggestion made in 1981 [9J. This involves acombination of the asymmetrical room conceptand that of strategically sited RAM in an elliptical

9.2.3 RF anechoic screened chambers

9.2.3.1 Partial solutions

The most obvious way of suppressing the standingwaves inside a screened chamber is to cover all ormost of the reflecting surfaces with radioabsorbent material which can significantlyattenuate the reflected waves and prevent modalpatterns from forming. Unfortunately this solutionhas two clear disadvantages:

(i) The cost is extremely high largely owing tothe material cost.

(ii) The working space in the room can besubstantially reduced by the absorber andthus a larger and more expensive chamber isrequired.

The full RAM-lined large chamber solutions areonly used in industries where the investment canbe justified, such as the aerospace or automotivesectors. Small to medium sized chambers whichare more widely used in the electronics industryare often not equipped with RAM on costgrounds unless mandated by appropriate teststandards. This has led to a number of low-costschemes being proposed [9, 15-17J which usesome RAM in special configurations to mi tigatethe worst effects of standing waves onmeasurement accuracy.

One concept [15J is that of placing absorberonly on the back wall of the room, and buildingan absorbing hood around the EUT as shown inFigure 9.9. Such a configuration reduces thesharp peaks and troughs infield strength

mInImise the standing-wave problem in screenedrooms without the introduction of large volumes ofcostly radio absorbent material. It has beensuggested [12J that asymmetrical shaped chamberswhere few of the reflecting surfaces are parallel canbe helpful in controlling the allowable chambermodes, resulting in a reduction in typical fieldvariability in a rectangular chamber from +20 to-40 dB to less than ± 10 dB.

The specification and purchase of a screenedroom suitable for EMC testing is an importanttask [13J and involves considerations which liebeyond the immediate issues of size, shieldingperformance, delivery and cost. I t is important toconsider the through-life operating costs of theenclosure as daily use of this expensive hardwarewill inevitably result in damage and maintenanceproblems. These could be costly if the room andparticularly the doors and door seals are badlydesigned or of poor q uali ty. Salati [14J detailscost-effective maintenance for shielded enclosuresand suggests appropriate repair procedures.

Screened chambers with nonparallel walls, barnesand angled planes are all a ttempts to eitherminimise the amount of RAM required, or in thepast to compensate for poor attenuationproperties of RAM available at that time. Withthe development of high-quality broadbandpyramidal RAM made by impregnating a carboncarrying latex into a flexible polyurethane foamplastic [18J it is possible to produce simplerectangular chambers with good reflectionattenuation at an acceptable cost.

Anechoic chambers can be divided into two kinds:

>w::O0 ........-co00-0ZOI~1--o~zOwO0::0::1-0OOwoz...JwWwU:o::u

Cf)


9.2.3.2 Full RAM solutions

(i) Full anechoic facilities with quiet zones of upto 1-3 m across and reflectivities as low as-35 dB. These are specialist chambers usedmainly for measuring polar diagrams ofantennas mounted on vehicles, aircraft andspacecraft.

(ii) Semianechoic chambers that reduce standingwaves to manageable proportions such thatreliable EMC measurements may be made.These chambers sometimes have a reflectingground plane floor and simulate an openfield site for making CISPR, FCC typemeasurements [19J.

In the following text only chambers of the secondkind which are sufficiently anechoic to conductsuccessful EMC tests will be addressed. Theperformance of an EMC semianechoic chamber(SAC) can be gauged by reference to a typicalexample devised and built by IBM [20].

The screened chamber dimensions are10.5 x 6.5 x 4 m with HPY-24 pyramidal absorber[21] on three walls and the ceiling. Special thinferrite absorber NZ-31 [22J was placed on the farwall and door which was the closest to the EDTlocation. Standard EMC antennas such as biconicsand tuned dipoles were used to investigate the SACperformance. Figure 9.13 shows the improvementin chamber resonances after the room was equippedwith absorber. All the high-Q, peaks and troughsare suppressed leaving a chamber which is within± 1.5 dB of the equivalent open-range performanceat frequencies up to 230 MHz and within ±3 dB upto 950 MHz.

Kuester et al. [23J have produced a theoreticalmodel for calculating the reflectivity of absorberlined metal walls and this has been improved onby Gavenda [24]. He used the simplifying methodof electrical images to allow calculation of thefield strengths after multiple bounce reflections ina semianechoic chamber with partially reflectingsurfaces. The model still has considerablelimitations and cannot be used to completely designthe optimum SAC. However the model is helpful at

RAM column centre atsecond focus of ellipse

\SOURCE

* Location of RAM

Reproduced by permission of NPL/HMSO

Figure 9.12 Novel elliptical chamber design combiningasymmetric chamber and strategically sitedRAM (aj Focal source at 30 MHz - noRAM (b j Small amounts of RAM placedas shown

FREQUENCY MHz50

chamber. The scheme uses a RAM column sited atthe secondary focus. If the radiating antenna isplaced at one focus 'it can be seen in Figure 9.12athat the calculated field strength peaks at the otherowing to the range/phase invariance properties ofthe bounding ellipse. I t should then be possible toremove energy efficiently from the system withrelatively small amounts of RAM placed at thisimportant location as indicated in Figure 9.12b.

Figure 9.11 Suppression of screened room resonance withincreasing RAM loading at the room centre


80 80

60 60

> >(i):::1

~ -- coCD "0"0

80 80

60 60

(ii)

(a) (b)

80 80

60

> >~ ~- -CD co

(i)"0 (i) "0

::C Q ::r S](c) (d)

Figure 9.13 Antenna coupling plots made in IBM semianechoic chamber showing how absorber material suppressesscreened-room resonances (a) 30-80MHz (b) 80-130MHz (c) 130-180 MHz (d) 180-230MHz( i) Before RF absorber (ii) After RF absorber

Reproduced by permission or IBM

low frequencies where the absorber reflectivity is ofthe order of only -1 to -3 dB per bounce.

l'he performance of RAM is defined as the levelof reflected energy from a RAM covered surface ascompared with that reflected from the same areaof metal surface. The general construction of ablock of pyramidal broadband absorber is shownin Figure 9.14. The height of the pyramids variesfrom a few cm to 4 m depending on its desiredattenuation at low frequencies. Typical normalincidence reflected attenuation as a function ofpyramidal absorber electrical thickness [25] isshown in Figure 9.15a with the off-normal reflectivi ty shown in Figure 9 .15b. Figures for the reflectivity of a typical high-performance pyramidalRA~1 as a function of frequency and pyramidheight are given in Table 9.2.

FOAM PYRAMIDS

/

FOAM BASE

Figure 9.14 Construction ofpyramidal foam RFabsorber

Reproduced by permission of' Emerson and Cuming

162 A HANDBOOK FOR EMC TESTIN"C AND MEASUREMENT

Reprod uced by permission of Emerson and Cuming

0.01 !o-----...Io----..I------ _o 10 20 30 40 50 60 70

CHAMBER REFLECTIVITY ( - dB )

Figure 9.16 Measurement uncertainty as function ofchamber reflectivity

(0

~ 10

~z

~UJ

~ 1.0:::>IzUJ~UJ0::~ 0.1«UJ~

100 ------...-----....----.--.,.---.,,------.

tivity of -20 dB is required at the lowest frequencyof interest. By reading off the electrical length ofthe absorber for -20 dB reflection from Figure9.15a and selecting the 150-in-high absorber inTable 9.2 one can calculate that an anechoicchamber fitttJd with this material would fulfil the± 1.5 dB measurement uncertainty criterion atfrequencies as low as 16 MHz.

Semianechoic rectangular chamber groundreflection EMC test ranges with reflections whichare sufficiently controlled to enable testing to FCC,CISPR and VDE requirements at 3 m are commercially available [19]. Typical calibration curvesrelating chamber performance to theoretical opensite range attenuation can be seen in Figure 9.17.

50.......-----r-----T---~--..

~ 30:>~ 20w..JU.~ 10

O'--_--L_---L__--.L._--L._..L-~......L_~.........

1 1.5 2 3 4 5 6 7 8 9 10THICKNESS IN WAVELENGTHS ( 0 I A)

(b)

Figure 9.15 Performance graphs for pyramidal foamabsorber (a) Generalised reflectivity ofeccosorb VHP at normal incidence( b) 0fJ-normal reflectivity of eccosorbVHP. Eccosorb VHP is manufactured byEmerson and Cuming Ltd

co 40"0

50_---w'----r----r----.---r----r-~ .,....,

0.01 0.1 1 10THICKNESS IN WAVELENGTHS ( D / A)

(a)

~ 30:>i=frl 20..JU.

~ 10

Lawrence [18J derives a graph which relates fieldmeasurement uncertainty in an anechoic chamberto absorber reflectivity. This is reproduced inFigure 9.16 and indicates that for measurementuncertainties of 1.5 dB, absorber which has a reflec-

Table 9.2 Example of the reflectivity ofpyramidal RAM as function of thickness and frequency

Guaranteed maximum reflectivity in dB

Material 120 200 300 500 1 3 5 10 15 24MHz MHz MHz MHz GHz GHz GHz GHz GHz GHz

VHP-2 -30 -40 -45 -50VHP-4 -30 -40 -45 -50 -50VHP-8 -30 -40 -50 -50 -50 -50VHP-12 -35 -40 -50 -50 -50 -50VHP-18 -30 -40 -45 -50 -50 -50 -50VHP-26 -30 -35 -40 -50 -50 -50 -50 -50VHP-45 -30 -35 -40 -45 -50 -50 -50 -50 -50VHP-70 -30 -35 -40 -45 -50 -50 -50 -50 -50 -50

Non standardVHP-110 -35 -40 -45 -50 -50 -50 -50 -50 -50 -50VHP-150 -40 -45 -50 -50 -50 -50 -50 -50 -50' -50

Eccosorb VHP is manufactured by Emerson and Cuming Ltd. Suffix number is pyramid height in inches.


chamber in which to carry out EMC testing, someassis tance is at hand. l'he commercial manufacturers have wide experience of actual installationsand the obtainable performance; they can adviseand are often able to offer turnkey packages whichwill minimise the risks to the customer. A number ofreview papers have been written dealing withchamber design criteria [26, 27J and chamberjanechoic material selection [28J.

1000200 30020 30 50

50 _-----,r----r---r----r---~-.,..__-_y_-_,

45

~ 40

~35~ 30~ 25w 20

~ 15

~ 10Cii 5

9.2.3.4 Conclusion - anechoic chambers

Figure 9.18 Narrow angle tapered chamber ensuresdirect and reflected rays stay almost in phase

r------------I ANGLE POSITIONER-r.=======~~

II MODE TUNER

I

9.2.4 Mode-stirred chambers

The problem of controlling, or at least minimising,the measurement uncertainty caused by multiplestanding waves inside a reflective screened roonlhas been tackled in another way which is complementary to the 'absorption' approach. Thisalternative technique deliberately maximises thenumber of possible reflection modes which can besustained within a highly reflective chamber in aneffort to expose theEUT to all possible fieldstrength values at each frequency of interest inthe band being investigated.

This concept is known as mode stirring or tuningand in the mid 1970s seemed to be a promising lowcost alternative to semianechoic chambers for EMCtesting. Much has been written with regard to thisconcept [29-32J and a full understanding of itstechnical basis is not a simple matter. The methodoffers some potential advantages for EMC susceptibility testing in addition to lower cost than for asemianechoic chamber. The main advantageresults from the wave polarisation being randomlyvaried in the isotropic homogeneous field whichexists in the mode-stirred chamber [33J. With theE UT immersed in such a field there is no need toreorient it and repeat the test up to three times tocover all polarisations.

The reverberating chamber, as it is sometimescalled, is also capable of providing efficientconversion of source RF power to high fieldstrength for performing tests on large objects orcomplete systems. There is then the clear potential

DIRECT AND REFLECTEDWAVEFRONTS IN PHASE

TAPEREDANECHOIC CHAMBER

SPECULAR REFLECTION POINTFOR REFLECTED RAY

d~dr

ed~er

Reproduced by permission or Rayproor

9.2.3.3 Tapered anechoic chambers

This design of chamber usually permits lowerfrequency operation at a given reflectivity levelthan for a similar sized rectangular design. Thesource antenna is mounted at the apex of thetaper section and for taper angles of 30 degrees orless the specularly reflected waves from the sidesof the taper are at grazing incidence to wallsresulting in little path difference between thedirect and reflected radiation [18J . Under theseconditions an almost unperturbed wavefrontpropagates down the taper to the cubical workingvolume as shown in Figure 9.18. This style ofchamber is not particularly suitable for EMCmeasurements and is not widely used. I t is moreappropriate to antenna and other measurementswhere a low reflectivity quiet zone rather than asemicontrolled large working space is preferable.

FREQUENCY MHz

Figure 9.17 Typical calibration curve ofactual siteattenuation against theoretical site attenuation(Performance ojsemianechoic chamberdesigned to simulate open-area test site)

For managers and EMC engineers who may befaced with the responsibility of specifying andselecting a screened anechoic or semianechoic

Figure 9.19 Example of mode-tuned enclosure systemJorEMC measurements (NBS-US.fl)

Reproduced by permission or NBS


to reduce test time and cost as secondary benefits ofusing a mode-stirred chamber.

A typical chamber configuration is shown inFigure 9.19 with the mode stirrer constructed inthe form of multiple irregular shaped and sizedpaddle wheels which are fixed to the walls orceiling and driven by stepper motors to ensure themaximum number of possible chamber modes areactivated. The validity of this method depends onthe maximum number of chamber eigellmodesbeing excited with a known mode density as afunction of frequency. The maximum number ofmodes are generated when the chamber is largecompared with the frequency and this technique isvalid for frequencies of 200 MHz or above inchambers of the order of 5 x 5 x 3 m. Theoptimum design criteria for reverberating charnbersare to make the volume as large as possible and theratio of the squares of linear chamber dimensions asnon rational as possible [33J.

Reverberating chambers can be operated in twoways:

The curves relate to the maximum and mean fieldstrength recorded at each frequency when thechamber is mode tuned. It has been suggestedthat mode-stirred or mode-tuned chambers canbe used for radiated emission testing [34J, but thepractice has not been taken up widely in theEMC test community. Indeed, mode-stirredchambers have not been as widely used forsusceptibility testing as predicted, probably owingto the difficulty in relating the measurementsmade in this way to the results obtained frommore conventional test methods.

Some measurements have been made in anattempt to correlate susceptibility results obtainedusing mode stirred and anechoic chambers, whichshowed that the EUT response was less in the reverberating chamber than in the traditional anechoicchamber test [35]. There are however, exampleswhere these chambers have been used successfully,such as testing shielding effectiveness of cables andconnectors to MIL STD l344A method 3008 asreported by Crawford et al. [36J at NBS.

FREQUENCY GHz

When the systems to be tested are very large, such asequipment for communication centres, computer ortelephone switching installations, it may not bepossible to provide economically conventional electromagnetically enclosed testing facilities such asthose previously described. Depending on theamount of screening attenuation and minimuminternal reflectivity required to carry out theparticular tests, a number of unconventionalchamber solutions may be considered whichinclude large 'chicken wire' cages, metallised airinflated structures and underground caverns.

The chick-wire cage can be built from wood anda metal wire mesh to provide large enclosedvolumes at low cost. The penalties includeachieving only moderate attenuation of around40 dB at VHF and no suppression of internalstanding waves.

Metallised air-inflated structures are capable ofproviding moderate to large volumes with floorareas up to the size of a couple of tennis courts.They are expensive to purchase but can be movedto new locations and re-used many times.Shielding is relatively poor 20-40 dB but thisapproach is an option for testing large systems.

To my knowledge underground caverns havebeen used for a number of EMC tests whichinclude spacecraft and commercial computersystems. This approach can afford good screeningof better than 90-100 dB at most frequencies, andwith the addition of a small amount of RAM thecavern can closely simulate the EM performanceof an open test site. The working volume is

9.2.5 Novel fatilities

20

AVERAGE FIELD

--- Field from calibrated 1 em. dipole-- Field calculated from receive antenna

51....---------~------------i.0.2

E 35

>CD"'0

oill 25iI()

a:I()

~ 15ill

45r---------------,.-----------,

(i) Mode tuned where the paddle wheel is steppedslowly through many positions within onecomplete rotation at a given test frequency.The time for which the paddle blades arestationary is determined by the time it takesthe EDT to respond to the imposition of anew stimulus.

(ii) Mode stirred where the paddle wheel is movedcontinuously during the test at a givenfrequency. In this case the rotation rateshould be slow enough for the EUT torespond to the changing wavefield conditions.

Typical field strength variability with frequencyfor the NBS (US National Bureau of Standards)chamber is shown as a function of frequency inFigure 9.20 as measured by two different sensors.

Reproduced by permisssion or NBS

Figure 9.20 Maximum and average electric fieldstrengths generated inside the NBS chamber(empty) mode tuned)


usually inexpensive to hire or purchase as italready exists as a byproduct of mineralextraction. This makes the use of largeunderground sites, which can have floor areas thesize of a football pitch, very cost-effective fortesting large distributed systems which may havemany interconnecting cables.

The main drawbacks to using a facility like thisare the usually limited shaft or roadway access,the possibly high humidity environment and thesafety and evacuation considerations. Anunderground facility may be an ideal solutionwhen a project is subject to costly multiple EMrequirements such as EMC, NEMP and Tempest.

a standard test under the ambient conditions ateach test site as this may be reflected in test costs.In such conditions measurements may need to berepeated and taken close in to the EDT (at 3 m).This can lead to difficulties with extrapolation tomeasurements made at other distances on anothersite. Because of this difficulty, a problem couldarise if the measurements have to be checked by alaboratory working on behalf of the regulatoryauthority which is blessed with low ambients andcapable of measuring at 10 or 30 m. I t is thereforepreferable to use a test facility where the ambientsare low and measurements can be made directlyat 10 or 30 m.

\

ANTENNAPOSITIONER

o == t---+-----I~-EUT-

NON-CONDUCTING RECEIVING ANTENNALOW DIELECTRIC CONSTANT \WEATHER-PROOF BUILDING

~========:::::::? Automatic

antennaheightpositioning1-4m.

EUT SUPPORTEQUIPMENT

USN & MAINS POWER ARTIFICIALCONDITIONING FILTERS GROUND PLANE

Figure 9.21 Example oj open-range test facility

9.3.3 Testing procedure

To make more consistent and reliable measurements, the ground at an open-air test range iscovered with a large metal plate or mesh to providea known constan t conductivi ty and dielectricconstant resulting in predictable RF reflectionproperties. A typical mesh would extend for60 X 30 m to give a lower working frequency of20 MHz [37]. The mesh size d is determined by therequirement d < AI10 where A is the wavelength atthe highest frequency at which the range will beused. This is typically 1 GHz and so the mesh sizeshould be less than 3 em. It is important not ·toobstruct the ground plane with buildings or testequipment which can cause RF reflections, and forthis reason the receivers and antenna positioningequipment are often located close to the receivingantenna but under the ground plane. A similarprovision can be made for the mains supply andsupport equipment for the EDT. An example of auseful site configuration is shown in Figure 9.21.

The details of a suitable measurement site arespecified in the various regulations which apply tothe equipment being tested, and these must bec'onsidered carefully by the site operators. In the

9.3.2 Test site

9.3 Open-range testing

9.3.1 Introduction

Open-site RF measurement is the traditional andstraightforward approach to making measurementsof the EMI characteristics of equipment. Thetechnique has been used for testing both civil andmilitary electronic equipment. Open range testingbecame the norm for testing commercial equipmentwith the advent of. the VDE and FCC regulationsfor example, concerned "with the measurement ofradiated emission levels from household appliances,office equipment and information technologydevices such as personal computers.

The test specifications and regulations governingthe construction and calibration of open test rangesare dealt with fully in Chapter 2. The aim of thissection is to explore the construction, certificationprocedure and operation of these facilities.

The ideal test site consists of an obstruction-freeground area and hemisphere above it. The siteshould be located well away from sources of electromagnetic noise such as power lines or air conditioning plant containing electric motors,thermostats etc. Ideally, the site should be locatedin the countryside well away from all industrialEM noise and broadcast transmitters. Somescreening of TV and VHF radio transmitterscould be afforded by a ring of hills covered withtrees which would surround the ideal test site. Itis not always economically viable however forcompanies to set up a test facility at some distancefrom their city based customers to take advantageof the low EM ambients. The alternative is toattempt to make sensitive radiated emissionmeasurements in the presence of high level andrandomly changing ambient signals, and this canbe extremely difficult and time consuming.

Potential customers of such facilities shouldenquire about the average tim.e taken to complete


diameter = 2d ----to-!


rrhe purpose behind the detailed E UT configuration/layout and the height scanning of the receivingantenna required by many test standards is tomeasure the maximum interference generated bythe equipment at each frequency of interest.

The EU1' shown in Figure 9.21 is placed eitherin contact with the ground plane or at a distanceof 0.8 ill above on an insulating table if it is a freestanding item. All cables should be of the typeand length specified for the EDT being tested. Ifthe cables are very long they can be bundled into30-40 cm-Iong bunches at the centre of the cable.

During testing the EDT is exercised in one of itsoperational modes, the receiver is tuned to afrequency within the band of interest and theantenna is moved through a range of heightsusually from 1 to 4 m above the ground and themaximum signal level is recorded. This isnecessary to measure the signal strength from theEU1' at that frequency for the minimum siteattenuation. (This is considered later whendiscussing site calibration.)

The EUT must be rotated on a turntable, or thereceiving antenna can be repositioned at variouscompass points, to detect the maximum emissionas a function of azimu th angle if the Eurf cannotbe rotated. The structure of the radiation fieldemanating from an EDT is usually complex withmany peaks or deep nulls at various azimuth andelevation angles for each frequency being radiated.

The ability of open-field measurement methodsto measure the maximum fields at each frequencyof interest has been studied in some detail[38, 39J. I t is difficult to see how the accuracy ofopen-site tests can be improved without morefrequent sampling of the emitted radiationpattern from the EUT and thus further increasingthe protracted testing time.

The polarisation of the receiving antenna ischanged at each frequency and location tomeasure the worst-case emission field strength. Allsignal strengths should be measured using both anaverage and quasipeak detector function on theEMI receiver to fully cornply with CISPR 22. Thefrequency is then changed and the test repeateduntil the band from 30 to 1000 MHz has been fullyexplored and the maximum field strengthsrecorded. The operating mode of the EDT is thenchanged and the entire test sequence is repeated.

If the recommended tuned dipoles are used asthe receiving antenna, their length must bephysically adjusted at each frequency above80 MHz (for CISPR 22 and frequencies below80 MHz a fixed length equivalent to the 80 MHztuned length is used down to 30 MHz). Otherspecifications such as FCC part 15 require thedipole to be tuned over the whole frequency band.

Clearly, the test time required for the multiplica-

RECEIVINGANTENNA

Area within the ellipse must be freeof all RF reflecting objects

(bj

Figure 9.23 Minimum requirements for alternative testsite (a j Minimum alternative measurementsite (b j Minimum size of metal groundplane. D d + 2 m, where d is themaximum test unit dimension;W == a + 1 m, where a is the maximumaerial dimension; L == 3, 10, or 30 m

----------......../' /"'-

/\ ~m \

I ~3m 3m BII ~~ ·EUT I\ TEST AE~AL I

\ SURROUNDING BOUNDARY II'" \'-------_.-/'

(aj

context of this text on EMC testing consider as anexample the requirements specified by CISPR 22(testing of IT equipment) which call foradherence to CISPR 16 (specification of radiointerference measuring apparatus andmeasurement methods). (Details contained insection 3 of the CISPR 16 Document.)

The size of obstruction-free area is determinedby the measurement distance (3, 10 or 30 m) witha shape specified in Figure 9.22. The groundplane often extends to cover the full obstructionfree area to give the best results. For test siteswhere compliance is not possible, a minimum setof requirements for free area and ground planeare given and can be seen in Figure 9.23.

Figure 9.22 Requirements for open-area test site. EUTto antenna distance d can be 3, 10 or 30 m


Reprod Llced by permission or NBS

5 .....- ........- .......---.~-...-- __- ......--__- ...

459MHz 1GHz143MHz

97MHz 311 MHz 1GHz

44MHz

J: 430MHz

l-I<=>iIi 3IW...J0 2c-o

110

:I: 4l-IoW 3IW....J

~ 2Ci

20 25 30 35 40 45 50RELATIVE INSERTION LOSS dB

(a)5....--........- .......----,~- .....-...,..----r--....--....

1 .....- ......- ......- .......~-.......~..............~-- .....- .....10 15 20 25 30 35 40 45 50

RELATIVE INSERTION LOSS dB

(b)Figure 9.25 Calculated open-area site attenuation with

frequency, height and polarisation(a) Horizontal polarisation, (b) Verticalpolarisation. Source: M a and Kanda,NBS-USA

amplitude and phase changes introduced into thereflected path by the increased path length and theadditional phase change on reflection will producea systematic variation of received power as theheight of the receive antenna is changed. The siteattenuation at a given frequency j, range d,transmitter antenna height H t and polarisation isthe minimum value measured at a given H r wherethe contributions from the direct and reflectedpaths reinforce each other. Examples of calculatedsite attenuation as a function of receiving antennaheight, at a number of frequencies for bothhorizontal and vertical polarisation, have beenmade by Ma and Kanda [37J and are shown inFigure 9.25.

By selecting the minimum attenuation from aheight scan at each frequency for horizontal andvertical polarisations at the measurement distancesof 3, 10 and 30 m, one can plot a suite of siteattenuation curves shown in Figure 9.26 [40]. Itcan be seen that the attenuation values (for theNBS open range) vary from 20 dB at 30 MHz toabove 40 dB at 1 GHz for 10m separation, andthat the relationship between attenuation andseparation at any frequency is approximately 20 dBper decade. The calculated and measured values ofattenuation agree well at most frequencies.

Site attenuation is defined as the ratio of the inputpower to the transmitting antenna to the measuredpower at the output of an identical receivingantenna operating with a given polarisation whenboth are located at known positions on the site asin Figure 9.24. By measuring this attenuation as afunction of frequency it is possible to validate thesite for testing by comparing it with a referencesite which is usually operated by the governmentmetrology agency such as NBS in the USA orNPL in the UK.

It can be seen from Figure 9.24 that the powermeasured at the receiving dipole is a vectorcombination of the contributions from the directpath and the reflected path from the ground. The

9.3.4 Site calibration

Figure 9.24 Open-area test site calibration setup(horizontal polarisation). Site attenuation,A == 10 log PI.N / POUT (max) for fixedvalues of d == 3, 10 and 30 m and withPOUT ( max) selected from measurementswith receive antenna height H r == 1 - 4 m

tion of these measurement parameters is enormousand simplifying procedures have to be adopted inpractical tests. The height positioning of theantenna and the rotation of the EUT can beautomated with motor positioners. 'The frequencybands can be rapidly scanned using automaticreceivers and the data quickly accumulated andlogged using a control computer. Only thosefrequencies where the measured signal strengthapproaches the specification level need attract thefull rigour of the test procedure to ensure that themaximum field strength is measured.

All these measurements must be made in thepresence of ambient signals which ideally must beat least 6 dB below the specification limit. At mosttest sites this is rarely the case and computeraided signal identification is often used torecognise, mark and ignore some of the morestable ambient signals from broadcast transmitters.

Broadband antennas such as biconics and logperiodics are permitted under most testingregulations and their use, together with scanningreceivers, can lead to a much more reasonabletesting time.


9.3.5 Measurement repeatability

The performance of well constructed open-rangetest sites has been widely reported, for example theIBM Endicott facility [41 J, and general designstudies have been carried out by DeMarinis [42J.

9.3.5.1 Reflections from objects

An indication of the effects of reflections at theboundary of a 10 m measurement site can begauged by calculating the modifications to thenormal site attenuation curve when a large metalpIa te with its long axis parallel to themeasurement axis is introduced on the siteboundary. The errors are reported [42J to be± 1.5 dB with the plate at 15 m and ±0.5 dB whenthe plate is 60 m away from the measurementaxis. Measurements have also been made withtrees 15 m from the edge of the range area whichintroduced errors of 1-2 dB at certain frequenciesaround 80 and 180 MHz [42J. From these data itwould seem that reflections from objects outsidethe stipulated flat object-free area can contributebetween 1 and 2 dB to measurement uncertainty.

9.3.5.2 Weatherproof covers

Some open test ranges provide a weatherproofshelter over the test area in which the EUT ismounted so that testing is not compromised bybad weather. The construction of these sheltershas been investigated as a possible source ofmeasurement error due to reflections from theirwalls and roof. The use of various materials inshelter construction has been investigated [42Jand crude measurements of reflection coefficientshave been made and are given in Table 9.3. Thisshows that pinewood is a poor rna terial to usewhile plastic and glass fibre sheets have lowreflection coefficients.

However, Dash and Straus [44J show thatdielectric constant and thickness, not conductivi ty, maybe the key factors in determining thesuitability of materials for EMC test site shelters.Wood has a relatively low dielectric constant of

Cable and connector losses and VSWRPosition of EUT cables and peripheralsNonuniform field strength/range relationship .

In August 1985 a study (CISPR/B/WG/2) analyseddata gathered from a number of regulatoryauthority test sites on a single item of computingequipment measured during the previous year.Dash [43J suggests that, as expected by some EMCpractitioners, the correlation between measurements from the various test sites was. poor, with astandard deviation of 8.5 dB. This means that theemission field strengths at anyone site can only bepredicted to an accuracy of around 30 dB (to a95% certainty) based on the figures from anyother site. I t is therefore clear that although thesite attenuation may be measured quite accurately,this does not mean that the EMC tests that arecarried out on the sites are equally accurate.

103

102 103

FREQUENCY MHz

102

FREQUENCY MHz

Source height: 2m.Vertical polarisation •

30m

10m3m

Source height: 2m.Horizontal polarisation

to 60"'0CJ)

~ 50...J

5 40~0:: 30UJCJ)

~ 20~

~ 10Z~ 0

101

(a)

~ 60CJ)

~- 50...J

5 40~ 30mffi 30CJ)

~ 20 10m~:::>~ 10 3mz~ 0

101

(b)

The estimated worst-case uncertainty in definingthe range attenuation as calculated by the NBS[37J is ± 1.2 dB. The worst case differencebetween measured and calculated site attenuationis given as 1 dB for horizontal and 1.9 dB forvertical polarisation. These figures may be truefor a carefully controlled government establishment range charged with setting standards forindustry, but published evidence suggests that theeveryday problems encountered at industrial testsites make this accuracy difficult to achieve.

1-'he following factors affect the measurementaccuracy on an open site:

Reflections from objectsNonconductive weather cover structuresAntenna impedance and VSWR changes withheightAntenna size field averagingBalun VSWR and lossesDifferences in commercial antennas

Figure 9.26 Calibration of open-area test site (siteattenuation against frequency) (a) Verticalpolarisation) (b ) Horizontal polarisation)• Measured data, - Calculated data.Source height 2 m


Table 9.3 Examples oj RF reflection coefJicients ojnumber oj materials commonly used in EUTcovers on open-area test range

1.3-2.5 [45J but plastics have values from 1 to 5and must be used with care and only in thin sheets.

Site attenuation measurements in the presenceof an EDT shelter made from radome plasticrnaterials [46] has shown some evidence of achange in the maxima and minima of the heightscan pattern (Figure 9.25). This effect is notnoticed when these data are processed into therange attenuation plots, as only the minimumattenuation figure is used. The presence of theplastic shelter has altered the physical position ofthe minimum attenuation path on the range. Anoverall estimate of the errors introduced by a welldesigned weather cover are of the order of ± 1 dB.

Material

Yellow pine plywoodFir plywoodWaferboardDrywall or sheetrockAcoustic ceiling materialComposite wall w/fir plywood,

sheetrock, fibreglass insulation,and cedar clapboard siding

Composite wall as above, but withasbestos roof shingles

Composite technology plastic andglass fibre wall with foam insulation

Reflectioncoefficient

0.450.270.250.230.090.3

0.27

0.03

HEIGHT OF THE CENTRE OF VERTICAL HALF-WAVE AERIALABOVE THE GROUND (in wavelengths)

0.25 0.3 0.4 0.5 0.6 0.7 0.75100

a 90

ui 800Z«

70t-enCi5

60wa::z 500i=« 40CS«a:: 30

o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0HEIGHT OF A HORIZONTAL HALF-WAVE AERIAL

ABOVE THE GROUND (in wavelengths)

Figure 9.27 Variation oj radiation resistance with heightabove groundJor vertical and horizontalpolarised ha1jwave dipole

Reproduced by permission of RSGB

above the ground over the frequency range 30-300 MHz has been shown be ±2 dB or more [48].

9.3.5.4 Antenna size field averaging

This has been discussed in Chapter 6 in relation tothe difference in field strength measured on an opentest site with a point source at 3 m, when using ashort CISPR 80 MHz dipole/or biconic ascompared with a 30 MHz FCC tuned dipole.Figure 9.28 shows the antenna sizes in relation to

5 10 15 20 25 30dB (arbitrary scale)

E FIELD MEASURED AT A POINT(source is at a distance of 3m.)

Figure 9.28 Field averaging with large dipoles. Fieldexposed to: A == CISPR/bicon,B == 30 MHz dipole

W>°co-~ ~

Size of 30 MHz.... :; 4mZ 0 tuned dipole____- (J)° (I)o.."C.... - Size of CISPR 80 MHzZ EW e 3m tuned dipole or biconic~-W E antenna0::Cl')

~:::>-C/)W~Z

2mWe:(~...J

LLo..

80MH~ A

B0°.... z:c:::>0° 1m_0::

~o

5m9.3.5.3 Antenna impedance changes withheight

The change of antenna impedance with height canbe thought of as being due to the interaction withits image created by' the ground. The closer theantenna is to the ground the greater is the mutualimpedance effect. The resistive component of theimpedance of a vertical and horizontal halfwavedipole antenna above the ground is shown inFigure 9.27 [47J. The vertical dipole is wellbehaved at a height greater than half awavelength, but the resistance of the horizontaldipole is still changing from 60 to 80 ohms when itis a full wavelength above the ground. If theantenna impedance changes there will be amismatch with its balun, connecting cable andthe receiver input. Thus the voltage across thereceiver input port will be unknown and this willintroduce an error into the measurement. Forexample, the measurement uncertainty due toantenna mutual impedance/VSWR changes in ahorizontally polarised biconic antenna 1 to 4 m


the field profile as a function of height [49J. Thelarger 30 MHz dipole averages the 17 dB fieldvariation along its length and indicates a fieldstrength which is 8.5 dB lower than the actual fieldstrength at a height of 1 m. The shorter CISPR80 MHz dipole and the biconic only have a 3 dBfield variation with length and make a reasonablyaccurate measurement of the real field strength.

9.3.5.5 Balun VSWR and losses

Chapter 6 concluded that the typical VSWR for acommercial biconic antenna was between 1.5 and4: lover the frequency range 30 to 300 MHzcompared with a Roberts tuned dipole of less than1.5: 1. Clearly the higher VSWR of the commercialantenna will result in greater measurementuncertainty as the antenna impedance varies withheight. The loss in a typical commercial balun for abiconic antenna is in the range 0.5--2 dB (30300 MHz) vvhich reduces the minimum detectablesignal in the band compared with that obtainablewith the Roberts tuned dipole balun which has aloss of only 0.1-1 dB (30 MHz-l GHz).

9.3.5.6 Differences in commercial antennas

Work carried out at Hewlett Packard open testranges [50] has shown that range calibrations canvary by up to ±6 dB when carried out using variouscombinations of log-periodic antenna pairs. Theantennas are all manufactured to the same designby a single company and differ only in serialnumber. The errors are usually confined to a fewfrequencies between 300 MHz and 1 GHz wheresharp response variations are observed over ahundred MHz or so at up to ±6 dB and may be themanifestation of antenna/cable matching problems.

l'hese effects are not observed when using tuneddipoles for range calibration, but these are slow touse and a wideband antenna is preferred. Onesolu tion is to fit 3 or 6 dB pads at the an tennaoutput when using complex wideband antennassuch as the log-periodic or biconic to help controlVSWR problems. The effect of a simple fix suchas this is a reduction in measurement sensitivity:the preferred solution is to produce widebandantennas with high-quality baluns with closemanufacturing tolerances. Investigative work suchas this on the properties of antennas, indicatesthat the EMC test engineer can take nothing forgranted when attempting to make accurateradiated emission measurements and should makeevery effort to carry out measurement checks.

9.3.5.7 Cable and connector losses/VSWR

Investigations carried out by workers at DigitalEquipment Corporation in the USA [51] have

shown that significant errors of up to 10 dB canbe introduced by the change in position of theantenna cable during a test. This effect has beenattributed to two causes. T'he first is the cableacting as a parasitic element close to the antennawhich modifies its properties, and the secondmore serious cause is due to leakage between thecable and antenna elements due to imperfectcurrent balance and leaky connectors. Broadbanddipoles have shown relatively poor balun isolationresulting in 6-10 dB errors. Again, largedifferences in balun performance were discoveredin similar biconic antennas with different serialnurnbers [51] . Some of these effects can bereduced by placing a common mode ferrite chokeover the outer of the antenna coaxial cable tosuppress unbalanced current flow.

9.3.5.8 EDT cables and peripherals

Most of the 30-40 dB possible uncertainty betweenradiated emission measurements on the same EUTat different open sites [43] cannot be accounted forby a combination of the 1-1-0 dB errors resultingfrom the issues discussed. The ANSI C63 studygroup was set up in 1985 to examine the problemof variable cable layouts in FCC MP-4 andCISPR 22 tests on IT equipment. The wording inthese standards regarding placement of cables andperipherals calls for their placement to find theworst-case emission levels. This allows each testsite to place the EUT components in differentpositions in search of the worst case emissionconfiguration. This will result in differentradiation patterns and different test results. Theworking group therefore considered that certaincombinations of definite fixed cable placementsand peripherals should be prescribed. Thisapproach sacrifices the ability to find the absoluteworst-case emission levels but increases the testrepeatability. The details of suggested cable andperipheral placement are inappropriate for listinghere but are given by Dash [43].

9.3.5.9 Nonuniform field strength/rangerelationship

Within the CISPR 22 standard, para 9.4 relates tomeasurements in the presence of high ambientsignals. It permits measurements to be made atreduced distances from the EUT to increase themeasured signal level in an attempt to overcomethe masking ambient signals from broadcast transmitters and the like. The regulation permits thescaling of the specification limit in a linearmanner with measurement distance.

This procedure is highly suspect whenconducted in an EM radiation field of complex

9.4 Low-level swept coupling andbulk current injection testing

9.4.1 Introduction

These two techniques are distinct but complementary. They have evolved together over the last 10years in the UK, primarily in conjunction withimmunity clearance of aircraft [53J, military hardware and automobiles. The two techniques havebecome popular throughout the EMC communityand are being taken up in the USA and elsewhere.Several bulk current injection (BCl) tests have beenincorporated into the UK DEF STAN 59-41 EMCrequirements for triservice military equipment.

The low-level swept coupling tests are normallycarried out on complete systems to establish the bulkor common mode induced current spectrum on acable loom for a given external EM field as it isswept in frequency from about 5 to 400 MHz. Thisinduced current can then be scaled with externalfield strength to derive the expected current in thecable loom which would be induced by the specifiedfield to which the system must be immune. Thiscurrent can then be injected into the loom in theBCl test at levels equal to or above those corresponding to the specified immunity level. By directly

EMC TES1' REGIMES AND FACILITIES 171

9.3.6 Comments on open-site testing

Government regulatory agencies such as the NBScan construct open-range test sites for makingradiated emission measurements at 3, 10 and 30 mwith site-attenuation performance close to thetheoretical values. The open-range radiatedemission test methods as defined by regulationssuch as FCC, ClSPR and VCCl are timeconsuming to carry out fully, and the presence ofEM ambient signals is an additional problem.

The foregoing discussion relating to open-rangeemission testing shows that the practising EMCtest engineer and customers need to be aware ofthe sources of uncertainty that can undermine themost strictly regulated test regime. Theseproblems all stem from the attempt to makeextremely detailed and difficult electromagneticmeasurements of complex unknown wavefields byusing simple test equipment and quick procedureswhich result in affordable testing.

EMC testing provides an indication of emissionlevels from an EDT when measurements arecarried out to a standard procedure. I t is not anattempt to make full and accurate measurements,with a close estimate of associated uncertainty,irrespective of the time and cost involved.

'The overarching aim of con trolling the increasein EM emissions from electronic equipmentprobably justifies these less than rigorous buteconomically acceptable test methods.

1000

Measuredcorrection factor

\ EJITEMI1 METER

hr

100FREQUENCY MHz

Measured conversion factors to normalisemeasurement distances of 30 m and 10 m to3m

Typical open-area test site calibration oj'range attenuation (a) Site attenuationparameters (b) Site attenuation for 3) 10and 30 m (horizontally polarised) .ht == 1 m) hr == 1 to 4 m (D == 3 m)D == 10m)) hr == 2 to 6m (D == 30 m)

Figure 9.30

Figure 9.29

(a)60

50ht = 1m. hr = 1 to 4m. (0 = 3m., D = 10m.)

hr = 2 to 6m. ( 0 = 30m.)CD D=30m"C 40z0

~ 30:::>zw 20

~10

020 100 1000

FREQUENOY MHz

(b)

configuration where the far-field boundarydistances for components at different frequenciesare not well known. The site attenuation curvesas measured with two similar dipole antennas doshow a simple field strength/range scaling law.However, work carried out in Japan by theMatsushita Company [52J to derive the siteattenuation curves using EDTs with realisticradiation patterns and wavefield impedances hasshown that the 20 dB/decade scaling law isinadequate. The test set up is shown in Figure9.29a and the range a ttenua tion curves in Figure9.29b. The correction curves from 3 to 10 and30 m are shown in Figure 9.30 and differ from theassumed 10 and 20 dB by up to 8 dB at 80 MHz.

CD 30t---~-----~--------------"CZo 20

~:::>z 10w /J= Measured correction factor« Or--- ---.L ~

20


injecting current into the cables it is possible tosimulate to some extent high-level free-field radiatedsusceptibility tests but using only a fraction of the RFpower required to drive large antennas. This meansthat it becomes possible to carry out technicallymeaningful immunity tests on large systems at aneconomic cost.

9.4.2 Low-level swept coupling

Consider an electromagnetic wave incident on acomplex system as shown in Figure 9.31. Thediagram shows the possible coupling routes bywhich EM energy can interact with and propagatethrough the structure and its apertures to thesemiclosed metal equipment box which containscircuit boards populated with susceptiblecomponents. The size of box apertures, circuitboards and components dictates that they will beelectrically short at wavelengths much longer than

their physical dimensions and exhibit poor couplingefficiency. These direct wave to componentcoupling paths are therefore secondary at all butthe highest frequencies beyond about 0.5 GHz.

The main coupling path is provided by theconducting system structure and the extendedcable runs within it. This route predominates forall frequencies up to about 400-500 MHz forsystenls which are between 2 and 10 m long. Thesimplified coupling scheme showing only thestructural and cable coupling route is given inFigure 9.32 where the unknown individualtransfer processes within the structure and cablesare represented by a series of complex frequencydependent impedances.

At present no real credence can be given to E- andH-field measurements made in the restricted spacesinside cOlTIplex systems such as aircraft [54] andautomobiles using triaxial sensors or other probes.This situation, together with the limitations in

1

-- WAVE FIELD+SYSTEM STRUCTURE & CABLE COUPLlNG+SUSCEPTIBLE IBOX or BOXES~

MAIN COUPLING PATH UP TO ~ 400 MHz I I

II .. i RF curr?nt 1\ Internal mutual Multi-eonductor I RF curren~ delivered I~F wove Incident pattern In ,\ impedance transmission t~ suscephble tJ.ox I

I field on system s!ructu~e coupling to line response to via sy.stem cabling.structure diffraction wiring loom coupled energy (Dominates suscep- I

fields tibility up to 400MHz)

SECONDARYCOUPLING

/' PATHS ""

Direct pick-upby exposedwiring loom dueto aperturesin structure.

Figure 9.31 Schematic ofpotential RF coupling paths into system

TEST POINT uTP1 u

-current in rnA

CIRCUIT BOARDS

BULK RF CURRENT ( located .inside thePROBE susceptible box)

WAVEFIELDPARAMETERS

IE =VIm ~AVEFIELD TO BULK CABLE CURRE1TH =AIm TRANSFER FUNCTION ( mAl VIm)

IZ=opO=W/m2

! STRUCTURES & CABLES I

I

II 2 - 10m long ..

RF CURRENT FLOW

Za /INCIDENTWAVEFIELD

IMPEDANCE NETWORKrepresenting each step in coupling path

Figure 9.32 Main structure and cable dependent coupling path (up to ~400 MHz)


FIBRE OPTIC WIDEBAND~ELEMETRY SENDER

FIBRE OPTIC

CONTROL PLOTIERCOMPUTER

C/P ON VIDEO CABLE

100 200FREQUENCY MHz

C/P ON DISC BUS CABLE

TRACKINGGENERATOR

Whole vehicle low-level swept-jrequencycoupling test

Figure 9.36

E E

-- ~ 2

~ E

C/P ON COMPUTER MAINS LEAD

~

zai=() OI:.:L-. ~ """""""Z 0 100 200cr FREQUENCY MHzex:fi C/P ON PRINTER BUS CABLE

~ 4f-'«ex:l-

t?

~~2~6 E()

o -o 100 200

FREQUENCY MHz

CURRENT PROBE AROUNDCABLE OF INTEREST

Figure 9.35 Examples of coupling transfer functions forvarious cables on personal computer system.( CIP == current probe)

The test set up for n1easuring low-level sweptcoupIing to a system under test is shown in Figure9.36. The first step in the test procedure is togenerate a levelled field at the test location [55].This is done by placing the fibre-optically coupledfield sensor at the location where the E UT will beand then generate in the control computer a set ofamplitude values for the tracking generatorattenuator for which a constant field strength isproduced. The same values will be used whenradiating the swept signal to ensure that the EUTis subjected to the calibrated levelled field. Thedistance from the broadband antenna to the EUTshould be sufficient to ensure that the EUT is inthe far field at all frequencies of interest.

A current probe which is connected to a fibreoptic transmitter is placed around the requiredcable loom close to the connector on the

STANDARD FIELDMEASURING SENSOR

Used to "pre-calibrate"

the field at the test~object location. (Derivea power table andstore in the control == ~"'" ~...........computer.) ~ .......~

FIBRE OPTIC LINK S~C~~~~ _D~;T~CE

"/"'""-BROADBA~NDANTENNA ..... ",""FIBRE OPTICRECEIVER

If ~10 - 100 W L----+-A

U ~j

EQUIPMENTUNDER TEST

FIBRE OPTIC TX

---DATA FIBREcarrying bulk current signal

to fibre optic receiver

Typical bulk current induced into cables inaircraft. Incident field strength 10 Vimhorizontal polarisation

Measuring bulk cable current with isolatedcurrent probe and fibre-optic transmitter

CONTROL FIBRE TO~F.O. TRANSMITTER

CURRENT PROBEmeasuring bulk cable current

\

0.0.

c:(30

EI-Zilla:a::::::> 20()

ill..JCDc:(()

Q 10w():::::>Q

~

05 25

Figure 9.34

Figure 9.33

40---------r----.....----------~

computing multiconductor transmISSIon lineresponses, leads to severe problems in tracing theRF energy through the coupling chain to thesusceptible box.

I t is possible to measure the incident E- and Hwavefield cOlnponents and to determip.e waveimpedance, polarisation and angle of incidence onthe structure. I t is also easy to make an undisturbedmeasurement of the bulk or common-mode cablecurrentat the test point 1'P l, close to the connectoron the box of interest which contains the susceptiblecircuits. See Figure 9.33. If the incident fieldstrength is kept constant while the frequency ischanged we are able to measure a swept frequencycoupling response of the system under test.

For example, the induced current in an aircraftcable harness for a 10 V 1m external field ispresented in Figure 9.34 [54] and showsmaximum coupling at a frequency correspondingto the resonant electrical length of the structurelcable harness. Coupling factors for various cablesconnected to a personal computer are shown inFigure 9.35 and are typical of a wide variety ofequipments, having peak values of a few mA/V1m.

A HANDBOOK FOR EMC TESTING AND MEASUREMENT174

-10

«co -20"0

UJ...J -30co«()

~ -40I-zUJ -500::0::::>() -60QUJ()

-70::>Q~

-80

FRONT OF VEHICLE

FREQUENCY = 160 MHz

FRONT OF VEHICLE

FREQUENCY = 60 MHz

Figure 9.38 Wavefield coupling polar diagrams withcurrent probe monitoring pickup in enginemanagement cable harness--V polarisation--H polarisation

60 MHz (1st resonance) and 160 MHz (wellabove 1st resonance) for both vertical andhorizontal polarisation. Notice how at 160 MHzthe maximum coupling to a cable which is beingmeasured (located under the bonnet), occurswhen the field is incident on the front of thevehicle. 1'his is not seen at 60 MHz where thewhole vehicle is resonant.

The use of spot-frequency coupling polardiagrams can be well illustrated with an exampleof a less complex structure than a saloon carwhich has many different sized door and windowapertures. Figure 9.39 shows the couplingresponse from external field to a cable loom in amiss.ile. The physical structure of the missile canbe represented by a short fat electrical dipole withthe centre load termination being the impedanceof the fore body to motor body mechanical joint.Generating the polar diagram at the frequency ofmaximum coupling produces the response shown

-70 '---II------ll-----L_....-L._--'-_---'-_......._ .......... ............

40 50 60 70 80 90 100 110 120 130FREQUENCY (MHz)

Figure 9.39 Example of RF current induced in systemmain harness as function offrequency.Swept frequency field strength == 20 Vim

0

« -10co"0I-z -20UJ0::0::::> -30()

UJ...J

-40co«()

0 -50UJ()::>0 -60~

20 40 60 80 100 120 140 160 180 200FREQUENCY MHz

Figure 9.37 Typical induced cable current in completevehicle

susceptible electronics box and the controlcomputer reproduces the levelled field as thefrequency is scanned from about 5 to 400 MHz.The received fibre optic signal is converted backto an analogue electrical form and fed to thespectrum analyser which is driving the trackinggenerator providing the stimulus signal. Thespectrum analyser raw data are corrected for thecurrent probe transfer impedance and the truecable induced current is displayed as a function offrequency on the computer monitor. Typicalinduced currents in the looms of a vehicle areshown in Figure 9.37 for an incident levelled fieldstrength of 20 Vim. Note that the maximumcoupling value is 1.5 mAIVlm at 50 MHz(-30 dBA at 20 V1m) for this vehicle.

The production of a field to curren t couplingcurve enables the EMC engineer and productdesigner to have an overview of the electromagnetic coupling properties of the EDT. I t iscommon for coupling curves to be of the formshown in Figure 9.37 where the coupling increasesfrom a low level at low frequencies (where theEDT is electrically short) up to a maximum atnear structural resonant frequencies and then toshow a number of higher order coupling responsesin the region where the EDT is electrically long.

I t is interesting to investigate and understandthe physical properties of the EDT whichinfluence the coupling. 1'his can be done byselecting a frequency of interest, such as 60 MHzin the case of the coupling curve shown in Figure9.37 (which is in the frequency range ofmaximum coupling) and rotating the EDT whilerecording the induced current. In this way, a'pick-up' polar diagram can be produced, theinterpreta tion of which can often reveal thoseaspects of the EUT structure which aredetermining the system coupling. Figure 9.38shows these polar diagrams for a saloon car at


Figure 9.41 Example oj automatic bulk current injectiontest corifiguration

TAIL

HEAD ON

PICK UP PROBE\

\EUT MALFUNCTION LINE(automatic indication)

Operator intervention via keyboardwhen EUT malfunction is observed.(If no automatic link is possible.)

that when theprobes are

9.41, and asthe measuredto the current

- Onset of failure- - - Release of failure

-10

J:- -20U«:fen~:::

-30!;(cn..... 0::z::::> -40w O0::00:: 0::JZ -50uo~p=

~~ -60Ur!o...J

-70we:(u:2::JI-0::::>

-80~w

-90

Figure 9.42 Example oj injected current level as JunctionojJrequency at which microprocessor vehicleengine management system is disrupted

20 40 60 80 100 120 140 160 180 200

FREQUENCY MHz

control computer records the current flowing inthe loom as measured by the pickup currentprobe via the spectrum analyser. The current'readings are then corrected for pickup currentprobe transfer impedance. The injected current isreduced and the current at which the EDTreverts to normal operation (if it does) is recordedautomatically by the computer. The process isrepeated as each new frequency is tested until thewhole band of interest has been covered.

An example of the output of an automated BCItest on a microprocessor based electronics unit isshown in Figure 9.42 where failures occur for bulkcable currents of around 10 rnA flowing in thernulticond uctor unscreened cable.

Experiments [60J have showninjection and pickup currentpositioned as shown in Figurespecified in DEF STAN 59-41,cable current is very nearly equal

Figure 9.40 Coupling polar diagram oj system atprimary resonance Jrequency shows dipolelike response

In Figure 9.40 which is almost perfectly dipolar.This shows that the missile has minimumcoupling when the field approaches head or tailon and has a maximum repose when it isbroadside on. This is indeed the response of adipole and so this experiment confirms that themain coupling feature and electrical structuralmodel of the missile is that of a simple dipole.Such information can be used to suggest couplingreduction schemes and is very useful in assessingthe electromagnetic compatibility of systems intheir expected operational environment.

9.4.3 Bulk current injection

When the indilced bulk cable current coupledfrom an external wavefield has been determinedusing the LLSC technique, the scaling factor fromthe LLSC test level to the specified immunitylevel can be applied to the measured inducedcurrent. This level can then be injected into theloom under test via a high power currentinjection probe and the EDT observed for signs ofmalfunction. Much has been written about theadvantages and technical justification of thistechnique [56-59J and it is not possible to reviewall the work here. Space permits only a briefexplanation of the key points involved in BCImeasurements. The test setup for an automatic orsemiautomatic bench test on an EDT is shown inFigure 9.4l.

The control computer sets the frequency andadjusts the power level to the injection currentprobe. The forward power is monitored andrecorded for reference. The injected current isincreased until the EDT malfunctions and the


WHIl'E, D.R.J.: 'A handbook series on electromagnetic interference and compatibility, vol. 2, Testmethods and procedures'. Don White Consultants,Germantown, Maryland, USA, pp. 3.7-3.23

2 FARSI, M.: 'EMIjRFI shielding: theory andtechnique'. Interference Technology Engineer'sMaster, 1988, pp. 228---238

3 WHITE, D.R.J.: 'A handbook series on electrornagnetic interference and compatibility, vol. 3'. DonWhite Consultants, Germantown, Maryland, USA,1973, p. 10.75

4 Low-frequency cylindrical screened chamber14 m dia. x 12 m high, constructed for GEOSspacecraft programme 1973-6. BAe Dynamics Ltd,Filton, Bristol, UK

5 MIL STD 285: Attenuation measurements forenclosures, electromagnetic shielding for electronictest purposes, Method of, 25 june 1956

6 CARDENAS, A.L.: 'The examination of standardsfor testing RF shielded enclosures', EMC Technol.,jan-Feb 1988, pp. 26-38

7 CARTER, N.J. and THOMPSON, j.M.:'s.usceptibility testing of airborne equipmentThe way ahead'. 2nd symposium and technicalexhibition on Electromagnetic compatibility) Montreux,1977, pp. 83-88

8 STUCKEY, C.W., FREE, W.R. andROBERTSON, D.W.: 'Preliminary interpretationof near field effects on measurement accuracy inshielded enclosures'. Proceedings of IEEEsymposium on EMC, 1969, pp. 119-127

9 BENHAM, G., COLEMAN, C and MORGAN, D.:'Development of an elliptical anechoic chamber'.Final report BT11868 of contract NPL196j0060,1981, BAe Dynamics, Filton, Bristol, UK

10 SHEPHERD, D.R. and GOLDBLUM, R.D.:'Exploiting shielded room characteristics to providea low cost, semiautomated test system for EMS andEMV testing'. Mikrowellen Mag., 1982, 8, (6),pp. 714-718

11 Cavitenna AT2000, Accessories for RF testing.Amplifier Research, 160 School House Road,Souderton, PA 18964-9990, USA

12 EDWARDS, D.J. and BURBIDGE, R.F.: 'Anexperimental investigation into the performance ofasymmetric screened rooms for electromagneticmeasurements'. Proceedings of IEEE symposium onEMC, 1990, pp. 114-118

9.5 References

specification is exceeded. This conveys far moreinformation than a simple traditional radiatedsusceptibility no fail test at the specified fieldlevel. The BCI and LLSC tests can permit thecareful optimisation of EMC safety margins abovethe specified limit for production items, bygathering information on the statistics ofvariability of system coupling from the LLSC testand on the failure current as measured by theconvenient BCI test.

200

Bulk cable currentrequired for unitmalfunction

\Induced bulk cablecurrent for "X" VIm(coupling measurements)

50

40

«E 30t-ZWa:0: 20:::>0

10

00 50 100

FREQUENCY MHz

Figure 9.43 Deriving susceptibility safety margin frombulk current injection and bulk currentcoupling measurements

in the box under test for bulk current injection,and that this is also true for direct illumination bya wavefield. This gives confidence that the resultsobtained via BCI tests can be related tooperational exposure of the equipment to EMfields. Lever [61] has shown good correlationbetween BCI system-induced failures and thoseexperienced during 'normal' radiated susceptibility measurements on automobiles. The BCItechnique has also been adapted for use indevelopment testing and assessment of a widerange of electronic systems including ITequipment [62]. In this case the injection probe isconstructed from two inexpensive ferrite C-coresand the stimulus is provided by a pseudorandomsequence generator which provides a broadspectrum output.

Having described them separately, the LLSCand BCI measurement techniques can now bebrought together to show how cost-effectiveimmunity testing can be carried out on largecomplex systems such as aircraft, automobiles andcommercial vehicles for which standard radiatedsusceptibility free-field testing may be prohibitively expensive.

The LLSC coupling data are scaled by the ratioof the test field (1-10 v1M) to the specifiedimmunity field (say 200 Vim) and the scaledinduced current is plotted as the lower curve inFigure 9.43. The results of the BCI test, which area set of cable currents as a function of frequencyat which the EDT is compromised, are plotted onthe same graph as the upper curve in Figure 9.43.The margin of safety between the availablecurrent in the looms for the specified field leveland the required current for a failure is a directmeasure of the degree to which the immunity

13 LAHITA, M.J.: 'RF shielded enclosure purchasingconsiderations'. Interference Technology Engineer'sMaster, 1988, pp. 37 and 386

14 SALATI, B.D. and CHAPMAN, C.J.: 'Maintenanceof aged modular shielded enclosures'. InterferenceTechnology Engineer's Master, 1988, pp. 38-388

15 BUCELLA, T. and SANCHEZ, G.: 'A low-costRFI susceptibility testing system'. Proceedings ofIEEE symposium on EMC, 1981, pp. 31-35

16 DAWSON, L. and MARVIN, A.C.: 'Alternativemethods of damping resonances in a screened roomin the frequency range 30 to 200 MHz'.

17 BAKKER, B.H. and PUES, H.F.: 'A noveltechnique for damping site attenuation resonancesin shielded semianechoic rooms'. Proceedings ofIEEE symposium on EMC , 1990, pp. 119-124

18 LAWRENCE, B.F.: 'RF anechoic chamber testfacilities'. EMC Technol., April-June 1983, pp. 32-38

19 Indoor ground reflection EMC test range, System86/3. Rayproof division of Keene Corp, Norwalk,Connecticut 06856, USA

20 NICHOLSON, J.R.: 'Perform 'open field' measurements in a shielded enclosure'. 2nd symposium andtechnical exhibition on EMC, Montreux, June1977, pp. 413-418

21 'Ultra high performance flexible pyramidalabsorber'. Technical bulletin 8-2-2, 1-74, Emerson& Cuming Inc.

22 'Thin-film absorber for 50 MHz-1 GHz'. Technicalbulletin 8-2-17, rev 6/75, Emerson & Cuming Inc.

23 KUESTER, E.F. and HOLLOWAY, C.L.:'Improved low-frequency performance of pyramidcone absorbers for application in semianechoicchambers'. Proceedings of IEEE symposium onEMC, 1989, pp. 394-399

24 GAVENDA, J.D.: 'Semianechoic chamber siteattenuation calculations'. Proceedings of IEEEsymposium on EMC, 1990, pp. 109-112

25 Eccosorb VHP RAM. Grace electronic materialscatalogue, p. 1-100. Emerson & Cuming (UK) Ltd,838 Uxbridge Rd, Hayes, Middlesex, UB4 ORP, UK

26 MISHRA, S.R. and PAVLASEK, Z.T.J.F.: 'Designcriteria for cost-effective broadband absorber-linedchambers for EMS measurements'. IEEE Trans.,1982, EMC-24, (1), pp. 12-19

27 NICHOLS, F.J. and HEMMING, L.H.:'Recommendations & design guides for theselection and use of RF shielded anechoic chambersin the 30-1000 MHz frequency range'. Proceedingsof IEEE symposium on EMC, 1981, pp. 457-464

28 TSALIIOVICH, A.: 'RF absorber qualificationcriteria and measurement techniques'. Proceedingsof IEEE symposium on EMC, 1990, pp. 361-366

29 MENDEZ, H.A.: 'A new approach to electromagneticfield strength measurements in shielded enclosures'.Wescon record, Los Angeles, CA, Aug. 1986

30 LIU, B.H., CHANG, D.C. and MA, M.T.:'Eigenmodes and the composite quality factor of areverberating chamber'. NBS tech. note 1066, Aug.1983

31 CORONA, P.: 'Electromagnetic reverberatingenclosures: behaviour and applications', Alta Freq.,1980,49, pp. 54-158

32 BEAN, J.L. and HALL, R.A.: 'Electromagnetic


susceptibility measurements using a mode-stirredchamber'. Presented at IEEE symposium on EMC,1978

33 MA, M.T. and KANDA, M.: 'Electromagneticcompatibility and interference metrology'. NBStechnical note 1099 section 8

34 ROE, M.J.: 'An improved technological basis forradiated susceptibility and emission specifications'. Presented at IEEE symposium on EMC,1978

35 CRAWFORD, M.L. and KOEPKE, G.H.:'Comparing EM susceptibility measurement resultsbetween reverberation and anechoic chambers'.Proceedings of IEEE symposium on EMC, 1985,pp. 152--160

36 CRAWFORD, M.L. and LADBURY, M.J.:'Mode-stirred chambers for measuring shieldingeffectiveness of cables and connectors: Anassessment of MIL STD 1344A method 3008'.Proceedings of IEEE symposium on EMC, 1988,pp. 30-36

37 MA, M.T. and KANDA, M.: 'Electromagneticcompatibility and interference metrology'. NBStechnical note 1099 section 2.22

38 DVORAK, T.J.: 'Fields at a radiation measuringsite'. Proceedings of IEEE symposium on EMC,1988, pp. 87-93

39 MISHRA, S.R. and KASHYAP, S.: 'Structure ofEM field at an open-field site'. Proceedings of IEEEsymposium on EMC, 1988, pp. 94-98

40 FIZGERRELL, R.G.: 'Site attenuation'. Proceedingsof IEEE symposium on EMC, 1985, pp. 612-617

41 MADZY, T.M. and NORDBY, K.S.: 'IBMEndicott EMI range'. Proceedings of IEEEsymposium on EMC, 1981, pp. 17-21

42 DeMARINIS, J.: 'Studies relating to the design ofopen-field EMI test sites'. Proceedings of IEEEsymposium on EMC, 1987, pp. 115-126

43 DASH, G.: 'Computing equipment standards - Anupdate on cable and peripheral placement'.Proceedings of IEEE symposium on EMC, 1987,pp. 332-337

44 DASH, G. and STRAUS,!.: 'Studies on the use ofwood in open area test sites. Proceedings of IEEEsymposium on EMC, 1988, pp. 295-303

45 'Reference data for radio engineers'. (Howard W.Sams, 1975, 6th edn.) pp. 4.28-4.31

46 MAEDA, A.: 'Note on EMI measurement at openfield test site (6): Effect of EU1-' shelter'.Proceedings of IEEE symposium on EMC, 1990,pp. 241-246

47 'Radio communication handbook' (RSGB, London,5th edn.) p. 12.56

48 BENNETT, W.S.: 'Antenna to ground planemutual coupling measurements on open-field testsites. Proceedings of IEEE symposium on EMC,1988, pp. 277-283

49 BRENCH, C.E.: 'Antenna differences and theirinfluence on radiated emission measurements'.Proceedings of IEEE symposium on EMC, 1990,pp. 440-443

50 BENNETT, W.S.: 'Radiated EMI measurementreproducibility'. Proceedings of IEEE symposiumonEMC, 1987, pp. 90-93


51 DeMARINIS, J.: 'The antenna cable as a source oferror in E1\1I measurements'. Proceedings of IEEEsymposium on EMC, 1988, pp. 9-14

52 T'KEYA, S. and MAEDA, A.: 'Note on EMImeasurement at open-field test site'. Proceedings ofIEEE symposium on EMC, 1985, pp. 593-599

53 CARTER, N.J.: 'The application of low-level sweptRF illumination as a technique to aid aircraft EMCclearance'. Interference Technology EngineersMaster, 1990, pp. 236-252

54 PANKHURST, R.V.: 'The coupling of electromagnetic interference into aircraft systems'. Proceedingsof IERE conference on EMC, 1980, pp. 117-128

55 CARTER, N.J., REDMAN, M. and WILLIS, P.:'Validation of new aircraft clearance procedures'.Proceedings of IEEE symposium on EMC, 1988,pp.117-124

56 AUDONE, B., FERRERO, G., GIORCELLI, L.and VECCHI, G.: 'Critical examination of bulkcurrent injection techniques: Theoretical aspects'.Proceedings of IEEE symposium on EMC, 1988,pp. 109-115

57 OBERTO, G., BOLLA, L., ROSTAGNO, G. and

BARARDO, R.: 'Critical examination of bulkcurrent injection techniques: Experimentalcomparison'. Proceedings of IEEE sympoSiUll1 onEMC, 1988, pp. 101-107

58 KERSHAW, D.P. and WEBSTER, M.J.:'Evaluation of bulk current injection technique'.Presented at IEEE symposium on EMC, 1990, (latesubmission)

59 NEWTON, P.M.: 'Aircraft testing in the electromagnetic environment'. Proceedings of ERAseminar on DEF STAN 59-41, December 1990,ERA Technology, Leatherhead, Surrey, UK

60 BURBIDGE, R.F., EDWARDS, D.J., RAILTON,C.J. and WILLIAMS, D.J.: 'Aspects of bulkcurrent immunity test'. Proceedings of IEEEsymposium on EMC, 1990, pp. 162-168

61 LEVER, P.H.: 'Development of a system levelbench test for the automotive industry'.Proceedings of IEEE symposium on EMC, 1990,pp. 270-275

62 MARSHALL, P.C.: 'Convenient current injectionimmunity testing'. Proceedings of IEEE symposiumon EMC, 1990, pp. 173-176

Chapter 10

ElectroDlagnetic transient testing

10.1 Introduction

10.1.1 Transient types

Over the last 20 years traditional EMC testing hasconcentrated on radiated and conducted emissionand susceptibility testing. This is carried out byexamining or generating signals as a function offrequency, and this activity is sometimes referredto as operating in the frequency domain. Somediscontinuous signal tests have also been carriedout as part of EMC work with specifications suchas MIL STD 461. These spike tests simulateswitching transients which may be induced ontopower lines when a neighbouring unit is operated.Other additional power-line surge and drop-outtests have also grown in importance over the years.

Now that EMC design is becoming a sophisticated and integrated part of electronic design, onesees that the effects of both continuous interference and transient effects are having to bedesigned out at the same time, and usually by thesame design team, as solutions for one type ofinterference may compromise those for another.Cost-effective protection across the whole field ofRF interference demands that a coherent set ofdesign techniques are employed to combat bothcontinuous and transient interference with theminimum component count and cost.

In some cases, the solutions to transientproblems using nonlinear voltage clipping devices,such as zener diodes or spark gaps, are not usedat all in controlling continuous interference.However, to be effective their location in thecircuit and their position on the circuit board canbe crucial, but this may compromise the optimumdesign for reducing continuous interference. Thedesigner must be able to produce a design whichis the best cost-effective compromise solution forall the continuous and transient requirementscalled up in the range of standards which theproduct must meet.

For transient signals these standards can includeconsideration of:

Power-line spikesPower-line surges and drop ou tsESD -- electrostatic dischargeNEMP - nuclear electromagnetic pulseLightning strike

179

The first two have been covered as part of thecontinuous interference tests, as they havedeveloped alongside traditional EMC testing andare considered together with continuous interference, often in the same standards. The remainingthree areas are dealt with in this chapter.

10.1.2 Continuous and transient signals

Before discussing these topics in detail it is relevantto point out some key differences between theconventional continuous interference EMC testingand that required to demonstrate compliancewith 'transient' specifications. There are twoaspects which .are different:

The instrumentation is differentThe conceptual models needed to bestunderstand the types of signals are different.

The instrumentation used for continuous andtransient measurement is different because it isrequired to capture and present signal data indifferent ways. Spectrum analysers or scanningEMI receivers are needed to look at thecontinuous interference as a function of frequency[1]. The limits for conducted and radiated interference (and susceptibili ty) are all drawn onfrequency plots. Therefore all the instruments aredesigned to produce data in that form. Theseinstruments generally do not preserve and presentphase data on the signals being measured, onlyamplitude is recorded after being measured witha given IF jvideo bandwidth and filter shape.

Measurement of fast transients cannot be madesensibly using scanning receivers. Fast oscilloscopes or digital transient recorders must be usedto capture the waveform [2]. Certain aspects ofthe waveform such as amplitude, risetime, PRF,etc., are then con1pared with the limits set out inthe applicable specification. Most transient testsare susceptibility tests and the instrumentation isprimarily used to confirm the waveform beinginjected into the EDT while it is monitored forany malfunction.

Test engineers and equipment designers willfind that when dealing with both continuous andtransient phenomena it is vitally important to beable to hold two conceptual models in their mindsand to be able to switch easily between them.


These are the frequency domain concept and thetime domain concept.

The frequency domain is useful for representingthe spectra of continuous signals. The form of thespectrum is fixed (within the observation time) anddoes not change or evolve with time. A spectrumcan be considered to have two components,amplitude and phase, see Figure 10.1. All thedetails of the signal can be recorded if both theamplitude and phase data are measured, but it isusual in EMC testing, when using a spectrumanalyser or EMI receiver, to measure only theampli tude of the signal as a function of freq uency.This results in some limitations with regard tospectrum manipulation (as will be seen) and it isnot usually possible to unambiguously reconstruct

the signal waveform with only the ampli tude of thespectrum. Thus conversion from the frequencydomain (spectrum) to the time domain (waveform)is unfortunately normally barred.

The time domain contains only a record ofsignal amplitude as a function of time withrespect to a time zero, as would be seen on anoscilloscope screen. It displays no frequency dataabout the signal directly but a time-domainrecord can yield this information by theapplication of the Fourier transform [3-5J.

10.2 Fourier transforIlls

10.2.1 Introduction

-10 ............._ ......._ .......-...._......_ .............- ...o 10 20 30 40 50 60 70 80

FREQUENCY MHz

PHASE PLOT

o 10 20 30 40 50 60 70 80FREQUENCY MHz

Figure 10.1 Example of signal spectrum showing bothmagnitude and phase components

10.2.2 The transform

Fourier series, Fourier transforms, Laplacetransforms and convolution theory are all linkedmathematical ideas which shed light on the relationship between the frequency and timedomains. The subject is wide and detailed, withwhole texts such as that by Bracewell [3J devotedto its explanation. Clearly, in this chapter there isno space to do other than give some of theconcepts involved and ·to state the formulas whichapply. Understanding Fourier transforms is soimportant to being able to understand bothfrequency and time-domain data that the readeris urged to undertake additional study byrecourse to the References.

The central idea behind the Fourier series!transform is that within linear systems a complexperiodic signal or response can be constructed bythe superposition of a number of simple harmonicfunctions. This is illustrated in Figure 10.2 wherea complex signal is presented in the time domain

The Fourier transform technique was developedby].B.]. Fourier (1768-1830) to assist in solvingproblems in heat conduction [4J, but is sopowerful that it has become very widely used byphysicists and engineers and can be applied to awhole range of problems. In the present sphere ofinterest it is the means by which one cantranspose data between the frequency and timedomains. This will allow EMC designers and testengineers to view information contained in asignal from the point of view of both domains andthus achieve maximum understanding of it. Theexamination of prominent ampli tude peaks in aspectrum and their comparison with dominantwaveform features can lead to an insight into thetype of circuits which are generating the interference signal. This is particularly useful whencarrying ou t design or diagnostic EMC testing.

AMPLITUDE PLOT

The magnitude trace is the only oneproduced by a scanning spectrumanalyser.All phase data is lost using such adevice

~ 30wo::::>.-.I-~Z 8 20OC/)«~~e:E :t: 10:::)-ea:~

t;w0- 0CIJ

W...JoZ«w~s: 0::E::::>a:t;w0W

+1t

-1t

ELECTROMAGNETIC TRANSIENT TESTING 181

AMPLITUDE OFWAVEFORM COMPONENTS f--=-;T--·I

r1'-l' I

TIME---""

FREQUENCY

o"SPECTRUM"OF INDIVIDUALFREQUENCY

~COMPONENTS

FREQUENCY DOMAINOBSERVATION

WAVEFORMBEING ANALYSED

~DOMAIN~ 11MEOBSERVATION

Figure 10.2 Relationship between time and frequencydomains

where f (t) represents the waveform in (1) and Anare the amplitude coefficients, n the harmonicnumber, and w the angular frequency.

(1), all the simple harmonic components requiredto synthesise the complex shape are laid out infrequency order (2) and the spectrum is thensimply the view along the frequency axis (3).

Consider the example of a complex apparentlydiscontinuous signal such as a square wave. It canbe constructed by the linear superposition of aDC term and a large number of harmonicallyrelated cosine/sine waves as shown in Figure 10.3together with the general expression for theFourier series. A pictorial representation of theFourier series for a pulsed waveform is shown inFigure 10.4 with each line representing acomponent in the series. The first few lines of thepulsed waveform spectrum can be seenrepresented in the frequency domain (3) in Figure10.5. The individual cosine waves which make upthe complex time-domain waveform are shown in(2). They are depicted as all having the samephase with regard to the starting point on thetime axis. I t is therefore dictated that

" 3

1~

" ::J

~EQUE~CYFrequency domain view(Spectrum analyser)

Spectral lines

I2T 7'

FREQUENCYfRectangular pulse and Fourier series spectrum

wo:::>I-:Ja..~<C

INDIVIDUAL COSINEWAVES

Reproduced by permission or Hewlett-Packard

Figure 10.4

10.100

LAn cos nwtn=O

f(t)

10.2.3 Introducing phase

Figure 10.5 Prequency and time domains (cosine wavesonly)

Unfortunately, this expression does not allow us tocompute the time domain waveform from aknowledge of the amplitude and frequency of thelines in the Fourier spectrum as shown in Figure10.5(3), because it is assumed that all the simpleharmonic components are cosines with the samephases. In general, the phases will not be thesame and so an extra term is introduced:

10.200

LAn cos (nwt+cPn)n=O

J(t)Reproduced by permission or Hewlett Packard

Sum of Fundamental,Fundamental 2nd Harmonic

Average value and average valueof wave 2nd Harmonic /

1(0) ".

f(f) = 00 + :I: an cos n 0) f + :I: bn sin nO)fn=1 n=1

Figure 10.3 The Fourier series: building up a pulse


Incorrect - arbitrary waveform.(No phase data)

where c/Jn is the phase of the nth component.But

and

1 Joo .f(t) == - F(w) elwtdOJ (indirect transform)2n -00

10.4

10.2.4 Fourier transform expressions

The Fourier transform pair is expressed usually inone of three ways [3J and is given here as statedin Reference 8:

F(w) = J~ooj(t) e-iWldt (direct transform)

10.3

wheref(t) is the time varying function and F(OJ) isthe frequency spectrum. A good library ofwaveforms and Fourier transforms is available [8Jwhich can aid the EMC engineer in analysingspectra or predicting the spectrum of a givenwaveform.

Present-day firmware embodiments of theFourier transform are very fast and can produce a1024-point FFT in a fraction of a second. Whilethe manufacturers take care to prevent signalaliasing [2J and carefully select window optionssuch as Hanning and Hamming [2J in addition toa simple rectangular 'boxcar' function, the expertuser of such advanced instruments will need tohave studied the effects of signal convolution [3Jwith the measurement instrument time window tobe able to detect instrument/measurement artifacts.

10.2.5 Impulse response

Consider a simple network (a resonant filter)shown in Figure 10.7. I ts performance is usuallydetermined by a scanning CW technique using avector network analyser (for amplitude andphase) giving rise to the filter frequency responseas either amplitude and phase plots againstfrequency or sometimes in the form of a Smithchart.

The complementary time-domain technique isto determine the filter impulse response bysubjecting the input port to a very fast stepfunc tion waveform with a rise time tmin' Thisminimum rise time is equivalent to the maximumfreq uency which has been scanned in thefrequency domain method. The filter will respond(ring in this case) in a characteristic way which isobserved from the waveform at the output port.This waveform is called the impulse or stepresponse of the filter and contains time detailsresolvable down to the limit set by tmin' Theimpulse response and the frequency response arerelated by the Fourier transform pair.

Now consider calculating the output from thefilter for a given input signal and proceed in two

'Correct' waveform - reconstructedusing phase information.(Sine &Cosine terms in the Fourier Series)

ARBITRARY

7

Figure 10.6 Reconstruction of the true waveform jromspectrum

AMPLITUDE SPECTRUM ONLY(no phase data)

wCl::)I-

~ "CORRECT"

~ ~SES

FREQUENCY ~

cos (OJt + c/J) == cos c/J . cos OJt - sin c/J . sin OJt

in which cos c/J and sin c/J are constants that can berepresented by a and b:

cos (n OJ t + c/Jn) == an cos n w t - bn sin n w t

and this result leads directly to the expression forthe Fourier series given in Figure 10.3.

If only the amplitude of the cosine and sineterms are known (as measured by a spectrumanalyser) this is equivalent to losing the phaseinformation which is required to synthesise thetime-domain function f(t) and it cannot bedetermined unambiguously. By assigning differentvalues of phase to the spectral lines in view (3) ofFigure 10.5 a range of arbitrary waveforms canbe produced in addition to the correct waveform,but all of which have the same amplitudespectrum, as illustrated in Figure 10.6.

This explanation has been made to illustrate howpowerful the technique of Fourier analysis can be intransposing between the frequency and timedomain, and also to show that it is not possible tomake rigorous use of the transformation with onlyamplitude data in a spectrum as generated by aspectrum analyser or EMI meter. I t is possiblethough to use the full power of the Fouriertransform when suitably digitised waveform dataare processed using digital fast Fourier transformroutines which are now firmware-based and selfcontained within modern instruments [6, 7J.


SWEPT FREQUENCYNETWORK ANALYSER SOURCE

NETWORK FREQUENCY RESPONSE(AMPLITUDE & PHASE)

Single frequencyscanned to Fmax

J~-"

Fmax

I

PHASE..... ,~

W ,0::::>t-:JQ.

~~

DC FREQUENCY

OUTPUTINPUT

STEP OR IMPULSEGENERATOR

A FREQUENCY DOMAINMEASUREMENT

NETWORKUNDER TEST

A TIME DOMAINMEASUREMENT

::E ::Ea: a:f2 0

III tl, a: a:

,~ ~ ~~ NETWORK

IMPULSE RESPONSE

t

---SLOPEIIt

m1n"

tmin is related to Fmax Fastest slope in responseis slower than IItll

min

Figure 10.7 Network measurement infrequency and time domains

INPUT SIGNAL SPECTRUM(phase not shown)

FREQUENCY RESPONSE OF NETWORK(phase not shown)

SPECTRUM OF OUTPUT SIGNAL(phase not shown)

OUTPUT WAVEFORM

wo:::J

~ fI--t-+---1r----1"--':--+~+.:>Iitrc"...T,-IM-E::E«

wQ:::JI::Jc..::E«

F3(OO)

FREQUENCY

F2 (00)

NETWORK IMPULSE RESPONSE

wo:::J~ IL+-..........~~I-'-~...,.,....,__-c..::E«

wo:::>I:::Jc..:E«

x(MULTIPLY)

...... , ......

TIME

F1 (00)FREQUENCY

INPUT WAVEFORM

w0:::>I-:::Jc..::E«

t~~a::u..:::JCf)OZu..«a::

I-

w0:::>I-:::Jc..::E«

f1 (1) f2 (t) f3 (t)(CONVOLVE)

Figure 10.8 Multiplication in frequency domain and convolution in time domain

complementary and connected ways, one in thefrequency and one in the time domain. Considerthe input signal defined by the frequencyspectrum F 1(w) in Figure 10.8. This can bemultiplied by the network frequency response

F2 (w) (linear amplitude) to yield the outputspectrunl F3 (w) in the usual way. If the outpu twaveform is required, it can be calculated fromthe output spectrum (amplitude and phase) bythe application of the Fourier transform.


f (t) narrow range of some variable. All measurementsof physical quantities are limited by the resolvingpower of the instrument and convolution theory isthe underlying mathematical description of thesmearing process.

u

9 (t)

F (t)

feu)

" ,"'.-g(t-u)

/w. f ( u ) g ( t - u ),,',

F (t) 0fJ......., f f ( u ) 9 (t - u ) du' ......... -011

f(x) *g(x)

10.2.7 Advantages of time-domainmanipulation

Convolution and manipulation of data in the timedomain is very irnportant if engineers anddesigners are to be able to understand the significance of complex waveforms and their interactionwith electrical networks. This ability is useful inconsidering for example the impact of a unidirectional NEMP step pulse shown in Figure 10.1 Oaon a target system, and in relating it to theringing waveform which is specified for NEMPcurrent injection into a target EDT in MIL STD462 CS10. See Figure 10.10b.

It should be clear that the situation is exactly thatdescribed in Figure 10.8 where the excitation pulse ismodified by the impulse response of the target

10.9 Representation of convolution process

Reproduced by permisssion or McGraw-Hill

10.2.6 Convolution

The output waveform can also be calculatedby convolving the input waveform with

the network impulse response. Convolution is amathematical process with wide application andis described by Bracewell [3J. The convolution oftwo and g(t) is given by

a:wtii~

~o>

Approximately- 50 kVIM

E (t) = A (e-at - e-pt)

/

J~oof(u)g(t - u)du 10.5

"'V 10 nanosecondsTIME---............

(a)

Figure 10.10 Representation of an exoatmosphericNEMP pulse (a) Double exponential(b) US MIL STD 461 CS10 waveformfor injection into pins/cables on an EUT.Ip == 1.05 Imaxe-nft/D sin (2nft) whereIp == common mode pin current in amps,f == frequency, Hz, t == time, s,D decay factor

and is written asf (t) *g (t). The process can be seenby inspecting Figure 10.9 where two samplefunctions of t are drawn out. For a given value of tthe function g(t u) is multiplied by f(u)andintegrated over u. The value produced is the valueof the convolu tion integral at that value of t and isrepresented as a single value of F(t) by a line witha heigh t proportional to the area under theproduct curve. rThe output function F(t) isevaluated by the above process for all values of t ofinterest. rThis process is sometimes called thecomposition product or the superposition integraland at first sight it may seem difficult to evaluate,but is amenable to solution by simple computernumerical techniques. I t has wide applicationbeyond the filter example in Figure 10.8.Convolution is useful in describing the action of anobserving instrument when it makes a weightedmean measurement of some quantity over a

IZW0::a::::>

a. U- .s

a.

(b)

TIME


system, leading to the ringing type waveforms thatare conducted along cables into the EUT. Riad [9]compares the advantages to be gained by solvingtransient problems in the time domain with thoseoffered using the frequency domain, and using bothin a hybrid fashion with Fourier transforms as aconnection. This is illustrated by rigorouslyderiving the 'impulse response of a simple RCnetwork using each of the three approaches. Hepoints out that obtaining solutions to problems inthe time domain is not widely taught but offersadvantages which are self evident to those engineersprepared to learn the techniques involved. Themain advantage of formulating problems in thetime domain is that nonlinearities can be properlytreated, as can systems which change with time.

10.3 ESD - electrostatic discharge

10.3.1 Introduction

hygroscopic additives which draw in moisturefrom the air. I t has the same resistivity as pinkpoly but persists for longer; for about five years.

Third generation - conductivepolypropylenes

This material has carbon black incorporated intothe resin and has a surface resistivity of 105 ohmper square. The antistatic properties arepermanent and the substance has a higher rigidityand dimensional stability than earlier materials.

Fourth generation - fibre impregnatedmaterials

These materials are impregnated with a randomnetwork of fibres that provide structural reinforcement and static charge dissipation. They offerpermanent antistatic properties, a high resistanceto sloughing and to chemical agents.

This material owes its antistatic properties to

Potentially damaging electrostatic discharge toelectronic components and equipment caused bycontact with human beings, tools and officefurnishings has been of concern since at least theearly 1970s [10]. The ESD problem can beconsidered in two areas:

First generation pink polythene

Developed in the 1970s it has a surface resistivity ofbetween 109 and 10 12 ohm per square, but it hadlow stiffness, poor dimensional stability, short-livedelectrical properties and was easily damaged bysolvents.

(i) ESD to vulnerable components such asMOSFETs in manufacture, storage, handling,transport and assembly into products.

(ii) ESD to finished products such as desktopcomputers in the operational environment.This is caused by the electrical charging ofhumans and furniture etc. in dry airconditioned offices fitted with carpets andchairs made from synthetic materials.

I t is estimated [10] that the cost of ESD damage inthe USA alone was around ten billion dollars a yearin 1988, and that this may increase as devicesbecome more sensitive with increasing scale ofintegration. Control of ESD in the first category,the production phase, is amenable to controlthrough the implementation of careful proceduresand the use ofspecial antistatic bags and containers.

Four generations of antistatic materials havebeen produced:

10.3.2 The ESD event

Human beings can acquire electrostatic chargewhen walking or shuming along a carpet by aprocess known as the triboelectric effect. Twodissimilar materials such as wool in the carpetand rubber on the soles of shoes can exchangecharge which then builds up on the person. Thecharged individual then moves towards a piece ofelectronic equipment such as a personal computerand discharges the stored energy to it via a fingertip or a metal object such as a ring, pen or tool ofsome kind. The resulting spark to the caseproduces a fast risetime current which is injectedinto the equipment and can disrupt or damagethe device.

Depending on the two materials and the rate atwhich they are rubbed together, ch~rge will buildup at different rates on the two objects. Theaffinity for materials absorbing charge can beranked in a table known as the triboelectric series,

The second category of ESD problems, whichaffects completed electronic products in theoperational environment, cannot be easilycontrolled by procedure. Resistance to ESD mustbe designed into the product by careful choice ofcomponents, their positions on circuit boards, theuse of electrostatic and electromagnetic shields,protection of circuits backing onto connector pinsby the use of fast switching limiters, the choice ofsuitable materials for product cases and thecareful design of case joints, edges and apertures.

To design in ESD protection the designer mustunderstand the nature, magnitude and frequencyof occurrence of ESD events and how they aregenerated in the operational environment.

antistatic polypropylenesSecond generation


Table 10.1 Sample common materials arranged intriboelectric series

16

15

14

13

> 12~

w 11(')

~ 10....J0 9>w 8(')0: 7~J:

60

5WOOL

4 /3

2


1a--.--&...-.a..---a._01---+---a._..a.-.......a---a._.......

5 10 20 30 40 50 60 70 80 90100 RHO/o

Figure 10.11 Typical voltages to which humans willcharge as function of relative humidity.Maximum values of electrostatic voltageto which operators may be charged whilein contact with the materials shown

circuit as shown in Figure 10.12 and has beenadopted by the lEG for lEG 801-2 and by theDOD for MIL STD-M-385IO. ~1ore complexmodels have been proposed to account for fasterrisetimes of the leading edge of the pulse and toproduce multiple discharges, which are bothsometimes observed under normal officeconditions. A crude description of the worst-casewaveform and its frequency conten t resulting fromthe simple RC model discharge is given in Figure

AirHuman handsAsbestosRabbit furGlassMicaHuman hairNylonWoolFurLeadSilkAluminiumPaperCottonSteelWoodAmberSealing waxHard rubberNickel, copperBrass, silverGold, platinumSulphurAcetate rayonPolyesterCelluloidOrIonPolyurethanePolyethylenePolypropylenePVC (vinyl)KELFSiliconTeflonNegative

Positive

~20dB

/perdecade

Typical assumed worst-case ESDwaveform

time

10Hz

Discharge model ,....., 500 ohms

----+~20 kV ,.....,200 pF

*

10 MHz ~ 200 MHzFREQUENCY

~~ Awo-I«0::tOWDen

!z Aw~40A

:::>ow

"0::«J:oeno

Figure 10.12

and an extract from the US DOD- HDBK-263(1980) is given in Table 10.1.

Once the charge begins to build up on a persondue to the triboelectric effect a second effectbegins to take place which bleeds off charge to thesurroundings. This discharging effect depends onthe humidity of the air: thus ESD problems canoccur more frequently in air conditioned officeswhere the humidity is low. The voltage to whicha person becomes charged depends on the amountof charge acquired and the self capacitance of theindividual. In general, the larger the surface areaof the body the larger is the self capacitance [11 J.Jones [12J gives data for the voltages to whichhumans can be charged when in contact withvarious materials as a function of relativehumidity, see Figure 10.11.

The basic human ESD model IS a simple RC


Probe tip inductance

--Ground returninductance

.... Azw~ 40A::::>()

woex:c(:r:()(j)

Btime

metallic objects such as rings. Events with leadingedges rising as fast as 300 ps were observed, as weremultiple discharges at modest voltages of 3-4 kVwhen metallic objects were in the discharge path.Transition regions were observed at voltages of 68 kV between variable very fast risetime and highcurrent events to the more repeatable conventionalslower (> 1 ns) pulses.

In some tests [13], fast pulse peak currents of170 A have been observed at discharge voltages of4kV. It has been shown by experiment that thevariety of ESD pulses is considerable andargument still continues about the need tosimulate this variability, particularly with regardto the fast-pulse phenomena. Throughout theinvestigations however it is clear that the worstcase events (high current and fast risetime) areobserved when the discharge to the EDT takesplace through a metallic object.

The multitude of ESD events that occur in theoperational environment can be simulated bythree types of model [14].


Figure 10.14 ESD probe circuit from first edition oflEG 801-2

10.3.3.1 Human body model

People are the primary source of ESD events andthe normal, slow discharge phenomenon ( lO15 ns risetime), can be simulated by the RGnetwork as sho~:t;l in Figure 10.12. The lEe 8012 (1984) standard calls for alSO pF capacitanceand the specified simulator circuit is given inFigure 10.14.

FUNCTIONALEARTH

150 n DISCHARGE150 PF ELECTRODE

CAPACITOR DISCHARGINGRESISTOR

\ Rd

100Mn

16.5 k V Cs

CAPACITOR CHARGINGRESISTOR

"'" Rch

10.3.3.2 Charged device model

Device leads, frames and packages can becharged triboelectrically, just the same ashumans and can be discharged from the surfaceof the device to ground via the pins or otherconductive parts of the device under test. Thecharge vol tage and discharge energy will dependon the position and orientation of the chargeddevice with respect to ground. In general theESD pulse produced with the charged 'devicemodel has a much faster risetime than that for

10.3.3 TypesofESD

There are a number of types ofESD which give riseto different voltage and current pulses to whichvictim devices or equipments may be exposed. Thevariability and fineness of detail which can occur inESD pulses has been explored by King andReynolds [13], amongst others. They measuredmany hundreds of discharge events for both directfinger contact and conduction through small

Figure 10.13 Ground return inductance impairs desiredwaveform

10.12 [11]. I t can be seen that the risetime is of theorder of 1 ns with a tail about 100 ns long.

In a portable ESD simulator test equipmentthe achievable risetime is governed by theinductance of the tip and ground return leadsshown in Figure 10.13. Risetime is importantbecause it is known to affect the susceptibility ofan EDT to ESD of a given voltage. Thusproducing an ESD simulator with a requiredand repeatable risetime is an important goal ofsimulator design.

When the energy stored in the capacitor of thesimple RG model in Figure 10.12 is dischargedinto a short circuit a current of up to 40 A canflow into the EDT. In nature both positive andnegative polarities can build up on human skin.However there is no clear evidence [11] thattesting with either polarity produces differenteffects in electronic EDTs. Most simulators andstandards, such as lEG 801-2 (1984), specify onlypositive voltages.


Air discharge method

the human body model and can be substantiallyless than 1 ns [14J. Bhar [14J conducted aliterature survey which showed that devices suchas a 64 K DRAM have an ESD damagethreshold of 2 kV when tested using a simulatorbased on the human body model, but whichdropped to only 850 V when testing with thefaster pulse produced by the charged devicemodel. I t was observed that generally hurnanbody ESD caused junction damage to the devicewhereas charged device ESD causes dielectricbreakdown [15, 16J.

10.3.3.3 Field-induced model

This simulates the effect of charge separation andsubsequent discharge on a device when it isexposed to an external static electric field. Themechanism of charge separation and subsequentESD on an integrated circuit placed in a fieldgenerated by a potential gradient between thecharged object (not the IC in question) and theground is described by Unger [1 7J. The fieldinduced ESD model may be visualised as shownin Figure 10.15 with the device lying in the

~+~+ +

CHARGED _

OBJECT

&t~~jj+~ C

l ----~~~I~~~~~~~~~;;--- I 1 Iiii =r=cc 2

Cg/ ++++++++++++++++++++++++ =r:::

EQUIVALENT CIRCUIT

c1 ___}'igure 10.15 Induced ESD on electronic devices

Cj == Capacitance between charged object(+) and IC) C2 == IC capacitancebetween top and bottom surfaceC3 == Capacitance between lower plate

and bottom surfaceCo == C2 + C3 == IC capacitance tog;ound) R t Resistance oj the test circuit)Rd and Cd == Resistance and capacitanceoj the device

external electrostatic field of the charged object.The discharge pulse is fast, rather like that for thecharged device model and parameters have beenvalidated by Enoch and Shaw [18, 19J.

10.3.4 ESD-induced latent defects

Experimental work has been carried out [20, 21 Jin exploring the effect of ESD which does notmodify or destroy the device at the time ofapplication, but does cause an operational failureat a later time. This is called the latent defectmechanism. The existence of such a mechanism isof considerable concern as devices which mayhave experienced ESD events will not havedetectable failures and may be incorporated intoequipments which then subsequently fail. Thisleads to expensive field diagnostics and equipmentrepair with the attendant poor reliability imagewhich the product may then acquire in the mindof the users.

It has been reported that CMOS ICs fail in alatent manner owing to gate oxide rupture [20].Gate oxide shorts can change character withnormal use and sometimes partially recover, butsometimes degrade and cause intermittent orpermanent failure [21 J. There has been somesuggestion that latent ESD effects could bebeneficial to device operation [22J as dischargepulses have a 'hardening' effect which reducedsubsequent sensitivity to ESD damage.

10.3.5 Types of ESD test

ESD testing is conducted using two differenttechniques which can be applied directly orindirectly:

}direc t tes tindirect test

Contact discharge method

There was some controversy in the late 1980sabout which was the more suitable type of test,some preferring the reality of an air discharge atthe expense of repeatability, while others felt thata reliable contact test was more important fortesting mass-produced items such as computers,even if it did not simulate all the phenomenaobserved in operational situations. If the questionof the need to simulate the fast subnanosecondprepulse is also considered, it is evident that ESDtesting was an area for lively technical debate,which still continues to some extent even thoughstandards such as IEC 801-2 have been re-issuedin draft to reflect some of the technical concernswhich had been raised.


10.3.5.2 Contact discharge test

To overcome some of the unpredictable aspects ofthe air discharge test, workers [12] have proposedthe introduction of a contact discharge simulatorwhere the EHT is switched to the probe tip(which is held in contact with the EUT surface)via a suitable closing switch. A circuit diagram ofthe contact discharge tester as specified in the

event from the body finger model giving rise tothe slow pulse shown in Figure 10.1 7. This test iswidely used the world over, but the knowledgegained during testing has led to the realisationthat it has some deficiencies [12]. These include

The test did not always simulate EDT failuresobserved in operational useThe severity of the test varied with relativehumidityEquipment could fail at low voltage levels, butpass at higher ones.

The test procedures for discharging to objects nearthe E UT were not well defined, yet this indirecttest is important for items with plastic cases andIi ttle RF shielding.

I t had been shown [23] that the pulses producedby air discharges were heavily dependent on therate and direction of approach of the simulator tipto the EUT. Further, complex corona dischargephenomena have been observed [24] which lead toa reduction of the actual probe tip voltage fromthe expected value by as much as 10kV /s forcorona current of 1 f.1A. Thus during the approachtime of the probe to the EUT the voltage could fallfrom say 20 to around 10kV resulting in anundertest. There are advanced air dischargesimulators which compensate for this effect byinternal tip voltage feedback circuitry [24].

Despite some of these problems· the air dischargemethod is reported [25J as being specified by thefollowing standards in addition to the originalIEC801-2:

EIA PN 1361: The Electronic IndustriesAssociation (USA) produced a draft standard in1981 which focused on voice telephone terminalsrequiring body finger and body metallic modelsto be simulatedECSA: Exchange Carriers Standards Association(USA) adopted IEC801-2 with regard to centraloffice telecommunications equipment in 1984 andrequires both direct and indirect discharge testsNEMA DC33: National Electrical Manufacturers'Association (USA) issued a draft standard in 1982which focused on residential equipment andappliances. It was never formally released butcalled for both direct and indirect air discharges.

DISCHARGEELECTRODE

Rd

150 Q

150pF

CIRCUIT DIAGRAM

Rch

100MQ

16.5 kV Cs

Figure 10.17 ESD probe first edition of IEC801-2(1984)

I

~ 1-----.~ 0.9a:::::>()

UJ

~ 0.5 ....-.....-+-

«I()C/)

is 0.1

DISCHARGE CURRENT WAVEFORM

1~. 50 ± 0.02 _

AIR DISCHARGES

ep8

/___BODY OF GENERATOR

DETAIL OF PROBE TIP

FUNCTIONAL----------11...-. .... EARTH


Figure 10.16 Hand-held air discharge testing

10.3.5.1 Air discharge test

This has been the standard test for a number ofyears and relies on the discharge taking place inthe air between the tip of the simulator probe andthe surface of the EUT as shown in Figure 10.16.The design of the ESD simulator circuit andshape of probe tip, as defined for example inIEC80 1-2 (1984) attempts to replicate the ESD


CURRENT I

NEW DISCHARGE WAVEFORM

In addition to the second version of IEC80 1-2, anumber of other standards call for a contactdischarge test and some also permit air dischargeswhere appropriate, as the contact method is notsuitable for injection onto the joints and seams inthe plastic case of an EUT.

ECMA TR40: The European ComputerManufacturers' Association published a report in1987 which changed the approach from airdischarge to contact discharge ESD testing. It stillpermits air discharge tests onto plastics cases near toconducting parts. The contact discharge test is alsopermitted using a metal foil which is connected tothe close-by metal part/case.ANSI: The American National StandardsInstitute committee ASC C63.4 ScI began workin 1985 to define a standard for testing productsand adopted the contact discharge methodalongside the air discharge test. The method usedis at the discretion of the tester [25]. Both directand indirect tests are stipulated and the bodymetallic and mobile furnishing models are used.Draft 5 (1989) of the standard has been producedand is directed to all electronic equipment.SAE AIR 1499: The Society of AutomotiveEngineers (USA) has produced several ESD specifications over the years, with this one being thelatest. It calls for either air discharge or contactdischarge and both body metallic and mobilefurnishing models are used. The SAE have alsomade a proposal to ISO (International StandardsOrganisation) calling for air discharge testingusing a body finger model to be applied toautomobile subsystems and components.

time60ns

Tip is in contact with EUTSUrl~ _

---I===J------,r----e==t---+~ -Iof---''I/. FACEOFEUT

D~~~~~GE II

10%

very fast

risetime --- fr

I at 60 ns .... - .....,..--

I peak: 100%

90%

I at 30 ns

-f-7~~~:SHARP POINT

FOR CONTACT DISCHARGES

DISCHARGE TIP OF NEW GENERATOR

DISCHARGE RETURNCONNECTION

CIRCUIT DIAGRAM OF NEW GENERATOR

Figure 10.18 Details oj new IEC801-2 ESD test

Reproduced by permission oCBSI A more complete list of standards relating toESD can be found in Chapter 2 and Appendix 1.9.

second version of IEC80 1-2 is shown in Figure10.18.

The most appropriate reliable fast risetimeswitch was found to be a vacuum relay. Theprobe tip is now pointed rather than rounded toenable contact to be made through paint to themetal surface of the E UT. The revised specification also stipulates a more complex waveformwhich has a fast event before the main dischargewhich can be seen in Figure 10.18. I t has beenreported [25] that the fast-edge events occurunder operational conditions only 3-5 % of thenumber of actual events. However, Wood [26Jsuggests that although the .fast spike may beinfrequent and contains little energy it is 'largelyresponsible for voltage failures and the corruptionof IT equipment'. The slow pulse which followsthe spike contains most of the energy and isconsidered to cause current failures and lead tocomponent damage.

10.3.5.3 Indirect ESD tests

In these tests either an air or contact discharge ismade to a conducting plane in the vicinity of theEUT. This plane can be vertical or horizontaland in several positions around the EUT; seeFigure 10.19. When the indirect discharge occursto the metallic plane the fast-varying currentsthat are set up on the plane produce electromagnetic radiation which .couples to the EUT and isdeemed to be important in generating faults in it[27]. The EUT is almost certainly in the nearfield of the radiation for most of the frequencieswhich make up the radiated pulse. Thus thecoupling to the EUT is very complex but involvesmagnetic and electric induction fields andradiative coupling. The repeatability of indirecttests has been investigated [27J and it has beenfound that there are three test parameters whichaffect the outcome. These are


Table 10.2 IEC801-2 (second draft)

10.3.7 ESD test voltage levels

approval' test it is important that no sensitiveareas are overlooked, otherwise there may beproblems in the field. I t is estimated that between3,000 to 5,000 pulses need to be applied to a unitsuch as a desktop computer to ensure all aspectsof ESD susceptibility have been covered. Thequality control verification testing can be carriedou t wi th perhaps as few as 200 to 500 pulses.

kV248

15Special

Test voltageair discharge

kV2468

Special

Test severity levels

Test voltagecantact discharge

1234

x(1)

Level

These will be contained in the applicable specification relating to the type of product being testedand the country into which it is being sold. TheIEC80 1-2 ESD standard is widely usedthroughout the world and the levels specified inthe new draft are given in Table 10.2. Theselection of a severity level for a test depends onthe type of materials and environmentalconditions that are to be found surrounding theEUT in normal operational use. The IECrecommended ESD levels are to be assessed fromTable 10.3.The four severity levels from the lEe 801-2(1984) standard are carried forward and a new

Figure 10.19 Indirect ESD test for table-top equipment

• The location of the discharge on the flat plate;discharging on the edges of the plate ratherthan to the centre, makes a factor of twodifference in the voltage at which computerEUTs are prone to fail.

• The position of the simulator ground returnwire; this return wire appears to be asignificant radiator and its routing thereforechanges the field around the EUT. I ts effect isto alter the disruption or failure threshold bythe equivalent of a few kV.

• The location of the radiating plate groundwire; this has been shown to have a smalleffect on the disruption levels of an EU1-' andis less significant than the two former ones.

TYPICAL POSITION FOR DISCHARGETO VERTICAL GROUND PLANE ( VGP )


\.

ESD test voltage and environmental conditions

Table 10.3 Selection of test severity levels (IEC801-21984)

Test severity levels are selected in accordance with themost realistic installation and environmental conditions.For other materials, for example, wood, concrete,ceramic, vinyl and metal, the probable level is notgreater than class 2.

10.3.6 Number of discharges per test

Jones [12J suggests in line with the revisedIEC80 1-2 that at least ten discharges (fivepositive, five negative) should be applied to eachtest point selected. Others [25J indicate thatmany manufacturers/test houses use about 50discharges per unit area. This may be defined asthe side of the case for example, or as dischargesper square metre. Other testers may use as manyas 10,000 pulses to test an EUT to make a validstatistical analysis of its ESD withstand capability[28J taking into account aspects such as EUTcycle time.

I t is necessary to use more pulses to perform anengineering characterisation of a product than toperform quality control sampling. When theengineering team carry out the ESD 'type

Class Relative Antistatic Synthetichumidityas low as

%

1 35 x2 10 x3 50 x4 10 x

Maximumvoltage

kV248

15


x-level in Table 10.2 is permitted, which can beagreed between the manufacturer and user [12].For air discharges, the levels remain the same inthe revised standard. However, because thecon tact discharge test has been shown to bemore s'evere at the higher voltage levels, thevoltages differ at the two highest levels. Contactand air discharges are compIementary, notalternative, and should be applied as follows.

Contact discharges should be applied to areasof the EDT which are accessible to anoperator in normal use and customermaintenance.Air discharges should be applied to case seams,slots, air vents and keypads, etc. where thedischarge could penetrate to internalconducting components and which cannot beadequately tested in the contact dischargemode.

10.3.8 Assessing EDT performance

There are four categories of EDT performancethat need to be monitored during an ESD test:

(i) Normal operation within specified limits(ii) Temporary degradation/loss of function

which is self recoverable, known astransienterrors

(iii) Temporary degradation/loss of functionwhich requires operator correction(correctable errors) or system reset (noncorrectable errors)

(iv) Degradation/loss of function which is notrecoverable due to component or softwaredamage, or loss of data, known as harderrors.

The detailed statement of pass/fail criteria basedon the voltages at which the above performancelevels are recorded will normally be contained inthe test plan approved for the product in question.

Dash [11] suggests some acceptance criteria forcomputer-like products, given in Table 10.4.

ESD testing is now accepted as an integral partof the wider EMC testing field. I ts importance is

likely to grow rather than diminish, as experienceto date suggests that there may be other morecomplex aspects to ESD discharges and theirinteraction with equipments, which may berevealed as oscilloscopes and waveform digi tisersbecome faster and permit closer inspection of thephenomenon.

10.4 Nuclear electromagnetic pulse

Many readers involved with the electromagneticcompatibility of commercial electronics mayhave no direct need to understand the effect ofthe radio freq uency pulse prod uced by a nuclearweapon on their equipment. I t is suggestedhowever that this section con tains a goodexample of a transient RF pulse and how itinteracts with equipment. Many of the generalpoints made earlier in the chapter aboutconvolution and equipment impulse response areillustrated.

10.4.1 Introduction

Enrico Fermi suggested that electromagneticeffects would be observed in the first nuclearexplosion in 1945 [29]. In 1954 Garwin at LosAlamos estimated the parameters of the pulse thatwould be radiated by an asymmetric gammasource in the exponential growth phase of theNEMP signal. In 1956 other workers in the USAstudied the possibility of using the EMP pulsefrom a nuclear weapon to detonate magneticmines. The first serious attempt to understand theimpact of NEMP on possible target equipmentsgot underway with the Minuteman missileprogramme in 1960.

The electromagnetic pulse is only one of anumber of effects produced by a nuclearweapon. In order of promptness following thedetonation, the others are gamma, optical andx-ray pulses, EMP, neutrons, thermal effects,blast/overpressure, dust, debris and fallout.Details of weapon effects are of course stillclassified, but readers involved in this field are

Table 10.4 Suggested tolerablepercentage errors for different test charge voltages

Test voltage Transienterrors Soft errors Hard errors

Correctable Non-correctable

kV DID DID DID DID

5 0 0 0 010 50 5 0 015 100 15 5 0.20 100 100 100 0


referred to courses such as those run by RM CS[30]. In this text it is intended only to acquaintthe reader with a broad view of issues relating tothe NEMP effect, methods of simulating it, andbasic test methods which are used alongsidethose specified for EMC in documents such asMIL STD 461BjC.

10.4.2 Types ofNEMP

The nuclear electromagnetic pulses to whichelectronic equipments may be subjected areproduced by several mechanisms, of which thegamma-ray mechanism is perhaps the mostimportant [29]. The EMP generation mechanismand the resulting pulse depend on the height ofthe weapon burst above the earth.

Surface burst: Occurs at an altitude of less than2 km and produces an EM field similar to that oflightning [31]. The prompt effects, however,dominate at this range.Air burst: Occurs at heights of 2-20 km and issometimes called an endoatmospheric detonation.Both the prompt weapon effects and an EMP areobserved.lligh-altitude EMP (HEMP): Sometimes referredto as an exoatmospheric detonation which occursat heights in excess of 40 km and produces themost general EMP effects over a large area.

For space-based electronic systems the promptradiation effects can produce an internallygenerated EMP known as SGEMP (systemgenerated EMP) by direct interaction withstructure and components.

What follows concentrates on the exoatmospheric EMP effect because of its potential impacton all types of civil and military electronicequipment over a wide geographical area.

Rudrauf [32J reports that if a 1-5 megaton devicewere detonated at an altitude of 300 km above theBa7J of Biscay the resulting EMP would contain10 1joules and would produce a pulsed fieldstrength of up to 50 kV jm over almost all ofWestern Europe. Although the pulse would lastonly a few hundred nano seconds the instantaneouspower would be some 500,000 million megawatts.

10.4.3 Exoatmospheric pulse generation

This has been widely described [29, 30, 32-34J andis treated very briefly here. The gamma pulse fromthe detonation will build up with a risetime of afew nanoseconds and propagate in all directions.That portion of the flux travelling towards theearth will interact with the upper layers of theatmosphere as shown in Figure 10.20. The gammarays produce a flux of Compton recoil electrons\,yhich constitute an electric current density with arisetime of the same order as the gamma pulse.The time-varying current density then gives rise tothe radiated EMP.

Each Compton electron receives about 1 MeV ofenergy from the gamma interaction and canproduce about 30,000 electron ion pairs along itstrack in the air. These secondary electrons makeno appreciable contribution to the EMP drivingcurrent but they make for electrical conductivi tywhich limits the amplitude of the EMP andinfluences the waveform [29].

Compton electrons deflectedby Earth's magnetic field

UPPERATMOSPHERE~ ~ ~IDEPOSX~ ~~~ Gamma rays interact with neutral

particles in upper atmosphere toRADIATION REGION produce energetic electrons by

:I EARTH "Compton II collision

Gamma ray energyfrom explosion

Figure 10.20 Generation of nuclear exoatmospheric electromagnetic pulse Reproduced by permisssion of leT Inc.


400 500

'0 = 10 MHz

'0 =10 MHz

100 200 300TIME (ns)

(b)

100 200 300 400 500TIME (ns)

(a)

2.0~ 1.6I-

1.2zwex:: 0.80:::::>0 0.4l-S 0U0:: -0.40I- -0.8ex::0 -1.2I 0C/J

320------------.------~ 280wc:> 240~...J 200o> 160l-S 120o0::o 80

Z 40wc.. 0 a...-_........__-'--_---ll--_........__....

o 0

12

10

«~

I- 8zW0::0::::::> 600w0::::> 40~

2

00 2 3 4 5 6

TIME ( in micro seconds)

Reproduced by permission of' Wiley

Figure 10.23 Voltage and current induced into7.5 m-Iong monopole e.g. antenna tower orstreet lamp, by NE.MP stimulus(50 kVj m) (a) Open-circuit voltage for7.5-m monopole (b) Short-circuit currentfor 7.5-m monopole

Reproduced by permission of' ICT Inc.

Figure 10.22 Example of currents induced into longoverhead power lines by exoatmosphericNEMP

~1MHz~50MHz1GHz

FREQUENCY~10ns

TIME

.........ill

~50 - 100 kV 1m

~ --~:r:IC)Zilla:ICf)

o..Jillu:

10.4.4 NEMP induced currents

(aj (b)

The exoatmospheric pulse shown in Figure 10.21can induce enormous currents into long wireantennas such as overground power lines andrepresents a major threat to electrical generation,distribution and control equipment. An estimateof the induced current in such a power line isgiven by Ghose [34J and shown in Figure 10.22.Currents induced into the structures of systemssuch as aircraft, missiles, radio towers and othersimple forms which can be represented byelectrical dipoles or monopoles with a reasonableQ, (of around 5-10) are usually found to have thecharacteristics of a damped sinusoid.

Results of the calculation of the induced opencircuit voltage and short-circuit current in anexample monopole [33J (7.5 m high and resonantat 10 MHz) are shown in Figures 10.23a and band illustrate the general damped sinusoidresponse. Other calculations and measurements[35-37J have been made to explore the inducedcurrents in cylinders, wires inside leaky cylindersand surface currents on other metallic bodies ofrevolution. All indicate that the usual inducedcurrent waveforms are bidirectional damped

Reproduced by permission of' ICT Inc.

The waveform which propagates to the surfaceof the earth is roughly represented by a doubleexponential pulse [34J of the type illustrated inFigure 10.21, with a characteristic risetime andfalltime, and a peak amplitude of about 50 kVjm. The spectrum can be obtained by Fouriertransform of the pulse waveform and is alsoillustrated in Figure 10.21 showing that thernajority of the energy in the pulse is confinedto frequencies below a few MHz [30J. I t ispossible that in future new phenomena mayemerge which lead to pulses with enhancedhigher freq uency components. Additional pulseshapes and peak amplitudes derived from theother types of NEMP generation are given byRickets et al. [33].

Figure 10.21 NEMP exoatmospheric pulse (a) pulsewaveform (b) pulse spectrum


10Hz

NOMINAL F-111AIRCRAFT/FREQUENCYRESPONSE

/PULSE SPECTRUMOF HPD SIMULATOR

(AFWLUSA)10-2 '-- ---L. ----IIL-L.- .....

1MHz

wo:::>.....:Ja...~« 10-1

LU>~..JLUex:

10.4.5 NEMP testing

NEMP testing can be divided into component,equipment and system testing; each, type of testrequires different stimulation equipment anddifferent instrumentation. The order just givenreflects the complexity of the test and the costwhich can increase almost exponentially from a

10 MHz 100 MHz

FREQUENCY

Figure 10.25 NEMP simulator pulse spectrum andfrequency response of F111 aircraft

form would be preserved. The Fourier transformof the true current density waveform is shown inFigure 10.24b. This induced current densityspectrum shows a resonant peak at about 6 MHzwhich corresponds to the main ringing in thecurrent waveform and is related to the electricallength of the aircraft fuselage.

The tests on the F III were carried out by Kunzand Lee [39J in the USA using a horizontal dipolesimulator at the US Airforce WeaponsLaboratory, which attempts to simulate theNEMP pulse. Its idealised frequency spectrum isgiven in Figure 10.25. After recording manymeasurements of the damped sinusoid type atvarious locations on the aircraft a picture of itsmain resonant properties could be built up andthe envelope of many spectra such as that inFigure 10.24b was constructed, and is shown inFigure 10.25. This inverted-V curve is anexample of the general coupling property ofsystems with resonant, medium Q, metallicstructures and has been confirmed elsewhere [41 J.

All systems that have a frequency response ofthis resonant form will have an impulse responsein the form of a damped oscillatory wave. This isexemplified in Figure 10.26 which shows theinduced current in the skin of a missile [40J,which is a high-Q, high-aspect-ratio metallic tubeand behaves like a fat electrical dipole. I t isbecause so many real induced current waveformsare damped oscillations that the current injectionNEMP tests called up in standards such as MILSTD 462 CSI0/ll specify damped sinusoidwaveforms.

Measured

400 600

TIME (ns)

(aj

200o

5...-------r-----r----,..---,-----,

4

3

2

1O~_I_-I-___f-""-~~r---~~ .......r__~

-1

-2

-3

-4

-5

I2Wa::a:::::>Oq>2052~CfJ_LL (J)

OW E>-~~> <il: J

Wow::Et=

(bj

}-'igure 10.24 Examples of NEMP induction in aircraft(aj Induced current waveform( bj Induced current spectrum

sinusoids produced in response to the unidirectional NEMP incident pulse.

Damped-sinusoid induced-current responseshave been measured and calculated for completesystems such as fighter aircraft [38, 39J andmissiles [40]. Figure 10.24a shows the inducedcurrent density parallel to the fuselage of an FIllaircraft, measured on the underside just in frontof the wings [38J. The plot is actually the timederivative of the current density and needs to beintegrated over time to yield the actual currentwaveform.

I t is common practice to record and presenttime-derivative data as these are what is actuallymeasured by the special wideband Band D dotsensors fitted to the EUT. The time derivativeexaggerates the higher frequency components inthe actual signal. If the necessary integration iscarried out at the sensor, problems of signal breakthrough, dynamic range and signal-to-noise ratiomay be encountered. The integration of the timedifferential signal can be better carried outdigitally in the microprocessor of a digitising oscillo'scope at the recording site.

If the current density differential record given inFigure 10.24a were integrated to yield the truecurrent density waveform, the fast variationswould be suppressed, but its damped oscillatory

I2~ 1000a:::::>o252CfJ E1--

~ ~ 100a:: <OJa:: 50«@ 30

o 20 ........------------I"-----..JIL..-~..L-:-'~5 1 10 20~ FREQUENCY MHz


Reproduced by permission of' BAe Dynamics Ltd.

incident pulse can be measured by instrumentswhich are either attached directly to thecomponent in a noninvasive way, or connectedexternal to the jig. The jig design may be based ontransmission-line techniques, the RF properties ofwhich are well understood, so that the pulsewaveform delivered to the component is notdistorted, and that the absorbed, reflected powerand component dynamic impedance can bedetermined by measurement external to the jig.

Many types of active and passive componentshave been measured over the years and a sounddatabase exists in various government andindustrial establishments. Component tests resometimes conducted with a simple squaretopped 1 IlS pulse [34] or a damped sinusoid.Rickets [33] gives examples of upset levels foractive components and damage levels for activeand passive components in terms of watts for a1 IlS pulse, see Figure 10.27. Much useful data onin tegra ted circuit disruption and damage due toRF energy are contained in a report byMcDonnell Douglas [42]. Figure 10.28,reproduced from their report, shows the peakpower needed for burnout of a typical bipolaroperational amplifier integrated circuit as afunction of pulse duration. Figure 10.29 gives themeasured pulse energies [34] required to failvarious typical electronic components. Noticethat the most sensitive devices require less than1 IlJ to destroy them.

400

Ringing frequency is characteristic/Of object length

o

.30Resonant frequency is located where there is

--Llimited energy in NEMP spectrum (~30 MHz ),.20 so induced current is low.

.20

«IZWa:~ .10uz~ 0owu5 .10~

10.4.5.1 Component testing

This is carried out by a direct injection method ontothe component terminals when mounted in asuitable jig. The jig must accommodate thecomponent in such a way that the true RF power!energy absorbed by the component from the

100 200 300TIME! NANO SECONDS

Figure 10.26 Example of current induced in short objectsuch as missile by NEMP

few hundred pounds/dollars for a component testto many millions for tests on large complexsystems such as an aircraft or ship.

Figure 10.27 Thresholds fortypical active and passiveelectronic components

WATTS

TRANSFORMERSINDUCTORS

WATTS

100,000"""-

-106

WIRE WOUNDRESISTORS

10,000 '-

1-105

POWERSCRsPOWER DIODES.

CARBONRESISTORS

1,000 ~

1-104

HIGH POWER TRANSISTORSZENER DIODES

MEDIUM POWER '-103

TRANSISTORSJ FETs

H. V. RECTIFIERS

LOW POWER TRANSISTORS '-102

SIGNAL DIODESLOW POWER

SWITCHING DIODES --10

TTL LOGICLINEAR ICsMaS LOGIC -1

PAPER / POLYESTERCAPACITORS

FILM RESISTORSCERAMIC-MYLARCAPACITORS

TANTALUMCAPACITORS

100 i-

10--

1.0""-

0.1--- LINEAR les

TTL LOGICDTL LOGIC

0.01-i-

MICROWAVE MIXER DIODES -0.1 0.001 MOS LOGIC-

Reproduced by permission of'Wiley

DAMAGE THRESHOLD WATTS (1Jl S PULSE) DISRUPTION THRESHOLDWATTS (1Jl S PULSE)


Reproduced by permission of leT Inc.

Figure 10.29 Examples ofpulse energy required todamage electronic components

0.01 .a.-__...I.-..__...&...-__--'--__--4

0.001 0.01 0.1 1 10 100

PULSE DURATION mSEC

Figure 10.28 Estimate ofpeak power required to burnout bipolar op. amp. Ie as function ofpulse duration.

These NEMP equipment tests are described brieflyto illustrate a few of the more important testrequirements and instrumentation which shouldbe used.

CS 10: Conducted susceptibility, dampedsinusoidal transients, pins and terminals (pininjection), 10 kHz-I00 MHz (MIL STD 462notice 5 Navy).CS 11: Conducted susceptibility, dampedsinusoidal transients, cables, 10 kHz-l 00 MHz(MIL STD 462 notice 5 Navy).CSI2: Conducted susceptibility, common-modecable current pulse, interconnecting and power(MIL STD 462 notice 6 USAF).CS 13: Conducted susceptibility, single WIrecoupled pulse (MIL STD 462 notice 6 USAF).RS05: Radiated susceptibility, electromagneticpulse field, transient (MIL STD 462 notice 5 Navy).

Specifications for NEMP testing usually onlyapply to military equipments, but therequirement may be extended to cover certainitems of commercial electronics such as telephoneand telecommunications systems, power conditioning and distribution and computer systems. Inthe UK, DEF STAN 00-35 describes all environments which are relevant to military equipments,including natural ones such as lightning and alsoincludes a section on NEMP.

Design advice is available in DEF STAN 084with regard to techniques for hardeningequipment against NEMP. Testing requirementsfor equipments are exemplified in two technicalstandards which are DEF STAN 59-41 in the UKand MIL STD 461Cj462j463 in the USA.Probably MIL STD 461 Cj462 represents the bestexample of a set of NEMP related equipment testswhich are embodied within a wider framework ofgeneral EMC testing. It is for this reason that theNEMP tests called up in this standard areexamined more closely.

The structure of MIL STD 461 C is complicatedand different sections relate to tests on differentclasses of equipment which are procured for useby the Army, Navy and Air Force. It is notintended here to define the exact applicability ofthe various NEMP tests but rather to indicate thetypes of tests a'fld their limitations when appliedto equipments. The NEMP-related tests in MILSTD 461C consist of two types. The conductedsusceptibility tests CS 10 to CS 13 involve theinjection of damped-sinusoid waveforms into pins,wires and cables. The radiated susceptibility testRS05 requires equipment and interconnect cablesto be subjected to a unidirectional doubleexponential simulation of the free-field weaponpulse.

104

NO BURN OUT

PULSE ENERGY Jl J101 102

100

(fJ 10

~~0:W~oa..~

l1i 0.1a..

______ POINT CONTACT DIODES

-- BI POLAR OP. AMP. ICs

r-OW POWERTRANSISTORS--------

HIGH POWER TRANSISTORS

SWITCHING DIODES -

ZENER DIODES

RECTIFIER DIODES --

RELAY CONTACTS ----Itw CARBON RESISTORS -

10.4.5.2 Equipment testing

Some equipments may be installed in the'final systemsuch that the only NEMP induced energy to whichthey are subjected comes along cables or otherconducting pipes or hoses. In this case currentinjection using damped sinusoids is the appropriatetechnique. Other equipments may be exposed toboth current injection via cables and directly to theNEMP wavefield. In this case both a dampedsinusoid current injection test and a radiated testusing a simulated NEMP pulse are both needed.

Much has been written on designing equipmentto withstand NEMP attack and the data oncomponent hardness are important in this context.Many protective measures can be taken [33, 34] toreduce the ingress of pulse energy to that whichcomponents can survive. These include screeningcables, shielding equipment, fitting transientclipping devices and filters. The first real test ofmany of these interconnected design featuresoccurs at the equipment or box level.

Reproduced by permission of McDonnell Douglas


GROUND PLANE BENCH

DAMPED SINUSOIDGENERATOR

Figure 10.32 MIL STD 461 CS-11 NEMP inducedcable current injection test setup

ISOLATIONTRANSFORMERTEST SAMPLE

____LISNS

TEST INSTRUMENT or- TERMINATION BOX

\ALTERNATIVE COMMON-------

MODE FILTER

~DAMPED SINUSOIDGENERATOR COUPLING DEVICE

~GROUNO PLANE BENCH

Figure 10.30 Test configuration,lor US MIL STD461 CS-10 NEMP pin injection test

10.4.5.3 System testing

Testing large systems for hardness against NEMP(or for EMC) is a difficult and expensiveundertaking. I t is only carried out on key systemswhich must be shown to be able to survive thehigh-level electromagnetic pulse. Such systemsmight include long-range bomber aircraft, nucleararmed missiles, fighter-bomber aircraft, ships,command and control centres, mobile tacticalcommand and control facilities, telecolnmunications facilities, and key power generation and transmission facilities. There are two key interrelatedproblems with testing systems such as these:

dealt with in different ways depending on thenature of the cable. The specification should beconsulted for details. As with CS 10 the injectionsignal generator is calibrated prior to use to setthe maximum test levels.CS 12 and CS 13: The test configurations andinjected waveforms are similar to those for CS 10and CS 11. The peak current as a function offreq uency is shown in Figure 10.31. The pulses inCS 12 are applied at the rate of 1 per second for aperiod of 5 minutes, after which the polarity isreversed and the test repeated. In the case ofCS 13 the pulses are applied at one pulse persecond for not less than 1 minute and a minimumof 50 pulses must be applied for each polarity.RS05: The EDT is set up in a 100 ohm parallelplate line as shown in Figure 10.33. The lineshould have sufficient separation between theplates such that the EDT occupies less than halfof the available height. The EDT shall have afootprint which is smaller than the useable areawhich has been previously mapped out during acalibration exercise. The field strength as afunction of time is shown in Figure 10.21 andsimulates the standard exoatmospheric pulse. TheEDT is exposed to a minimum of ten pulses ineach of three orientations at pulse repetition ratesof between one per second and one per minute.

100

CS10CS11/

0.63 MHz

\10

0,01 '--__--L..__---L ""--__-J

0.01

0.160.1

(j)a..~«Jz

~~x ::::> 0::« ~::::>~ X'O

...-I « w~Cl

o~

zo~~oo

0.1 1 10

FREQUENCY ( MHz)

Figure 10.31 MIL STD 461 .NEMP current injectionlimits

CS 10: The test setup is shown in Figure 10.30. Thepin/terminal under test is accessed via a breakoutbox using a short « 25 cm) low-resistance wireand the damped-sinusoid current is indirectlyinjected into it via an inductive (ringingfrequencies < 10 MHz) and capacitive (ringingfreq uencies > 10 MHz) probe. Alternative directcoupling techniques can be used with theagreement of the EDT procurement authority.l'he test waveform is that shown in Figure 10.1 Oband the value of the peak current at variousfrequencies is taken from Figure 10.31. The testshould be conducted at frequencies of 0.01, 0.1, 1,10, 30 and 100 MHz and at other EDT criticalfrequencies. Ten pulses of both positive andnegative polarity should be injected at a rate ofbetween 1 per n1inute and 1 per second.CS 11: The test configuration is shown in Figure10.32 and the test waveform, maximum current,test frequencies etc. are all similar to CS 1O. Thescreens around cables which are to be tested are

100


PARALLEL PLATE LINE

~~,.-_-_-_-_-_-_-_-_A_====::=::~--r jlOn view)

/LOAD

TRANSIENT PULSEGENERATOR

/ '

FRONT IUSABLE TEST VOLUME

POWER LlNE-

-CABLE (in conduit)SHIELDED ENCLOSURE:---------,

TEST SAMPLEINSTRUMENTATION

POWERSOURCE

Figure 10.33 Test setup for MIL STD 461 RS05 NEMP radiated susceptibility test

(i) They are large 20-300 m (or greater) in extent.Some systems such as communications centresare fixed and must be tested in situ.

(ii) Simulating the correct NEMP waveform atthe high field strength required over such alarge target system. This calls for thegeneration and precise shaping of a veryhigh-voltage pulse (up to 5 MV) that isapplied to the large radiating elements ofthe simulator.

The technical and cost constraints which flow fromthese two problems mean that it is not possible tobuild a single, large all-purpose facility which cantest a whole inventory of systems. It hastranspired that individual simula tors have beenbuilt to test specific types of systems. NEMPsimulators fall into two technical types:

(a) Bounded-wave devices, where the energy isconstrained somewhat in the simulatorstructure and, around the system under test.

(b) Free-field simulators, where the wave isgenerated and allowed to propagate freelyinto free space. With such a simulator it ispossible to test a system such as an aircraftin any orientation or even in flight (atsubthreat levels).

In general, the field strengths produced in thebounded-wave devices can be at full threat level(50 kV1m), whereas the larger volumes covered byfree-field simula tors necessitate a reduction in

maximum field strength except close In to theradiating elements.

10.4.5.4 Bounded-wave NEMP simulators

These may be of the following types:

• Parallel transmission lines made from anumberof closely spaced wires with a tapered sectionleading to the parallel-line working volumeand than via an opposite taper to a matchedload resistor. See Figure 10.34a for a diagramof the simulator referred to in MIL STD 462RS05 for testing large systems. As an example,Figure 10.34b shows a schematic of the ALECSparallel-wire simulator (95 ohm line) at AirForce Weapons Laboratory, Kirtland AFB,USA, used for testing the B-1 bomber [34].The working volume of this facility is27 X 15 x 13 m cube. The peak voltage acrossthe line is about 1.7 MV, the peak E-field is upto 125 kV 1m and it can fire one pulse perminute. Another example of a parallel-linesimulator (125 ohm line) is ARES, which is aDefense Nuclear Agency facility at KirtlandAFB. It has a large working volume of40 x 33 x 40 m cube and can produce fieldstrengths of up to 100 kV 1m [34].

In the UK, PETS 1 and 2 simulators atAWE Aldermaston are parallel-wire lines andlike ALECS and ARES produce verticallypolarised waves. It is much simpler to build avertically polarised simulator than one which


(b) Reproduced by permission of leT Inc.

TRANSMISSION LINE ANTENNA

WORKING VOLUME

10.4.5.5 Free-field NEMP simulators

ANT-II0 [43] is shown in Figure 10.35b. Thisdevice can be equipped with a pulsed powersource of up to 2.5 MV for full threatsimulation. A facility called HEMP which is adouble- tapered line with the wires forming anelongated diamond in plan view is located atFort Hauchuca in the USA. Each taper is 68 mlong and the intersection is 15 m high and 24 mwide. I t can produce a field strength of around26 kV 1m from a 400 kV pulser. In ,France,Aerospatiale and Thomson CSF havedeveloped a large relocatable offset singletaper wire simulator for testing ground installations such as communications facilities [32].I t can illuminate an area of 40 x 60 m with afield strength of up to 50 kV1m. A number ofother bounded-wave simulators existincluding TEFS and SIEGE in the USA.Details of these and other facilities exist inpublished references [33, 34, 44, 45].Simulators in France such as CYTHARE,SIVA, SIEM 2, PEGASE andCORNEMUSE are listed by Rudrauf[32].

Simulators of this type have been constructedmainly in the USA from horizontal dipoles,resistively loaded dipoles and vertical conicalmonopoles. The TEMPS (transportable EMPsimulator) is an example of a 300 m horizontaldipole device. It can produce about 50 kV/m ata distance of 50 m normal to the centre of thedipole. Large horizontal dipoles have beenconstructed similar to that shown in Figure10.36. These simulators sometimes have a 15 MV switched pulser located at the centre of thedipole, high above the system under test and canprovide near full threat pulses. Two 1000 ft longwire simulators have been built in the USA.Such simulators with up to 100 kV pulsers canradiate a field of about 1 kV 1m at 100 ft from theantenna.

A vertical dipole facility formed from aresistively loaded wire conical monopole above aground plane has been bu:ilt in the USA. It is27 m high and is driven by a 4 MV pulser locatedat the apex of the inverted cone, see Figure 10.37a and b. Finally, one of the largest simulators hasbeen constructed on a floating barge for theNEMP testing of complete ships. The EMPRESSsimulator in the USA is a vertically polariseddipole using a large vvire monocone antenna asshown in Figure 10.38.

There are regular symposia and conferences[46] dealing with all aspects of high-voltage pulsegenerators, NEMP and lightning with specialistsections on simulators.

(aj

SHIELDED ROOM

MARX GENERATOR ---+---~n.

is horizontally polarised, as the lowerconductor can be laid on the ground for thevertical simulator. However, the actualNEMP from a weapon release may containpredominantly horizontal polarisation andthis is more difficult to simulate with facilitiesin close proximity to the ground. Thesimulator and test target must be raised abovethe ground by a significant fraction of awavelength at a few MHz if a horizontallypolarised pulse is not to be distorted byinteraction with the conductive ground. Thelarge TRESTLE simulator at AFWL,Kirkland has two 5 MV pulsers producing ahorizontal E- field from two vertical sets ofwires on a 61 x 61 m wooden platform whichis 36 m above the ground. 1'he cost of suchfacilities is enormous, many millions of dollars,but if NEMP hardening is to be proven atsystem level, such simulators are necessary.

• Tapered transmission lines, ,again constructedfrom wires but with no parallel workingsection. An example is shown in Figure 10.35aof a line referred to in MIL STD 462 fortesting large systems to method RS05. Adrawing of a large 20 m high double-tapersimulator manufactured by Elgal EM-IIOI

------- LOAD/TAPER ----r-95 12.75 m

~L..- ~__--I---.- -&--oh_m~

25 m / t E (t)

PERFORATED 0 H (t)BANK PSU VOLUME METAL BASEPLATE

+

TOPV=--II~MULTIPLE WIRES i

(bj

Figure 10.34 Large system NE'MP test simulators(aj Large parallel-wire line (b j USAALECS parallel-Loire NEMP simulator


System under testsituated under the wires

(aj

~.

//~TANKSAND OTHER VEHICLES

(bj

Figure 10.35 Large NEMP simulators (aj Double-taper NEMP simulator (bj Elgal ANT-110 double-taperNEMP simulator

(b) Reprod uced by permission or Elgal

Figure 10.36 Example of horizontal dipole NEMPsimulator

Reprod uced by permission or Eigal

10.5 Lightning itnpulses

10.5.1 Lightning environment

The physics and electrical engineering associatedwith the study of lightning and its impact onelectrical equipment is a distinct technical field,with many workers contributing to the currentstate of knowledge. In the space available here itis only possible to introduce the reader to thesubject in ,the context of EMC testing electronicequipment to withstand the large impulsiveelectrical transients which lightning creates.

This natural electrical phenomenon has beenstudied scientifically since Benjamin Franklin,who remarked in 1752 that 'the clouds of athunder-gust are most commonly in a negativestate of electricity, but sometimes are in a positivestate'. The details of how the charge separation


(bjFigure 10.37 (a) Conical monopole above ground plane

(b) Vertical dipole NEMP simulator

mechanisms operate and the complex microstructure of lightning discharge formation are stillbeing studied and argued over [47J .

While the debate concerning the physics of the

== 20 kA== up to 150 kA

== 40 J1s== 20-200 J1S

== 2 J1S== 1-10 J1s

Other data .may be obtained from References 48and 49.

Risetimeaverage risetimevalue range

Durationaverage durationvalue range

Peak curren taverage valuevalue range

EM radiationfrequency ofmaximum radiation == 10 kHz

Occurrencestorms worldwide atany instant == 2000daily occurrence == 5000

phenomenon has continued, it has been necessaryto collect actual data on the nature of lightningstrokes and their frequency of occurrence aroundthe world such that protection against typicalstrikes can be engineered into equipments. Thesedata have been analysed statistically to yield anominal description of the lightning dischargeevent which has been used as the basis for anumber of standards and specifications. One suchis DEF STAN 00-35 which gives the standarddescription of the discharge as:

10.5.2 Defining the dischargeAerospace vehicles may experience different directdischarges from ground equipment and this isreflected in the standards which each type ofequipment must meet. Figure 10.39 shows thestandard double exponential form for the transient

27m

-------39m------

(ajPULSER ( 1 - 7 MV )

WIRE

/CONICAL WIRE ANTENNA

( Z = 60 to 75 Q )

(b) Reproduced by permission of ICT Inc.

Figure 10.38 7MVEMPRESS verticalpolarised 'dipole' VPD forwhole-ship testing.Manufactured by MaxwellLabs, USA

Reprod uced by permission or Maxwell Labs.


Reproduced by permission of BAe Military Aircraft Ltd.

3.5 kA

100 kA\

7kA/

O : 0 400 A\ :~ : ~: (}----·u I

a I a I Q=35C: Q= 120C IQ=5CI Q= 10C : TIME (ms)I I I I I I

2J..ls 100J..ls 5ms 58ms 355ms 380ms

«~

IZWa:a:::>o

Figure 10.42 Model of lightning strike waveformshowing peak current, duration and charge

in the SAE AE 4L standard, and shown in Figure10.41 encompasses the parameters of mostlightning strikes, and is divided into the fourcomponents A to D.

Statistical studies of a great number of lightningstrikes have been carried out by NASA [53Jincluding both positive and negative discharges.This has led to a model being defined whichaccounts for 98% of lightning strikes. Theidealised curren t profile is similar to that in theSAE standard and is shown in Figure 10.42 [54].It gives the current at various times during thepulse along with the charge transferred duringthese times. Details of current lightning standardsapplicable to a range of equipments and situationsmay be found by consulting

200 kA./

Figure 10.41 Lightning test waveform (SAE-AE4L).A: initial stroke, peakamplitude == 200 kA ± 10%; actionintegral == 2 x 10 6 A2 s ± 20%.B: intermediate current, maximum chargetransfer == 10 coulombs, averageamplitude == 2 kA ± 10%. C): continuingcurrent, charge transfer == 200 coulombs± 20 %, amplitude == 200 - 800 A.D: restrike, peak amplitude == 100 kA± 10%, action integral == 0.25 x 10 6 A2 s± 20 %, duration == < 500 /lS

....Zwa:a::::>()

TIME1< 5 X 10-3 sec I ( not to scale)

2 X 10-6 sec 0.25 sec < T < 1 sec

TIME

DURATION

~.:¥..Zill

~ 10::::>o

IZUJ0:0::::::>()

SUBSEQUENTRETURN

FIRST RETURN STROKE STROKES

STEP-WISECONTINUING CURRENT DART LEADER

Figure 10.40 Pictorial representation of the componentsin lightning flash

20""-----11

Q)

"0c.>en~,g:eo

Figure 10.39 Parameters of typical lightning pulse whenit strikes Earth

current pulse observed in a ground strike withaverage risetimes and fall times and peak currentssuggested in DEF STAN 00-35, and cited by Keiser[50J. A 2 J.1S risetime and 50 J.1S falltime doubleexponential pulse with a worst-case peak current of200 kA has been specified in an early MIL-B-5087B[51 J for aerospace systems and a pulse with similarrate of rise of current (100 kAj J.1s) is suggested fortesting aerospace equipment by SAE [52].

The discharge in a natural lightning event hasbeen shown to be a complex phenomenon givingrise to a number of components as shown inFigure 10.40. The whole event may last for asecond or so and is commonly perceived as thelightning flash. The testing waveform stipulated


r--VLF ---+·1.......--HF -----I,,~I-·-VHF - FREQUENCY

10.5.3 Effects on equipment

conductors, magnetic forces leading to hightransient pressures, and fuel ignition. The indirecteffects include high transient magnetic field, upsetground voltages across the aircraft, unwantedoperation of electrical power trips, and electromagnetic shock excitation of the aircraft structureresulting in oscillatory currents.

This last topic is of particular concern inconnection with the assumed risetime of thelightning stroke as it attaches to a prominent pointon the aircraft. Fast risetimes can occur during theattachment/discharge. For shorter risetimes thanthe normally specified 2 J1S (where 90% of theenergy is below 10kHz [60J), which have beenreported as being down to 200 ns [61 J, thelightning pulse begins to have significant energy atfrequencies which can resonate with the aircraftstructure (5 to 20 MHz) and cause it to ring. Thisproduces a damped sinewave oscillation similar tothat generated by NEMP which can then coupleinto cables and thus into avionics, which increasingly contain large quantities of sensitive digitalintegrated circuits, and can lead to disruption offunction or component burnout. The indirecteffects of lightning on aircraft are covered in RAEtechnical memorandum FS(F)457 which isreferred to by Carter [58].

Revised lightning specifications relating to aircraftsystems and avionics call for testing using a multiplestroke waveform. There is one initial return strokewith a peak current of 200 kA followed by as manyas 23 re-strikes of 50 kA distributed over a period ofup to 2 seconds with spacings between any twopulses of 10 to 200 ms [59]. As with ESD measurements, it would seem that the exploration of theactual physical events using faster detection andrecording equipment reveals faster risetimephenomena, which have previously beenunobserved. The slower risetime oversimplifiedmodels of ESD and lightning which have been usedto test equipments may explain some of the discrepancies which occur between the EUT performancein the test laboratory and that actually experiencedin the operational environment.

FAA AC 20-53AFAA AC 20-136Do 160/ED14B (section 22)MODAVPl18

N

~ NEMP-E LIGHTNING ( 2 / 40 lls )

~ / --7', Fast rising lightningCi5 ~10 kHz ~1MHz' / pulse (200 ns)aJ CLARKE 60 ',. ( PODGORSKI 62 )

o COMMUNICATIONS~ ,~ \~ \~ \~ \~ \

Lightning can influence equipment in two ways.The obvious means is by a direct strike where thelightning channel attaches to the system at someprominent point and leaves by some other pointto complete the discharge path to ground. Effectsmay also be produced in nearby equipmentindirectly by electromagnetic fields radiated bythe rapidly changing voltages and currents in theligh tning channel.

The spectrum of a lightning pulse contains lessenergy at HF and VHF than an NEMP pulse, ascan be seen in Figure 10.43. Most of the energy inthe lightning pulse is confined to VLF. Directstrike and indirect effects can be consideredseparately for two broad categories of equipment:aerospace systems and ground equipments.

There are reported to be a number of newlightning standards under development (1991)[55J which include

AC 20-53BDo 160/ED 14C (sections 22 and 23)EUROCAE:

Lightning test standard (WG31-SG3)Lightning threat definitionLightning zoning standard

SAE Orange BookCLM-R163MIL STD 1757AMIL STD 1795

Figure 10.43 Comparison oj RF spectra oj NEMP andlightning pulses

10.5.3.1 Aerospace systems

Include civil and military aircraft, high altitudemissiles, and space systems transiting theatmosphere. This is a major specialised field in itsown right [56-59J and is not pursued further here,other than to point out some of the issues which areof concern. The direct effects of a ligh ting strike onan aircraft can include: arc burnthrough of highresistance parts, acoustic shock, heating of

10.5.3.2 Ground equipment

Particularly power distribution and telephonesystems, radio communications and generalindustrial, commercial and domestic electronicequipment. Lightning effects on these equipmentsare of more general interest to the majority ofdesigners and test engineers involved with EMC.

Consider the simplified situation shown in Figure10.44 where lightning discharges are taking placeover an industrialised area with power and telecommunications systems feeding two interconnected equipments. These could, for example, be a

ELECTROMAGNET,IC TRANSIENT TESTING 205

10.6 Transients and general powerdisturbances

10.6.2 Examples of power supplyimmunity standards

The CS06 spike test in MIL STD 461/2 has beenadopted as a good test for both military andcommercial equipment.

10.6.1 Importance of power transients

All the disturbances previously mentioned cansometimes occur on power lines as a result of localloading or switching problems rather than fromexternal sources such as lightning. While theundesirable properties of equipments generatingswitch-off spikes and being susceptible toincoming transients are addressed in militarystandards, commercial equipment has in the pasthad to operate in the presence of similar disturbances but without the comprehensive immunityachieved by rigorous design in compliance with aset of standards.

Early studies by IBM (1974) showed that mostpower supply related problems with regard tocomputer equipment were due to the existence oftransients on power cables. Other measurementsby AT&T and the US Navy have deduced thatvol tage sags and dropou ts were the primarycauses of computer disruption. Taken together,this suggests that transient suppression and theprovision of internal stored energy in theequipment are two necessary design features ofmodern digital electronic equipment.

current will be lower. The ground currents willmanifest themselves by developing large potentials,Vg in Figure 10.44, between electrical grounds atvarious places, and this will drive surges down thepower and communication return lines leading todata corruption or damage to interface circuits.

Lightning is considered to be a significant causeof power sags and transients. The peak transientvoltages can reach several kV [50J before arcingthrough the insulation of commercial or domesticpower wiring at levels of around 10kV [60].Gerke [64J asserts that 'in too many cases, powerdisturbances (to electronic equipment) areoverlooked or ignored by the manufacturer in thehope that someone else the user or the powercompany will solve the problem (of transientfailures of their equipment)'. However, not all thepower line transients or other effects are caused bylightning. A resume of these other effects is alsogiven by Gerke and includes power sags, surges,outages, harmonic distortion, frequency deviation,superimposed transients and wideband noise.

POWER GENERATOR

--Vg_..··.....

NJ:......« 10-1I:-ZWa:: 10-2a::::>0

10-3

1

/~L.v. P~ER L1NE~LARGECURRENTSWI~

GROUND VOLTAGE FLOW INTO ~DIFFERENTIAL DUE TO THE GROUND ~HEAVY CURRENT FLOW > lime

SURGES, TRANSIENTS ANDDROPOUTS DUE TO VARIOUSFORMS OF LIGHTNING STRIKE

Figure 10.44 Types of lightning discharges leading todirect and induced effects on equipment

10 r--.,...----r---r----r--------

10 100 1k 1Ok 100k 1M 1OM

FREQUENCY (Hertz)

Figure 10.45 Typical lightning spectral density

couple of desktop computers connected by amodem link via a telephone line. Cloud-to-clouddischarges result in a radiated electromagneticpulse which can couple into power cables andresult in a power line transient. A direct strikefrom a cloud to the equipment is rare [60J and ifthe building in which it is situated is protectedaccording to the advice contained in BS6651 thelikelihood of a direct strike is even more remote.

Discharges from cloud to ground that do notstrike an equipment directly are obviously morecommon than those that do. They can produceradiated electromagnetic field strengths of 6 kV 1mat a distance of 1 km from the lightning channel[62]. The measured spectrum of frequenciesproduced by a typical discharge can be seen inFigure 10.45 [63J. Note that this confirms thatlightning contains less high-frequency energy thana NEMP transient.

The peak lightning channel current is limited bythe ground resistance in the particular geographical area where the strike occurs. In lowresistance coastal marsh areas the current may begreater than 30 kA whereas in high-resistance areaswhere the ground is predominantly granite the


Examples of other relevant standards include

ANSI/IEEE STD C62.41 (1980) formerly IEEESTD 587: Guide for surge voltages in low-voltageAC power circuits, essentially a lightning damagespecification [64]. I t defines surge/transientvoltages and currents up to 6 kV and 3 kA. Thereare two classes of equipment: Class A, thoseconnected further than 10m from the entrance ofthe power cable into a building, and Class B,those connected within 20 m of the powerentrance. Figure 10.46 shows the applicabletransient waveshapes and limits.ANSI/IEEE STD 446 (1987): Recommendedpractice for emergency and standby power in theUSA. It gives recommended power limits forcomputer systems as shown in Figure 10.47.Notice that the transient ampli tude increases withdecreasing pulse width. Occasionally, levels canreach 400 % of the line voltage for a 20 JiS longtransient.ANSI/IEEE STD 519 (1981): 'A guide forharmonic control and reactive compensation ofstatic power converters'.IEC 801: Part 4 of international standard (whichapplies to European equipments) concerned withpower transients, including those induced by

VOLTAGE

V peak Vp

0.9V p0.9V

P

0.5Vu

0.1 Vp TIME TIME

(aj 60% of V peak (bj

CURRENTIp

0.91p

0.5Vp

'------~~----;------.TIME

(c)

Figure 10.46 Examples of transient waveshapes on LVpower lines (ANSIIIEEE C62.411980)(a) Category A and B voltage and current( b) Category B only) voltage( c) Category B only) current

Location Waveform Amplitudecategory

A: Inside building (a) 0.5 ,us-lOO kHz 6kV> 10 m fromentrance

B: Inside building (b) 1.2 x 50,us 6kV< 20 m from (c) 8 x 20 ,us 3kAentrance (a) 0.5 ,us-l00 kHz 6kV

500A

3000/0wC)«I-...J

~ 200%I-ZW()

ffi 100%0..

0% '--_-'---_~----I_...&_ ----I ~

0.001 0.01 0.1 1 10 100 1,000

TIME IN CYCLES ( 60 Hz )

Figure 10.47 Limits on transients and dropouts for powersupplies connected to computer systen1S.ANSIIIEEE-STD 446-1987recommended power limits

lightning. Part 5 deals with surges in electricalpower systems. Typical waveshapes are given inFigure 10.48. This is probably the standard whichmust be met by most new commercial electronicequipment.IEC 60-2: 'High-voltage test techniques' IEC 60-2,gives a standardised 1.2/50 JiS lightning impulsevol tage transien t and 8/20 JiS current surge pulsesimilar to that for ANSI/IEEE C62.41 Cat.B [65J.BS 5406 (1988)-EN6055: Concerned with disturbances to supply systems caused by householdapparatus and similar equipment. Part 2Harmonics, Part 3 Voltage fluctuations.BS 66~2 (1985): Guide to methods of measurementof short-duration transients on low-voltage powerand signal lines.BS 2914 (1979): Specification for surge divertersfor AC power circuits.

10.6.3 Summary

In this brief introduction to the nature of transientsand disturbance to power distribution systems ithas only been possible to acquaint the reader withthe broad categories of effects and show someexamples of applicable standards and specifications. With the fast growing implementation ofdigi tal microprocessors in to industrial, commercialand domestic equipment this area of electromagnetic compatibility is becoming very much moreimportant than it has been in the past.Consequently, EMC design and test engineersmust be aware of this somewhat overlooked aspectof electromagnetic interference engineering, andnot allow an otherwise compatible equipment tobe compromised by these conducted transientsand other power line phenomena.


course of mathematics for engineers and scientistsvolume 5'. (Pergamon)

5 FEYNMAN, R.P.: 'Lectures in physics'. (AddisonWesley) Vol. 1, Chapters 25, 47 and 50

6 7200 series digital oscilloscope 1990 catalogue.LeCroy Ltd, 28 Blacklands Way, AbingdonBusiness Park, OX14 1DY

7 TD230 1/TD 130 1 digitiser systems. 1990 productcatalogue. p. 162, Tektronix UK Ltd, FourthAvenue, Globe Park, Marlow, Bucks SL7 1YD

8 'Spectrum analysis - pulsed RF'. Application note150-2. Nov. 1972, Hewlett-Packard, Winnersh,Wokingham, Berks, RG 11 5AR

9 RIAD, S.M.: 'Instructional opportunities offered bythe time domain measurement technology' . inMILLER, E.K. (Ed.): 'Time domain measurements in electromagnetics'. (Van NostrandReinhold), Chapter 3, pp. 72-94

10 SCHAFFER, R.: 'Choosing an ESD container:Materials and mechanics'. Interference TechnologyEngineer's Master, 1988, pp. 122-128

11 DASH, G.R.: 'Designing to avoid static - ESDtesting of digital devices'. Interference TechnologyEngineer's Master, 1985, pp. 96-110

12 JONES, B.: 'Improvements to the ESD testing ofequipment;. Presented at IEEE symposium onEMC) 1990

13 KING, W.M. and REYNOLDS, D.: 'Personal electrostatic discharge: Impulse waveforms resultingfrom ESD of humans directly and through smallhand-held metallic objects intervening in thedischarge path'. Proceedings of IEEE symposiumonEMC) 1981; pp. 577-590

14 BHAR, T.N.: 'Electrostatic discharge modelssimulating ESD events'. Interference TechnologyEngineer's Master, 1988, pp. 112-118

15 ENOCH, R.D., SHAW, R.N. and TAYLOR, R.G.:'ESD sensitivity of NMOS LSI circuits and theirfailure characteristics'. Proceedings of EOSsymposium on ESD) 1983

16 TAYLOR, R.G., WOODHOUSE, J. andFEASEY, P.R.: 'A failure analysis, methodology forrevealing ESD damage to integrated circuits'.Proceedings of EOS symposium on ESD) 1984

17 UNGER, B.A.: 'Electrostatic discharge failures ofsemiconductor devices'. Proceedings of 19th international symposium on Reliability physics) 1981

18 ENOCH, R.D. and SHAW, R.N.: 'An experimentalevaluation of the field-induced ESD model'.Proceedings of EOS symposium on ESD) 1984

19 SHAW, R.N. and ENOCH, R.D.: 'An experimentalinvestigation of ESD induced damage to integratedcircuits on printed circuit boards'. Proceedings ofEOS symposium on ESD, 1985

20 GAMMILL, P.E. and SODEN, J.M.: 'Latentfailures due to ESD in CMOS integrated circuits'.Proceedings of EOS symposium on EMC) 1986

21 HAWKINS, C.F. and SODEN, M.J.: 'Electricalcharacteristics and testing considerations for gateoxide shorts in CMOS ICs'. Proceedings of IEEEinternational conference on Test) 1985

22 CROCKETT, R.G.M., SMITH, J.G. andHUGHES, J.F.: 'ESD sensitivity and latency effectsof some HCMOS integrated circuits'. ProceedingsofEOS symposium on ESD) 1984

max. 30%

1Tr = 20j..lS ± 20% lT1 = 8j..ls ± 30% IEC 60 - 2Tf = 1.25 x T1

1Tr =50j..lS ± 20% lT1 = 1.2j..ls ± 30% IEC 60 - 2Tf = 1.67 x T1

WAVE SHAPE OF A SINGLE PULSEINTO A 50 n LOAD

Repetition period (depends -----/SINGL~ PULSEon the test voltage level). IN BURST

I I 111

'1 11I.li:I 'I,'I !il l

~15 ms~ Burst durationII Burst period 300 ms -----,

..tsons ± 30%~""...._---'"

FAST TRANSIENT BURST

V

1-+---~.....UJC) 0.9-+-----.-

~-Jo~ 0.5+---4----4------.:JIIl~CJ)-J:::>0.. 0.1

V

V

(aj

Typical transient burst and surgewaveforms (lEG 801) (aj lEG 801-4transient burst (b j lEG 801-5 surgewaveform


10.7 References

GLEDHILL, SJ.: 'Spectrum analysis' in BAILEY,A.E. (Ed.): 'Microwave measurements'. (PeterPeregrinus, 2nd edn., 1989) Chap. 13

2 'Digital signal processing'. Technical tutorial 1990catalogue, LeCroy Ltd, 28 Blacklands Way,Abingdon Business Park, OX14 1DY

3 BRACEWELL, R.N.: 'The Fourier transform andits applications'. (McGraw-Hill, 2nd edn.)

4 CHIRGWIN, B.H. and PLUMPTON, C.: 'A

t- I5 1.0+--:----r:-....a t- 0.9

~~lr ~ 0.5+---4-+.,..-----~ouJ:CJ)

V

§ 1.0+----1W........a ~ 0.99;«atz-J~ g 0.5

o 0.3

0.1O~+-++-~-----------


23 DAOUT, B. and RYSER, H.: 'The correlation ofrising slope and speed of approach in ESD tests'.Presented at 7th symposium on EMC, Zurich, 1987

24 RICHMAN, P., WElL, G. and BOXLEITNER,W.: 'ESD simulator tip voltage at the instant oftest'. Proceedings of IEEE symposium on EMC,1990, pp. 252-257

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39 As Reference 38, part 240 THURLOW, M.H.: 'Evaluation of EMP coupling

of linear systems by swept CW methods'.Proceedings of lEE colloquium on Protection ofcommunications equipment against EMP and otherhazards, March 1985, digest 1985/28

41 MORGAN, D.: 'Analysis of EMC susceptibilitydata on 23 systems'. Internal report (Class), BAeDynamics Ltd, Filton, Bristol, UK

42 'Integrated circuit electromagnetic susceptibilityhandbook'. Report MDC E 1929, McDonnellDouglas Astronautics Company, St Louis, Missouri63166, USA

43 Elgal EMP and HV Systems, P.O. Box 494,Karmiel 20101, Israel

44 GILES, j.C.: 'Simulating the nuclear electromagnetic pulse'. Mil. Electron./Countermeas.., August 1977

45 RUDY, T., BERTUCHEZ,j. and WARMISTER,B.: 'NEMP simulation and tests in Switzerland'.Third symposium on EA1C, 1979

46 Pulsed power conferences. IEEE Electron DevicesSociety, 1987 Washington, 1989 Monterey, USA

47 WILLIAMS, E.R.: 'The electrification of thunderstorms'. Sci. Amer. Nov. 1988, pp. 48-65

48 'World distribution and characteristics ofatmospheric radio noise'. CCIR Report 322, 1964

49 HORNER, F.: 'Analysis of data from lightning flashcounters'. Proc. IEE,july 1967, 114, p. 916

50 KEISER, B.E.: 'Lightning protection'. InterferenceTechnology Engineer's Master, 1985, pp. 66-68

51 MIL-B-5087B: 1978 'Bonding electrical andlightning protection for aerospace systems'

52 SAE ARP: 1978 'Lightning test waveforms andtechniques for aerospace vehicles and hardware'

53 NASA SP 8084: 1974 'Surface atmosphere extremes'54 PERROTON, G.: 'How to protect hardened

facili ties from very high level transients induced onpower line by lightning or EMP'. Proceedings ofIEEE symposium onEA1C, 1985, pp. 1-4

55 JONES, C.C.R.: 'Latest lightning standards'.Presented at BAe electromagnetics seminar, BAeMilitary Aircraft Ltd, Warton, UK, Nov. 1990

56 BISHOP, j.: 'EMP and lightning testing of avionicequipments with an introduction to transient effectsto aircraft'. Proceedings of colloquium on Protectionof communications equipment against EMP and otherhazards, March 1985, lEE digest 1985/28

57 DARGI, M.M.: 'Lightning protection testing of fullscale aircraft to determine induced transient levels'.Proceedings of international conference on Lightningand static electriciry, September 1989, University ofBath, UK

58 CARTER, N.J. and HOBBS, R.A.: 'Lightning qualification testing for UK avionic equipment'. Proceedingsof international conference on Lightning and staticelectriciry, September 1989, University of Bath, UK

59 WILES, K.G.: 'Lightning protection verification of fullauthority digital electronic control systems'.Proceedings of international conference on Lightning andstatic electricity, September 1989, University of Bath,UK

60 CLARKE, G.J.: 'The hybrid barrier: protectingagainst EMP and lightning'. InterferenceTechnology Engineer's Master, 1989, pp. 162-170

61 FIEUX, R.P. et al.: 'Research on artificiallytriggered lightning in France, IEEE Trans. P AS-97(3), 1978, pp. 725-733

62 PODGORSKI, A.S.: 'Composite electromagneticthreat'. Proceedings of IEEE symposium on EA1C,1990, pp. 224-227

63 HAR1', W.C. and MALONE, E.W.: 'Lightningand lightning protection'. Don White Consultants,Gainesville, Virginia 22065, USA, 1979

64 GERKE, D.: 'Power disturbances and computerisedequipment'. Interference Technology Engineer'sMaster, 1990, pp. 302-309

65 FREY, O. and HAEFELY, E.: 'Electromagneticcompatibility testing of electronic components,subassemblies, measuring instruments and systems'.EMC Technol., 1982, pp. 78-83

Chapter 11

Uncertainty analysis: quality control andtest facility certification

11.1 Introduction

The repeatability of EMC testing depends onmany factors that affect the measurement result.Some factors are not well understood or notdocumented in terms of the nature andmagnitude of their contributions to the totaluncertainty. The treatment of measurementuncertainty involves the use of statistics toestimate the probable uncertainty and associatedconfidence level with regard to a particularmeasurement or set of measurements. Statisticsand probability theory is a considerable subject inits own right and far too large to deal withadequately in a book such as this, which isprimarily concerned with the varied aspects ofEMC testing. Useful summaries of the maintheories and analytical techniques are given in anumber of texts [1-5J and are recommended tothose particularly interested in this aspect ofmathematics, or those who are determined toachieve a sound grasp of the basics beforeconsidering how to apply them to the understanding of EMC measurement uncertainty.

The application of statistics to experimentalmeasurements has been dealt with specifically byBox et al. [6J and Topping [7], and a good understanding of the subject can be achieved via thesetexts. Early work carried out in the BritishCalibration Service by Dietrich [8J resulted in thedevelopment of a statistical approach to combiningmeasurement uncertainties based on the assumptionof probability distributions for all the contributingcomponents. In 1977 a useful code of practice wasproduced by Harris and Hinton [9J for thetreatment of uncertainty in electrical measurements.

Test engineers should always endeavour toestimate the errors or uncertainties associatedwith the measurements which they make.However, a superficial and incomplete knowledgeof the statistical treatment of measurementuncertainty may lead to an unwarranted beliefthat an error estimate is correct. In such circumstances it is possible to become complacent andassume that because some uncertainty analysis isbeing done that the measurement results are nowmore reliable in some way. This can occasionallylead to a less searching approach to testing on thepart of engineers. This trap is particularly easy to

209

fall into in the case of EMC testing where, as willbe seen, there are a great number of factors whichaffect the final measurement uncertainty, but forwhich there may be no sound theoretical or experimental knowledge of the distributions of individualerror terms.

What follows considers the general treatment oferrors in electrical measurements and then putsthis into context as part of the whole qualitycontrol process for EMC testing. Finally, how theUK calibration and test laboratory accreditationservice (NAMAS) regulates measurement andtesting activities to ensure sound practice isexamined.

11.2 Sotne definitions

Terms that are commonly used in the analysis ofmeasurement uncertainty [7, 10, 11 J have specificmeanings which should be understood prior to ageneral consideration of the statistical treatmentof uncertainty.True value: the actual value of the quantity beingmeasured. This is unknowable via measurement.I t is approximated in practice by the value of thequantity measured that is established by traceability to national standards [1 OJ.Measured value: the result of conducting a fixed testprocedure to determine a quantity of interest. Inthe statistical treatment of errors its value isusually denoted by x.Error: the difference between the measured valueand the true value.Mean value: the result of computing the arithmeticmean of a number of measurements. Usuallydenoted by x.Uncertainty: this term quantifies the indeterminacyin the measurement process by stating the rangeof values within which the true value of thequantity being measured is estimated to lie [10].This is normally expressed in terms of ± anabsolute number or a % of the mean value.Confidence: because the limits of uncertainty of ameasurement cannot be known absolutely it isnecessary to qualify them with a statement ofconfidence. This is expressed as a percentage; forexample, a confidence value of 95% means thatthere is only a 1 in 20 chance or probability thatthe true value lies outside the stated uncertainty.


Random error probability distribution: the result ofplotting the frequency of occurrence of a measuredvalue as a function of the value. If a large number ofmeasurements of a quantity are made using thesame procedure and under the same conditions, butwith small random variations, it is found that theenvelope of the probability distribution approxirnates to the 'bell-shaped curve known as theGaussian or normal curve. This normal curve wasfirst derived by de Moivre in 1733 when dealingwith the problems associated with the tossing ofcoins. It was also later obtained independently byLaplace and Gauss. I t is sometimes known as theGaussian Law of Errors because it was applied tothe distribution of accidental errors in astronomicaland other scientific data. The Gaussian curve ISderived from the expression

where A) hand m are constants. The shape of thecurve is given in Figure 11.1. Topping [7Jsuggests that 'it is difficult to overstate theimportance of the Gaussian error curve Instatistics as its importance is like that of thestraight line in .ge~metry'. Definitions of a widerset of terms concerned with uncertainty andstatistics can be found in the UK NAMASinformation sheet NIS 20 [11 J.Systematic error: one which consistently biases themeasurement (and therefore the mean of nmeasurements) in one direction away from thetrue value [12J. Examples include a panel meterwith the resting place of the needle not alignedwith the zero mark or an instrument calibrationfactor which may be incorrect.

Most uncertainties in real measurements resultfrom a combination of systematic and randomerrors which are introduced via a number offactors. I t is common to treat these two types oferrors separately as systematic errors that may beknown (within a given uncertainty, by referenceto a national standard) and can be removed fromthe measurement by a correction factor, whereas

y==A (x m/ 11.1

the uncertainty resulting from random errors cannot. It will be shown that random errors can becombined with systematic errors to yield a singlevalue for the uncertainty of a measurement for agiven confidence level.

11.3 MeasureDlent factors

Consider a typical EMC measurement of the RFcurrent flowing in a multiconductor cable asdepicted in Figure 11.2. There are three mainareas where uncertainty can be introduced intothis measurement.

(i) The factors affecting the physical/electricalenvironment of all components of the test.These are sometimes referred to as thecontrol quantities.

(ii) The factors involved with the sensor and itslinkage to the quantity being measured.These are known as the coupling factors.

(iii) The calibration of the measuring instrumentand the way it is used.

This classification of uncertainty factors followsthat given in NAMAS NIS 20 [11] and isillustrated in Figure 11.3. As an example, some of

CABLE POSITIONP

CURRENT PROBETRANSFER IMPEDANCE ( T I )

Figure 11.2 Example of uncertainty factors in currentprobe EMC nzeasurementy

ACONTROLFACTORS

A

BC

0---"""4IIIIn I

SENSORDATA

MEASUREMENTINSTRUMENT

FACTORS

1CALIBRATION

DATA

RESULTR

Figure 11.1 Normal error curve y Ae_h2

(x-m)2

m xFigure 11.3 Classification of contributions to uncertainty

in measurement

211

The nature of random variables in measurementcan be demonstrated by reference to Figure 11.4.A number of measurements are made of aquantity and these are recorded as arrows. Thearithmetic-mean value of the set of measurements is also shown. This, however, will notnecessarily coincide with the mean or centralvalue for the whole population of possiblemeasured values. Such a value will be the meanvalue of the Gaussian or norlTIal distributioncurve relating to the quantity under investigation. Hinton [7J defines the difference betvveenthe mean of the me:lsuremen t set and thepopulation mean as an expression of the randomcomponent of uncertainty. I t is not possible tocorrect for the uncertainty in troduced byrandom errors; all that can be done is to state avalue of uncertainty and an associatedconfidence level.

In practice, it is found that a relatively smallnumber of contributions to random uncertaintywhich are of similar range can be combined toproduce a distribution which is close to thenormal or Gaussian curve, even if theirindividual probability distributions are notnormal (i.e. entirely random) [1 OJ. This isimportant in calculating the uncertainty in EMCmeasurements where there are a large number offactors con tributing to the final uncertain ty withmany of them not having purely random distributions.

Pure random variables or measurement errorsare those which are independent of other errors

itsandfactor

Control factorsStability of source and load parameters on theends of cablePosition of current probe along the wire (1)Exact lay-up of wires in the bundle (w)Angle of current probe to cable axis (a)Height of cable and probe above ground plane(h)Degree to which centre of cable bundle is offsetfrom centre of current probe (0)Position of the connecting cable from probe tometer (p)Grounding of meter (g)Stability of the power supply to meterNormal physical environmental factors such astemperature, humidity, etc.

Coupling factorsCurrent probe transfer impedance as a functionof frequency (1'1)Connector and cable losses as a function offrequency (-C1, C2 and A)Current probe, cable and meter impedancesresulting in VSWR for given cable length(L)

Measuring instrumentCalibration correctionuncertaintySignal-to-noise ratioOperator accuracy in setting up the meter andin reading and recording the result (R)

UNCERTAINTY ANALYSIS: QUA.LITY CONTROL AND TEST FACILITY CERTIFICATION

the factors that affect the measurement 11.4 Random variablesuncertainty for the current-probe measurementin Figure 11.2 in each of these arelisted:

VALUE OFQUANTITY

IN SYSTEMATIC

I.FACTTS .1I I

II THE CAliBRATION

LABORATORY VALUE

I I ASSUMED WORST CASERECTANGULAR

II UNCERTAINTY

DISTRIBUTION

THE TRUE VALUE(unknowable by measurement)

INDIVIDUALMEASUREMENTVALUES IN AMEASURED SET OF n

____error for __-__a ~,

.-1 ~ measurement no. 3

RANDOM UNCERTAINTY IN THEMEANS OF THE MEASURED SET

I :

MEAN VALUE OF ALL I CALIBRATION FACTOR ---JMEASUREMENTS r SYSTEMATIC UNCERTAINTY -IWITHIN THE~ (can be removed) JPOPULATION I

I MEAN VALUE UNCERTAINTY

NORMALPROBABILITY .DISTRIBUTION

~

(J)W:J-J

~LLoW()Zwa::a:::J()()oLLo>-()zW:JawfE ..-

f'igure 11.4 Illustration of measurement uncertainties

212 A HANDBOOK FOR EMC TESTING·AND MEASUREMENT

(i.e. not related or linked to the same causalfactors) and produce additive effects due toindependent random causes. The mean value of nmeasurements is

where x == mean value, n == number of measurements, and Xi == value of the ith measurement.For a random distribution of the variable x it ispossible to derive meaningful quantities known asthe variance and standard deviation which for asample population are given by Reference 10 as

FREQUENCY OF OCCURRENCEOF MEASURED VALUES OF x

20"--.........--20"-+--.........

......-o----t--a--~ 1 STANDARD DEVIATION

...---68.3%---10+.------95.5%------tlIoI

....-------99.7%-------..

x

% CONFIDENCE THAT THE TRUEVALUE OF x LIES IN THIS RANGE OF x

....---+---+-30---+----30"+----+--..... 3 STANDARD+---+---+----~---+__-_+_-~ DEVIATIONS

11.3

11.2

where a 2 == variance of the sample anda == standard deviation. Each time a set of nsamples is taken the value of a will changeslightly. The best estimate of the standarddeviation for a whole population of results basedon a single sampIe of n measurements is

Figure 11.5 Standard deviations and percentageco1?fidence for normal distribution of values

Table 11.1 Relationship between K and % populationfor normal distribution

and the difference between aest and a is onlysignificant when the number of measurements n issmall. This standard deviation relating to theuncertainty in a quantity may be calculated from aset of measurements, or estimated. The probabilitydensity may follow a normal or Gaussian distribution, a log-normal, Poisson, binomial or other distribution [12]. Information regarding the nature ofthese and other types of probability distribution canbe found in Meyer [13J and in a useful NATOrestricted document [14].

Standard deviation is useful because it is anindicator of the range of measurementuncertainty for a given distribution. In Figure11.5 it can be seen that for the Gaussian ornormal distribution there is a 68.3% probabilitythat any particular measured value will be withinthe range ±a of the population mean. Further,95.5% will be within 2a and 99.7 % within 3a.Defining the multiplying factor for a asK, thensome further useful relationships between K and0/0 of population within K a limits can be seenfrom Table 11.1 The uncertainty U whichcorresponds to a specified % probability orconfidence factor can now be defined as

There is a relationship between the standarddeviation and the parameter h (sometimes calledthe precision constant) in eqn. 11.1. I t can beshown [7J that

The value of the standard deviation a for a givenpopulation mean determines the tigh tness of thedensity distribution. For example, in Figure11.6 it can be seen that as a decre~ses thetightness or precision of the error distributionincreases.

11.9

11.8

11.7

h

yin

1 2-a2

A

%population within limits

5068.3

.9595.59999.7

( )2;:2__1_ e- x-x 20"y - 0"J2i[

and so it is possible to rewrite eqn. 11.1 as

and that A in eqn. 11.1 is taken as

0.675al.Oa1.96a2.0a2.58a3.Oa

11.6

11.5

11.4

U == Ka

[

In

n - 1 6 (x

aest

- a[_n ]~n - 1

aest

so that

lJNCERTAINTY ANALYSIS: QUALITY CONTROL AND 1'EST FACILITY CERTIFICAl'ION 213

11.4.1 Student's t-distriblltion

11.11

11.10t

== (Jest Vn

11.5 System.atic uncertainty

In determining uncertainty it isnecessary to consider the measuring apparatus,

where (Jest is the estimate of the standard deviationderived from eqn. 11.4 and t are the valuesspecified in Table 11.2 for given confidence levelsand as a function of n, the number of measurements made. I t is clear that as n tends to infinitythe mean for the set tends to the popula tion meanand Um tends to zero.Finally, in connection with random distributions,the standard deviation (Je of the combination of anumber of random distributions each expressedby a standard deviation (J, is the root sum squareof the contributions:

For small samples the normal distribution mayunderestimate the probabilities for largedeviations [13]. Assuming that the density distribution function for the population is Gaussian,then to calculate the random uncertainty of afinite sample of measurements to a givenconfidence level, it is necessary to use Student's tdis tribution [13, 15J. The random uncertainty ofthe mean of a sample of measured values is

x .....o

(21t )~

y

·12 (21t)-'2

y t.-----------

When the measurement of a quantity is only madea few times the mean value for the set is unlikely tobe the same as that for the whole population ofmeasurements which could be made. Thus thereis an additional uncertainty associated with themean for the set, as shown in Figure 11.2. Thisuncertainty decreases by the square root of thenumber of measurements made but can besignificant for small values of n, say less than ten.

Figure 11.6 Normal distribution curves for differentvalues of (J

Table 11.2 Studenfs t-distribution table

Specified confidence level

~ 0.500 0.683 0.950 0.955 0.990 0.997

2 1.000 1.84 12.7 14.03 0.817 1.32 4.30 4.53 9.924 0.765 1.20 3.18 3.31 5.84 9.22

....... 5 0.741 1.14 2.78 2.87 4.60 6.62ven

6 0.727 1.11 2.57 2.65 4.03 5.51v~ 7 0.718 1.09 2.45 2.52 3.71 4.90.......

.S 8 0.711 1.08 2.36 2.43 3.50 4.53en 9 0.706 1.07 2.31 2.37 3.36 4.28.......~

10 0.703 1.06 2.26 2.32 3.25 4.09ve 11 0.700 1.05 2.23 2.28 3.17 3.96v$....i;:) 12 0.697 1.05 2.20 2.25 3.11 3.85enrj

13 0.695 1.04 2.18 2.23 3.05 3.76ve 14 0.694 1.04 2.16 2.21 3.01 3.694--f0 15 0.692 1.04 2.14 2.20 2.98 3.64$....i

16 0.691 1.03 2.13 2.18 2.95 3.59v..D-e 17 0.690 1.03 2.12 2.17 2.92 3.54

;:)18 0.689 1.03 2.11 2.16 2.90 3.51Z19 0.688 1.03 2.10 2.15 2.88 3.4820 0.688 1.03 2.09 2.14 2.86 3.45~r----ex: 0.675 1.00 1.96 2.00 2.58 3.00 Values of't'


a

If there are a number of uncorrelated systematicuncertainties with assumed rectangular distributions with values ± al a2 a3 ... an the combinedstandard deviation of the contributions is

There are two cases that must be treatedseparately vvhen combining systematic andrandom uncertainties: when a dominantsystematic term exists, and when no dominantsystematic term exists. When a dominantsystematic term ad exists in eqn. 11.13 thenUs == K (Js may exceed the arithmetic sum of the

11.14

I

U= ad + [U/ 2 + U;] 2

n

Us> L amm=l

11.7.1 Contributions to measurementuncertainty

There are two main problems to be overcome ingenerating worthwhile uncertainty statements toaccompany EMC measurements. The first is thesheer number of variables involved in thesecomplex measurements and which must be investigated and segregated into control, coupling orinstrument factor categories. There may be up to50 identifiable contributing sources ofuncertainty in, for example, a radiated emissionmeasurement [12]. This contrasts with a muchmore simple measurement of quantities such astemperature or weight etc. where there may beless than a dozen significant sources ofuncertainty. As a rule of thumb, used to simplifymatters, one might ignore uncertainty contributions that are smaller by a factor of ten than thelargest contributor.

The second problem relates to obtaining ameaningful understanding of the nature of theprobability distributions attaching to each ofthese uncertainties. This can be done either byexperiment, where each contributing factor isisolated and studied in a series of careful and

If this is the case, the dominant systematicuncertainty contribution should be broughtoutside the RSS (root sum square) addition. rrhusthe correct total uncertainty is

where U~, == K (J~ and (J~ is obtained from eqn.11.13 without including the dominant term ad. Ur

is the random uncertainty. When no dominantsystematic term exists the random and systematicuncertain ties can be combined by the RSSmethod as expressed in eqn. 11.11.

For all EMC test engineers and designers ofelectronic equipment that may be subject toEMC testing it is advisable that they shouldconsult the references listed in this chapter andgain a working understanding of the statisticalbasis for the treatment of measurement errors asoutlined to understand and have confidence inthe results of EMC tests.

11.7 Uncertainties in EMCn1.easuren1.ents

semirange values of the individual contributions.This can be checked by

11.13

11.12

aT + a~ + a~ ... a~

3

11.6 COInbining random. andsysten1.atic uncertainties

the operational procedure and the item under test.Systematic bias in a number of individualmeasurements can be detected by plannedvariation of the measurement conditions andprocess, and by averaging the results. The biascan then be corrected and thus removed froIncontributing to the total measurementuncertain ty.

The calibration of a measurement instrumentwith respect to a national standard will revealany systematic factor which the instrument ormeter may contain, see Figure 11.4. This can thenbe corrected or allowed for by applying thecalibration factor specified in the certificate ofcalibration. The calibration laboratory value andthe rnean of the population as shown in Figure11.4 can then become aligned.

The correction factor given in the calibrationcertificate has an error associated with it which isthe systematic uncertainty and is assumed to havea worst-case rectangular distribution. Thisuncertainty, and any other systematic uncertainties which can be identified and quantified, arefirst grouped together in accordance with theirknown or assumed probability distributions. Ifonly the limits of the systematic uncertainties areknown it is safest to assume they have rectangulardistributions [10]. They are then combined andthe standard deviation is added to that of therandom uncertainty to yield the full measurementuncertainty.

It has been shown [8J and quoted [10, IIJ thatthe standard deviation (Js of a systematicuncertainty having a rectangular distributionwith limits of ± a is

UNCERTAINTY ANALYSIS: QUALITY CONTROL AND TEST FACILITY CERTIFICATION 215

probably time-consuming controlled experiments,or by examination of the theory relating to thefactor in question.

The experimental EMC uncertainty database isgenerally considered to be poor [12], and as thegeneration of definitive data is expensive forcommercial test houses they tend to do only thatnecessary to sa tisfy the requirements of thenational accreditation authority, NAMAS in theUK. A progressive programme to provideindustry with a well researched EMCmeasurement uncertainty database could beundertaken by the national metrology authority,or possibly by combining efforts with those ofother nations to provide a really comprehensivefoundation for the treatment of EMCmeasurement uncertainty.

11.7.2 Identification of uncertaintyfactors

Now examine a typical EMC radiated emissionmeasurement made in a semianechoic screenedchamber and attempt to identify the moreobvious sources of uncertainty as an example ofthe complexity of uncertainty analysis in EMCtesting.

Other test regimes such as open-site testing orbulk-current injection will have factors contributing to measurement uncertainty that areparticular to them, and no general uncertaintyanalysis can be developed to cover widelydiffering tests methods. Each must be considered

separately when combining the uncertaintiesrelevant to that test regime, although in manycases a common database can be used forcomponents such as VSWR mismatch, polarisation and antenna positioning uncertainties.

Slightly different uncertainty analyses will resultfor variations in a basically common test methodwhen there are detailed variations required byparticular test standards. For example, considertwo open-range radiated emission tests, onespecified by FCC and the other by VDEjCISPR.The field averaging uncertainty (Chapter 10)in trod uced by virtue of the size of the tuneddipole antenna will be different for the two testsat frequencies below 80 MHz owing to the CISPRrequirement that the dipole length remains tunedto 80 MHz for frequencies below this, whereas theFCC requires the dipole to be tuned down to30 MHz.

Returning to the uncertainty analysis of atypical radiated emission test in a screened room,consider a standard measurement of radiatedemissions from an EDT at 1 m as shown in Figure11. 7. The factors that contribute uncertainty tothe measurement in the groups identified earlier,in line with NAMAS NIS 20, i.e. control factors,coupling factors and instrument factors are listedin Tables 11.3-5. There is no absolute determination as to which group some uncertainty factorsshould be placed in and some control factors mayalso be considered as coupling factors. Thedefinition used in this example is that couplingfactors are those which influence the measurement

EMI METER

CABLE

ATTENUATION "A"

AMBIENT SIGNALPENETRATION

SWRATLOAD

\ r-----[2]~-...

___SCREENED ROOMI.;!ANTENNAV ANGLE

~T:NNACALIBRATION

FACTOR(AF)

SWRCONNECTOR

/I\. ..-------f'<'-L--;---..CABLE ~----~

~--t-.-...-POLARISATION

ANGLE P

"-...RAM PERFORMANCE

Penetration of ambients through shielded room

y

~P '~ reflections from

- ower Angle subtended bycable - EUT & cables walls, floor &

position ...... / ceilings

+- / - -----, rAnglesublend~- ~- 0x+ ~I by EUT d - ------------..:

I ----- -=---:-______ SWR ANTENNAI _------- _______

EUT GROUNDING - ANTENNAICONDITIONS _ ----MUTUAL IMPEDANCE

-=- I _t= TO ITS REFLECTIONS -------- M----r---- · NEARFIELD

Sign~1 I BOUNDARY?

cable 1 TYPE & CONFIGURATIONposition OF GROUND PLANE BENCH

EUT positionPEUT

POWER LINE NOISE(external)

SIGNAL CABLEEXTERNAL NOISE

Figure 11.7 Sources of measurement uncertainty in typical radiated emission test


of the electric fieldimmediate region ofIneasurement instrument.

from within theantenna to the

Table 11.5 Instrument uncertainties

EMI meter calibration..... TTl-,i-·L1.,""'"i-'" calibration shifts are assumed to be

uncertainty11.7.3 Estimation

For each of the identified the EMCtest engineer needs to estimate the level of randomand uncertain ties and express these interms of a standard deviation for a particular typeof function. In some cases thefactors may rise to a contribution to bothrandom and types of uncertainty.

The total uncertainty for the random and1J'-'-'J.A.A."""c"'",,-",, errors can be calculated by "-'''-'J.LI.U'",,-LI..I.. .I.''F

the standard deviations for each as shownif they are substantially independent of each

even if the density functions are notGaussian. This is a useful property of the centrallimit theorem [13]. Depending on whether thereis a dominant systematic contribution, therandom and systematic uncertainties can beaccumulated according to eqns. 11.11 or 11.14.

I t is unreasonable to carry out a detaileduncertainty analysis for all EMC tests taking intoaccount all of the possible influencing factors suchas those listed, for the radiated emission test in ascreened room. The experienced EMC testengineer should be able to rank the uncertainties

correctedEMI meter stability with

humidity/altitudeIn1pulse generator (internal gain stabilityEMI meter input VSWR various input

attenuator uvl,L.LL.LC::,~

EMI meter overload (saturation and harmonic/intermod distortion)

EMI meter spurious signalsEMI meter linearity NB and BBEMI meter bandwidths and shape factorsEMI meter detector weigh ting functions against

pulse repetition frequencyNB and BB range

Instrument operator related uncertaintiesScan speed/bandwidth choicesMeter reading errors/computer system data

recording errorsIn-built software errorsEMI meter parameter recording errors

(e.g. atten. settings, bandwidth records2/10 dB amplitude scale confusion etc.)

Misnumbered test runsIncorrect recording of amplifiers

in line to EMI meterProcedural or· data recording errors at

shift handover times in test facili ty

IJ '-'-d,A.A.,-"," '- •• '-~ calibration shifts are

Table 11.4 Couplingfactors

Antenna correctionAntenna cable positionExternal noise on antenna cable

Path loss due to angle subtended by EUT andcables

Multipath reflections from screened room walls,floor and ceiling

Screened room sizeScreened room absorber efficiencyPosition of EDT and antenna in room

assumed toAntenna sizeAntenna polarisation (elevationAntenna azimuth angleAntenna polar response (beamwidth and spotAntenna balun efficiency (differential/common-

mode rejection)Antenna \lSWR

Measurement antenna connector loss andVSWR

Measurement cable VSWR and lengthMeasurement cable lossMeasurement cable layoutMeasurement instrument connector loss and

VSWR

Antenna correction factor

EUT orientationEUT

humidityEU1" groundingEDT signal cable layoutEUrr signal cable penetrationsEDT power cable layoutEUrr power cable LISN terminations

Ground-plane ratioGround-plane heightGround-plane conductivi tylintegri tyGround-plane bonding to screened room wallIngress of external noise through screened room

walls, through penetration panels and alongEUT power and signal cables

Antenna heightAntenna distance from EUTAntenna mutual impedance to screened room

Table 11.3 Control factors

UNCERT'AINTY ANALYSIS: QUALITY CONrrROL AND TEST FACILITY CERTIFICATION 217

• Polarisation uncertainty: White [12J showsthat the uncertainty in measuring a maximumE-field due to an element of cross polarisationis composed of an offset mean of -6 dB andan uncertainty with (J == 6 dB. This comesabout because the probability of making ameasurement, other than for perfectly alignedpolarisation, must always result in a lowerthan true reading. Assuming that a dipole-likeantenna has nulls at -20 dB below the peakresponse and that all polarisations of wavesfrom the EU1' are equally likely, it can beshown that the figures quoted above arerealistic even though the distribution is notGaussian.

• VSWR mismatches: If a typical biconicantenna has a VSWR of 3: 1 it can be shown[17J that the uncertainty can be representedby (J 2.9 dB. For a receiver with an inputVSWR of 2: 1 there is another additionaluncertainty of 1.9 dB. Taken together thecombined uncertainty will be represented by(J == 3.5 dB.

• Antenna calibration uncertainty: There is verylittle reliable information regarding theuncertain ty associa ted with the use ofcommercial passive antennas. I t is clear thatvariations in the manufacturers' calibrationcannot be~ .ignored as these calibration curvesmust usually be used. I t is very difficult tocalibrate one's own antennas by comparisonto industry reference standards or to calibratethern by generating known fields. The NPLpublished uncertainty [20J in calibration forantennas below 1 GHz is ±1 dB excludinglarge loops (assuming a rectangular distribution for systematic uncertainty) ,(J 0.5 dBapprox. This is almost insignificant, but whenother factors such as ageing in use, looseelements, worn connectors are included,White [12J suggests it is prudent to allow for(J 3 dB.

• Antenna cable/balun effects: Uncertainties dueto antenna cable, antenna element andantenna cable and poor balun effects havebeen estimated by DeMarinis [21 ] tointroduce uncertainties with a maximumvalue of up to 12 dB. Assuming this to berepresenting a 3(J case one may estimate thestandard deviation to be around (J == 3-4 dB.

• Field averaging: Estimates of worst-caseuncertainty for field averaging introduced intoan open-range measurement with a tuneddipole at 30 MHz have been produced byBrench [22]. He gives values of -8.5 dB lowerthan would be expected from a pointmeasurement. If some systematic correction ismade, for say half this range, it may be

Figure 11.8 Coupling between antennas in shielded roomas function offrequency -' with 1 m spacing*Coupling normalised by r~ference to opensite antenna coupling values

(j)

(j) ~ 50-----------.....,.-....-----..,...---.,~~ 40ww 30

~~ 20Zz 10w w co 0t.=:=======:=:II~t-'I,..,PI~~'O -10

~~ -2000 -30~ ~ -40~~ -506~ L..-__---I_......... ....._~-_-~---

GoZ

introduced by the various factors so that only themajor ones need to be included in the formalstatistical analysis. This will greatly reduce thework needed to arrive at a satisfactory estimate ofmeasurement uncertainty. The NAMASexecutive [11 J require EMC test houses toformally present, during a quality audit surveillance visit, at least one such analysis carried outon a test during the last year.

Examples of the magnitude of the uncertaintyassociated with the most important factorsrelating to the radiated emission test in the screenroom follow. Strictly, one should not use thelogarithmic term decibel to represent anuncertainty range, as figures should be expressedas a linear factor. However it has becomecommon practice in test laboratories to use the dBand it is widely quoted in References 12 and 17.

• EUT placement: Very dependent on the typeof EUT and its radiation pattern. Defaultvalue (J == 6 dB.

• Screened room reflections: Dependent on sizeof room, location of antenna and EUT andanechoic damping. For a room with noanechoic lining and below first resonance(J 2 dB [16, 17J. Above first resonance andup to 5x this frequency (J == 7 dB [16, 17]. Anexam.ple of the difference between EUT toantenna coupling for an open site and ascreened room, given the same test configuration, is shown in Figure 11.8 [18, 19]. It canbe seen that the worst-case differences are upto ±40 dB. I t would be interesting to analysethe density distribution of the occurrence ofdifferences to see if it was normal, log-normalor another well known type. For a screenedroom with RAM lining giving 6 dB reflectionloss at 30 MHz it is possible to reduce thereflection uncertainty to (J == 3 dB above30 MHz and (J == 2 dB above 100 MHz [1 7J .


11.8.1 NAMAS

11.8.2 NAMAS and measurementuncertainty

The broad policy and details of statistical methodsto be used by accredited laboratories whenassessing measurement errors are contained inpublications NIS 20 [11 J and B3003 [26].NAMAS requires that test laboratories shall havesufficient understanding of measurement, suchthat their measurements, or statement ofcompliance or noncompliance 'can be defendedbeyond all reasonable doubt'.

NAMAS policy aims to minimise doubt whichcould result from a difference of opinion with

The authority responsible for test/calibrationlaboratory accreditation in the UK is theNational Measurement AccreditationNAMAS, formed in 1985. The organisationdeveloped from the National Testing LaboratoryAccreditation Scherne and the Bri tish CalibrationService. NAMAS document MIO [24J, 'Generalcri teria of competence for calibration andlaboratories' replaces the BCS EO 102 'Approvalcalibration laboratories' and the NATLAS Nldocument. '[he principles on which documentM lOis based are consistent with those given inISO Guide 25: 'General requirements for thetechnical competence of testing laboratories', andEN4500I: 'General criteria for the operation oftesting laboratories', which standard calls up therelevant requirements of BS 7581: 'Measurementand calibration systems'.

Laboratories accredited NAMAS meet therequirements of ISO 25 and EN4500I and areconsidered as meeting the requirernen ts concernedwith the adequacy of calibration and testingcontained in IS09000, EN29000 and BS5750series of specifications, relating toassurance in manufacture and similarDetails of contact names and telephone numbersfor staff in the NAMAS executive are available[25].

NAMAS does not accredit laboratories toapprove products [11]. This is the role of theappropriate national certification bodies such asthe DTI Radio Communications ])ept in the UK,the FTZ in Germany and the FCC in the USA.The reports produced by accredited competenttest laboratories concerning the EMCmeasurement of a product is usually the basis onwhich the certification body will grant approvalfor sale and operation.

11 ..8 Test laboratory llleasuretnentuncertainty

11.7 Estima te of total urlcertainty

White [12] a comprehensive account ofestimating EMC measurelnent uncertainties onthe basis that most probability density functionsfor contribu ting factors are largely unknown ornot documented, and over a limited range of ±(J

may be represented by a log-normal distribution.l'his is a distribution where the random variablex has a logarithm which is normally distributed[13]. Thus White gives standard deviationsexpressed in dEs and sun1med according to eqn.11.11. While this is certainly convenient, onemust be careful to establish its validity in eachparticular circumstance.

Combining the estimates of standard deviationsgiven earlier for the major factors which contributeto measurement uncertainty of our example test ina screened room, using eqn. 11.11 (assuming nototally dominant contribution) we arrive at avalue of (Jtotal == 10.6 dB. This is higher than thefigure given by Dash [23J, produced by analysingactual test data from open-range sites derived frommeasuring the same EUl' at a number ofgovernment facilities in different countries. l'hisshould not be surprising, as the extra contributionto uncertainty from standing waves in the shieldedroom case will increase the total measurementuncertainty. The standard deviation quoted byDash for the open site is 8.5 dB.

The example calculation of uncertainty in themeasurement of radiated emissions fron1 an EU'fat 1 m in a screened room has been carried out asan illustrative exercise to demonstrate the natureof factors and to suggest reasonablemagnitudes their standard deviations. 1-'hereader should take the values suggested as beingindicative only, and should carefully evaluate thestatistical distributions and estimate standarddeviations for contributing factors with referenceto particular test configurations and operating

reasonable to set a value for (J 2 dB at< 80 MHz falling to < 1 dB above 80 MHz.The 1 dB value is relevant to screened roomtesting where only short dipoles are used.

• EM! meter uncertainty: This is generallyquoted as having a worst-casevalue of ±2 dB. This would' lead to anestimation for the standard deviation of< 1dB.

• Near-field uncertainty: White [17] quotesvalues for the uncertainty in a 3 m open-rangeFCC test with a dipole at 30 MHz as being(J < 1 dB. It is unlikely that the figure V\Tillbe so low for MIL S1-'D measurements madeat 1 m in a screened room.

UNCERTAINTY ANALYSIS: QUALITY CONTROL AND TEST FACILITY CERTIFICATION 219

respect to the compliance or otherwise of aparticular product. Laboratories are thereforerequired to be able to make corrections for errorwhere appropriate, and having done so, toevaluate the overall uncertainty when carryingout testing. The role of NIS 20 [11] is to enablelaboratories to understand the NAMAS requirements with regard to uncertainty in measurementand to be able to evaluate the total uncertaintyusing a statistical approach.

Test reports must carry a statement of totalmeasurement uncertainty and therefore laboratories are required to have a documented policyin line with NIS 20 on measurement uncertainty,which guides staff in the preparation of suchstatements. Laboratories must produce and meeta NAMAS-approved laboratory schedule of limitsof error and uncertainty (known as Schedule A),for all quantities for which measurement accreditation is claimed.

The applicant laboratory decides the level ofuncertainty for which it wishes to be accredited,but the limits cannot exceed those specified inappendix 4 of NIS 20 and each limit set out inSchedule A must be justified by an uncertaintybudget and calculation. At least one uncertaintyevaluation for an accredited test must beavailable for inspection each year during theNAMAS surveillance visit. Within the UK,NAMAS prefers that the uncertainty in a giventest be evaluated at the 95% confidence level.

This is a convenient point to look briefly at thestatistical approach which is widely adopted tothe statement of compliance of a sample of massproduced items with the interference limitsstipulated in EMC standards. The statement ofcompliance is made after the test, or possiblymultiple tests, of a batch of units, with the resultsbeing analysed according to a set of statisticalrules.

11.8.3 Limits and production testing

In the wider field of sample testing for productapproval, the CISPR publication 16 (section 9)[27J requires that 'compliance of mass-producedappliances with radio interference limits should bebased on the application of statistical techniquesto assure the consumer with an 80% degree ofconfidence that 80% of the appliances being investigated are below the specified limit'. This is the socalled 80% /80 % rule and details of the statisticaltechniques are given in Reference 26.

Dvorak [28] gives data on practical measurements made on samples (up to ten) of anglegrinder and hand-drill electric power tools anddiscusses the statistical techniq ues used to reach astatement of compliance. He suggests that very

Ii ttle of value has been published on a rationalengineering approach to the problem of settinglimits to ensure interference control. However,this is crucial if technically derived limits are tobe implemented within realistic economicconstraints for mass produced products. Absolutelimits (no interference from any product shallexceed a stated value) are not unreasonable forsmall numbers of specialist devices, but astatistical analysis is far more appropriate to theEMC clearance of mass produced items. Many ofthe equations given in Section 11.4-11.6 areapplicable to the calculations which need to beperformed.

Compliance with a limit L can be judged from

xn + Kan < L

where x == arithmetic mean of n samples, K is afactor related to confidence level for a givensample size, and an == standard deviation of thesample of size n. To generate a high degree ofmanufacturer confidence that the testing carriedout on equipment does lead to a crediblestatement of compliance, it is necessary to testsamples of up to ten randomly selected examplesof the product and perhaps to repeat the testingof the sample a few times [28J. This willinevitably be rather time consuming and thereforeexpensive, but should lead to the absoluteminimum of product compliance rejectionswithout sound cause. An early CISPRRecommendation [29] gives the procedure to beused for testing appliances in large-scaleproduction.

11.9 NAMAS requirem.ents forlaboratory accreditation

11.9.1 Requirements for accreditation

Companies which conduct EMC testing and wishto gain NAMAS accreditation must be preparedto submit to the requirements set out in NAMASdocuments M 10 [24] and M 11 [30] . Theprincipal requirements follow.

The laboratory must be legally identifiable.Paying due fees to NAMAS.Giving such undertakings as NAMAS mayreqUIreBeing subject to reassessment every 3-4 years,and surveillance visits every year, with thepossibility of unscheduled surveillance visits.The laboratory cannot make use of unreasonable or irresponsible subcontracting.The laboratory must abide by the law of theland.The laboratory must be impartial, and the


staff shall be free from any commercial,financial or other pressures which mightinfluence their technical judgment.The laboratory must afford the client whoseproduct is being tested reasonable cooperation so that the performance of thelaboratory can be monitored.The laboratory must afford NAMASreasonable accommodation and co-operationto enable their representative to monitorcompliance with the regulations.Key staff in the laboratory must have jobdescriptions and be aware of their roles andthe extent and limitations of their responsibilities. Specification of the qualifications,training and experience necessary for the keymanagerial and technical staff shall becontained in the job descriptions. Thelaboratory shall normally use only staff whoare permanently employed or on contract tothe laboratory.There shall be a documented policy to ensurethat new and existing staff maintain relevantacademic expertise, technical skills and professional expertise.ManageriaJ staff shall have the authority andresources needed to discharge their duties.The laboratory must have a documentedpolicy and procedures to ensure the protectionof proprietary rights and confidentialinformation concerning products being tested.The laboratory should have restrictedphysical access for unauthorised individuals.There must be named laboratory and qualitymanagers with responsibility for all workundertaken. The quality manager must havedirect access to the highest level ofmanagement which is concerned with thecompany policies which affect the laboratory.The laboratory shall have nominatedauthorised signatories recognised by NAMAS.A q uali ty system shall be formalised in aquality manual which meets the requirementsofNAMAS document M16 [31]. The manualmust be maintained up to date.The laboratory must establish and maintainprocedures to control the distribu tion,updating and retrieval of all documentationthat relates to the calibration or tests itperforms. Amendments to the documentationshall be subject to strict management control.The laboratory quality manager must planand ensure that periodic internal audits ofprocedure and operation are carried out.They shall be conducted as frequently asnecessary to achieve the required level ofquality control. The period between auditsshall not exceed 1 year.

The laboratory shall normally use onlyequipment that is owned by, or on long leaseor loan to the laboratory. It shall be suitablefor all the tests for which it is to be used andmust be capable of achieving the accuracyrequired. I t should be of established design ifpossible and be operated only by namedauthorised staff. Each items of equipmentshould be uniquely identifiable.i\n up-to-date calibration history of equipmentreq uiring calibration shall be maintained andavailable for inspection.The laboratory shall, wherever possible, ensurethat any data recording and analysis computersoftware is fully documented and validatedbefore use.The standard of accommodation and facilitiesshall be such as to facilitate properperformance of the tests. Effective monitoringand control of environmental factors shalltake place. Due attention shall be paid tofactors such as mains voltage, electromagneticinterference, humidity, temperature, dust andvibration levels.The laboratory shall have an effectivedocumented system for identifying test items.There must be a systematic and documentedrecord of all information of practicalrelevance to the tests performed. All recordsmust be maintained for a period not less than6 years.

11.9.2 Advantages of laboratoryaccreditation

The examples of the requirements that a NAMASaccredited EMC test laboratory must meet arestringent and involve the laboratory in sometime, trouble and cost. The accreditation may betaken as an indication that the requirements setout are being met by the laboratory on a dailybasis.

For a manufacturer reqUIrIng EMCconformance testing the use of an accreditedlaboratory offers a number of advantages in termsof confidence in the q uali ty of test reportsobtained. A major one is the acceptance of thirdpart certification by customers, avoiding the needfor each customer to carry out a separateassessment. A list of accredited laboratories (atthe time of writing) is given in NAMAS ConciseDirectory M3 [32].

Not all EMC tests need to be carried out inNAMAS approved facilities; adequatedevelopment or preconformance testing can bedone in other laboratories, or by the manufacturers themselves. Under the terms of the ECharmonisation directive 89/336/EMC self certifica-

UNCER1'AINTY ANALYSIS: QUALITY CONTROL AND TEST FACILITY CERTIFICATION 221

tion of product EMC performance by manufacturers using their own facilities is permitted. Itmay be in the interests of self certifying Inanufacturers to obtain NAMAS accreditation for EMCtesting if they wish to convince customers they areworking to the highest q uali ty standards, or offera testing service to other companies.

NAMAS-accredited laboratories may becomethe core of competent bodies referred to in theharmonisation legislation who can assess theEMC performance of a new equipment bystudying the technical file which is built upduring the development programme. If theassessment is posi tive it is possible to certifycompliance with the relevant standards calledup in the legislation wi thou t recourse to a full/ /EMC test. Thus the generation and submissionof the technical file to a NAMAS/ liboratorycould be the preferred route to EMCcompliance if the equipment is large, difficultand expensive to test.

The electrical measurements division of theNational Physical Laboratory and the NAMASexecutive are approachable and will help in theunderstanding and interpretation of technical andquality requirements which relate to EMC testingin the UK. EMC test facility managers andengineers, together with managers anddevelopment engineers in firms producingproducts which require EMC clearance, shouldstudy carefully the NPL/NAMAS documentationreferred to in this chapter to appreciate theframework within which EMC compliance testingis carried out in the UK.

11.10 References

SPIEGEL, M.R.: 'Probability and statistics'(McGraw Hill, 1988, SI edn.)

2 FELLER, W.: 'An introduction to probabilitytheory and its applications' (Wiley, 3rd edn.)

3 BEAUMONT, G.P.: 'Probability and randomvariables' (Wiley)

4 HARPER, W.M.: 'Statistics' (Macdonald & Evans,London, 1965)

5 TUCKWELL, H.C.: 'Elementary applications ofprobability theory' (Chapman & Hall, London)

6 BOX, G.E.P., HUNTER, W.G. and HUN'TER,j.S.: 'Statistics for experimenters: An introduction to design, data analysis and model building'(Wiley)

7 TOPPING, j.: 'Errors of observation and theirtreatment' (Chapman & Hall, London)

8 DIETRICH, C.F.: 'Uncertainty, calibration &probability' (Adam Hilger, 1973)

9 HARRIS, LA. and HINTON, L.J.T.: 'Theexpression of uncertainty in electrical measurements'. BCS guidance publication 3003, issue 1,1977

10 HINTON, L.J.T.: 'Uncertainty and confidence inrneasurements' in BAILEY, A.E. (Ed.):'Microwave measurements' (Peter Peregrinus,London, 2nd edn., 1989) Chap. 25

II NAMAS information sheet NIS 20. NAMASExecutive, National Physical Laboratory,Teddington, Middlesex, TWll OLW

12 WHITE, D.R.J.: 'A handbook series on electromagnetic interference and compatibility, volume 2 EMI test methods and procedures'. Don WhiteConsultants, Germantown, Maryland, USA, Chap. 5

13 MEYER, S.L.: 'Data analysis for scientists andengineers' (Wiley)

14 'Statistics, probability and reliability' . NATO ES/CISG/71, draft issue C 1986, Chap. 4

15 GOSSETT, W.S.: 'Student's 't' distribution'.(Statistician to Guinness Brewery, UK)

16 Shielded enclosure performance measurernentprogram. APG contract DDAD05-77-9551, USA

17 WHITE, D.R.J. and MARDIGUIAN, l\r1.: 'Errorsin EMC compliance testing and their control'. DonlvVhite Consultants Inc, Gainesville, Virginia 22065,USA

18 HEIRMAN, D.N.: 'Education and training of theindustrial regulatory compliance test team'.Proceedings of IEEE symposium on EMC) 1981,pp. 365-368

19 FREE, W.R. and S'TUCKEY, C.W.:'Electromagnetic interference methodology, communications equipment'. Technical report ECOM0189-F, NTIS AD 696496, Oct. 1969

20 'RF and microwave measurements'. NPL 00/1.5K/NI/9/90, National Physical Laboratory,Teddington, Middlesex, TWll OLW, UK

21 DE MARINIS,j.: 'The antenna cable as a source oferror in EMI Measurements'. Proceedings of IEEEsymposium on EMC) 1988, pp. 9-14

22 BRENCH, C.E.: 'Antenna differences and theirinfluence on radiated emission measurements'.Proceedings of IEEE symposium on EMC) 1990,pp. 440-443

23 DASH, G.: 'Computing equiplnent standards -- Anupdate on cable and peripheral placement'.Proceedings of IEEE symposium on EN/C, 1987,pp. 332-337

24 NAMAS accreditation standard: General criteria ofcompetence for calibration and testing laboratories,MI0. NAMAS Executive, National PhysicalLaboratory, Teddington, Middlesex, TWll OLW

25 'NPL points of contact'. NPL0004/8K/Nj/6/90,National Physical Laboratory, Teddington,Middlesex, TW11 OLW, UK, pp. 36,37

26 'The expression of uncertainties in electricalmeasurements B3003'. NAMAS Executive, NPLTeddington, Middlesex, 1'W11 OLW (Related toRef. 9)

27 'Specification for the radio interference measuringapparatus and measurement methods, section 9:Statistical considerations in the determination oflimits of radio interference'. CISPR 16

28 DVORAK, T.J.: 'The problems of limits interpretation in type approval testing and productioncontrol'. Proceedings of IEEE symposium on EMC,1981, pp. 264-268


29 'Compliance with limits in large scale production'.CISPR Recommendation 19, publication 7

30 'Regulations to be met by calibration and testinglaboratories'. Publication M 11, NAMAS Executive,NPL, Teddington, Middlesex, TWll OLW, UK

31 'The quality manual: Guidance for preparation'.

Publication M16, NAMAS Executive, NPL,Teddington, Middlesex, TWll OLW, UK

32 'NAMAS concise directory'. Publication M3,August 1990, edn. 5, NAMAS Executive, NationalPhysical Laboratory, Teddington, Middlesex,TWll OLW, UK

Chapter 12

Designing to avoid EMC problems

12.1 Intrasystetn and intersystetnEMC

Electromagnetic compatibility is likely to play anincreasingly important role in both electronicengineering and commercial law during the 1990s[1]. rrhe overuse of the EM spectrum, the rapidinclusion of microprocessors into a very widerange of industrial, commercial and domesticequipment, and the increased awareness ofpossible effects of EM energy on biologicalsystems will bring this abou t. Electromagneticcompatibility has emerged from the world ofmilitary and space systems manufacture, tocontribute to the performance of everydayelectronic equipment and the profitability ofcompanies engaged in its supply.

There are two kinds of electromagnetic compatibility which a system or equipment designer mustconsider: intrasystem compatibility, which isconcerned with EMI internal to an equipment,and intersystem compatibility, which is concernedwith EMI external to the equipment. Thisinvolves compatibility with other equipments orsystems in its immediate electromagneticenvironment and the safeguarding of the widerEM spectrum.

12.1.1 Intrasystem EMC

Internal electromagnetic compatibility isimportant for the equipment to function properlyin its own right. Without careful EMC design,circuits can interfere with each other and thisleads to unreliable and degraded operation,unnecessary expensive field support engineeringand possibly a loss of customer confidence in theproduct. Electronic designers have a greatnumber of problems to consider and sometimesonly recognise EMC when it becomes a problem,usually close to the end of the design stage whenit is far too late to effect low-cost solutions. In thepast, electronic designers have sometimes designedwith only the obvious product design goals inmind, and have not recognised the need for aformal approach to EMC to be considered as anintegral part of the design process. Thusequipments have sometimes been produced wherethe margin between successful operation anddegraded performance due to a lack of internal

223

EMC has not been quantified. Productionvariability can then lead to EMC problemsmanifesting themselves in some proportion of theunits manufactured.

Design is an iterative feedback process whichinvolves continued interaction between requiredperformance and technical and financialconstraints. I t involves the synthesis of systemarchi tecture and the development of strategies andtechniques by which the product performancegoals can be met in the optimum cost-effectivemanner within the imposed project constraints.

In general, it is the designer who is technicallyresponsible for every significant aspect and resultof his design. But EMC problems can often arisefrom obscure causes which may not be significantin terms of the design strategy adopted to meetthe primary design goals. The designer can callfor assistance in the form of an EMC specialist,possibly an external consultant who will providethe background knowledge and expertise toadvise the designer and his team about aspects oftheir proposed design which may lead to EMCproblems. Together they can then work out a setof modifications which improve the EMCperformance of the design without compromisingthe original design goals.

Whilst this approach does work, it necessarilysets the EMC expert apart frorn the main designeffort and can cast him in the role of theopposition, an extra qualification on the designwhich is being developed. In these circumstancesEMC engineering tends to con tinue as adiscipline outside that of mainstream electronicdesign and the situation is repeated again andagain, as EMC knowledge stays with the EMCexpert and is not transferred and integrated intothe product design team. To do so requirescompany management to realise that EMCproblems and solutions do not fit neatly intowatertight design compartments, nor can it bedealt with at a single level of technical staff andmanagement. EMC is a vital part of productdesign and the best way to ensure success is to setaside time and funds to train existing designers andto recognise the value of using multidisciplineElVIC skills as just one part of the formal designprocess. See Figure 12.1.

Consultants can still occasionally be brought into supplement the EMC knowledge required for


Figure 12.1 EMC is just one important product designfeature

yELECTRICAL I I REQUIRED H SOFTWARE

DESIGN r------i EQUIPMENT DESIGNPERFORMANCE

12.1.2.2 EMC control plan

The evolution of a satisfactory EMC design thatcan meet the contractual requirements should beensured by the control afforded by adequate documentation, design reviews and for large projects,the referral of key design decisions to an EMCcontrol board. The top-level control plan willspecify how the design is to be controlled, bywhom and with what means.

EMC expertise is required to establish and agree asatisfactory contract for the equipment to beproduced. In the simplest case this may onlyinvolve interpreting the technical and cost impactof the legal requirements flowing from EEC/89/336 and appropriate technical standards forequipment to be sold within the EEC. For

/equipment to be sold worldwide, carefulassessment of the multitude of country andtrading block EMC standards and regulationsmust be made to establish equivalences andunderstand which regulations take precedence.Only when this has been completed can theworst-case composite specification be defined andits impact reflected in the contract and price.

12.1.2.1 Contractual assistance

12.1.2.3 System specification

The contractual requirement for EMC will beinterpreted into design requirements in the formof system and subsystem EMC budgets as shownin Figure 12.2. These documents can be simple inthe case of a commercial product that only has tomeet, for example IEC 801, or may be complexin the case of a military equipment or largecommercial system. The subsystem specifications

earliest stage in the development process with flairand invention.

The existence of a specified external EMenvironment in the relevant standard is thestarting point for the intersystem EMC designer,who knows what performance must be met. Inthe case of designing purely for internal compatibility, one must attempt to specify both theemission and susceptibility levels for theequipment and set an affordable margin ofcompatibili ty.

Designers of military equipment are familiarwith the formal control of the EMC design andcertification process as required by the relevantMIL or DEF standards. Much of the approach istransferable to commercial equipment designteams without imposing unnecessary technicalconstraints or costing too much.

The general approach is outlined next.

ENVIRONMENTALENGINEERING

12.1.2 Design for formal EMCcompliance

With the advent of these mandatory regulationsconcerned with the electromagnetic compatibilityof almost all commercial electronic equipment,EMC has come to be associated by someprincipally with those procedures and practiceswhich will ensure compliance v\lith standards.l'his is perhaps a narrow view and fails torecognise the reliability and other performance/quality benefits which good intrasystem EMCdesign can bring to the product to increase itscompeti tive edge in the market.

A successful EMC design requires more thanslavish adherence to a set of design rules andpractices, useful though these are. Designers musthave the knowledge to design in EMC from the

certain tasks, but the long term goal of thecompany should be to access education andtraining programmes [2-5J to build an EMCdesign knowledge base within the normal designteams.

The need for management to provide trainingsupport for staff to ensure good EMC design,especially for companies producing commercialelectronics, is sometimes not understood. Thismay stem from the multifaceted nature of EMCand the difficulties which arise in quantifying thebenefit to be gained by a formal EMCprogramlne being carried out by the design team,when compared with the investment which isneeded for their training and support.

The situa tion has been a little clearer forcompanies in the military and aerospace sectorswhich have been contracted to meet EMCstandards, and formally demonstrate productcompliance. Legally enforceable EMC standardsare being introduced in the UK in the first half ofthe 1990s to regulate the EMI from commercialelectronic products. When this happens, and thepenalties for noncompliance are known, the costbenefit in pursuing good EMC design will bemuch more obvious.

DESIGNING TO AVOID EMC PROBLEMS 225

EMC DESIGNBUDGET CONSTRAINTS

SIMPLE EMICOUPLING MODELS

Figure 12.2 Top-level EMCdesign process

EMC TECHNICALDATABASE

(Case histories!Expert system/Consultants)

EMC TECHNICAL APPROACH- a design methodology for

cost-effective balancedhardening

CO-ORDINATED EMC DESIGNAND TESTING PROGRAMME

CONTROLLED BY EMC CONTROLAND TEST PLANS

EMC DESIGNHANDBOOKS

- detail technicaloptions

will reflect the need of the product to meet theimposed external EMC specifications and theneed to achieve internal compatibility.

Clear understanding of the technical aspects ofEMC is important at this stage as the apportionment of the contractual and internal EMCrequirements down to subsystem level affects thecost of optimum design for compliance withimposed standards and correct equipmentfunctioning. The systems engineers or designersmust have a good understanding of EMChardening methodologies [6-8J and be capable ofrunning trade-off studies between various possiblehardening solutions to select the best overallsystem philosophy.

12.1.2.4 Design handbooks

Often design engineers engaged on EMC will makeuse of a wide range of general design handbooks toaid their work. Some are produced by the Military[9-12J and various books and notes [8, 13-15Jhave been written which include information onEMC design. Although some of the basictechniques are reviewed in this book, it is notintended to deal in depth with EMC designtechniques, as the subject is well covered elsewhere.

Companies which have a longstanding need forcornpetence in EMC design often produce theirown handbooks for the guidance of electronic and

other engIneers. However, these are usuallycompany confidential documents as they containthe results of hard-learned lessons in the field ofEMC design.

Design handbooks will typically include data onthe following topics:

Bonding: types, corrosion, vibrationGrounding: LF/HF/VHF solutionsFiltering: passive, lumped/distributed, absorbativeEMC aspects of mechanical design: box

fabrication, seams, materialsShielding: materials, penetrations and aperturesCable design: coupling, screening, routing,

segregationConnectors: type, use, shielding, connection to

cable screensCircuit board design: ground planes,

decoupling, loop areas, inductance and straycapacitance

Component placement and orientationIC logic selection: speed, noise emission and

susceptibili tyPower supplies: regulation, noise emission,

susceptibili tyOptoisolators and fibre opticsTransformers: mains, audio, video, "RFCommon-mode/differen tial-mode interfacesInterface circuit protectionElectrical safety requirements.

Figure 12.3 Block diagram of the generalised EMIsituation

The configuration of these design elements into acomplete EMC equipment design is a taskrequiring considerable theoretical and practicalknowledge and experience. In a real system thetransfer of EM energy can occur at subsystemlevel (wi thin the system), or between the systemand its EM environment and between the systemand other systems. Depending largely on thedistances between the coupled elements, thecircuit impedances and the frequency of the EMenergy being coupled, the coupling paths can beconsidered to fall in to three broad types, as shownin Figure 12.3. All possible paths or combinationsof paths must be addressed in a competent EMCdesign.

l

eg. own subsystem,Iown system, othersystem, the generalEM environment

{real coupling USUallY}

involves parallel pathsleg. own subsystem, Iother system, externalEM environment


12.1.2.5 EMC computer modelling

Extensive EMC system-level computer modelshave been constructed in the past for military andspace programmes such as SEMCAP [16] (systen1and electromagnetic compatibility analysisprogram) but the use of such a program isperhaps inappropriate to all but the largestprojects. Smaller versions have been producedwhich are better for smaller projects, but thesecomplex software packages have not beenuniversally adopted as the best way of estimatingpotential EMC problems.

Other EMC system software has been developedlargely in the US, and is described by Keiser [13].These models include IPP-1 (interference predictionprocess) and IEMCAP (intrasystem electromagneticcompatibility analysis program) which determinewhether a coupling path exists between any twonominated subsystems/units as shown in Figure 12.4.Appropriate transfer function or coupling models arethen used to determine the level of potential incompatabilities between the various subsystems.

The incompatibility identification process can bevisualised in the form of a set of matrices as shownin Figure 12.5. The matrix is composed of columnsof subsystems defined as ABC D etc. which are

EXAMPLE SYSTEM (transmitter)

POWER AMPLIFIERSUBSYSTEM ICI

OSCILLATORSUBSYSTEM IBI

POWER SUPPLYSUBSYSTEM IN ~-.:...._----..

Figure 12.4 Illustrativeexample oj intrasystemcoupling paths

PROBABLE COUPLING PATHS

LIST OF CONDUCTED EMISSIONSLIST OF RADIATED EMISSIONS

LIST OF CONDo SUSCEPTIBILITYLIST OF RAD. SUSCEPTIBILITY

SUBSYSTEM 'A'cCULPRIT

(Psu)

CERE

SUBSYSTEM 'Bl

cCULPRIT(oscillator)

CERE

SUBSYSTEM IA'vVICTIM

(Psu)

cSRS

SUBSYSTEM'SSvVICTIM

(oscillator)

CSRS

SUBSYSTEM 'C'cCULPRIT

(PA & antenna)

SUBSYSTEM ICIV

VICTIM(PA & antenna)

CULPRITS --- COUPLING PATHS ---- VICTIM

NOTE: SUBSYSTEMS ARE TAKEN TO BE COMPATIBLE WITH THEMSELVES


The diagonal line representsself-compatibility at subsystemlevel.

Susceptibilities divided intoCS • conducted susceptibilityRs(E) - E field radiated susceptibilityRs (H) - H field radiated susceptibility

Each cell contains details of frequency,level and modulation characteristics ofpotential incompatibilities betweeneach combination of subsystems.

Radiated susceptibilityhas two components

I""uscepfibiJity I Rs(E field) \componentI Rs(H field)"- '~l\SUE SYSTEMS AS VICTIMS (CS)

Av Bv Cv Dv -I-~-"':""~ -- Nv

Ac "" {w

""Q. BeenI-~

Ce ""a...J:::>0

""en Dc<en:e I

""w Iti)-. I

""enm I ~

:::>en

""~Ne t i"---

')

Conductedshas on/yone

p'igure 12.5 Intrasystem compatibility matrix

considered to be potential culprits capable ofemitting unwanted electromagnetic energy byconduction or radiation to other subsystems. Therows in the matrix contain actual or estimatedsusceptibility data for each subsystem/unit as avictim of EM interference. The frequency andamplitude details of any potential incompatibilitiesbetween any subsystems/units are recorded in theappropriate matrix elements.

For clarity, three rna trices can be used: one forconducted interference incompatibilities and oneeach for E- and H-field radiated incompatibili tiesas shown in Figure 12.5. The subsystems or unitsare assumed to be self compatible and thus the1:1 diagonal line in the matrix is void.

Many EMC system level programmes can be usedto 'cull' the many possible frequency matches whichwill be found between culprits and victims, on thebasis of expected incompatibility level. Thuspotential subsystem incompatibilities can beranked into 60 dB problems, 30 dB problems orrelatively trivial problems of less than 10 dB. Thisanalysis can be very useful in directing the attentionof the design team to the key EMC issues at a veryearly stage in the design. Detailed analysis of seriousincompatibilities can then be carried out withcomputer models such as NEC [17J and EMAS[18J or other finite element/difference/method ofmoments or other electromagnetic codes.

Whatever computer software packages are used

to carry out system-level EMC assessments, agreat deal of reference data and a number of casehistories are usually used as background materialto support the current assessment. I t is thereforeimportant that the designers and managers ofprojects with an EMC content, formally andcarefully, record data generated on their projectand make them available to future projects bycontributing to a company wide EMC database.

In recent years, some design teams haveattempted to codify EMC system and detaileddesign knowledge in the form of EMC expertsystems which can be run on personal computers[19, 20]. This technique may prove to be morepractical than the earlier large models and moresuitable as a tool for equipment designers ratherthan EMC specialist engineers.

12.1.2.6 Test plan

The requirement for this document would bewritten by the EMC project management team inthe case of a large project and would require thedesigners and EMC test specialists to define whattesting is required at the various stages of theequipment development. The plan itself would beproduced by the designers and EMC testengineers. The test plan would include state,mentson the following tests.

Risk reduction tests: These short tests would be


EMC:

NEMP:

Transients:

Lightning:

12.2.2 Determining EMC hardeningrequirement

The system hardening requirement for anequipment which must meet a full range of interrelated electromagnetic specifications may have toaccommodate the following features.

Radiated susceptibilityRadiated emissionsConducted susceptibili tyConducted emissionsImported transient immunityExported transient generationRadiated susceptibilityConducted susceptibili tyDirect effect immunityIndirect effect immunity

In addition, some equipments or systems may besubject to extra requirements, such as that forsecure communications (Tempest). If all theserequirements have to be met simultaneously tosome degree, as for example with the modernmilitary aircraft, the task of reconciling thediffering technical solu tions to meet these requirements can be considerable.

The generation of typical system hardeningrequirements is demonstrated in what follows withreference to radiated susceptibility and emissions.When evaluating conducted emissions and susceptibilities the situation is more straightforward andsimple transmission-line models can be used.

legal obligations relating, to product sale, exportand use, the system-level EMC requirement canrange from simple to very complex. If complex, itis necessary to consider a hierarchy of requirements and define areas of precedence for certainspecifications and standards for the product as awhole. For example, electrical safety requirementsare usually paramount.

Once the customer/contractual/legal/export/licence for operation and electrical safety requirements have been defined, it is then necessary toestimate the probable levels of RF emissions andsusceptibilities of the proposed design. These datacan then be compared with the requirements, andemissions reduction requiremen t or hardeningrequirement (for immunity) can be defined.

These EMI immunity hardening or emissionreduction requirements must then be apportionedto the various subsystems such as power supplies,mother board design, clock and processor circuits,cable harnesses, interface boards/circuits andmechanical design of the case or container, basedon the need to achieve these req uirements for theminimum cost and impact on the optimumequipment design.

12.2 Systetn-Ievel EMC requiretnents

12.2.1 Top-level requirements

Depending on the nature of the product or systemand any specific clauses relating to EMC in thecontract to purchase, together with any EMC

undertaken to resolve a design question in favourof one technique or another. They are carried outat a very early stage of the design and are in someways the most important tests which will becarried ou t on the project. They will set thecourse for all future EMC engineering solutionsand may be concerned with deciding on agrounding and bonding philosophy or selectingthe types of integrated circuits to be used.

Development tests: These semiformal tests wouldbe required at certain well-defined stages of thedeveloping design to check that the EMI predictionswhich will have been made are being achieved.Such tests may be conducted before design drawingsare formally adopted and other elements of theoverall design are allowed to proceed on the basis ofwhat has already been achieved.

Occasionally additional informal developmenttests may need to be conducted by the designer toquickly verify the EMC benefits of a particular circuit, shield or technique. Such tests need not be doneby submitting the problem to a test house, external orinternal to the company. They are best done on thebench by the designer responsible for that element ofthe design who may be assisted by test personnel andmay need to use some of the specialist equipmentsuch as current probes and spectrum analysers.

Conformance tests: The test plan for these testsis usually written by the EMC test laboratorypersonnel in conjunction with the design team. Itrequires intimate knowledge of the standards andspecifications which have to be met and thelimitations of test techniques which are specified.

In certain circumstances, on small projects forexample, a consultant may be called in to liaisewith the external approved test laboratory, tohelp generate the test plan and to oversee thetesting which is carried out. The consultant mayalso be in a position to advise the designer as tothe validity of the results obtained and to pointout improvements which should be made to thedesign if the equipment has failed the test.

On large projects the certification testing mayinvolve formal demonstration of the equipment orsystem functioning in its operational environment,either at its place of installation or on a speciallyprepared EMC test range. Often these finalproving trials are witnessed by a representative ofthe customer's organisation and may be importantin obtaining a stage or final contract payment.


Figure 12.6 Example of specification for composite RFenvironment in which system must operate

------~:fj)CURRENT INDUCED

IN CABLEHA/m

k

CABLE~---"" 7.5m 10ng-----!

....--~--~ E VImZ (waveimpedance)

--~--

COMPLEX RF CURRENTSFLOWING IN BOXSTRUCTURE

12.2.3 Simple coupling models

FIELD INCIDENTON BOX AND CABLE

Figure 12.7 Example of box-and-cable pickup

apertures and a simple dipole model may be usedfor field-to-wire coupling.

This section demonstrates how simple considerations of key physical aspects of a system, such asits size and the length of conductors inside it, orcables attached to it, can be used to predict itscoupling properties to external fields. Designersworking on projects which have access to sophisticated computel' models will of course make fulluse of them, bu t they may also carry out calculations using simple models to crosscheck theirresults.

Many examples of simple models exist [13, 14,21, 22J which enable predictions of certain aspectsof EMI problems to be made. These include

Common-mode coupling of fields into the boxcable-box loop area for balanced andunbalanced systems

Differential-mode coupling into box-cable-box loop area

Differential-mode coupling into coaxial cablesCapacitive coupling between circuitsInductive coupling between circuitsCoupling rejection of twisted pair cablesShielding effectiveness of coaxial cablesShielding effectiveness of metallic screensFilter performance modelsActive component rectification modelsAttenuation of EM waves through buildings.

The selection and combination of these simplemodels to 'scope' EMI problems in a systemrequires experience. Methodologies derived byWhite [22J and implemented in the form ofcomputer software can be of assistance. Asexamples of simple models, the circuits on whichthe capacitive and inductive coupling models arebased are shown in Figure 12.8. The field -to-cablecommon-mode coupling circuit is shown in Figure

10GHz1GHz

EXCURSIONS TO ACCOMMODATEINTERNAL COMPATIBILITY (due toradiating antennas in the system I

for example)

10MHz 100 MHz

FREQUENCY

COMPOSITE FIELD STRENGTH SPECIFICATION

1MHz

3COMPOSITE EXTERNALRF ENVIRONMENT

0.1 SPECIFICATION

100

30

1000 .------------....----------.-----,

E

>:c~C)zwa:~enCl...JWu::

Large projects may use complex computermodels to define a system hardening requirementgiven the specifications of the external electromagnetic environment and an estimate of theinternally generated interference. For mostprojects, EMC systems engineers or designers canuse a number of simple models to yield a worstcase estimate of the hardening requirement. Thesophistication of the models available will dependon the expertise of the design team and thefinancial resources within the project.

An example follows of the use of simplemodelling tools to establish an early estimate ofthe scale of the EMC design task on a project.Such information guides the technical andmanagement framework within which the electromagnetic design engineering of equipment iscarried ou t.

Consider Figure 12.6 which shows a systemEMC radiated immunity requirement for projectX. This will have been obtained from the EMCstandards and specifications called up in thecontract, with an additional element derived fromthe engineering estimate of the potential interference levels likely to be self generated within theequipment. This intrasystem compatibilityrequirement luay consider, for example, the localfields due to high power RF radiation from anantenna which may be associated with theequipment being developed.

To calculate the hardening requirement it isnecessary to estimate the worst-case coupling fromthe specified field to the equipment being designed.Two features of the equipment tend to dominatethe EM field coupling: the length and type of cableruns, and the characteristics of equipment boxesand conductive structures in which they aremounted together with any apertures, as shown inFigure 12.7. A number of models can be used toevaluate the worst-case coupling via the box and its


Reprod ueed by permission of ICT Inc.

difference in means correspondsto 'average shield performance'

( UNSHIELDED WIRE) I PICK-UP PROBABILITY14-----'---- .-/ DISTRIBUTION FUNCTION

EM wave couples

into loop area

Figure 12.10 Probability density functions for shieldedand unshielded wires

Overlap in distributions indicates that occasionally pick-upin a shielded wire is as great as in an unshielded one.

Reprod uced by permission of ICT Inc.

Figure 12.9 Two-box EMI problem

COMMON-MODE COUPLING INTO BOX-CABLE-BOX-GROUND LOOP AREA-10---r----r----r--_-....-- _

Ico -20"0

~ -30

~ -40::J0.. -50:::>8 -60woo:EZo:E:Eo()

Reprod ueed by permission of McDonnell Douglas

showed little effect on the pickup probability distribution function, which has a log-normal form witha standard deviation of between 3 and 6 dB.

Shielded wires of course will normally tend topick up less than unshielded wires, but the distribution means may not be as widely spaced as theassumed shielding figure (of perhaps 30 dB) wouldsuggest. Practical measurements of real cableswith imperfect shield terminations, poor braidsetc, show that the worst-case pickup from ashielded cable can be comparable with pickuplevels for an unshielded cable. See Figure 12.10.

SHIELDED WIREPICK-UP PROBABILITY

cF- DISTRIBUTION FUNCTION

wozW0:0:::::>oooLLo

~co«CDo0:O'~_~__----1......_--1 I0000-~.......~--..........~

(Receivednoise)

Cross talk capacitance

"---~~--~--Mutual impedance

s

(Source ofnoise/EMI)

(Source ofnoise/EMI)

(b)

Figure 12.8 Typical equivalent circuits used to modelcable crosstalk above ground plane

(a) Circuit representation of capacitive couplingbetween parallel wires over ground plane

Vv/Vc = ( -; 1 ) = wCw<zlm1 +J wCcv< vlm

for f « 1/2nCcv< zlm whereW = 2nfxfrequency in HzCev = wire-to-wire coupling capacitance per

metre< v = parallel combination of victim source and

load impedances and wiring capacitanceCv to ground = l/(1/<vI + 1/<v2+ jwCvlm)

lm = cable length in metresCv = victim wire capacitance to ground return

per metre( b) Circuit representation of inductive couplingbetween parallel wires over ground plane

12.9 together with examples of the calculatedcoupIing response for various loop areas as afunction of freq uency.

Engineers at McDonnell Douglas [21] haveconducted many experiments on field -to-wirecoupling of representative avionics cables, includingsingle wires. They showed that the coupled powerfrom the field to the conductor depended on

wavefield frequencywire orientation to the wavewire geometrical factors: length, dialueter,

shape, rou tingterminating load impedance

In a real system there is little control over therouting and orientation of cables with respect toany particular subsystem and the internal field-tocable coupling can be considered as random.Measurements made by McDonnell Douglas usingdifferent lengths of cable and load impedances


Reproduced by permission of McDonnell Douglas

10GHz

cable/structureis electrically

long

1MHz 10MHz 100MHz 1GHzFREQUENCY

close to first cable or structure resonancetypically 5 - 50 MHz for average system

0.1 MHz

" Coupling for matched dipole (tuned length) is" unrealistic below first resonance length of~ cable, or system structure.

" " Intercept point at close to,, first cable or structure--------·1 resonanceElectrically short

mismatched

dipole model I P a ..!..

Paf 2 I f2Pa 1 Pa ')..}

i 2 ~20 dB / decade I

Icable/structure cable/structureis electrically is approximately

short resonant1....----

Maximum power inducedat close to first cable orstructure resonance

~3:OCO0:::"0LLOa......J:::>~OLLUJUJ

5~-3:a.....O:::zUJUJ3:0~~LL-0:::

10

i-wave dipole aperture = 0.131..'

.. ·0.16 m long wires• - 3.05 m long wires

-50~ --I ""'"

0.1 1

FREQUENCY GHz

·10wcr:::,)....cr::.....,We;'

~..s .20

~.!~§ -30LL oIt,.-~ ·40~

O~----------,r--------.....,.

Figure 12.11 Measured maximum effective apertures(Ae) of various wire lengths. The halfwave dipole is a reasonable upper boundfor these experimental data

The McDonnell study showed that the pickupcould be characterised by calculating the effectiveaperture of the wire by dividing the power pickedup and delivered to the load by the wavefieldpower density:

12.1

where P == power picked up by the wire (watts),Pd == EM-field power density (W/m2

), andAe == effective aperture of the wire (m2

).

Measurements of the maximum effective apertureof real wires produces a plot as shown in Figure12.11 for which a least squares fit is close to thatfor the theoretical aperture for a matchedhalfwave dipole, which is given by

12.2

where A == wavelength. This matched-dipoleformula is not, however, useful at low frequencieswhere the length of a halfwave dipole would begreater than the actual length of any cable orsystem structure. Thus the resonant matcheddipole model should be abandoned at frequenciesmuch below the fundamental cable or systemstructure resonance.

At such frequencies the condition for a tunedhalfwave dipole is not physically achievable in thereal system and the model dipole becomesdetuned and electrically short, with a risingreactive source impedance still driving the fixedload impedance. Under these conditions thepower delivered to the load tends to fall withfreq uency owing to the mismatch [13, 21].

From the McDonnell Douglas study two thingsmay be deduced:

(i) Complex coupling in real systems is besttreated in a statistical fashion without

Figure 12.12 Composite wavefield to structure/cablemodel: inverted V

making an attempt to calculate individualcoupling path coefficients for the enormousnumber of possible coupling combinations.The exception to this approach arises whenthere is a clear dominant coupling pathfrom a strong interference culprit to asensitive victim subsystem. In such a casethe coupling to the particular cable shouldbe calculated individually.

(ii) The simple matched dipole formula can beused to crudely estimate the worst-case fieldto-cable coupling. For frequencies below thecable first resonance a limit must be appliedto the matched dipole model and a roll offwith frequency can be used resulting in thecombined curve appearing as an inverted-Vas shown in Figure 12.12.

12.2.4 Susceptibility hardening case study

12.2.4.1 Cable hardening requirement

Consider the equipment shown in Figure 12.7 witha box of electronics of 1 m side connected to anunshielded cable approximately 7.5 m long, whichwill have its first resonance in the region of20 MHz. Let the specified RF field strength inwhich this equipment must operate be as shownin Figure 12.6 and reproduced as curve A inFigure 12.13. As an example of a generalcoupling model, use the simple matched-dipoleworst-case pickup model at frequencies where thecable is electrically long (above 20 MHz). Onecan estimate the induced power shown in Figure12.13 as curve B (shown dashed above 1 GHz).

The coupled power at low frequencies rolls off


1 VIm

10 VIm

zo:r:~""00-Z~

ClUJ 0...Ja:w~ .... a.U-(J)CJ)

1000 VIm

z

~o ~:::>10dB m'20dB ~

UJ...JCO«()

~SYSTEM CABLES

SIMPLE WORST CASE HARDENING REQUIREMENT

CURVE 101

CURVE IBI

/ ...-~-------- ""...-------- I, ~CMOS---- ~... --'1'" • • TTL,.,......... -- / . .-BURN OUT ( linear & digital •integrated circuits ) ~ ~L1N IC

~ ~. ~

~ -CMOS DEVICES_ ~ .~(leVelchange)~ ........~ CURVE'E'~

TTL DEViCES... • _.-- _./

(Ievelchange) _---.....---------/LINEAR ICs

30

20

10

~ 0"0

g, -10~

() -20c::ffi -30

~ -400..

-50

-60

-70

f-Zw~w coc::5

"0

a 80wc::

700zZ 60w0 50c::«:c 40w...JCD 30«0~ 20w.... 10CJ)

>-CJ) 0

Figure 12.13 Development ofcable hardening equipment

1 MHz 10 MHz 100 MHz

FREQUENCY

10Hz 100Hz

with frequency from the value at 20 MHz (close tothe first cable or structural resonance). Applyingthis roll off to the specified field strength (curve A)in this frequency range generates curve C. Becausethe system power and data cables in which thepickup is occurring are not specially designed totransfer RF signals, there is an additional cableloss above 1 GHz which should be introduced asshown in curve D. This modifies the pickup (curveB) above 1 GHz and results in curve E. The finalsimple estimate of maximum power picked up inthe cable and delivered to circuit (loads) inside thebox is given by curves C and E.

For specific system configurations, other simplecoupling models, such as the box-cable-box model(referred to earlier [22J) may be appropriate andcan be used in a similar vvay to predict the powerextracted from a field and delivered to apotentially susceptible circuit in the box.

The delivered power may now be comparedwith susceptibility levels for the active electronicdevices used in the equipment to yield thehardening requirement.

Various susceptibility levels can be de.fined fordifferent types of active device such as linearbipolar ICs, linear MOSFET's, TTL logic,

CMOS logic etc. [21 J. These are shown as thefamily of dotted/dashed lines in Figure 12.13. Acomposite worst-case curve could be produced forall IC types used in the equipment, for bothfunctional upset and circuit damage. This couldthen be used to generate the hardeningrequirement by comparison with the pickuppower.

In this example the most severe susceptibilitycurve for linear I Cs has been chosen andsubtracted from the power pickup curve (C andE) to give the system cable hardeningrequirement which is the lower curve in Figure12.13.

12 ~ 2.4.2 Equipmen t case hardeningrequiremen t

The process that is undertaken to derive theapproximate worst-case hardening requirementfor the equipment case (Figure 12.7) is similar tothat for generating the cable hardeningrequirement. The fundamental simple model willof course be based on the screening performanceof the case material [13, 22J. However,penetration through a well constructed metalcase will be dominated by leakage throughjoints, slits and other intended apertures.Microwave EM energy will radiate throughsuitably sized apertures directly to thesusceptible device and associated wiring/PCBtracks. The transmission of radiation throughshield imperfections can be modelled aswaveguide apertures or slot antennas.

The preceding example illustrates the use ofsimple approximations to quickly assess the levelof EMC design required for a particularequipment which helps to direct early designeffort to where it is most needed.

12.2.5 Emission suppression requirement

The foregoing has demonstrated how simpleconsiderations can lead to a rough assessment of asystem hardening requirement for system susceptibili ty. Similar considerations of voltage swingsand current flows generated by active componentsaround circuit board conductors can lead to anestimate of the likely radiated signal strength fromthe equipment at a particular distance (whichmay be that stated in the emission standards, say1, 3, 10 or 30 m). A rough spectrum of the signalsgenerated by digital and other circuits canquickly be estimated from their waveforms byusing Fourier transforms or transform look-u ptables.

It must be emphasised that these techniques areonly a rough guide to setting the system


hardening and suppression requirements. Somedesigners will have access to more sophisticatedcomputer models which will lead to a moredetailed understanding. For those who do nothave these software tools, the approach outlinedhere may be useful in demonstrating that someprogress may still be made.

12.2.6 System hardening flow diagram

A process for deriving a set of EMC systemhardening requirements is summarised in Figure12.14 where a simple flow chart is presented. Akey decision to be made during the process iswhen to cease calculation and to beginpreliminary assessment testing in order to furtherreduce the engineering risk in the EMC design.

A general rule of thumb is based on thehardening requirement being greater or less than30 dB. If the initial calculations indicate that theEMC hardening or suppression required is lessthan 30 dB, further calculation effort to refine theestimate may not be as useful as a quick andsimple test. The simple calculations are generallyonly accurate to about 10-20 dB and the testsshould be made to generate information about theactual pickup or emission levels within the system.(Bear in mind that these measurements will alsobe subject to some uncertainty.) However, theextra practical information will undoubtedly helpto guide the design process.

If the initial system-level calculations show ahardening requirement greater than 30 dB theengineering design on which they are basedshould be re-examined and various mitigationapproaches involving specific design featuresshould be tried until the hardening requirement iscloser to 30 dB. If the calculations show that thehardening requirement is close to zero, then tosave development time it may be worth riskingthe cost of a system or preconformance testagainst the specified requirement.

12.2.7 Subsystem apportionment andbalanced hardening

Once the system hardening requirement has beenestablished for radiated emissions, susceptibility,conducted interference and transients, it is thennecessary to apportion the overall requirementdown to subsystem or uni t level. The details willbe specific to the system in question and the taskis not necessarily an easy one. Success reliesheavily on the experience of previous design cases,and expert system software which incorporatesthis knowledge can be of help here.

EMC is achieved through a number of technicalmeasures applied at various levels from component


Figure 12.14 Typicalhardening task flow chartfor system susceptibility

SYSTEM.....-----4 DESIGN

INPUTS

Determine.HARDENING

REQUIREMENT

HANDBOOKComponentsusceptibilityinformation orexpert EMC

system

Perform moredetailed

VULNERABILITYANALYSIS

RefineSYSTEM

HARDENINGREQUIREMENT

Reproduced by permission or BAe Dynamics Ltd.

greater than 30 dB

( usual case)

YES YES PICK UPLEVELknown?

less than 0 dB

( rarely achievable)

( sometimes achieved)

detailedDESIGN

TECHNIQUES& MODELS

HANDBOOKcoupling information

or expert system

COUPLINGMODELS

selection and positioning on a circuit board up toefficient global shielding, for example afforded bya metallised plastic case which might surround apersonal computer. For cost-effective EMCmeasures which meet the system hardeningrequirement without reducing the systemfunctional performance, balanced hardening isnecessary. This req uires careful selective apportionment of the EMC requirements for eachelectronics box, unit or subsystem, including thecabling design/installation and the mechanicaldesign of the hardware boxes and vehicle orsystem structure.

The apportionment must be made on thegrounds of what is technically achievable for anaffordable cost based on using complementaryEMC design techniques at the various levels ofconstruction. Table 12.1 indicates some of thesetechniques and the levels at which they can beapplied.

Often an effective hardening programmerequires a careful combination of screening,filtering, grounding, bonding, isolating, frequency

planning and component selection. Generally thetechniques must be used together if they are to beeffective at all levels where they are employed.For example, there is no point in placing a noisycircuit in a well-designed screened box if theinput and output cables are not filtered to thesame standard. Equally, one would not allow ahigh-power RF transmitter to operate at afrequency which was known to be in-band toother equipment in the system which is sensitiveto that frequency. Bandpass filters would be usedat the Tx and Rx subsystem ports in conjunctionwith frequency planning to minimise unwantedinterference.

EMC techniques such as these must beemployed in a coordinated way across the wholesystem being designed. The process should beunder the direction of the chief designer whocontrols and monitors the EMC tasks fromcomponent selection to the final system test toachieve the best EMC performance for theminimum cost, weight, space and impact onsystem reliability and maintainability.

Table 12.1 EMC hardening techniques

EMC technique Applied at

Component selection Circuit levelI C type selectionComponent placementComponent screeningComponent filterIngComponent-case groundingSpectrum limiting: fast-edge sluggingUse high signal levels for good noise

immunityFrequency management: selecting

oscillator, Tx, Rx, IF, frequenciesetc.

Use low-power circuits wherepossible - for low emissions

Bandlimit RF inputs and outputs tominimise spurious, intermod. andoverload signals

Large RF ground plane Board levelUse RF layout techniques for

digi tal boardsMinimise track loop areaMinimise HF signal currentEmploy loop-area compensationClose mounting of ICs to PCBUse of surface mount componentsDecoupling chip capacitorsBoard groundingBoard edge/section filtering: with

chip capacitors or lossy ferrites/pastes

Screened box with minimum Box levelapertures

Box with RF compartmentsUse of RF gasketsScreened cablesCables not routed in corners of box

or behind slots and aperturesBoard ground planes well-grounded

to boxBox grounded to structureFil tered connectorsConnectors with 360 0 backshells for

shield terminationNo unfiltered cable penetrations

12.2.8 Staff support for EMC

The design, manufacture, and field service personnelall have a role to play in ensuring successful EMC ofa product. I t is not just the responsibility of theelectrical designer and his immediate team. The


EMC technique Applied at

Screened power and signal cables Subsystem levelUse lossy filter lineSubsystem grounding and bonding

planCable routing: minimise cable runs

and loopsCable grounding scheme: single

point, both ends, multigroundUse isolated power supplyPositioning of subsystem components

on structureUse differen tial interfacesUse signal ground referenceTransformer coupleUse optoisolators on key interface

linesEmploy fibre optic signalling

Use global shield System levelEnsure all structure components are

bondedMinimise structure penetrationsUse waveguide beyond cutoff

access ductsEmploy correct RF gasketsUse system grounding planUse single RF ground if appropriate

Break up electrically resonantstructures

Use RAM or lossy coatingsCover windows, air vents etc. with

suitable RF screeningMinimise exposure to external high

power RF fields during operationSite equipment away from sensitive

receIvers

mechanical engineers, wiremen, installers andservice teams must all contribute. This willnecessitate these staff having an understanding ofwhat EMC is and how it is achieved and maintained.

Many company managements are unprepared forthis aspect of meeting the EMC requirements which


are increasingly being placed on their productsthrough measures such as the EC Directive 89/336 onEMC. It is probable that proper EMC training foreach type of staff involved in designing and supporting equipment will be necessary in the future.Prospective EMC engineers should ensure that theyreceive professional training before engaging in thisfascinating bu t sometimes difficult engineering field.

12.3 Specific EMC design techniques

This book is concerned primarily with EMCtesting; the higher level aspects of product orsystem design to meet EMC requirements havebeen discussed in some detail so that the readercan appreciate the nature of the overall EMCdesign task. This clearly involves more thanspecifying a mains filter connector, fitting a fewchip capacitors on a PC board, or spraying theinside of the plastic equipment case with metal. Itis not the intention to detail all the techniquesmentioned for achieving good EMC or to give thebasic formulas that would be needed to enable adesign. Many other books and papers have beenwritten [8, 11-15J on the whole range of designtechniques and it is suggested that these and otherreferences are consulted for information on thefollowing topics:

Shielding theory [8, 13, 14, 23, 24JSurface transfer impedance [25JCable screening design [8, 13JCable crosstalk coupling, capacitive and

inductive [8JTransmission line theory [26JFilters, types and application [8, 13, 14JRF grounding and bonding [8, 13, 14JCorrosion control of bonds [23, 24JGasketting [13, 14,23, 24JFrequency planning [8JPCB design [8, 27-33J

Many of the references cover more EMC designtopics than are listed. The books and papersthemselves also have extensive references andbibliographies. With these it should be possiblefor the reader to begin to explore the full range ofEMC design techniques and their applications.

12.4 References

HILLARD, D.E. et al.: 'Social and economic implications of EMC: A broadened perspective'.Proceedings of IEEE symposium on EMC, 1990,pp. 520-525

2 OTT, H.W.: 'EMC education - the missing link'.Proceedings of IEEE symposium on EMC, 1981,pp. 359-361

3 PEREZ, R.J.: 'First year graduate level course InElectromagnetic compatibility'. Proceedings ofIEEE symposium on EMC, 1990, pp. 232-239

4 RILEY, N.G. et al.: 'A university post-graduatecourse in EMC'. Proceedings of IEEE symposiumonEMC, 1990, pp. 240-242

5 HERIMAN, D.H.: 'Education and training of theindustrial regulatory compliance test team'.Proceedings of IEEE symposium on EMC, 1981,pp. 365-368

6 'A structured design methodology for control ofEMI characteristics'. Presented at IEEEsymposium on EMC, 1990 (late submission)

7 SULTAN, M.F. et al.: 'System level approach forautomotive electromagnetic compatibility'.Proceedings of IEEE symposium on EMC, 1987,pp. 510-520

8 WHITE, D.R.J. and MARDIGUIAN, M.: 'E1\I1Icontrol methodology and procedures'. ICT ISBN0-944916-08-2

9 NAVAIR AD1115 (obsolete but very useful).Department of Defense, Washington DC, USA

10 NWS 1000, part 1. Ministry of Defence, UK. Chap.5, section 10

11 'Electromagnetic compatibility design handbook'.US airforce systems command, 1975, DR 1-4

12 'Electromagnetic (radiated) environment considerations for design and procurement of electrical andelectronics equiprnent'. MIL-HDBK-235, DoD,Washington DC, USA, june 1972

13 KEISER, B.: 'Principles of electromagnetic compatibility'. (Artech House, 3rd edn.)

14 DUFF, W.G.: 'Fundamentals of electromagneticcompatibility'. Interference Control Technologies,Inc, Gainesville, Virginia, USA

15 'The achievement of electromagnetic campati bili ty' .Report 90-0106, ERA Technology, Leatherhead,Surrey, LT22 7SA, UK

16 JOHNSON, W.R., COOPERSTEIN, B.D. andTHOMAS, A.K.: 'Development of a spacevehicle electromagnetic interference Icompati bili tyspecification'. NASA contract 9-7305, finalengineering report document 08900-6001-TOOO,TRW systems, Redondo Beach, CA, USA, june1968

17 BURKE, G.J. and POGGIO, A.J.: 'NEC-2numerical electromagnetics code - Method ofmoments: A user-oriented computer code for theanalysis of the electromagnetic response of antennasand other metal structures'. NAVELEX 3041,Washington, DC 20360, USA

18 'EMAS electromagnetic code'. MacNeal-Schwendler Co Ltd, 85 High 81., Walton onThames, Surrey KT12 IDL, UK

19 PRICE, M.J.S. and MACDIARMID, I.P.:'Developing an expert system for EMC design'.Proceedings of IEEE symposium on EMC, 1988,pp. 331-336

20 LOVERTI,j. and PODGORSKI, A.S.: Evaluationof HardSys: A simple EMI expert system'.Proceedings of IEEE symposium on EMC, 1990,pp. 228-232

21 'Integrated circuit electromagnetic susceptibilityhandbook'. Report MDC E]929, phase 3,

McDonnel Douglas Astronautics Company, StLouis, Missouri 63166, USA

22 WHITE, D.: 'Montreux workshop on applicationsof programmable calculators and minicomputersfor solutions of EMI problems'. Don WhiteConsultants Inc., 14800 Springfield Rd.,Germantown, MD 20767, USA

23 'Design guide to the selection and application ofEMI shielding materials'. Tecknit EMI ShieldingProducts, Cranford, NJ 07016, USA, 1982

24 'EMI shielding engineering handbook'. ChomericsEurope, Globe Park Industrial Estate, Marlow,Bucks SL7 lYA, UK, Jan. 1987

25 RICKETTS, L.W., BRIDGES, J.F. andMILLETTA, J.: 'EMP radiation and protectivetechniques'. (Wiley)

26 SMITI-I, A.A.: 'Coupling of external electromagnetic field to transmission lines'. (Wiley)

27 VIOLETTE, M.F. and VIOLETTE, J.L.N.: 'EMIcontrol in the design and layout of printed circuit


boards'. EMC Technol.) l\1arch-April 1986, pp. 1932

28 MARDIGUIAN, M. and WHITE, D.: 'Printedcircuit board trace radiation and its control'. EMCTechnol. Oct. 1982, pp. 75-77

29 MARDIGUIAN, M.: 'Prediction of EMI radiationfrom PCBs', R F Design, July/August 1983, pp. 2636

30 KOZLOWSKI, R.: 'Follow PC board designguidelines for lowest CMOS EMI radiation'. EDN)May 1984, pp. 149-154

31 COOPERSTEIN, B.: 'Radiation from printedwiring boards'. Xerox Corporation, 701 S AviationBlvd. El Segundo, CA 90245, USA

32 WAKEMAN, L.: 'Transmission line effectsinfluence high speed CMOS'. EDN, June, 1984,pp.171-177

33 POLTZ, J. and WEXLER, A.: 'Transmission lineanalysis of PC boards, VLSI Syst. Des., March1986, pp. 38-43

Chapter 13

Achieving product EMC: checklists forproduct development and testing

13.1 Introduction

13.1.1 Chapter structure

This chapter is written as a set of checklists in theform of flow diagrams which engineers andmanagers can use to assist in generating their ownEMC product development and test programiues.It highlights the importance of the issuespreviously discussed and relates them in terms ofan overall EMC programme.

13.1.2 Example adopted

To focus the discussion, assume the point of view ofa manager in an electronics company manufacturing personal or small business computers whohas recently heard about EMC and the possibilityof the European Harmonisation Directive havingsome impact on the company's business. Assumethat the manager has no particular backgroundin electromagnetic engineering, and that there isno one to turn to in the company who has directexperience of EMC.

The issues that would need to be considered aredealt with in the form of a top-down appraisal,starting with general questions such as 'What isEMC and how does it affect my operation?'(including assessment of relevant standards).Consideration is also given to the issues involvedin setting up an in-house test facility to self certifynew products. Specifically, the discussion centreson the following broad issues:

Where to obtain information about EMCDetermining EMC requirements for newproductsDeveloping an approach to EMC designSetting up an in-house test facility

13.1.3 Personal computers andinformation technology

Personal/business computer products represent alarge part of the electronics industry andchoosing this relevant example permits thediscussion of EMC issues to be focused forincreased clarity. The sales of such equipmentsinto the home and office have increased rapidly

238

In the last few years, with more market growthin prospect. The existence of this large marketsupports the choice of the personal computer(PC) as a good example with which to considerthe task of coping with EMC productdevelopment for the first time. The general issuesaddressed will also apply to manufacturers ofother electronic products including industrial,scientific and medical equipment and householdelectrical/electronic appliances. The specificationsand details of test methods however will bedifferent for each class of products as has beenpointed out in the preceding chapters.

13.2 Inform.ation about EMC

Managers and engineers need information on anumber of aspects of EMC to plan and properlyexecute a product development and certificationprogramme. A check list of possible useful sourcesof information follows.

13.2.1 Customer sources

The primary motivation for a manager In anelectronics company to understand EMC is theneed to meet the expectations of his customers. Inturn, their needs may be driven by regulatoryauthorities which implement national or international legislation on EMC. Manufacturers canonly sell and customers will usually only buyproducts which meet the requirements of theregulatory au thori ty, and in that sense theregulations are the main driver. Customers mayalso have requirements for aspects of EMC whichare not covered by mandatory governmentregulation. For example, enforceable regulationscovering PCs and other IT products in somecountries may only relate to their EM emissionperformance, but the customer/operator may alsobe concerned about reliable operation in a hostileRF environment and thus be interested inequipment performance with respect to radiatedor conducted susceptibility

Among the first questions the productdevelopment manager must ask are:

Who are my main customers?- Which countries are they in?

ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESTING 239

Do my customers demand EMC performance?Are their requirements legislation driven?Are their requirements operationally driven?Do they demand special EMC performancewhich it would be uneconomic to provide inthat sector of the market?

The information gained will lead to a number ofspecifications being cited depending on theproduct type and customer countries. Forexample, to sell worldwide, the PC manufacturermay have to meet the requirements of

FCC part 15j (RF emissions) + voluntaryimmunity standards for USA customers

VCCI (RF emissions) + any special voluntaryimmunity standards for customers in Japan

EN 55022 (RF emissions) for customers in EuropeEN 55101-2 Immunity to ESDEN 55101-3/4 Immunity to radiated and conducted

EMI for customers in Europe

There may also be specific single customer EMCrequirements which must be taken into account ifthe manufacturer sells high-cost customisedsystems.

In consultation with the legal and sales andmarketing departments, the example productmanager in the PC company must attempt todraw up a precedence hierarchy for EMC specifications which are to be met by the new product,depending on volume sold, technical difficulty ofcompliance, plans for future market, competitors'EMC policy, etc.

13.2.2 Regulatory authorities

The specifications cited will be contained withinEMC standards produced by various bodies suchas the CENELEC, IEC, CISPR,VDE, BSI,SAE, IEEE, etc and called up by the clauses inthe appropriate regulations. In the UK, the DTIRadiocommunications Division is the responsiblelegislative authority and produces papers forguidance on EMC [1--4]. 1'he Department havean active awareness campaign (1993 onwards)designed to provide industry with a great deal oftechnical information on EMC. At the time ofpreparing this text information can be obtainedfrom

DTI Implementation and interpretationof regulations

Manufacturing Technology DivisionDepartment of Trade and Industry, Room 1/112151 Buckingham Palace RoadLondon SW1 W99SSo171 2 15 1403.

Notified bodiesAppointed by the DTI and operate the typeexamination procedure on their behalf for certainclasses of equipment e.g. radio transmitters

Radiocommunications Agency, Room 106Waterloo Bridge HouseWaterloo RoadLondon SE 1 3UA0171 215 2084concerning RF matters

DRAARE Fraser RangerFort Cumberland RoadEastneyPortsmouth P04 9LJ

Air Traffic Service StandardsAviation HouseGatwick AirportGatwickWest Sussex RH6 OYR

Other sources:Contact point for type approval of radio transmitters:Radiocommunications Agency, Room 514AWaterloo Bridge HouseWaterloo RoadLondon SE1 8UA

For copies of the EC directiveAlan Armstrong Ltd2 Arkwright RoadReading RG2 OSQ01734 751771

Progress on harmonised standardsBSI2 Park StreetLondon W 1A 2BS0171 629 9000

BSI Standards Sales Dept.BSILinford WoodMilton Keynes MK14 6LE01908 221 166

Technical help to exportersBSI Standards01908 226 888

For information relating to the EC commiSSioncontact the London office: Tel. 0171 222 8122

For information relating to the EuropeanParliament contact the London office: 0171 2220411


Information relating to EMC quality assuranceand laboratory accreditation in the UK can beobtained from

NAMASNational Physical LaboratoryTeddingtonMiddlesexTW110LW0181 943 71400181 943 7094

Information is also available concerning RFmeasurements and calibration of EMC antennasfromDivision of Electrical ScienceNational Physical Laboratory0181 943 7175For details of services and staff at NPL consultReference 5

EMC contacts for military equipments are usuallyarranged through MOD PE for the specificequipment in question. Helpful advice can usuallybe obtained fromDr N Carter, Mission Management 5RAE FarnboroughHants, GU14 6TDparticularly on matters relating to aircraft. A list ofUS EMC contacts for military systems is given inthe ITEM handbooks (see 13.2.7).

13.2.3 Industry sources

Information on EMC matters can be obtainedfrom trade associations. In the example of a PCmanufacturer this would include ECMA(European Computer Manufacturers Association).

ECMARue DuRhone 114CH-1204 GenevaSwitzerland22-353634

Association of the Electronics,Telecommunications and Business EquipmentIndustries (EEA)8 Leicester StreetLondon WC2H 7BN0171 437 0678

Information on EMC standards, design techniquesand testing is also available from

ERA Technology LtdCleeve RoadLeatherheadSurrey KT22 7SA01372 374151

ERA have produced 'Guidelines to national andinternational recommendations and standardsrelating to EMC' presented at a conference andexhibition 'EMC 91 direct to compliance',Feb. 1991

Manufacturers with an interest in Civil AircraftAvionics can contact the EMC Club,Secretary D.A. Bull,RAE FarnboroughMission Management 5Hants, GU14 6TD

Most NAMAS-approved EMC test houses willgive advice on standards, design techniques andtesting. A list of such facilities can be found in theNAMAS M3 document [6] and in Appendix 13.1

Competent Bodies will also offer advice on EMCdesign and testing to meet the EC Directive, a listis provided in Appendix 13.2

A list of over 40 organisations offering EMC testor consultancy services can be found in a specialEMC edition of New Electronics [7].

13.2.4 Equipment, component andsubsystem suppliers

There are a large number of EMC equipmentsuppliers for everything from screened rooms tospectrum analysers; they can be helpful withgeneral information which need not be strictlyrelated to their products. They are often amongthe first to appreciate the impact within theelectronics industry of new EMC legislation ortest methods. If one particular person does notknow the answer to a question, they will oftenknow of a contact who does. Any productmanager or manager setting up an EMC testfacility will find the representatives of equipmentsuppliers a good source of information on EMCmatters. An efficient way to make contact withfirms is to attend one of the national/internationalEMC conferences and exhibitions which are heldin most years. Electronic component andsubsystem suppliers may also be helpful inproviding specific product information if theirgoods have been designed with EMCperformance in mind.

13.2.5 Professional bodies andconferences

'The lEE and IEEE regularly sponsor national andinternational conferences on EMC. They are wellworth attending and valuable conferenceproceedings are published, usually quite quicklyafter the event. 'The IEEE also produce aspecialist journal of their transactions on EMC.

ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESl'ING 241

The Electromagnetic Compatibility Society of theIEEE is designated S-27 and ,has its headquartersat

345 East 47th StreetNew York, NY 10017, USA

The lEEElectronics DivisionTechnical Information UnitSavoy PlaceLondon WC2R OBLo171 240 187 1

The Society of Automotive Engineers has an EMCcommittee designated AE-4 with headquarters at400 Commonwealth DriveWarrendale, PA 15096, USA

The Electronic Industries Association have anEMC committee (G-46) at2001 I Street, NWWashington, DC 20006, USA

The dB Society is a special organisation for personswith considerable experience in the field of EMC.It is a worldwide organisation and the contactpoint in the UK is atMission Management 5Room 302, Q153 BuildingRAE FarnboroughHants, GU14 6TD

13.2.6 EMC consultants and training

Information available from EMC consultants canbe extensive, but is rarely free. There are agrowing number of consultancy organisationswithin the UK and they can usually be foundthrough company and trade directories. Someindividual consultants, including those withincompanies, may be contacted through

The Association of Consulting Scientists11 Rosemont RoadLondon NW3 6NG0171 794 2433

A number of companies offering EMC consultancyare listed in Reference 7, with about an additional30 offering consultancy along with other EMCservices. Using a consultant is a quick way to gaininitial EMC information about legislation,standards, design techniques and testing. It may beexpensive and a manufacturer who takes EMCseriously for the long term competitiveness of hisbusiness will probably wish to develop some inhouse capability. Consultants can still be broughtin as needed, to assist the in-house team in tackling

a particularly difficult EMC problem, or venturinginto a new market with its own EMC requirements.A list of organisations offering consultancy andtraining in EMC is given in Appendix 3.3

13.2.7 Electronics and EMC technicalpress

There are a number of specialist EMC publications and a few periodicals which have a specialinterest in EMC. Among these are

ITEM, Interference Technology Engineers'Master, directory and design guide for the con trolof EMI. Published by Robar Industries, Inc.R & B Enterprises Division20 Clipper RoadWest CosnshohockenPA 19428-2721USA

EMC Technology, Published by Don White,Interference Control Technologies5615 West Cermack RoadCicero, IL 60650-2290, USA

New ElectronicsFranks HallHorton KirbyDartfordKent DA4 9LL

DTI EMC awareness campaignEMC helpline (1993) 0161 954 0954Managed by Findlay Publications

EMC Awareness Campaign AdministratorFindlay PublicationsFranks HallHorton KirbyKent DA4 9LL

A list of useful publications (available 1993)related to EMC is given in Appendix 3.4.

13.3 Detertnining an EMCrequiretnent

Consider again the example of a PC manufacturerwho has just realised that EMC exists and that itwill probably apply to his future products but hasno experience and no contacts in the field ofEMC. The broad questions that should beconsidered were listed briefly in paragraph 13.2.1;these are now examined in the framework of alogical flowchart which will act as a checklist toenable the 'PC product manager' to get started indefining the system level EMC requirement for anew product.

242 A HANDBOOK FOR EMC TESTING·AND MEASUREMENT

~L----r----j.4--nE~lec;;rrtricairoilssdaff8ei\;ty;----'requirements

Any specificEMC requirements based

on operational environment

tImmunity to RF ESD, lightning&faults on mains power lines/transients - leads to low costof ownership of the equipment

Product manufacturerstipulated EMC requirementsfor reliable product.(eg. immunity requirementsadditional to those specifiedby customer/legislation.)

Customer's stated_____ EMC requirements

Compilation of allEMC requirementsfor the new product

EMC standards/legislation.......----1 relating to product type or

country.(Mainly for emissions)

tNationaVInternationalstandards called up withinlegislation

tProfessional bodiesconcerned with EMCStandards. (ENELEC, IECVDE,SAE,IEE,CISPR etc.j

Consider competitors' policy ron EMC~-----'

Consider cost ofimplementing EMC

. Consider marketingadvantages of lowerthrough-life cost of ownership

Generation ofEMC requirementhierarchy for productbeing developed

System levelEMC specificationfor new product

....._---1 Consider manufacturer'stechnical EMC capability

Export marques EMCrequirements considered

Marketing input: Salesvolume/Customer/CountrylEMCspec.

Consider company qualityimage is enhanced bygood EMC

Figure 13.1 Generation of EMC requirement for product development (Example: manufacturer of desktop computerproducts)

Figure 13.1 shows a typical flow chart forgenerating product EMC requirements. There isinsufficient space available here to discuss individually each item in the chart, but the short titlesact as a reminder to consider that topic as part ofthe checklist. Some insight into the central processof assessing individual customer requirements anddeveloping a system level hierarchy of EMC teststandards which must be met, is given by Barrettand Scherdin [8J, in the case of industrial controIsproducts produced by Texas Instruments. Theyfound that products had to meet up to 30 teststandards to satisfy all their customers' needs.After considering the issues shown in Figure 13.1they were able to narrow this down to aminimum of 15 test standards. In the case of a PCmanufacturer who intends to sell world wide theproduct may have to meet 5-10 standards tocover the full range of EMI emission andimmunity, ESD and power line transient requirements.

13.4 Developing an approach to EMCdesign

13.4. 1 Process flow chart

Once the system-level EMC requirement for a newproduct has been determined the product managerand her technical team must decide how toapproach the EMC design process as part of theoverall product development programme. There aremany ways to do this depending on the product, thecompany, and the individual talents and expertiseavailable. One example, which again acts as achecklist, is given in Figure 13.2. Two features ofthis flow chart meri t further discussion.

13.4.2 EMC strategy

There are two fundamental approaches to ~eeting

the emission regulations for products. The first hastraditionally been favoured by the military for


Technical resources:. tackle project in-house?

Use consultants?Do both?

t I

System level IEMC requirement

Develop an approachto system & subsystem ~EMC design

+Mainstream productdesign constraints

EMC STRATEGY:Containment?Source suppression?Tackle immunity beforeemissions?Do the minimum?

Figure 13.2 .Developing anapproach to EMC design

IN-HOUSEConsider:Staff expertiseTraining in EMCCapital equipmentLocation of EMCwithin companystructure

CONSULTANTSBalance of shortterm advantage tolong-term reliance.CostAccountabilitySelection of:

Technical design of'EMC' sub syfems(Technical plan)

Project EMCcontrol plan!management plan

their procurements and is based on the concept ofcontainment. 1--'hus the EMC subsystem designswill be based around the techniques of screening,filtering and shielding of cables with less attentionbeing paid to suppression of the interferencesignal sources. Such an approach is viable formilitary products which usually have stout metalcases and require strong semiarmoured cables,and where cost may not be the overriding consideration in product design.

EMC designers of commercial products, whichare usually lower cost and may have features suchas plastic cases and unshielded cables, havetended to suppress EMI at source whereverpossible by careful signal shaping/band limiting,the use of screened/filtered subsystems and lowradiation PCB designs. Staggs [9J suggests thataround 1984 the optimum balance for PC-typeproducts was to use a 50/50 strategy forcontainment and source suppression. Howeverwith the advent of very fast 32 bit chip setsrunning at 25 MHz or more (resulting in highpeak switching currents) source su ppressionbecomes more difficult and the balance shiftsmore to a 60/40 containment/suppression ratio.

13.4.3 Immunity first?

For IT products, current enforceable EMCregulations are limited to radiated and conductedemissions and therefore meeting these regulationswill have a very high priority in the system EMCrequirement for a new IT product. It should beremembered that the FCC in the USA has thepower to introduce immunity standards if thevoluntary system to ensure product EMIimmunity fails to work. In Europe there are a

number of published draft standards which willcover product immunity to EMI in future years.

The performance of the product in use can beseriously reduced by EMI if it is susceptible, andthis will result in a poor reputation for reliabilityfor the product and the company. The fieldservice and warranty costs can be high ifequipments are continually in need of repairowing to disruption or damage from EMI andESD/power transients. It has been estimated [9Jthat for a machine population of 20,000 unitsover five years the total gross savings in fieldrepair and warranty could be as high as $2.77 M,with the cost of implementing EMC at $42 perunit, this results in a net saving of almost $2 M.

For all these reasons it n1akes good sense for theproduct development team to insist that newproducts are designed with EMI immunity as ahigh priority. For the example of a fast 32 bit PCwhere the emphasis is on containment rather thansource suppression, this strategy becomes doublyattractive as much of the shielding, filtering andscreening needed for immunity reduction will berequired anyway for emission suppression. Staggs[9J comments that, 'by solving the immunityproblem first, and consequently lowering theemission levels, the incremental cost of specificemission solutions is relatively small'. For PC-typeequipments he estimates that 80% of emissionproblems will be solved by meeting immunitystandards first.

13.4.4 Example of EMC design process

The considerations taken into account Indeveloping the overall approach to achievingEMC for a new product (Figure 13.2) are


EMC test plans for- development- conformance- technical file

EMC project control plan- including the generationof a technicalconstruction file

System level EMC.....----t strategy:

containmentsource suppressionimmunity/emissionstechnical resources

FOR MINOR PROBLEMSMODIFY ONE OR MORESUBSYSTEMS

,If fail, re-examine1--__--11.... design of sub systems

Subsystem integrationand pre-conformancetesting

Sub system levelEMC specificationseg. for case, keyboard,interfaces, chipsets,PSU etc.

I___ ...I

Detailed EMC design activity on each sub system(including EMC development testing andpractical computer modelling)

Consideration ofbought-in items.(Some may haveEMC featuresbuilt-in)

Approach toand constraintson mainstreamelectronic design

Figure 13.3 Example of EMG1 design process

13.5 Technical construction file

enable the engineers to describe and visualise theEMI characteristics of the system being designed.The successes (and the failures) of such models topredict solutions to EMI design problems can beused to support building a computer based expertsystem which then becomes a valuable part of thedesign process itself and which is applicable to thedesign of the next generation of systems.

During the development of an approach to EMCdesign (Figure 13.2), and the subsequent desig!lprocess (Figure 13.3) a decision will have beenmade in most cases to opt for confirmation of asuccessful EMC design by one of two methods: byconformance testing, or by the submission to arecognised assessment body of a technicalconstruction file. A decision to test or submit atechnical file has implications for the EMCprogramme cost and time to market. I t dependson a number of factors related to the availabilityof appropriate EMC standards, product type,size, numbers made and installation features. If a

contained within the larger design process. Figure13.3 illustrates the issues that need to beaddressed to progress from the system-level EMCrequirement to a preproduction unit which isready for EMC certification.

Managers in small companies may shy awayfrom including mathematical modelling as part ofthe design process. If staff have the skill to usecomputer models and are able to interpret theresults so that competing design solutions can beevaluated, and only the best option tested andcompared with the predictions, the use ofmodelling can often be beneficial. If the decisionis made to use mathematical models the questionarises as to which models to use. They range fromthose req uiring large mainframe machines downto those based on simple assumptions (such as themodels described in Chapter 12) and whichrequire only a calculator or PC.

The question of the appropriate level of modelshas been addressed by Atkinson [10]. Heconsiders that practical and useful EMI modelslie somewhere between the big-machine modelsand the simple models used for scoping EMCproblems. He proposes the use of models based onspreadsheet techniques that can run on a PC and

13.5.1 Routes to compliance options


technical file is to be generated it will usuallycontain some test data, but not at conformancetest level. The requirement for the generation of atechnical file which would be acceptable to meetthe needs of the EC EMC directive 89j336 is nowexamined.

13.5.2 Circumstances requiring thegeneration of a technical file

An early document containing guidance to manufacturers for the preparation of a technicalconstruction file which would meet the requirements of the Ee EMC directive has beenprepared by working party WP-3 of NAMASWG4 [4]. It lays out the circumstances where itmay be appropriate to use a technical construction file in aid of the EMC certification ofproducts. These are:

(i) For apparatus for which harmonised ECEMC standards do not exist or are notappropriate.

(ii) For apparatus for which standards do existbut which are not applied in full. I t may bepossible for the manufacturer to claim thatthe fund amental requirements of thestandard are met without performing anyor all of the required tests.

(iii) For installations where testing toharmonised standards is not practicable dueto the physical properties of the installation.

(iv) For installations where testing of each installation is not practicable due to the existenceof large numbers of similar installations.

(v) Combinations of the preceding circumstances.

13.5.3 Contents of a technical file

Each of the four basic circumstances listed resultsin a slightly different requirement for the contentsof a suitable technical construction file. Take theexample of the second condition where thestandards are not applied in full. For thissituation the technical file shall contain thefollowing as a minimum.

• A general overview. Statements should bemade that seek to demonstrate why tests forcertain phenomena were not felt to benecessary. The key activity must be todemonstrate what special. properties theconstruction of the apparatus has whichrender unnecessary the required EMC tests inthe appropriate standard.

• Identification of apparatus. Typical contentsshould include

Brand nameModel number

Name and address of manufacturerjimporterPurpose of equipmentPerformance specification (EMC-relevant)External photographs of the equipment

• Technical description of the equipment.Typical contents should include, but notnecessarily be restricted to,

Full technical description including blockdiagram showing the interrelation of thefunctional areasSet of technical drawingsComponents list, with special regard tomicroprocessors, RAM, ROM, logicfamilies, oscillators, power supplies, motorsand relaysSignal data including frequencies,risetimes, switching curren ts, groundingschemesDescription of physical characteristics ofthe equipment, SIze, weight, powerconsumptionListing of available purchase options of theequipmentList of other equipment likely to beconnected to the device being assessedDescription of any design measures takenspecifically to control EMI and enhanceEMC performance of the productA copy of any assembly or installationmanuals supplied to the customer whichmay affect its EMC performance.

• Technical rationale. This must explain indetail why the harmonised ECEMCstandards have not been applied in full. Itshould be supported where possible byinformation derived from theoretical andpractical studies. Details of all tests performedshould be included. Explanation of thequality control procedures that apply to theproduct must be given to show that futuresamples of the equipment will also complywith the directive. The detailed contents ofthe technical rationale should include, bu t notbe limited to, the following:

The logical process used to determine thatcertain tests need not be performed. Foreach type of test described in thestandards and which has not beenperformed, a description and explanationof the results of any relevant developmenttests. Support for the decision not toperform certain tests with any theoreticalstudies which show that the apparatusmust inherently comply. The descriptionof any EMC design measures relevant tothe phenomenon being assessed in tests

246 A HANDBOOK FOR EMC TESTING AND MEASUREMENrr

which have not been performed. A detailedanalysis of the method of operation,relevance and expected effectiveness ofany EMC protection measures incorporated into the equipment.A list of all formal tests and reports whichhave been carried ou t.An account of how the EMC performanceof production samples will be verified, andhow the sampling levels will be chosen.An account of variant and build standardcontrol in production and an explanationof the procedures used to assess whether adesign change requires the apparatus tobe retested/requalified.

13.5.4 Report from a competent body

1'he technical construction file must be submittedto an organisation which is deemed by theregulatory authorities to be competent to judgethe performance of the apparatus in relation tothe harmonised EMC standards. Competentbodies are likely to include NAMAS approvedEMC test houses and some EMC consultancies.The report from the competent body produced asa result of assessing the technical file shouldinclude

Reference to the exact build standard of theapparatus assessed and a cross reference to thetechnical file.Statement on the work done to verify thecontents of the technical construction file.Comments on the procedures used by themanufacturer to ensure compliance with theEC directive for each phenomenon describedin the relevant standard for which formal testshave not been performed.Comments on any tests which were carriedout, and an analysis of test methods employed.Comments on q uali ty assurance procedureswhich the manufacturer intends to apply tothe product.

T'he report from the con1petent body can be usedby the manufacturer of the equipment to supporthis statement of compliance with the EC directiveon EMC.

13.5.5 1'esting or technical file?

I t would be incorrect to suppose that thegeneration of a technical file and its submission toa competent body for scrutiny is necessarily aneasy way of obtaining EMC clearance to Inarketa product under EC 89/336. The testing optionmay prove less costly in some cases and if suitabletest facilities have been booked in good time there

should be little extra delay in bringing theproduct to market. Experience will be gainedduring the 1990s as to when the technical filerou te as opposed to conformance testing is mostappropriate for product approval. For large, oneoff type equipments it maybe impossible to carryout approved tests and the technical file is thenthe only route to EMC clearance. In the years tocome all Inanufacturers and EMC practitionerswill watch with interest the developmentssurrounding this question.

13.6 Self certification

13.6. 1 Need for an in-house facility

The option to self certify products under the ECregulation can be very attractive to some manufacturers. If they originate a continuing series of newproducts or new variants of existing products,then it may be cost effective to develop an inhouse capability to carry out the necessary EMCconformance tests.

The particular list of EMC tests required bystandards will be product specific, and a manufacturer's in-house facility need not cover such a widerange of tests as would an all-embracing thirdparty EMC test house. The exact list of EMCtests with which the in-house facility must copewill have been determined during thedevelopment of the product design. rrhe basictypes to be considered are

emission tests -- conducted and radiatedimmunity tests conducted and radiatedscreened enclosure testingopen-area test siteESD and transient testing.

The technical details of all these types of tests havebeen covered in preceding chapters.

An additional consideration for defining therequirement of an in-house test facility is the size ofthe equipment to be tested. Generally, this is thekey cost driver, the greater the EUT size thegreater the cost of the test facility. The break pointcomes for EUTs about 1 m cube. If they are largerthan this the facility will be very expensive ifradiated immunity/susceptibility testing is to becarried out using semi-anechoic screened chambers.

Anyone considering the technical definition andfinancial investment required to develop an EMCfacility should consult as widely as possibleexisting facility operators dealing with theirproduct types to avoid making potentially costlymistakes. Time permitting, the safest route toconstructing the most suitable and cost-effectivein-house facility is to build it up in a series oflevels of capability.


13.6.2 Gradual development

There are four components which need to be considered when developing an in-house EMC facility:

The risks associated with these issues can bereduced by a gradual approach to developing anin-house EMC test capability which has threebasic levels:

Level 1:

Level 3:

Level2b:

Level2a:

Bench testing to serve product developmentDeveloping dedicated facilities i.e. anopen-area test site or a suitablescreened enclosure plus the associatedtest equipmentUpgrading these facilities to con-formance test standardsObtaining accreditation for thefacility.

This process is shown In Figure 13.4. Selfcertification of products is clearly possible atlevel 3 but may also be substantially risk-freeat level 2b if care is taken to operate in theprofessional manner req uired by N AMAS (atlevel 3).

rrhe information contained in this and otherbooks on EMC can be helpful in determining thetests/standards which must be covered (forvarious product types) and the technicalknowledge required to specify and use EMC testequipment in approved ways to measure therequired parameters of current, field strength andimmunity level, etc.

(i) Defining and obtaining the correct accommodation, facilities and test equipment toperform the range of tests specified in thechosen EMC standards.

(ii) Having sufficient well motivated andqualified test personnel available.

(iii) Putting in place sound workable operatingand q uali ty control procedures to guide thetechnical staff using the equipment to carryout the tests.

(iv) Ensuring the right amount of work for thefacili ty - too li ttle, and morale will fall,along with any continued investment instaff training and new equipment; toomuch, and the facility will be overloaded,resulting in potential errors in test work andpressure to compromise quality standards.In such circumstances the staff would haveno time for training and attendingconferences, etc. which would result in littleor no development of their skills. The latestinformation relating to test methods andequipment would not find its way into thefacili ty and benefit the conformance testingwhich helps get the products to market.

Bench-test capability for development testingincludes: conducted emissions

ESO & transientsnear-field radiatedemission probes

-------------------------LEVEL 1

THIS ROUTE FOREMISSION TESTING OF

COMMERCIAL PRODUCTS

THIS ROUTE FORMILITARY AND SOME COMMERCIALPRODUCT TESTING

Open-area test sitefor radiated emissiondevelopment testing

Screened room test facilityfor radiated emission andsusceptibility developmenttesting

------------------ LEVEL 20

Up-grade 'OATS toconformance teststandard.(Gain experience)

Introduce strict lab.procedures, calibra-

~ tion and quality ~

control to 'NAMAS'standard

Up-grade toconformancetest standard -------- LEVEL 2b(Gain experience)

Obtain accreditation:can now self-eertifyproducts withconfidence.(Sell spare capacity)

+

Obtain accreditation

H Seek NAMAS II--_~ (up-grade capabilityaccreditation I with higher susceptibility

field strengths and ---------LEVEL 3accommodate largertest objects)(Sell spare capacity)

+Figure 13.4 Steps to consider when setting up in-house.EMC test facility


13.6.3 Estilllates of facility cost

An estimate of the typical equipment cost to set upan EMC test facility with various capability levels

in line with those previously described1 to 3), is given in Figure 13.5. l'hese

estimates do not include the additional costs ofland or buildings which facilities would occupy.No cost has been included for the recruitment andtraining of EMC test personnel.

l'he of facilities is often a criticalwithin companies wishing to develop an

EMC test capability. There is usually a shortageof qualified and experienced staff in theUK and often only premiurn conditions willattract the best people. Developing the necessaryexpertise completely on the basis of existingcompany staff is also expensive due to the cost oftraining, which takes from 1 to 3 years.

13.6.4 Turnkey facilities

If time is pressing and a complete facility must be putin-house in a short time, it is possible to

contract a consultant or possibly an EMC equipment manufacturer to deliver an adequate turnkey

£0.5 M-£l M) within about 6-9months. Developing the requirement specificationfor such a programme would be the main activitywhich the purchasing company would have toundertake. I t is to use a second set of consultants to do this and to monitor the performanceof the ITlain contractor supplying the turnkey facility.

rrhere are two main weaknesses inherent in this

It is to be costly: the additional cost tocover risk factors could be substantial(10 (Yo ---20 % of the turnkey price).

When all the consultants have gone, thepurchaser is left with an impressive facilitybut perhaps without the necessary trainedstaff, facility operating procedures, orbackup support, unless they have beenincluded as specific req uirements in theturnkey package, at an extra cost.

l'he conclusion is clear: anticipate the EMCproduct development needs of the company, planfor a phased build-up of facili ties and expertise tomeet the need, recruit and/or train theappropriate in-house staff to run the facility.

13.7 Conclusion

Electromagnetic compatibility is a fascinating andrapidly developing field of electrical engineering.It poses important, interesting and wide rangingmultidisciplinary challenges to equipn1entdesigners. I t affords many technical challengeswhich must be met by engineers, to Inakeconsistent and meaningful measurements ofcomplex EMI quantities. The managernent andquality aspects of EMC work are equallyin1portant if the technical effort expended is to berealised in terms of added value to the productsbeing developed.

In its widest role, EMC is part of the growingrealisation that the benefits of industrialisation canonly be enjoyed to the full if one takes care tominimise the unwanted impact of the technologyemployed. For example, without EMC controls,what would be the net benefit of the advent ofpowerful desktop business computers if the electricalemissions _from them disturb and compromise eachother or the communications systems which carrythe data generated to other uses?

£10k £100k £500k £1 m £10m £100mI

!SIMPLE SPECTRLM ANALYSER & !URRENT PROBE(& NEAR-FIELD P~OBES FOR BENC~ TOP TESTING

-1rBENCH ~OP C.E. & E.S.D. ~ SOME TRANSIENT TESTING I I

I I I*SMALL SCREENED ROOM C.E. & E.S.D. -& TRANSIENTS & R>E> & R.S. (1 Vim to 1GHz)

I I I I I*O.A.T.S., R.E. UP TO 1GHz (SPE. ANALYSER OR CHEAP EMI METER)I I IlICOMPREHENSIVE CHAMBER-BASED FACILITY CAPABLE OFR.S. UP TO 10V/m & R.E. UP TO 1S-GHz & C.E. & ESD & TRANSIENTS

I I I*MULTI-CAPABILITY COMMERCIAL TYPE EMC TEST HOUSE FACILITY

I I I*HIGH CAPABILITY MULTI-PURPOSE FACILITYFOR COMMERCIAL & MILITARY TESTING OFSYSTEMS UP TO 4 X 3 X 2m

I I* A NATJONAL LEVEL TEST FACILITY FOR

A MAJOR MILITARY OR CIVIL PRODUCT

I IBENCH TOP

...::1---..... LEVEL 1DEDICATED FACILITIES CONFORMANCE TEST FACILITIES

... til LEVEL 2 ... LEVEL 3 .. ....MAJOR NATIONAL

FACILITIES---.........

13.5 cost against capabilify (1991 £ Sterling)


EMC will be a major area of expanding interestand influence in the decade of the 1990s aslegislation drives the commercial world to strictcompliance with national and interna tionalstandards.

Electromagnetic compatibility is then a branchof electrical engineering which has a clearcommercial and social impact, and its practitioners will con tribu te in a small way to thefuture quality of life in an environment whichincreasingly interacts with technology.

13.8 References

'The Single Market ElectromagneticCornpatibility'. HMSO, 2/90. Dd 8240964 INDYJl077NE 40M, Feb. 1990

2 'Electrical interference: a consultative document,The implementation in the UK of Directive 89/336/EEC on electromagnetic compatibility'. Availablefrom L.B. Green, Radiocommunications Division,DTI, Rm 106, Waterloo Bridge House, WaterlooRoad, London SE1 8UA

3 'The Single Market: EMC Update'. DTIRadiocommunications Division, Waterloo Bridge

House, Waterloo Road, London SEl 8UA, June1990 and March 1991

4 'Guidance document for the preparation of atechnical construction file as required by the ECdirective 89/336, A draft for discussion'. FromA. Bond, MT Division 4e, DTI, 151 BuckinghamPalace Road, London SW1

5 'Points of contact'. NPL0004/8K/NJ/6/90, NPL,Teddington, Middlesex, TW11 OLW, 1990/91

6 'Concise directory - M3'. NAMAS Executive, NPL,Teddington, Middlesex, TWll OLW, Aug. 1990

7 Special edition on EMC, New Electronics, October1990

8 BARRETT, J.P. and SCHERDIN, S.: 'Thedevelopment of EMC laboratory standards andprocedures'. Proceedings of IEEE symposium onEAfC) 1990, pp. 329-332

9 STAGGS, D.M.: 'Corporate EMC programmes'.Proceedings of IEEE symposium on EMC) 1989,pp. 320-325

10 ATKINSON, K.: 'Graphical EMI modellingspreadsheet'. Proceedings of IEEE symposium onEAfC) 1990, pp. 175-179

11 'Help is at hand - useful contacts for advice andtechnical information'. DTI EMC awarenesscampaign, . Findlay Publications, Franks Hall,Horton Kirby, Kent DA4 9LL, May 1993

Appendix 1.1

Signal bandwidth definitions

1 Narrowband/broadband signals NARROWBAND SIGNAL (CW)

Figure A1.1 Narrowband/ broadband signals

receiver input, increases with the bandwidth used.The shape factor of the IF filters becomesimportant as it affects the intercepted signalpower and therefore the indicated signal voltage.

'To derive a signal level that is universal, irrespective of the bandwidth used, the measurementmust be expressed in power or voltage per unitbandwidth. This is called the signal spectraldensity and for EMC measurements it is usuallyexpressed in /lV/kHz or /lV/MHz. When thecalibration of the sensor connected to the receiveris taken into account the measurements ofphysical quantities are expressed as dBIlV/m/kHzor dB/lV /m/MHz for radiated emission fieldstrengths and dB/lA/kHz or dBIlA/MHz forconducted emissions.

For pure broadband thermal n~ise where thesignal power at each frequency is phaseincoherent, the measured signal vol tage increasesas the square root of bandwidth used. Forcoherent impulsive noise (where there is a defined

FREQUENCY

Narrowband signalfrequency f1

Bandwidth 11kHz

Bandwidth 210kHz

/Bandwidth 3 30 kHz

I FREQUENCY1t

Wide bandwidth 10MHz

Wide bandwidth 1MHz

Wide bandwidth 100 kHz

Wideband impulse signalspectrum with manyfrequency components

3 dB BANDWIDTH

~ r--I I

OdB--

BROADBAND IMPULSE SIGNAL

6 dB IMPULSE BANDWIDTH

I r-I IW

..J«oenwCl::::>I::J0-~«

W,...Ji«oenWCl::::>.....:.Ja.~«

Measuring the amplitude of a narrowband signalwith a tuned radio receiver or EMI meter issimple, as once the receiver is tuned to the signalfrequency at say 11 MHz, the measured signalstrength is independent of the bandwidth used tomeasure it. This is because all the power in thesignal is always contained within the 3 dBpassband of even the narrowest IF (intermediatefrequency) and post-detector filters as shown inFigure Al.la.

This is not the case with broadband noise orimpulsive signals where the power in the signal isdistributed over a range of frequencies which ismuch greater than the receiver bandwidth whichis used to measure it. Figure Al.lb shows atypical impulse spectrum from a digital switchingwaveform which has frequencies out to perhaps100 MHz. It is represented as a series of frequencycomponents spaced at 1/ T (where T is the signalPRF) and a first null at lit (where t is therisetime /fall time). The signal power interceptedunder the IF filter bandwidth increases as thebandwidth increases. Thus the measured powerand indicated signal voltage referred to the

2 Measuretnent of narrowband andbroadband signals

Signals measured during radiated and conductedemission tests are often extremely complex,consisting of a mixture of individual signals frommany separate sources within the equipment.Some of these may emanate from stable oscillatorsfor example, producing a sinusoidal waveform ata single frequency. The frequency extent orbandwidth of this signal is small, perhaps lessthan 1 Hz, and it is clearly narrowband.

Other circuits which generate fast switchingwaveforms such as switched mode power suppliesor digital processing circuits produce signalswhich contain many frequency components whichmay be spread across tens or hundreds ofmegahertz. These. signals have a wide bandwidthwith the highest frequency being determined bythe switching-edge risetime/falltime, and thespacing of frequency components in the spectrumby the switching repetition frequency. Such asignal is referred to as broadband.

250

phase relationship between adjacent frequencycomponents) the measured signal voltageincreases proportional to bandwidth [1] . Somebroadband signals will not fall neatly into eithercategory and for these the change in measuredsignal voltage with a change in receiverbandwidth will lie between that for incoherentand coherent signals.

The precise defini tion and consistentmeasurement of broadband signals is a majorissue in EMC testing. Some EMC standardsovercome the difficulty by specifying the precisereceiver IF bandwidths which must be used formeasurement at different frequencies. This meansthat all measurements will be made in the sameway without the test engineer making a judgment

SIGNAL BANDWIDTH DEFINITIONS 251

as to whether the signal is narrowband orbroadband.

The discussion in the EMC community aboutnarrowband/broadband measurement seems to bemoving to a conclusion where most commercialand military standards will adopt fixedbandwidths. Those EMC test engineers who arerequired to measure broadband signalsnormalised to 1 kHz or 1 MHz bandwidth shouldexerCIse care with this aspect of measurementtechnique.

DUFF, vV.G.: 'Fundamentals of electromagneticcompatibility'. Interference Control TechnologiesInc., Gainesville, Virginia, USA, sections 2.6, 2.7,pp. 2.38-2.44

Appendix 1.2

UK EMC legislation(up to 1 January 1996)

1 General exclusions

Exclusions from. EC harm.onisedEMC regulations

rrhis information is taken from the DTIpublication 'Product standards electromagneticcompatibility (UK Regulations April 1993)'.

The EC EMC regulations do not apply to:

Apparatus for export to a country outside the EECwhere the supplier believes with reasonable causethat it will not be used in the UK or anothermember state.

Some installations are excluded where two ormore combined items of equipment are puttogether at a given place to fulfil a specific

objective, but not designed by the manufacturer (s) for supply as a single technical unit.

Spare parts, subject to regulation 14(2) of the ECEMC Regulations whereby nothing shall be takento affect the application of the regulations torelevant apparatus into which a spare part has beenincorporated. 'Spare part' means a component orcombination of components intended for use inreplacing parts ofelectrical or electronic apparatus.

Supply of apparatus to authorised representativeresponsible for complying with the regulations.

Second-hand apparatus is excluded, with theexception of such apparatus which has, since it waslast used, been subjected to further manufacture;and second-hand apparatus which is eithersupplied or taken into service in the community forthe first time having previously been supplied orused in a country or territory outside thecommunity. Second-hand apparatus means thatwhich has previously been used by an end user.

Electromagnetically benign apparatus isexcluded where the inherent qualities of theapparatus are such that neither is it liable tocause, nor is its performance liable to be degradedby, electromagnetic disturbances.


Active implantable medical devices within the

3 Apparatus wholly covered by otherdirectives

2 Specific exclusions


Apparatus for use in a sealed electromagneticenvironment so long as it is accompanied by instructions stating that the apparatus is suitable for useonly in a sealed electromagnetic environment.

Radio amateur apparatus which is not availablecommercially is excluded.

Military equipment defined as apparatus whichis designed for use as arms, munitions and warmaterial within the meaning of Article 223.1 (b) ofthe Treaty establishing the EEC (notwithstandingthat it may be capable of other applications), butdoes not include apparatus which was designedfor both military and non-military uses.

Title

The Wireless Telegraphy(Con trol of interferencefrom ignition apparatus)Regulations 1952The Wireless Telegraphy(Control of interferencefrom electro medicalapparatus)Regulations 1963The Wireless Telegraphy(Control of interferencefrom radio frequencyheating apparatus )Regulations 1971The Wireless Telegraphy(Control of interferencefrom household appliances,portable tools, etc.)Regulations 1978The Wireless Telegraphy(Control of interferencefrom f1uorescen t lightingapparatus)Regulations 1978The Wireless Telegraphy(Control of interferencefrom CB radio apparatus)Regulations 1982

1963 1895

1952 2023

1971 1675

1978 1268

1978 1267as amended

Statutory instrument

1982 635as alnended

252

meaning of Article 1.2 (c) of the Council Directive90/385/EEC.Medical devices covered by. the EC directive inpreparation (1993) and comprising anyinstrument, apparatus, appliance, material orother article, including software for the purpose of

(a) diagnosis, prevention, 'monitoring, treatmentor alleviation of disease, injury or handicap

(b) investigation, replacement or modification ofthe anatomy or of a physiological process

(c) control of conception.

4 Apparatus partly covered by otherdirectives

The EMC regulations do not apply to

Electrical energy meters as regards the immunity

UK EMC LEGISLATION 253

thereof (regulated by Council Directive 76/89/EEC).Spark ignition engines of vehicles in so far as theelectromagnetic disturbance generated thereby isliable to cause radio interference (such interference is regulated by Council Directive 72/245/EEC).Spark ignition engines of tractors in so far as theelectromagnetic disturbance generated thereby isliable to cause radio interference (such interference is regulated by Council Directive 75/322/EEC and as amended by Article 1 of 82/890/EEC).Nonautomatic weighing machines as regards theimmunity thereof (regulated by Council Directive90/384/EEC).rrelecommunications terminal equipment (TTE)to the extent that EMC requirements aredetermined by Council Directive 91/263/EEC.

Appendix 1.3

European EMC standards

Listed in Table Al.3.1 are commonly usedEuropean EMC standards, their applicability andequivalent national standards. In some cases anear equivalent USA standard is also identified.

Table Al.3.1 EMC standards (emissions)

Equipment

Generic emission

ISM

Radio and TV receivers

Household appliances

Lamps and lighting

IT equipment

Electrical supply networks

Vehicle igni tion systems

Motor cycles and vehicles with 3 ormore wheels

Vehicle brakes

Metrology (weights and measures)

EMC standards (immunity)

Generic immunity standard

Broadcast receivers

Industrial process control

IT equipment

Euro standard

EN50081

EN55011

EN55013

EN55014

EN55015

EN55022

EN60555

75j245jEC

UN ECE Reg.10

71j320jEEC, UN ECE Reg.13

EN50082

EN55020

HD481

EN55101

254

Equivalents

BS4809, VDE0871, CISPR11(FCC pt18)

BS905 pt1, VDE0872, CISPR13(FCC ptl5)

BS800, VDE0875, CISPR14(FCC pt15)

'BS5394, VDE0875, CISPR15

BS6527, VDE0871, CISPR22(FCC ptl5)

BS5406, IEC555

BS833, CISPR12

NW0320

BS905 pt2, CISPR20

BS666 7, IEC80 1-1, IEC80 1-2(BS6667 (1985)), IEC80 1-3IEC801-5

EUROPEAN EMC STANDARDS 255

Table A1.3.2 Existing EC harmonised standards: emission standards (up to 1993)Compiled from References 16 and 17 of Chapter 2

CENELECreference

Draft for public British Standardcomment

Equipment covered

90/20911DC[proposed rev]

EN50065-1 *

EN50081--1 *

EN55011*

EN55013*

EN55014*

EN55015*

Mains signalling on low voltage electricalins tallations.

90/26273 DC Any equipment in the domestic, commercialand light industrial electromagneticenvironment (class 1 environme'nts) - genericemission standard.

BSEN550 11: 1991 Limits and methods of measurement of radiointerference characteristics of ISM RFequipment (excluding surgical diathermyapparatus). Based on CISPRII

BS905: 1991: ptl Limits and methods of measurement of radiointerference characteristics of sound andtelevision receivers. Based on CISPR 13

BS800: 1988 Limits and methods of measurement of radiointerference characteristics of householdelectrical appliances, portable tools and similarelectrical apparatus. Based 'on CISPR 14

BS5394: 1988 Limits and methods of measurement of radiointerference characteristics of fluorescent lampsand luminaires. Based on CISPR15

EN55022*

EN60555-2*

EN60555-3*

88/27854DC

90/28296DC

BS6527: 1988 Limits and methods of measurement of radiointerference characteristics of informationtechnology equipment. Based on CISPR22

BS5406: 1988: pt2 Disturbances in supply systems caused byhousehold and similar equipment (harmonics).

BS5406: 1988 Disturbances in supply systems caused byhousehold and similar equipment (voltagefluctuations) .

Standards not yet adopted

IEC50 chap161 BS4727: ptl: Gp9 EMC definitions

* Denotes standards which have been referenced by the Official Journal of the European Communities and are thereforenotified for use in self certification.


Table A1.3.3 Existing*t and possible future EC harmonised standards: immunity standardsCompiled from References 16 and 17 of Chapter 2

CENELECreference

EN50082-1 *

prEN50082-2

Draft for public British Standardcomment

90/26272DC

91j21828DC

Equipment covered

Any in the domestic, commercial and lightindustrial electromagnetic environments (Class 1environment). Generic immunity standards.

Any in the industrial electromagneticenvironment.

EN55020*

prEN55101-2

prEN55101-3

prEN55101-4

HD481(IEC801)

HD481.1SI

HD481.2SI

HD481.3SI

IEC801

IEC801-3(Draft Rev)

IEC801-4:1988

IEC801-5(Draft)

IEC801-6(Draft)

91 j25187DC[proposed rev]

89j34172DC

89j34171DC

90/30270DC

90j29283DC

90j21076DC

90/27512DC

BS905: 1991: pt2

BS6667: 1985: pt1

BS6667: 1985: pt2

BS6667: 1985: pt3

BS6667: pt4

Sound and TV broadcast receivers andassociated equipment.

Information technology equipment.Electrostatic discharge.

ITE Radiated RF disturbances.

ITE Conducted RF disturbances.

EMC for industrial process measurement andcontrol equipment.

General introduction.

Method of evaluating susceptibility toelectros ta tic discharge.

Method of evaluating susceptibility to radiatedelectromagnetic energy.

Electromagnetic compatibility for industrialprocess measurement and control equipment.

Immunity to radiated RF EM fields.

Electrical fast transient/burst requirements.

Surge immunity requirements.

Immunity to conducted RF disturbances above9KHz

* Denotes standards which have been referenced by the Official Journal of the European Communities and are thereforenotified for use in self certification.

tup to 1993

EUROPEAN EMC STANDARDS 257

Table A1.3.4 Proposed product-specific EMC standards (introduction into EEC by 1996)

Industrial

1. Industrial measurement and control equipment2. Machine tools (electronic control of manufacturing machinery robots)3. Power electronics (convertors, rectifiers etc)4. Industrial electroheat equipment5. Electrical welding6. Industrial transport equipment (cranes)7. Power capacitors7a. Related filters8. LV switchgear and control gear9. Rotating machinery

10. FusesLV

Residential) commercial and L V professional1. Audio, video, audiovisual equipm.ent for domestic entertainment1a. Broadcast satellite receivers2. Audio, video, audiovisual lighting control equipment for professional use3. Domestic appliances and similar household appliances (including toys)4. Lighting5. Alarm systems (without mains connection)6. Mains signalling in low-voltage7. Building automation8. Small power electronics (power supplies)9. Lifts

10. LV circuit breakers and similar equipment11. Residual current devices12. Electronic switches

Information technology equipmentITE (including telecommunication terminal equipment)ISDN

Traffic) transportation1. Electric traction equipment2. Motorway communication equipment and traffic control equipment3. Electrical installation of ships4. Navigational instrumentation

Utilities1. HV switchgear and control gear (secondary systems)2. Protection equipment3. Telecontrol, teleprotection and associated telecommunication for utilities4. Measuring, metering and load control apparatus (electronic)5. HV fuses

Special1. Medical equipment2. Electrical and electronic test and measuring instruments (including scientific

instruments)3. CATV cable distribution equipment

CENELEC ref.

prEN55024ENV55102-2

prEN50082-2


Table Al.3.5 CISPR publications

CISPRICISPR2CISPR3CISPR4CISPR5CISPRI-6CISPR7CISPR8CISPR9CISPRI0CISPRIICISPR12CISPR13CISPR14CISPR15CISPR16CISPR17CISPR18CISPR19CISPR20CISPR21CISPR22CISPR23

RFI measuring set 0.15-30 MHzRFI measuring set 25-300 MHzRFI measuring set 10-150 kHzRFI measuring set 300 MHz-l GHzPeak, quasipeak and.average detectorsinclusive have been superceded by CISPR 16Recommendations of CISPRReports and study questionsCISPR and national limitsOrganisation, rules and procedures of CISPR (1981 and 1983)ISM limits and measurements (1975, 1976)Ignition limits and measurements (1978, 1985)Sound and TV receivers limits and measurements (1975, 1983)Household equipment limits and measurements (1985)Fluorescent lamps and luminaires limits and measurements (1985)RFI measuring sets specification and measurements (1977, 1980, 1983)Filters and suppressors measurement (1981)RFI of power lines and HV equipment part 1: description (1982)Microwave ovens measurements above 1 GHz (1983)Immunity of sound and TV receivers (1985)Interference to mobile radiocommunications (1985)II' equipment limits and measurement (1985)Derivation of limits for ISM

Appendi)( 1.4

Gerntan decrees and standards

Table ~41.4.1 German decrees relating to Ell-lC

Decree

\lfg 523/1969\lfg 1046/1984

\lfg 1045/1984\ffg 1044/1984

.A.pplicability

Individual permit for HF equipment - Class A DecreeISJ\tI and similar equipment (EDP, etc) - Class B general permit decree (includes selfcertification ) [specifies VDE08 71 ]General permit for household appliances [specifies VDE0875]EC directives implementation and harmonisation of legislation [specifies 82/499/EEC and82/500/EEC]

Examples of Germ,an VDE standards

VDE Ref.

'iDE0565

VDE0871

VDE0872VDE0873VDE0874\lDE0875

\lDE0876

VDE0877

\rDE0879

Relating to

Specification for RFI suppression devicesPart 1 CapacitorsPart 2 ChokesPart 3 Filters (up to 16 A)Part 4 Ceramic capaci torsLimits of RFI from RF apparatus and installationsPart 1 ISNIPart 2 EDPjITInterference suppression for radio and T\l receiversRFI from electrical utility plants and H\l systemsRecommendation for RFI suppressionRFI from appliances (frequencies below 10 kHz)Part 1 Household appliancesPart 2 Fluorescent lightingPart 3 i\ppliances with motorsInterference measuring apparatusPart 1 vVeighted indicationPart 2 Disturbance analyserPart 3 Current probesRFI NIeasurement proceduresPart 1 Interference voltagePart 2 Interference field strengthPart 3 Interference on power leadsRFI suppression for motor vehicles and engines

259


Table Al.4.2 Sun'lmary oj important test differences between USA (FCC) and Europe/Gern'lany (VDE)

Class A

Class B

Class C

Radiated testdistances

Conducted test

Magnetic fieldtest

Radiated test antennas

Radiated test cables

RFI testequipment

FCC

For commercial/office products.Manufacturer does RFI test for selfverification and labels product

For residential products only. RequiresFCC authorisation and FCC identifiernumber on device

Class A equipment can be tested atinstallation site for self verification.The FCC does not perform this test

3 m for Class Band 30 m for Class A,preferred. Distances less than 30 mallowed if data are correlatable.

450 kHz-30 MHz, Class A and B

N/A

Antenna height is varied from 1 to 4 mfor measurement distances up to andincluding 10m. For distances of 30 m theantenna is varied from 2 to 6 m

Interface cables are configured todiscover maximum emission

EMI receivers are preferred but spectrumanalysers are allowed. Spectrumanalysers are widely used in the US

VDE

VDE RFI test and FTZ certification ismandatory by law, primarily for systemsor low volume products. FTZ numbermust be on product and user registersdevice location.

Can test at VDE or self certify. Used forhigh volume and stand-alone products.German RFI declaration and ZZFregistration required for self certification.A 2 dB margin is required when one unitis tested.

Test is conducted by local postalauthority at place of installation. Class Cis for large 'one-of-a-kind' systemsinstalled in an industrial zone.

10m for Class B, 30 m for Class A(10 m, 470-1000 MHz).

10 kHz-30 MHz, Class B. 150 kHz-30 MHz, Class A. For Class B tests, whena reading is within 5 dB of the B limit,then rerun test with floating ground.

Performed per VDE0871 at 3 m from10 kHz to 30 MHz. If EU'I' fails at 3 mthen retest at 30 m for Class B.

Antenna height is fixed at 3 m for up to470 MHz and varied from470-1000 MHz for Class A (30 mdistance). Antenna height is varied from1 to 4 m for Class B (10m distance).

Interface cables are positioned at 1.5 mout from EUT and parallel with ground(see VDE0877).

EMI receivers with CISPR are highlypreferred. Spectrum analysers with QPand preselection are sometimespermitted. Spectrum analysers are rarelyused in Germany

Appendix 1.5

US EMC regulations and standards

Table Al.5.l FCC EMG~ regulations and information

Part 15Part 15jPart 18Part 68OST62OST55MP-4 Guly 87)

Standards for unlicensed or restricted radiation devicesCovers the limits for computing devicesStandards for industrial, scientific and medical equipmentSpecification for equipment connected to telephone networkUnderstanding FCC EDP regulationsOpen-field sites for FCC testingFCC measurement procedures for EDP

Table Al.5.2 Examples of other US EMC standards

SAE J-551 C [20]:

SAE J -1113 (1984).}SAEJ-1338 (1981)SAEJ-1407 (1982)SAEJ-1507 (1986)

SAE ARP-937 [21]:

SAE AIR-1147 [22]:

RTCA/DO-160B/C:

ANSI C63.2 [23J:

Vehicle ignition interference. This places limits of -2 dB flV/in/kHz (30-88 MHz)and 8 dB flV /m/kHz (88-400 MHz) at 10m from the vehicle.

Concerned with radiated susceptibility testing of whole vehicles (not aircraft),but could be applied to testing any large system

Jet engine EMI

Precipitation static radio interference from jet engine charging

Environmental conditions and test procedures for airborne equipment

Specification for EMI and field strength instrumentation

Details of other US commercial specifications can be found in Appendix 1.9 in the compendium of EMCstandards.

[References relate to Chapter 2]

261

Appendix 1.6

Gerntan, North Anterican and JapaneseEMC standards

Table Al.6.1 Comparison oj VDE) FCC and VCC] test procedures

Horizontal antennadistance (m)Class A

Class B

Vertical antennaheight (m)

Class A

Class B

Equipment under testarrangement

Cable

Cable terrnination

VDE

10 or 30100 (H-field)

103 or 30 (H-field)

1-4*

1-4*

Floor-mountedequipment is placed onnonconductive surfaceat 0.15m max heightduring test

Table-top equipment isin normal operatingposi tion on anonconductive teststand at 0.8 m height

For table-topequipment, cables arelaid horizontally for1.5 ill at table height,then allowed to dropvertically to floor. Forfloor-level equipment,cables are laidhorizontally for 1.5 m atheight where conductorleaves EUT but atminimum height of0.1 m

Each cable port has acable attached to it

FCC

3, 10 or 30

3, 10 or 30

2-6

1-4

Floor-mountedequipment is placed onfloor

T able-rnountedequipment is positioned40 cm from ground thatLISN is referenced toand at least 80 cm fromnearest other ground

Logic cables are movedto maximise emissionprofile

One of each type ofperipheral is connectedto the system

VCCI

3, 10 or 30

3 or 10

1--4

1-4

Floor-mountedequipment is placed asclose as possible to metalplane

Portable equipment isplaced on a non-metalstand 0.8 m above metalplane

Eurr arrangement isclose to conditions inwhich equipment is used

One of each type ofperipheral is connectedto the system

* If the maximum dimension of the equipment under test is smaller than 10% of the measurement distance, a fixedantenna height of either 1.5 rn or 3 m can be used.

262

GERMAN, NORTH AMERICAN AND JAPANESE EMC STANDARDS 263

Table Al.6.2 Canadian IT EMC limits

Limits of radiated radio noise emissions

Class .,1

Limits of conducted radio noise emissions

C?ass A

Frequency range

MHz

~ 30.000 ~ 88.000> 88.000 ~ 216.000

> 216.000 ~ 1000.000

Class B

Frequency range

MHz

~ 30.000 ~ 88.000> 88.000 ~ 216.000> 216.000 ~ 1000.000

Limits at 30 m

dB (reference 1J.1Vjm)

3034

37

Limits at 3 m

dB (reference 1J.1Vjm)

404446

Frequency range

MHz

~ 0.450 ~ 1.600> 1.600 ~ 30.000

Class B

Frequency range

MHz

~ 0.450 ~ 30.000

LimitsNarrowband Broadband

dB (reference 1J.1V)

60 7370 83

LimitsNarrowband Broadband

dB (reference 1J.1V)

48 61

Regulations SORj88/475 and CI08.8-M1983

Appendix 1.7

Electrical safety and electroIIlagneticradiation safety

Table A1.7.1 Typical regulations on product safety

Standard

IEC950 }EN60950CSA22.2 No950UL1950

ULl14 }UL478CSA22.2 No154CSA22.2 No220

IEC380 }VDE0806CSA22.2 No143BS6301

ZHl/618 (BG)*

IEC601-1 }VDE0750BS5724

IEC204 }VDEOl13BS2771

IEC65VDE0860IEC335VDE0700BS3456

Description

Safety of inforn1ationtechnology equipmntincluding electrical businessequipment

Older standards

Safety of office equipment

Telecom equipment

Safety of video displays andcomputers

Safety of medical products

Safety of industrialequipment

Safety of householdappliances

*BG Injuries Insurance Institute (Germany)

264

ELECTRICAL SAFETY AND ELECTROMAGNErrIC RADIATION SAFETY 265

Table Ai.7.2 Examples of standards limiting exposure of hutnans to RF radiation

NCMDRH US [4J Public Law 90-602: Limits for ionising and nonionising radiation

ANSI (1982) [35J (under revision): American National Standard C95.1 (Safety levels with respect tohuman exposure to radiofrequency electromagnetic fields)

NRPB (1986) Consultative document [36J

NRPB (1989) GS--11 [37J

DEF-STAN 59/41 (part 3) [3J

MoD (PE) Code of practice (1982): calls up STANAG 2345

DEF-STAN 05-74/1 (1989) [38J

INIRIC (1984) Guidelines: reissued in 1988 [39J

The EEC has introduced regulations regarding nonionising radiation limits under Articles 4 and 5 [40J

[References relate to Chapter 2]

Table Ai.7.3 N RPB GSii recommendations: derived reference levels for exposure to electromagnetic fields atjrequenciesbelow 300 GHz

Frequency Root-mean-square values

Electric field strength Magnetic field strength Magnetic flux density*

Hz Vim A/m mT

< 100 614,0001f (Hz) 1630 20.1-1 k 614/f (kHz) 163/f(kHz) 0.2/f(kHz)1--30 k 614/f (kHz) 163/f(kHz) 0.2/f~kHz)

0.03-1 M 6 (kHz) 4.89If(MHz) 6.10- /f(MHz)1--10 M 6141f (MHz) 4.89/f(MHz) 6.10- 3 /f(MHz)

10---30 M 61.4/f(MHz) 4.89/f(MHz) 6.10- 3 /f(MHz)

below 30 MHz: electric and magnetic fields are considered

Frequency Root-mean-square values

Electric field strength Magnetic field strength Power density

Hz Vim A/m W/m2

30-400 M 61.4 0.163 100.4-2 G 97.1v!f (GHz) 0.258v!f (GHz) 25,[(GHz)2-300 G 137 0.364 50

Frequencies above 30 MHz: electric fieldalternatives under far-field conditions

and power density are taken as equivalent

Appendix 1.8

Military EMC standards

Table Al.B.l Classes of equipment for determining applicable test requirements

Class

I

IA

IB

IC

ID

II

IIA

lIB

IIC

III

IlIA

IIIB

IIIC

IIID

IV

Description

Communication-electronic (CE) equipment: any item, including subassemblies and parts,serving functionally generating, transnlitting, conveying, acquiring, receiving, storing, processingor utilising information in the broadest sense. Subclasses are:

Receivers using antennas

Transmitters using antennas

Nonantenna CE equipment counters, oscilloscopes, signal generators, and other electronicdevices working in conjunction with classes IA and IB)

Electrical and electronic equipment and instruments which would affect mission success orif degraded or malfunctioned by internally generated interference or susceptibility to externalfields and voltages

Noncommunication equipment. Specific subclasses are:

Noncommunication electronic equipment: equipment for which RF energy is intentionallygenerated for other than information or control purposes. Examples are medical diathermyequipment, induction heaters, RF power supplies and uninterruptible power units

Electrical equipment: electric motors, hand tools, office and kitchen equipment

Accessories for vehicles and engines: electrically and mechanically driven and engine electricalaccessories such as gauges, fuel pumps, magnetos and generatorsApplicable only to accessories for use on items of Classes IlIA and IIIB.

Vehicles, engine driven equipment

rfactical vehicles: armoured and tracked vehicles, off-the-road cargo and personnel carriers,assault and landing craft, alnphibious vehicles, patrol boats, and all other vehicles intended forinstallation of tactical CE equipment.

Engine generators: those supplying power to, or closely associated with CE equipment.

Special-purpose vehicles and engine driven equipment: those intended for use in criticalcommunication areas such as airfields, missile sites, ships' forward areas or in support of tacticaloperations.

Administrative vehicles of basically civilian character not intended for use in tactical areas or incritical areas covered by Class IIIC, and not intended for installation of communicationequipment.

Overhead power lines

266

MILITARY EMC STANDARDS 267

Table A1.8.2 MIL STn 461 test requirements applicable to equipment classes

Equipment class

rrest IA IB IC ID IIA lIB IIC III III III III IV Description ofA B C D test method

RE01 • • • • 30 Hz to 30 kHz,magnetic field

RE02 • • • • • • • 14kHz to 10GHz,electric field

RE03 • 10 kHz to 40 GHz,spurious andharmonics

RE04 • • • • 20 kHz to 50 kHz,magnetic field

RE05 • • • 150 kHz to 1 GHz,vehicles and enginedriven eq.

RE06 • overhead powerline test

CEOI • • • • • 30 Hz to 20 kHz,power leads

CE02 • • • • • 30 Hz to 20 kHz,control and sig. leads

CE03 • • • • • • • • • 20 kHz to 50 MHz,power leads

CE04 • • • • • 20 kHz to 50 MHz,control and sig. leads

CE05 • • • • • • 30 Hz to 50 MHz,inverse filter method

CE06 • • 10kHz to 12.4GHz,antenna terminal

RSOI • • • • 30 Hz to 30 kHz,magnetic field

RS02 • • • • mag. induction field

RS03 • • • • 14kHz to 10GHz,electric field

RS04 • • • • 14 kHz to 30 MHz

CSOI • • • • 30 Hz to 50 kHz,power leads

CS02 • • • • 50 kHz to 400 MHz,power leads

CS03 • • 30Hz to 10GHz,intermodulation

CS04 • • 30Hz to 10GHz, rej.of undesirable sig.

CS05 • • 30Hz to 10GHz,cross-modulation

CS06 • • • • spike, power leads

CS07 • • squelch circuits

CS08 • • 30Hz to 10GHz, rej.of undesirable sig.


Table Al.8.3 Equipment and subsystem classes and applicable parts of MIL STD 461BjC

Class Description

A Equipments and subsystems which must operate compatibly when installed incritical areas, such as the following platforms or installations:

Applicable part

Al

A2

Aircraft (including associated ·ground support equipment)

Spacecraft and launch vehicles (including associated ground supportequipment)

2

3

A3

A4

A5

Ground facilities

Surface ships

Submarines

and mobile, including tracked and wheeled vehicles) 4

5

6

B Equipments and subsystems which support the Class A equipments andsubsystems but which will not be physically located in critical ground areas.Examples are electronic shop maintenance and test equipment used in noncri tical areas;

7

C Miscellaneous, general purpose equipments and subsystems not usuallyassociated with a specific platform or installation. Specific items in this class are:

Cl

C2

C3

Tactical and special purpose vehicles and engine driven equipment

Engine generators and associated components, uninterruptible power sets andmobile electric power equipment supplying power to or used in critical areas

Commercial electrical or electromechanical equipment

8

9

10

Table Al.8.4 Test changes between MIL STD 461Aand 461B

Table Al.8.5 [IK RRE 6405 classes of equipment

CE05RE06RE05RE04CE02CE04CS08RS04CE07

CS09

UM03

UM04

UM05

Deleted from 461 BDeleted from 461 BReplaced with UM03Deleted from 461 BCombined with CE01Combined with CE03Deleted from 461 BDeleted from 461 BPower leads, time dcnnain in trod uced461BStructure (comm. mode) introduced461BTactical and special vehicles (replacedRE05), introduced 461BEngine generators and related,introduced 461 BCommon elec. and mech. equipments,introduced 461 B

Class

A

B

C

D

Description

Receivers using antennas

rrransmitters using antennas

Nonantenna equipment, such ascounters, test equipment, lasers,computers, power supplies, digitalequipment, and other electronic devicesworking in conjunction with classes Aand B

Electronic equipment and instrumentswhich would affect mission success or

if malfunctioned or degraded byEM interference, SUCfl as autopilots,flight instruments, and control devices.

MILITARY EMC~ STANDARDS 269

Table Al.8.6 RRE6405 list oj tests

Identification Equipment EMC parameterclass

Aspect Frequency, Hz

RCE01 A-D DC power leads, current, AF 20 - 50kRCE02 A-D AC power leads, current, LF 10 k- 50 kRCE03 A-D All leads, voltage, narrowband 50 k-100 MRCE04 A-D All leads, voltage, broadband 50 k-100 MRCE05 A--D Signal and control leads, current 20 - 50kRCE06 A-D All leads, current, narrowband, HF 50 k-lOO MRCE07 A-D All leads, current, broadband, I-IF 50 k-100 MRCE08 A,B Antenna terminals, narrowband, SHF 10 k-100 MRCE09 A,B Antenna terminals, broadband, SHF 10 k-lOO MRCE10 A,B Antenna terminals, narrowband, 0.1 G- 40GRCEll A,B Antenna terminals, broadband, 0.1 G- IGRCE12 A,B Ant. term, narrowband, key down 10k- 40GRCE13 A,B Ant. term, broadband, key down 0.1 G- 40GRCEl4 A-D Transient emission Spikes

RCS01 A-D DC power leads 20 - 50kRCS02 A-D DC and AC power leads 50 k-400 MRCS03 A,C Two-signal intermodulation, HF 20 -100MRCS04 A,C Two-signal intermodulation, SHF 0.1 G- 40GRCS05 A,C Rejection of undesired signals, HF 20 -100MRCS06 A,C Rejection of undesired signals, SHF 0.1 G- 40GRCS07 A,C Cross modulation, HF 20 -100MRCS08 A,C Cross modulation, SHF 0.1 G- 40GRCS09 A-D DC and AC power leads SpikesRCS10 A-D Post-detector rejection 20 - 40G

RREOl A,B,C Magnetic field, LF 20 30kRRE02 A,B,C Magnetic field, HF 30k- 3MRRE03 A--D Electric field, narrowband, HF 14 k-lOO MRRE04 A-D Electric field, broadband, HF 14 k-100 MRRE05 A-D Electric field, narrowband, SHF 0.1 G- 40GRRE06 A-D Electric field, broadband, SHF 0.1 G- IGRRE07 B Antenna, spurious, narrowband 0.1 G- 40GRRE08 B Antenna, spurious, broadband 0.1 G- 40GRRE09 A-D Transien t emission Spikes

RRS01 A---D Magnetic field, LF 20 50kRRS02 A--D Magnetic field, induction SpikesRRS03 A-D Electric field, HF 14 k-lOO MRRS04 A-D Electric field, SHF 0.1 G- 40G

RMP01 B Occupied bandwidth, transmitter 1.0M- 40GRMP02 B Frequency tolerance, transmitter 1.0 M- 40 GRMP03 A,B Antenna pattern, far field 0.1 G- 40GRMP04 A Receiver, bandwidth 0.1 G- 40G


Table A1.B.7 RRE6405 EMC' grading by environlnental conditions

Grade

L

M

N

Shipboard

Equipment used outside themetal hull and/or metalsuperstructure of surfaceships, or above decks only inany ship which is of metallicconstruction, or in any shipwhich is not of metallicconstruction

Not used

Equipment used on boardsubmarines and within themetal hull and /orsuperstructure of surface ships

Air service

Equipment for use generallyin civil and military aircraft

Not used

Not used

Land service

Equipment sited not within2 m of another equipmentthat emits, or is susceptible to,electromagnetic radiation

Equipment sited not within15 m of another equipment

Equipment sited not within100 m of another equipn1ent

Table A1.B.B DEF STAN 59-41 EMC' tests

Testreference

DCEOIDCE02DCE03

DREOIDRE02DRE03

DCSOIDCS02DCS03DCS04DCS05DCS06DCS07

DRSOIDRS02

DMFSOI

Test description

Power line, current 20 Hz-ISO MHzSignal/controlline~current 20 Hz-ISO MHzExported transients, power lines

E-field radiation at 1 m 14kHz-18GHzH-field radiation at 70 mm 20 Hz-50 kHzE-field radiation, installed antenna 1--76 MHz

Power line, voltage 20 Hz-50 kHzPower, control and signal leads 50 kHz-400 MHzControl/signal, current 20 Hz-50 kHzImported transient susceptibilityExternally generated transientsImported long transient susceptibility 28 V systemsImported short transient susceptibility

H-field at 50 mm 20 Hz-50 kHzE-field, 14kHz-18GHz

Magnetostatic field

Service*applicabili ty

Land sea airLand sea airI-land aIr

Land sea airLand sea airLand

I-Jand sea airLand sea airLand sea air

aIrsea

LandLand

Land sea airI-Jand sea air

Land sea

* Check individual limits for actual frequency rangeDCE conducted en1ission DCS conducted susceptibility DRE radiated emissionDRS radiated susceptibility DMFS magnetostatic field susceptibility

Appendix 1.9

COlnpendiuln of EMC andrelated standards

With limited space this compendium cannot beexhaustive of all standards and specifications relatedto EMC and electrical safety worldwide. Regulationsand standards are changing frequently as the fielddevelops rapidly owing to technical advances,economic and political factors. However, this listcontains sufficient examples of EMC standards toprovide a starting point for further exploration.

1 UK British Standards (EMC)

BS 613 (1977): Specifies components and filterunits for EMI suppressionBS 727 (1983): Specification for RFI measurIngapparatus (CISPR 16)BS 800 (1988): Specification of RFI limits andmeasurements for household appliances, portabletools and other electrical equipment (CISPR 14),(VDE 0875)BS 833 (1985): Specification of RFI limits andmeasurements for electrical ignition systems ofinternal combustion engines (CISPR 12)BS 905 (1985): Specification of EMC for soundand TV receivers and associated equipmentPart 1: Specification of limits for RFI(CISPR 13), (VDE 0872)Part 2: Specification of limits for immunity(CISPR 20)BS 1597 (1985): Specification for limits andmethods of measuring electromagnetic interference from marine equipment and installationsBS 2316 Parts 1 and 2 (1981): General requirements and tests for radio frequency cablesBS 4727 Part 1 Gp 9 (1976): Glossary of technicalterms connected with radio interferencetechnology (IEC 50 Ch. 902)BS 4809 (1981): Specification of RFI limits andmeasurements for RF heating equipment(CISPR 11), (similar to VDE 0871)BS 5049 (1981): Methods of measurement of RFnoise from power supply apparatus workingabove 1 kV (CISPR 18)BS 5260 (1981): Code of practice for RFIsuppression on marine installationsBS 5394 (1988): Specification of limits andmeasurement methods for RFI characteristics offluorescent lamps and luminaires (CISPR 15),(VDE 0875)

271

BS 5406 (1988): Limitation of disturbances inelectricity supply networks caused by domesticand similar appliances equipped with electronicdevices (IEC 555)BS 5602 (1978): Code of practice for theabatement of RFI from overhead power lines(CISPR 18)BS 5750 Series: Quality assurance relatedstandardsBS 5783 (1984): Code of practice for handlingelectrostatic devicesBS 6021 Part 3 (1982): Specification for fixedcapacitors used for RFI suppression: Generalrequirements and methods of test (IEC 384-14)BS 6299 (1982): Measurement methods for passivesuppression filters and components (CISPR 17)BS 6345 (1983): Methods of measurement of RFIterminal voltage of lighting equipment(CISPR 15)BS 6491 (1984): Part 1 ESDBS 6527 (1988): Specification for limits andmeasurements of RFI characteristics ofinformation technology equipment (CISPR 22),(VDE 08781)BS 6651 (1985): Protection of structures againstlightningBS 6656 (1986): The prevention of inadvertentignition of flammable atmospheres by RFradiation (read with HSE GS21 HMSO 1983)BS 6657 (1986): Prevention of inadvertent ignitionof EEDs by RF radiation (read with OB. Procs42413 and 42202)BS 6667 (1985): Electromagnetic compatibilityrequirements for industrial process control

Part 1 General introduction (IEC 801-1)Part 2 Susceptibility to ESD (IEC 801-2)Part 3 Susceptibility to radiated EM energy(IEC 801-3)Part 4 Electrical fast transient/burst requirements (IEC 801-4)Part 5 Surge immunity requirements (draft in1990)

BS 6839 (1987): Part 1 - Specification forcommunication and interference limits andmeasurements for mains signalling equipment

Part 2 - Specification for interfaces for mainssignalling equipment

BS 3G 100 (1980): Part 4 section 2: EMI at radio


and audio frequency requirements forequipment for use in aircraftBS G229: Specification of environments forequipment for use in aircraft (DO 160)NWM 0320: UK National weights and measuresstandards on EMC for metrology equipment

2 UK British Stand·ards (electricalsafety)

BS 2754: Protection against electric shockBS 2771: Electrical safety of industrial machinesBS 3192: Electrical safety of radio and TV transmittersBS 3456: Household safetyBS 5458: Safety requirements for electricalindicating and recording equipmentBS 5724: Safety of mechanical equipmentBS 6301: Safety of telecommunications apparatus

3 UK NAMASjNPL docutnents

NAMAS M3: Concise directory of approvedcalibration and testing laboratoriesNAMAS MI0: General criteria of competence forcalibration and testing laboratoriesNAMAS MIl: Regulations to be met bycalibration and testing laboratoriesNAMAS M 13: Regulation concerning the use ofthe NAMAS logo/markNAMAS M16: The quality manual - guidancefor preparationNAMAS NIS20: Uncertainty of measurement forNAMAS electrical product testing laboratoriesNAMAS B3003: Expression of uncertainty Inelectrical measurementsNPL 0004/8K/NJ /6/90: NPL Points of contactNPL 007/J.5K/NJ/9/90: RF and mIcrowavemeasurement services

4 CISPR EMC standards

CISPR 1: RFI measuring set 0.15-30 MHzCISPR 2: RFI measuring set 25-300 MHzCISPR 3: RFI measuring set 10-150 kHzCISPR 4: RFI measuring set 300 MHz-IGHzCISPR 5: Peak, quasipeak and average detectorsCISPR 1-6: inclusive have been superseded byCISPR 16CISPR 7: Recommendations of CISPR (includingrecommenda tion 19: Limits in large-scaleproduction)CISPR 8: Reports and study questionsCISPR 9: CISPR and national limitsCISPR 10: Organisation, rules and procedures ofCISPR (1981 and 1983)CISPR 11: ISM limits and measurements (1975,1976)

CISPR 12: Ignition limits and measurements(1978,1985)CISPR 13: Sound and TV receivers limits andmeasurements (1975, 1983)CISPR 14: Household equip. limits and measurements (1985)CISPR 15: Fluorescent lamps and luminaireslimi ts and measurements (1985)CISPR 16: RFI measuring sets specification andmeasurements (1977, 1980, 1983)CISPR 17: Filters and suppressors measurement(1981 )CISPR 18: RFI of power lines and HV equipment

Part 1 description (1982)CISPR 19: Microwave ovens measurementsabove 1 GHz (1983)CISPR 20: Immunity of sound and TV receivers(1985)CISPR 21: Interference to mobile radiocommunications (1985)CISPR 22: IT equipment limits and measurement(1985)CISPR 23: Derivation of limits for ISM

5 Gertnan VDE EMC and electricalsafety standards

5.1 VDE EMC standards

VDE 0565: Specification for RFI suppression devicesPart 1 capacitorsPart 2 chokesPart 3 filters (up to 16A)Part 4 ceramic capacitors

VDE 0839: Teil 10 generators ESDVDE 0843: Part 2 ESD identical with IEC 801-2VDE 0846: Teil 11 test generators ESDVDE 0871: Limits of RFI from RF apparatus and

ins tallationsPart 1 ISMPart 2 EDP

VDE 0872: Interference suppression for radio andTV receiversVDE 0873: RFI from electrical utility plants andHV systemsVDE 0874: Recommendation for RFI suppressionVDE 0875: RFI from appliances (below 10 kHz)

Part 1 household appliancesPart 2 fl uorescent lightingPart 3 appliances with motors

VDE 0876: Interference measuring apparatusPart 1 weighted indicationPart 2 disturbance analyserPart 3 current probes

VDE 0877: RFI measurement proceduresPart 1 interference voltagePart 2 interference field strengthPart 3 interference on power leads

COMPENDIUM OF EMC AND RELATED STANDARDS 273

VDE 0879: RFI suppressIon for motor vehiclesand enginesDIN 43 305 pt. 302: Methods of measurement onradio receivers for various classes of emission:Methods of measurement for checking theimmunity from. interference fields of radioreceivers. Beuth Verlag, Berlin

5.2 VDE safety standards

VDE 0113: Safety of industrial equipment (IEC204)VDE 0700: Safety of household appliances (IEC335)VDE 0750: Safety of medical equipment (IEC601 )VDE 0806: Safety of office equipment (IEC 380)VDE 0806: Safety of household appliances (IEC065)VDE 0848: Hazards by electromagnetic fieldsZH 1/618 (BG): Safety of video displays, etc

6 European and internationalstandards

EMC standards and specifications in force inEurope and most countries in the world are listedin the following publication: 'Electromagneticcompatibility regulations and standardsworldwide', Netderlands Normalisatie lnstitut(UDC 621.391.82(100))

6.1 European harmonised standards

EN 29000: Quality assurance of calibration andtesting laboratoriesEN 45001: General req uirements for technicalcompetence of testing laboratoriesEN 50065-1: Mains signalling on low-voltageelectrical installationsEN 50081-1: Generic RFI emission standard forany equipment in the domestic, commercial andlight industrial electromagnetic environments(class 1 environments)EN 50082-1: Generic RF immunity standard forany equipment in the domestic, commercial andlight industrial electromagnetic environments(class 1 environment)EN 55011: Limits and methods of measurement ofRFI characteristics of ISM equipment (excludingdiathermy apparatus) (CISPR 11, BS 4809,VDE 0871, FCC Pt18: near equivalent)EN 55013: Limits and measurement methods forRFI characteristics of sound and televisionbroadcast receivers (CISPR 13, BS 905 Ptl,VDE 0872, FCC Pt15: near equivalent)EN 55014: Limits and methods of measurement ofRFI characteristics of household electricalappliances, portable tools and similar equipment

(CISPR 14, BS 800, VDE 0875, FCC Pt15: nearequivalent)EN 55015: Limits and measurement methods forRFI characteristics of fluorescent lamps andlurninaires (CISPR 13, BS 5394, VDE 0875)EN 55020: Immunity of sound and televisionbroadcast receivers and associated equipment inthe frequency range 1.5 to 30 MHz by currentinjection method (CISPR 20, BS 905 Pt2)EN 55022: Limits and methods of measurement ofthe RFI emission characteristics of informationtechnology equipment (CISPR 22, BS 6527)EN 55101-4: RF immunity of informationtechnology eq uipment (in draft)EN 60555-2: Disturbances in supply systems causedby household appliances and similar electricalequipment - harmonics (lEe 555, BS 5406)EN 60555-3: Disturbances in supply systemscaused by household appliances and similarelectrical equipment voltage fluctuations(IEC 555, BS 5406)EN 60950: Safety of information technology andbusiness equipmentHD 481: Immunity to RF, ESD, transients andsurges of industrial process control equipment.Basic standard. (IEC 801, BS 6667)HD 481.1 S1: General introductionHD 481.2 S 1: Method of evaluating susceptibilityto electrostatic dischargeHD 481.3 S 1: Method of evaluating susceptibilityto radiated electromagnetic energy

6.2 Other European standards

UN ECE Reg. 10: Control of RFI from motorcycles and vehicles with 3 or more wheels72/245/EEC: Suppression of motor vehicle ignition(CISPR 12, BS 833)71/320/EEC: Vehicle brakesUN ECE Reg. 13/05: Vehicle brakesEEC Directive 71/320/EEC (amended 85/647/EEC): Braking devicesEEC 451 7/79: Contains reference to ESDECMA Standard 47: Limits and measurementmethods for radio interference from EDPequipment. European Computer ManufacturersAssociation, Geneva, SwitzerlandECMA TR/40: ESD testing of IT equipment ---technical report

6.3 lEe standards related to EMC

IEC 50: International electrotechnical vocabulary,Chapter 161 on electromagnetic compatibilityIEC 96: Part 0 - Guide to the design anddetailed specification of RF cablesPart 1 - General requirements and measurementmethods for RF cables .


IEC 106: Recommended methods of measurementof radiated and conducted interference fromreceivers induced by AM, FM and TV broadcasttransmissionsIEC 478: Part 3 - RFI tests for DC stabilisedpower suppliesIEC 533: EMC of electrical and electronicequipment in shipsIEC 555: Disturbances in supply systems causedby household appliances and similar equipmentIEC 654: Operating conditions for industrialprocess measurement and control equipmentIEC 801: EMC requirements for industrial processcontrol instrumentation

IEC 801-1: General introductionIEC 801-2: Susceptibility to ESDIEC 801-3: Susceptibility to radiated EM

energy (draft)IEC 801-4: Electrical fast transients/burst

req uirementsIEC 801-5: Surge immunity requirements (draft)IEC 801-6: Immunity to conducted RF distur

bances above 9 kHz (draft)

6.4 lEe standards related to electricalsafety

IEC 065: Safety of household appliances (VDE0860)IEC 204: Safety of industrial equipment (VDE0113)IEC 335: Safety of household equipment (VDE0700)IEC 380: Safety of office equipment (VDE 0806)IEC 601: Safety of medical products (VDE 0750)

6.5 ISO publications

ISO 9000: Quality assurance aspects of testingISO Guide 25: Technical competence of testinglaboratoriesISO/TC22/SC3/WG3 paper N 260: Roadvehicles: Electrical in terference from electrostatic dischargeINIRC/IRPA (1988): Guidelines on limits ofexposure to radio frequency electromagnetic fieldsin the frequency range from 100 kHz to 300 GHz

7 US EMC regulations and standards

FCC Code of Federal Regs. Title 47 volumes 1-5(docket 20780)FCC Part 15: Electromagnetic compatibility ofRFdevices

Subpart A GeneralSubpart B Administrative proceduresSubpart C Radio receiversSubpart D General requirement for low-power

communications devices

Subpart E Low-power communications devices(specific devices)

Subpart F Field disturbance sensorsSubpart G Auditory assistance devicesSubpart H TV interface devicesSubpart I Measurement proceduresSubpart J Computing devices

FCC Part 18: Electromagnetic compatibility ofISM equipment

Subpart A General informationSubpart B Applications and authorisationsSubpart C Technical standards

FCC Part 68: Electromagnetic compatibility ofequipment to be connected with the telephonenetworkFCC Doc. OCE44: Open-field site calibration(horizontal polarisation)OST 62: Understanding the FCC EDPregulationsOST 55: Open-field sites for FCC testingMP4 Uuly 1987): FCC measurement proceduresfor EDPSAE J 551 C: Vehicle ignition interferenceSAE J 1113 (1984): Susceptibility procedures forvehicle components (except aircraft)SAE J 1338 (1981): Open-field whole-vehicleradiated susceptibility testingSAE J 1407 (1982): TEM cells for automotivesusceptibility testsSAE J 1507: Apparatus and test procedures forwhole-vehicle susceptibility testingSAE ARP 936: ~1easurement of capacitorsSAE ARP 937: Jet engine EMISAE ARP 958: Test antenna calibration practiceSAE Jl113 (1989) Part 5: Electromagnetic compatibility measurement, procedure for vehiclecomponents: Susceptibility to ESDSAE J1338SAE J1407SAE J1507SAE AIR 1147: Precipitation of static radio interference from jet engine chargingSAE AIR 1499: Recommendations for commercialEMC susceptibility standardsANSI C16: EMC aspects of communication andelectronic equipmentANSI C63: Measurement techniques for EMCC63.2: Specification for EMI and field strengthins trumentationC63.4 (1988) EMC: Radio noise emissions fromlow voltage electrical and electronic equipment10 kHz-1 GHzANSI C68: EMC aspects of high-voltage testingtechniquesANSI C95.1 (1982): Safety levels with respect tohuman exposure to RF EM fields (300 kHz100 GHz)ANSI April 1989 (draft 5): Guide for ESD test

COlvIPENI)IUM OF EMC AND RELA1'EI) STANDARDS 275

00-1,

electro-DEF STAN 00-10: Part 2, section 4

DEF STAN 00--35: I)EFDEF STAN 07-55 and AvP

Part 2 of service ""Y\'! T1 r/"'''''",,''"¥> "",.Y\ TO

MIL-B-5087: Bonding, electrical\.J",,-/vL~.'J.LJ. for aerospace systems

MIL-W -5088: installedMIL-E-6051D:electromagnetic v'JAJ.l. t--J(A, ... ~" OJ"LL~",

MIL-E-55301MIL-I-16910CMIL-I-6181DMIL-M-3851 O-ESDMIL-W-83575:design andAFSC DR 1-4:netic r>"1"Yl y-,.r-. t-~

MIL STD 46 interfer-ence characteristics requirements forMIL STD 462:characteristics, measurement ofMIL STD 463: Electromagnetic interferencecharacteristics, definitions systems of unitsMIL STD 469: EMC - radar engineering

MIL STD 481A: Antenna: method of calibration,(paraMIL STD 704: Electric power, aircraft characteristics and useMIL STD 826A: 'Transients: a 10 JiF '-'.....,I--J(A,'--'~ ...'J~

MIL STD 831: Preparation of test reportsMIL STD 833C: Involves aspects of ESDMIIJ STD 1310: and grounding methodson board for EMC and safetyMIL STD 1337: General suppression and system

requirements for electric handtools (ships)MIL STD 1344A: Method 3008 (1980): "--JJ"~~'~~'-A~'"J.;;;"

effectiveness of multicontact connectorsMIL STD 1377: Effectiveness of connectorand weapon enclosure shielding and filters inprecluding EM hazards tomeasurement of,MIL STD 1385: Preclusion of ordnance hazardsin EM fields, generalMIL STD 1512:

initiated,requirementsMIL STD 1541: for spacesystemsMIL STD 1542: EMC andments for space systems facilitiesMIL STD 1857:

10 UK tnilitary standards

9 US m.ilitary standards (EMC)

MIL S1-'D 188: Military communications systemstactical standardsMIL STD 188-125 (draft 1989): NEMPtestingMIL STD 220: Insertion-loss measurements ofcomponentsMIL STD 285: Attenuation measurement methodfor shielded enclosuresMIL STD 331A: ContainsMIL STD 454: General rpnlllll~""YY'lPY\t-C'

equipment

methodologies and cri teria for electronicequipmentUL 1950: Safety of IT and electrical businessequipmentMDS-201-0004 (1979): EMC standard for rnedicaldevices. A voluntary codeAAMI ( :' Heart pacemaker EMC voluntarystandardFDA Infant Apena Monitor StandardMandatory EMC regulation applied to thesedevicesNCMDRH (US Public Law 90-602, 1968): Saferadiation limits from TVs, x-ray machines,microwave ovens, etcIEEE S302: Standard method for measuring EMfield strength below 1000 MHz in radiowavepropagationIEEE S299: Measuring shielding effectiveness ofhigh-performance shielded enclosuresIEEE S473 (1985): Recommended practice for anelectromagnetic site survey (10 kHz-1 0 GHz)IEEE S518: Guide on the installation of electricalequipment to minimise interference to controllersfrom external sour'cesIEEE ad hoc committee, 1990: Proposed new codeof ethics, vol. no.3, p.5US Safety Standard NFPA-STD-70-1971: Electric

wiring and other devices

8 Exam.ples of other EMCregulations

veCI (1986) Japan: 1'he control ofnetic emissions from electronic dataand office machines (Voluntary Code establisheslimits and test methods)SOR/88-475 (1988) Canada: Radio interferenceregulations amendmentCSA C108.8-M1983 Canada: EMCproceduresCSA 22.2 no.154: Safety of and electricalbusiness equipmentFor further examples of worldwideregulations, see 1'able 2.1 in 2


Part 4 natural environments (lightning)Part 5 ind uced environments

DEF STAN 08-4: Nuclear weapons explosioneffects and hardeningDEF STAN 08-5: Superseding AvP 32 'Designrequirements for guided weapons'DEF STAN 59-41 (June 1988): Electromagneticcompatibility

Part 1 general req uirementsPart 2 management and planning proceduresPart 3 technical requirements (test methods

and limits)Part 4 open-site testingPart 5 technical req uiremen ts for special EMC

test equipmentDEF STAN 61-5:

Part 1 terminology and definitionsPart 2 ground generating set characteristicsPart 4 power supplies in warshipsPart 5 ground power supplies for aircraft

servIcIngPart 6 28 V DC electrical systems in military

vehiclesNWS 2: EMC aspects of cabling in ship installationsNWS 3 (1985): Electromagnetic compatibility ofnaval electrical equipmentsNWS 1000 (1986): Requirements for navalweapon equipment

Part 1, chapter 5, section 10 - Electro-

magnetic compatibility design guide for navalweapons platform (refers also to Part 2)

NES 529: Nuclear hardening guideNES 1006 (1988): Supersedes NWS 6, Radiofrequency environment and acceptance criteriafor naval stores containing EEDsBR 2924: Radio hazards in naval serviceBR 8541: Safety req uirement for armament storesfor naval useMVEE 595 (1975): Electromagnetic interferenceand susceptibili ty req uiremen ts for electricalequipments and systems in military vehicles(forms part ofDEF STAN 59-41)RRE 6405 (1974): Requirements for electromagnetic compatibility of electronic equipments

Part 1 guidePart 2 req uirementsPart 3 measurement techniq ues ,

AVP 118: Guide to electromagnetic compatibilityin aircraft systemsSTANAG 1307: (Implement by NESI006)STANAG 4234: RF environmental conditionsaffecting the design of material for use by NATOforcesMoD (PE) 1982: Intense radio frequencyradia tion code of practiceSTANAG 2345: RF exposure limits for personnelNRPB (1986): Advice on RF safetyNRPB GS11 (1988): Guidance on RF exposure

Appendix 2.1

Modulation rules

Table A2.1 MIL STD 462 (N3) modulation rules

For communications equipment

(a) AM receivers: modulate with 500/0 1kHzsinusoid

(b) FM receivers: When monitoring signal-tonoise ratio, modulate with 1 kHz signal and10kHz deviation

(c) SSE receivers: No modulation(d) Other equipments: As for AM receivers

Equipments with video channels:

Use 90-100% pulse modulation withduration of 2/B Wand repetition rate ofBW/1000, where BW == video bandwidth

Digital equipment:

Use pulse modulation with pulse durationand repetition rates equal to that used inequipment

Nontuned equipment:

AM at 50% with 1 kHz sinusoid

277

Table A2.2 DEF STAN 59-41 modulation rules

Susceptibility Modulationfrequency type

Ship and land service use:

20 Hz-50kHz CW

50 kHz- 1 GH:z (a) CW(b) AM 100% square

or sine at frequenciesin the range100 Hz-10kHz

(c) AM 100% square 1 Hz

200 MHz-18 GHz Pulsed CW, pulselength 1 j1s, PRF 1 kHz(for test DRS02 only)

For aircraft use:

20 Hz-50kHz CW

50kHz-2 MHz (a) CW(b) AM 100% square at

lkHzPRF

2MHz-30MHz (a) CW(b) AM 100% square at

1 kHz PRF(c) AM 100% square at

1-3 kHz PRF

30 MHz--1 GHz (a) CW(b) AM 100% square at

1kHzPRF

Radiated testing (a) CW1-18 GHz

150-225 MHz (b) A complex modulation580-610 MHz of pulse (1 j1s) and

0.79-18 GHz AM (0.5Hz)

Appendix 3.1

NAMAS-accredited laboratories

As given in the EMC Awareness Campaignbrochure (Reference 11 Chap. 13).

Bri tish TelecomProduct Evaluation Section (MC2)Materials and Component Centre310 Bordesley GreenBirmingham B9 5NF0121 771 6007

BSI TestingUnit 5Finway RoadHemel Hempstead HP2 6PT01442 233442

Chomerics (UK) Ltd.ParkwayGlobe Park Industrial EstateMarlow, Bucks SL7 1YB01628 486030

Cranage EMC TestingStable CourtOakleyMarket DraytonShropshire TF9 4AG01630 658568

GEC Marconi AvionicsEMC Test CentreMaxwell BuildingDonibristle Industrial ParkDunfermline KY11 5LB01383 822131

GPT Ltd.Environmental Testing GroupNew Century ParkPO Box 53,Coventry CV3 lHJ01203 562046

ICLWinsford EMC LaboratoryWest AvenueKidsgrove, Stoke on TrentStaffs ST7 1TL01782 771000 x3772

278

Lucas Aerospace Ltd.Power Systems DivisionMaylands AvenueHemel HempsteadHerts HP2 4SP01442 242233

Matra Marconi Space UKEnvironmental Engineering and Test LabAnchorage Road, PortsmouthHampshire P03 5PU01705 674554

National Weights and Measures LaboratoryStanton AvenueTeddingtonMiddlesex TW 11 OJZ0181 943 7242

Thorn EMI Electronics Ltd.Sensors DivisionManor Royal, CrawleyWest Sussex RHI0 2PZ01293 528787 x4985

Other NAMAS-accredited test laboratories (listedin the NAMAS Concise Directory) follow.Apologies to those laboratories which may nothave been included. Prospective users of anaccredited EMC test laboratory should consultthe most up-to-date NAMAS directory for details.

Categories of permanent laboratory:

0: Permanent laboratory where the calibration ortesting facility is erected on a fixed location fora period expected to be greater than three years.

I: Site calibration or testing performed by staffsent out on site by a permanent laboratory thatis accredited by NAMAS.

AQL-EMC Ltd. Cat 0 and I16 Cobham RoadFerndown Industrial EstatePooleDorset BH21 7PE01202 861175

British Aerospace (Dynamics) Ltd. Cat 0EMC Test House, EME DepartmentFPC065PO Box 5Filton, Bristol BS 12 7QW0117 9693 866 x6549

NAMAS-ACCREDITED LABORArrORIES 279

Chase EMC Ltd.St Leonards HouseSt Leonards RoadLondon SW147LY0181 878 7747

Cat 0 GEC Ferranti Defence Systems Ltd.EMC LaboratorySt Andrews WorksRobertson AvenueEdinburgh EH11 1PX0131346 3912

Cat 0

ERA Technology Ltd.Electromagnetic Compatibility DivisionCleeve RoadLeatherheadSurrey KT22 7SA01372 374151

GEC Avionics Ltd.Central Quality DepartmentAirport WorksRochesterKent MEl2XX01634 844400 ext 3540

Cat 0

Cat 0

Hunting Communication Technology Ltd. Cat 0Electromagnetic Assessment GroupRoyal Signals and Radar EstablishmentPershoreWorcestershire WR 10 2RW01386 555522

Kingston Telecommunication Laboratories Cat 0Newlands Science ParkInglemire LaneHullHumberside HU67TQ01482 801801

Appendix 3.2

Cotnpetent bodies

As given in the EMC Awareness Campaignbrochure (Reference 11, Chap. 13).

Assessmen t Services Ltd.Segensworth RoadTitchfieldFarehamHampshire P0155RH01329 443322

AQL-EMC Ltd.16 Cobham RoadFerndown Industrial EstateFerndownPooleDorset BH21 7PE01202 861175

BNR Europe Ltd.EMC Engineering CentreLondon Road,HarlowEssex CM179NA01279 429531

British Aerospace Defence Ltd.Dynamics DivisionDept 319, FPC 047, PO Box 5FiltonBristol BS 12 7RE0117 969 3866

British Telecom Research LaboratoriesEMC Engineering GroupMartlesham HeathIpswichSuffolk IP5 7RE01473 642319

Cambridge Consultants Ltd.Science ParkMilton RoadCambridge CB4 4DW01223 420024

CF Europe Ltd.Greencourts Business ParkStyal RoadMoss NookManchester M22 5LG0161 436 8740

280

Chase EMC Ltd.St Leonards HouseSt Leonards RoadLondon SW147LY0181 878 7747

Dedicated Microcomputers Ltd.1 Hilton SquarePendleburyManchester M27 IDL0161 794 4965

EMC Projects Ltd.Holly Grove FarmVerwood RoadAshleyRingwoodHampshire BH24 2DB01425 479979

ERA Technology Ltd.EMC DepartmentCleeve RoadLeatherheadSurrey KT22 7SA01372 374151

GEC Avionics Ltd.Central Quality DepartmentAirport WorksRochesterKent ME12XX01634 844400 x8097

Hunting Communications Technology Ltd.Electromagnetic Assessment GroupR.S.R.E. PershoreWorcestershire WR 10 2RW01386 555522

IBM (UK) Ltd.EMC LaboratoryPO Box 30Spango ValleyGreenock PA 16 OAH01475 892000

IBM (UK) Ltd.EMC LaboratoryH ursley ParkWinchesterHampshire S021 2]N01962 844433

Interference Technology International41-42 Shrivenham Hundred Business ParkShrivenhamSwindon SN6 8TZ01793783137

JRS Associates59 Titchfield RoadStubbingtonFareham PO 14 2JF01329 665549

Kingston Telecommunications Labs (KTL EMC)Newlands Science ParkInglemire LaneHullHumberside HU6 7TG01482 801801

MIRAWatling StreetNuneatonWarwickshire CV 10 OTU01203 348541

OFFERElectrici ty Meter Examining ServiceHagley HouseHagley RoadEdgbastonBirmingham B16 8QG

Radio Frequency Investigations Ltd.Ewhurst ParkRamsdellBasingstokeHampshire RG26 5RQ01256 851193

COMPETENT BODIES 281

Radio Frequency Investigation Ltd.Dunlop HouseDunlopAyrshire KA34BD01560 83813

Radio Technology LaboratoryWhyteleafe HillWhyteleafeSurrey CR3 OYY0181 660 8456

Serco Services Ltd.DMCS EMC FacilityVI BuildingRSRE North SiteLeigh Sinton RoadMalvernWorcestershire WR 14 1LL01684 592989

SGS EMC ServicesThe Industrial EstateSt Michael's WaySunderland SR 1 3SD0191 515 2666

TRL Technology Ltd.Alexandra WayAshchurchTewkesburyGloucestershire GL20 8NB01689 850438

Appendix 3.3

EMC consultancy and training

Organisations offering consultancy and training inthe UK in addition to competent bodies as givenin the EMC Awareness Campaign brochure(Reference 11 Chap. 13). See also list of organisations identified as shadow UK Competent Bodiesand list of NAMAS accredited laboratories. Mostoffer consultancy, some also provide training.

*University of HertfordshireSchool of Engineering 01707 279176

York Electronics CentreUniversity of York 01904 432323

Paisley University 0141 848 3401

*Northern Ireland Technology CentreQueens University of Belfast 01232 245133

*University of Central EnglandBirminghamFaculty of Engineering and Computer Technology0121 331 5000

*Napier UniversityEdinburghContact Alexander MacLeod 0131 444 2266

*Huddersfield UniversityManufacturing Advice Centre 01484 422288

*The John Moore U niversi tyLiverpoolSchool of Information, Science and Technology0151 231 2052

*University of PlymouthSchool of Electrical Engineering 01752 232588

*Highbury College of TechnologyDepartment of Electrical and Electronic EngineeringPortsmouth 01705 383131

*Staffordshire U niversi ty. Stafford 01785 52331

*university of GlamorganThe Technology CentrePontypridd 01443 480480

*Denotes regional electronics centre offering EMCconsultancy

282

Salford University Business Services Ltd.0161 736 2843

Cherry Clough Consultants 01457 871605

Higher Degrees:MSc in Electromagnetic CompatibilityUniversity of HullDepartment of Electronics Engineering01482 465891

U niversi ty of YorkDepartment of Electronics 01904 432323

MSc in Electrical and Electromagnetic EngineeringUniversity ofWalesDepartment ofElectrical andElectronic EngineeringCardiff 01222874000

lEEProfessional' DevelopmentStevenage 01438 313311

lEEDistance learning package, in conjunction withYork Electronics Centre 01438 313311

Society of Environmental EngineersBuntingford 01763 71209

ElmacChichester 01243 533361

Pera InternationalMelton Mowbray 01664 501501

Schaffner EMCReading 01734 770070

Sira Communications (Training)Chislehurst 0181 467 2636

Sira Test and Certification (Consultancy)Chislehurst 0181 467 2636

Surrey GroupStaines 01784 461393

Note: many other companies offer EMC traInIngand consulting; see current EMC trade press

Appendix 3.4

Useful publications on EMC

'UK Regulations' (SI 1992/2372) are availablefrom HMSO and its agents.

'EMC workbook'. DTI training aid available fromInterference rrechnology International0793 783137

'EMC directive and amending directive'. Centrefor European Business Information, Contact:Steve Terrell 071 828 6201Also published in the Official Journal of theEuropean Communities, ref: L139 23 May 1989

European explanatory document on the EMCDirective, List of competent bodies, Guidancedocument on the content of the technical construction file, Product standards booklet on EMCDTI, 151 Buckingham Palace Road, LondonSW1W 9SS

BSI 'EMC manual' 0908 226888

Bibliography and information pack 1991, lEEtechnical information unit, Coupland andFountain

WILLIAMS, T.: 'E~;[C for product designers'.(Bu tterworth-Heinemann)

MARSHMAN, C.: 'A guide to the EMCdirective'. (York Electronics Centre, EPA Press)

ELLIS, N.: 'Electrical interference technology'(Woodhead Publishing)

GOEDBLOED, J.: 'Electromagnetic compatibility'. (Prentice Hall)

JACKSON, G.A.: 'The EMC Directive A thirdstatus report'. Proceedings of EMC 92 conference.ISBN 0 7008 0431 5

JACKSON, G.A.: 'The achievement of electromagnetic compatibility'. ISBN 0 7008 0400 5

'Safety and EMC' bimonthly newsletter.Contact for all these: Publication Sales, ERATechnology 0372 374151

OTT, H.W.: 'Noise reduction techniques inelectrical systems'. (Wiley Interscience, 2nd edn.)

'Understanding, simulating and fixing ESDproblems'. Don White Consultants ISBN 0-932263-27-5

283

CORP, M.B.: 'Zzaap! taming ESD, RFI andEMI'. (Academic) ISBN 0-12-189930-6

DENNY, H.W.: 'Grounding for the control ofEMI'. Don White Consultants ISBN 0-93226317-8

GEORGOPOULOS, C.J.: 'EMC In cables andinterfaces'. ICT

KEISER, B.: 'Principles of electromagneticcompatibility'. (Artech) ISBN 0-89006-206-4

SCHLICKE, H.M.: 'Electromagnetic compatibility: applied principles of cost effective con trolof electromagnetic interference and hazards'.(Marcel Dekker) ISBN 0-8247-1887-9

VIOLETTE, WHITE and VIOLETTE:'Electromagnetic compatibility handbook'. (VanNostrand) ISBN 0-442-28903-0

Texts that include rigorous analytical explanationsof electromagnetic phenomena associated withEMC and which may be suitable for undergraduate or postgraduate study include:

PAUL, C.R.: 'Introduction to electromagneticcompatibility'. (Wiley, 1992)

GOEDBLOED, J.J.: 'Electromagnetic compatibility'. (Prentice-Hall, 1990)

A text specifically concerned with explaining theEuropean Community EMC Directive is

MARSHMAN, C.: 'Guide to the EMC Directive89/336/EEC' (EPA Press) 1992

EMC in the 1990s is a rapidly developing field andno one book can be said to adequately cover it.Therefore a number of publications need to beavailable to any reader who seeks either a broadpicture of the topic, or desires specific informationon a detailed aspect of it. I t is hoped that thecontent and references given in this book enablemost readers to acquire the information on EMCwhich they need.

An additional way of acquiring information andlearning new skills associated with EMC is via adistance learning programme such as that offeredby the lEE. Brief details follow.


tion, legislation, flowchart of actionsrequired by manufacturers and suppliersand relevant standards

Video 2 Relevant standardsVideo 3 Achieving compliance

Module 3:

Video 1 Introduction to EMC measurementsRF measurement fundamentals

Video 2 EM wavesRadiation mechanismsMeasurements and measurement systems

Video 3 Screened roomsOpen-field test sites

Video 4 Practical measurementsConducted emission measurementsRadiated emission measurementsRadiated immunityConducted immunityElectrostatic discharge

Electrom.agnetic com.patibility (withparticular em.phasis on EC Directive89j336jEEC)

An lEE distance learning video course producedby the York Electronics Centre, University ofYork.

The course consists of 12 videos, in four modules,total running time approximately 10 hours, withaccompanying course notes. I t is intended to be astand-alone training aid to be used either as anindividual teaching package, or with tutoredvideo instruction. The course identifies the rangeof disciplines involved in achieving electromagnetic compatibility and provides a sound basis forthe application of design procedures to achieveEMC and in particular to ensure conformity withthe EC Directive and the various CENELECstandards currently applicable. It has a modularstructure to cater both for designers of electrical!electronic systems and those involved in EMCmeasurement and testing.

Module 1:

Module 4:Video 1 Shielding and crosstalkVideo 2 EMC on a PCBVideo 3 Conducted interference

techniquesVideo 4 CAD for EMC

suppresslon

Video 1 EMC explanatory introduction.Covers the general nature of EMI,definitions, EMI propagationmechanisms, areas for concern e.g.ignition hazards, overview of EMCtesting, design for EMC and futuretrends Module 2:

Video 1 European Community Directive on EMC.Describes EC Directive including scope,essential protection requirements,compliance, certification and markingrequirements, obligations of administra-

In addition, three videos on EMC are available aspart of the DTI awareness campaign availablefrom:

Technical Video Sales (New Electronics) 01322222222.

The videos are titled: 'EMC an introduction formanagers and senior engineers', 'EMC design'and 'Routes to compliance (testing)'. The scriptsfor these videos were written by the author of thisbook.

Absorbing current clamp 67,69Accuracy in testing 42, 168, 214, 216, 21 7, 218

.Achieving product EMC 238PC/IT example 238

Amplifiers, see Power amplifiersAnalysers, see Spectrum analysersAnechoic screened chambers, see Screened roomsANSI 16,23,133,170,190,206,261,265,274Antenna

aperture 74balun 91, 170, 217bandwidth 84basics 72beamwidth 79biconic dipole 45, 48, 94, IIIbounded wave 45, 119, 199calibration uncertainty 21 7'cavitenna' 158conical logarithmic spiral 45,48,98, 113Crawford cell 45, 48, 123dipole, diode detection 48dipole, electrically short 92dipole, log-periodic 96dipole, tuned half-wave IIIeffective length of 83E field generator 115electrically short dipole 48factor, transmitting & 74fibre, optically coupled 45, 48for EMC 72, 73for radiated susceptibility 110free field 45, 48, Ill, 200gain 73GTEM cell 45, 48,110,126horn 45, 48, 100horn fed dish 48input impedance 84, 169log-periodic 45, 48, 96, 112long wire 45, 48, 118loop, active 106loop, calibration of 106loop, large 45loop, passive 105loop, small 45magnetic field 105monocone 48monopole 45, 48, 86, 88mutual impedance coupling 75parallel plate line 45, 48, 119, 199parallel plate line in screened room 122phase centre 75polarisation 83, 21 7radhaz monitor 48reflector 45, 103, 114ridged horn 48, 102

Index

rod 45size field averaging 169, 21 7spot size 81TEM cell 45, 46, 48, 119, 123, 126transmission line 48, 199tuned dipole 45, 48, 89, 90, 91, IIIwideband 96

Asymmetric TEM cell 126Automatic EMC testing 151

emission testing 152in the future 152susceptibility testing 152

Balanced hardening 233Balun antenna, see Antenna, balunBalun losses 170Biconic dipoles 94, III

bow tie 94commercial 94use of 94wire approximation 94

BPM (Deutsche Bundespost) 21British Standards (EMC) 18, 133, 239, 254, 271

emission standards 18for civil aircraft 18immunity standards 18BS727 33, 50, 93, 96BS833 33, 254BS905 coupling capacitor 58BS905 LISN 50

British Standards (power disturbances)BS2914206BS5406206BS6662206

British Standards (product safety) 264, 272Broadband/narrowband signals 250Bulk current injection (BCI) 171, 175

Capacitorcoupling RF 52, 56, 58clamps 45, 59distributed 45, 55, 59feedthrough 44,48, 51, 55for coupling to AC power circuits 54, 56for coupling to I/O and control lines 55, 59for use in BS6667 (IEC 801 pt 4) 53for use in BS6667 (IEC 801 pt 5) 53,57wideband 44

CENELEC EMC standards 18, 239, 254, 255, 256, 257Circulator 151CISPR £1\;11 meter detectors 21, 133CISPR standards 16, 17,24,68,91,92, 133, 136, 154,

160,166,169,170,171,215,219,239,254list of 258, 272

Civil and EMC standards 15,254

285


Canadian E1\1 Cstandards 25, 275limits 263

comparing EMC tests 19, 260, 262compendium of E1\'1 C and related standards 271derivation of commercial standards 17derivation of military standards 15ESD and transient standards 27European commercial standards 20, 254, 255, 256,

257, 273examples of EMC standards 16FCC requirements 23generation of CENELEC E1\:1 C standards 18German EMC standards 21, 259, 272, 273Japanese EMC standards 24, 275other US commercial standards (not FCC) 24product-specific UK military standards 33, 34range of standards in use 15service specific standards (military) 31UK/European commercial standards 18, 273UK military standards 31, 268, 269, 270, 275UK standards-commercial equipment 18,271USA commercial standards 23,261,274USA military standards 28, 275USA military standards (other than 461/2/3 and

6051D) 31Clamp-RF current 67,69Coaxial couplers 150Compatibility matrix 227Competent body

list of 280report from 246

Component burnout 196, 197, 232Component upset levels 232Conducted emission 48Conduction and induction couplers 44, 48Conformance test plan 40Consultants (EMC) 223, 241Consultants, list of 282Control plan, see EMC control planConvolution 183Couplers, coaxial, see Coaxial couplersCouplers, directional, see Directional couplersCouplers, distributed capacitance 55, 59Couplers, inductive 61, 67, 69CoupIers, waveguide, see waveguide couplersCoupling capacitor, RF 52, 57, 58Coupling, low-level swept, see Low-level swept couplingCoupling models (simple) 229Coupling, radiative 45Coupling to victims 5Crosstalk, capacitive and inductive 230Current clamp 67, 69Current probes

cable 44, 48, 61, 62, 172injection 65measurement method

principle of 62surface 44, 48, 66, 68transfer function 63

Definitions of EMC 1DEF STAN 00-35 203

DEF STAN 05-74/1265DEF STAN 59-41 7, 14, 33, 34, 49, 60, 64, 65, 154,

171, 175,265,277list of tests 270

Design (for EMC) 223approach to 242for contractual assistance 224for formal compliance 224handbooks 225hardening 235process 225

example of 243techniques 236

Detectors, see Receivers, EMIDetermining EMC requirement 241DI-EMCS-8020 40Diode detection dipole 48J)ipole

aperture model (coupling to cables) 231electrically short 92log-periodic 96, 112non-resonant 93Roberts 92tuned 89, IIItuned, commercial 91tuned, practical 90tuned, radiated emission testing 91

Direct connection devices 48Directional couplers 148Distributed capacitance couplers 55DO 160 114,204DTI 18, 218, 239, 252

EC 89/336 20, 41, 224, 246, 252ECMA 190,240,273ECSA 190EEC 20, 21, 224, 246, 252, 253, 254, 259, 265, 273EED 27E-field generator 115E-field levelling loop 117, 142E-field sensors 48EIA 23, 189, 241EMAS 227EMC

computer models 31control plan 224defini tions of 1design, see Design for EMCearly problems 7early problems with military equipment 9hardening requirement 228hardening techniques 235historical background 7information about 238

customer sources 238industry sources 240notified bodies 239other sources 239professional bodies 240regulatory authorities 239suppliers 240

legislation, see UK EMC legislationphilosophy of 12

requirernent, see Determining EMC requirementsensor groups 44serious problems with 10standards and specifications 14, 224

compendium of271strategy 242system specification 224technical disciplines 10test plan 227training 241

sources of 282useful publications 283

EMIcoupling to victims 5receivers, intended and unintended 6sources, broadcast 4, 5sources, continuous 3sources, intended 4sources, man-made 3sources, natural 3sources of 1sources, transient 3sources, unintended 4

Emission suppression requirement 233EN (European Norm) 19,21,41, 133, 136, 151,206,

218,239,254,255,256,257,273Equipment case hardening requirement 233ESD 27, 140, 179, 185,204

air discharge test 189charged device model 187contact discharge test 189direct injection 44, 48event 185field induced model 188human body model 187IEC80 1-2 new ESD test 190indirect injection 45latent defects 188nurnber of discharges required 191probe 187types of 187types of test 188vol tage test levels 191waveform 186

EUROCARE lightning standard 204European El\1C standards

existing and possible future 256existing with eq uivalents 254, 255proposed product specific 257

FAA 204FCC (Federal Comrrlunications Commission) 16,18,21,

22,41, 136, 160, 169, 170, 171,215,218,261,262, 274

FCC pt 15j 16,23,24,89,91,92, 154,239,254,260,261,262,274

FCC requirements 23Feedthrough capacitor 44, 48, 51Ferri te-cored loop 48Ferri te wand 48Fibre-optic transmitter 173Filters 136Fourier transforms 180

INDEX 287

Frequency meters 142FTZ (Central telecommunications office,

Germany) 18,22,218,260

German decrees and standards (EMC) 259GS11 265GTEM 45, 48, 110, 126

Hardening requirement, see EMC hardeningrequiremen t

Hardening techniques, see EMC hardening techniquesHERO 27High-impedance voltage probes 56HIRF 114, 154Historical background 7HPM 154Hybrid ring 150

IEC 1, 14, 16,27,206,239,264,273,274IEC 555 28IEC 80127,53,55,57,59,110,114,119,121,187,188,

206, 224, 254IEC 801-2 ESD test (new) 190lEE 240, 241, 282IEEE 16,23, 106,206,239,240,241,275IEMCAP 31,226Immunity first? 243Impulse generators 137Impulse response 182Induction windings 45, 48, 70Inductive clamp 45, 67, 69Inductively coupled devices 61Information about EMC, see EMC, information aboutINIRC 27,265Injection current probes 65, 175Instrumentation

for emission testing 130for susceptibility testing 142

Intersystem and intrasystem EMI 7, 223Intrasystem EMC 223, 226, 227Inverted 'V' coupling model 231IPP-1 226IRAC 23ISM (industrial, scientific and medical) 16, 17, 24,

25, 274ISO 218, 274Isolator 151

JTRC Uapan) 24

Lightning 179, 201, 228discharge, definition of 202effects on equipment 204

aerospace equipment 204ground equipment 204

environment 201LISN 44,48

BS3G100 50BS727 50, 52BS905 50, 53DEF STAN 59-41 51developmen t of 49direct injection 50


for testing commercial equipment 505 microhenry type 4950 microhenry type 49specification of 49

Lossesin baluns 170, 217in cables and connectors 170, 21 7

Low level swept coupling 172

Magnetic field probes 65Magnetic field susceptibility tests 10Magnetic induction tests 70, 107Marconi 7Mathematical modelling 11, 38, 39,44, 226, 227Matrix, see Compatibility matrixMeasurement devices for conducted EMI 48MIL-B-5087B 203MIL-E-6051 D 31MIL STD 461/2/3 7, 17,28,33,35,40,41,49,51,52,

56,60,64, 70,86,95, 105, 118, 119, 136, 154, 184,206, 277

MIL STD 461B 30, 268MIL STD 461C (NEMP) 31, 197MIL STD 461 equipment classes 266

equipment and subsystem classes 268test changes between 461 Band 461 C 268test requirements applicable to classes 267

MIL STD 83141MIL STD 1541 17,61Modelling, see Mathematical modellingModulation rules 277Modulators 147

arbitrary waveform generators 148built in to equipment 147req uirements for 147

Monopoleactive 88passive 86

MVEE 33

NAMAS 21,209,210,215,217,218,219,240,245,246, 247

accredited laboratories, list of 278advantages of laboratory accreditation 220document list 272requirements for laboratory accreditation 219, 220

Narrowband/broadband signals 250NASA 23, 157,203NATO 212NBS (NIST) 23,93, 163, 164, 167NCMDRH 23, 265, 275Near field/far field boundary 76, 217NEC model 39, 227NEMA 189NEMP 110,154, 179, 184, 192,204,228

bounded wave simulator 199exoatmospheric pulse 193free field simulator 200induced currents 194testing 195

components 196equipment 197system 198

types 193NES 1006 33Notified bodies 239NPL 93, 167,221

documents 272NRPB 27,265NTIA 23NVLAP 23NW0320 National Weights and Measures

Laboratory 19NWS 3 33NWS 1000 33

Open range test site 20,42,92, 165, 166, 167,246Open range testing 165

site calibration 167repeatability 168

antenna impedance changes with height 169antenna size field averaging 169, 217balun and VSWR losses 170,217cable and connector losses 170, 217differences in commercial antennas 170EUT cables and peripherals 170non-uniform field strength/range relationship 170reflections from objects 168weatherproof covers 168

testing procedure 165Ordnance Board 27Oscilloscopes, digital 138

Parabolic reflector 114Pick up on wires and cables 230, 231Polarisation of antennas 83Power amplifiers 144

frequency range 145gain 145gain compression 146harmonic distortion 146intermodulation distortion 146output protection 147power ou tpu t 145specifying 145TWT 147

Power disturbances 205immunity standards 206importance of transien ts 205

Power meter RF 141Preselectors, see Spectrum analysersProduct 25, 26

radiation hazards 26, 27, 265regulations typical 264safety mark \lDE and TUV 26

of electrical devices 25, 264, 272, 274Product specific UK military standards 33, 34Production uncertain ty limits 219Protection devices for amplifiers 148Publications on EMC 283

Quasi peak detectors 20, 133ANSI 20, 133CISPR 20, 133

Radiated emission antennas 48

Radiated imrnunity field strengths 114req uirements for civil aircraft 114req uirements for commercial products 114requirements for military 115

Radiated susceptibility testing 110antennas used 11 0standards requiring 110

Radiation hazards 26limits for exposure 27

Radiative coupling (EMC antennas) 45RAM 34, 42, 161Receivers, EMI 130

commercially available examples 134design of 130detectors 133

Al\l/FM 134average 134peak 133quasipeak 20, 133slideback peak 133

n1easurement uncertainty 21 7selectivity and sensitivity 131

Regulatory authorities 18,22,218,239Repeatability in testing 41, 168, 21 7Routes to compliance 244RRE 6405 33, 268, 269, 270RSRE 33RTCA 23,261

SAE 16,23, 190,203,204,239,241,261,274Scope of EMC activity 7Screened rooms/chambers 154

anechoic screened chambers 159full RAM solutions 160partial RAM solutions 159

attenuation of 155, 156, 157elliptical chamber 160enclosed chamber testing 154mode-stirred 163novel facilities 164reflections in 158standard shielded enclosures 155tapered anechoic 162

Self certification 246SEMCAP 226Signal bandwidth 250Signal modulators, see ModulatorsSignal sources

tracking generators 143sweepers 143synthesisers 142

Spectrum analysers 134operation 135preselectors and filters 136types 134

Spiral induction windings 45, 48, 70Spot size, antenna 81Staff support for EMC 235Standards and specifications for EMC 14

civil and military 15contents of 14the need for 14the need to meet 14

INDEX 289

Strip lines, see Antenna, parallel plate lineSurface current probes 66, 68Susceptibility hardening case study 231System hardening (flow diagram) 233System level requirements 228System specification for EMC, see EMC system

specification

Technical construction file 244circumstances requiring 245contents of 245or testing 246

'Technical disciplines in EMC 10chemical knowledge 11electrical engineering 10legal aspects 11mathematical modelling 11physics 11practical skills 12quality assurance 12systems engineering 11

Tempest 106, 154Test differences between

FCC and VDE 260FCC, VDE and VCCI 262

Test facilitycost of 248in house 246turnkey 248

Test plan, see EMC test planTesting 38

accuracy 42automatic 151conformance 39conformance test plan 40development 38preconformance 39regimes 154repeatability 41, 217to verify modelling 38

Time domain 181, 182, 183Time domain manipulation 184Training, see EMC trainingTransformers

audio 45,60directly connected 60high-voltage 60, 61injection 48

Transient injection 60, 62, 140Transients power, see Power disturbancesTransient recorder, digital 140Transient testing 179, 228, 246

transient types 179Triboelectric series 186Two box EMI problem 230TUV safety mark 26

UK EMC legislation 252UL (USA Underwriters Laboratories) 26, 264, 275U ncertC\inty analysis 209 .

combining random and systematicuncertainties 214

control factors 211, 216


coupling factors 211, 216definition of terms 209estimates of for EMC 216estimates of total uncertainty 218in EMC measurement 214measurement factors 210standard deviation 212student's 't' distribution 213systematic uncertain ty 213random variables 211uncertainty 212

VCCI 24, 154, 171,239,262,275VDE standards 17, 21, 24, 105, 106, 133, 215, 239, 259,

260, 262, 264, 272Voltage probes 44, 48, 56, 61Voltmeters AFjRF 141VSWR 121,144,168,170,215,217

Wave impedance in TEM cell 124Wavefield impedance 76Waveguide coupler 149

A handbook for emc testing and measurement

Engineering

Transcript of A handbook for emc testing and measurement