C041 Electromagnetic Interference WP Oct 07

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Electromagnetic Interference and its

Relevance for Structured Cabling Systems

www.commscope.com

Today, organizations rely heavily on their computer network, in fact few can efficiently

operate without the use of high technology electronics. Once the investment and the

commitment have been made, reliable operation of the electronics is essential. Upset of

a system cannot only be disastrous in terms of lost business or reduced productivity but

confidence in the system quickly declines and the user either operates around the

difficulty, thus not gaining the full commercial advantage, or the system is changed for

one with a reputation for better reliability, and additional costs are incurred.

The concern and drive to define standards for Electromagnetic Compatibility (EMC) came

from the enormous growth in electronic equipment such as PCs and LANs, and theircontinuing evolution to higher data rates. Even more importantly, there has been an

explosion in the use of portable electronics, adding an unforeseen hazard to the

communications environment.

Modern electronic systems, often computer or microprocessor based, will in general have an unwanted response to

high frequency signals occurring in the system by either conduction down interface cables from another system or

induction by an electromagnetic field. In addition, systems that make use of digital electronics will contain circuits that

are switching currents and voltages at high speeds and thus causing significant levels of high frequency

electromagnetic radiation.

This Application Note on EMC aims to raise understanding of EMC and the relevant standards, and in particular

addresses the relevance of EMC for cabling.

Electromagnetic Effects

Electromagnetic effects fall into two broad areas. The first concerns the susceptibility or immunity of electronic

equipment to upset or damage by electromagnetically induced voltages and currents. The second concerns the

problem of unwanted emissions of electromagnetic noise.

The present day use of electronics has expanded to such an extent that the reliability and freedom of systems from

upset can now have significant implications on safety, productivity, reliability, reputation and inconvenience.

In addition to an equipment’s or system’s susceptibility to electromagnetic radiation, it is important that significant levels

of electromagnetic emissions outside the normal required signals are not produced, either as a conducted or radiated

signal. This is now controlled by strict legislation in many countries.

Application Notes

October 2007


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APPLICATION NOTES ELECTROMAGNETIC INTERFERENCE AND IT’S RELEVANCE FOR STRUCTURED CABLING SYSTEMS

What is Electromagnetic Interference?

Electromagnetic Interference (EMI) is an all embracing term for the cause of temporary or permanent upset of electronicsystems by either the naturally generated or man-made electromagnetic environment in which the system is immersed,

or by the unintentional induced currents and voltages being fed to equipment via interface cabling from other

equipment.

EMI can also be produced inadvertently by electronic and electrical systems as a result of high frequency currents

flowing within the system. If a system is designed to withstand electromagnetic threats within its operating environment

and does not emit electromagnetic radiation above some pre-described level then the system design is said to have

achieved EMC.

It would be prohibitively expensive to protect every piece of equipment from all possible electromagnetic threats. A clear

understanding of the level of protection required, and an appraisal of the electromagnetic environment in which the

equipment will operate, are the first steps in the design process.

Emissions

Time varying electrical charge distributions and current flow produce electromagnetic waves. All electrical and electronic

equipment and systems contain conductors which support time varying currents and voltages and are therefore

producing electromagnetic radiation to a greater or lesser degree depending on the following factors:

• The magnitude of the time varying voltages and currents flowing

• The length of the conductors

• The rate of change of the voltages and currents

• The geometrical layout of the conductors in the system in relation to each other and the earth reference.

It should be noted that even direct current (D.C.) and very low frequency systems are capable of emitting significant

electromagnetic radiation since rapid changes of current and voltage occur during switching operations. This can

produce a transient interference source.

Susceptibility

In order to look at the reasons for the increasing susceptibility of modern electronics we must consider the huge

advances in miniaturization. These steps forward have led to a reduction in the amount of power required to carry out a

task of given complexity; this has meant that the voltages and currents used in the signalling within and between the

equipment has been greatly reduced. Unfortunately, this makes the interference signals more significant by comparison.

In addition to the reduction in power levels the miniaturization has resulted in the closer proximity of one area of

equipment with another, thus increasing the chance of interference.

An additional contributory factor to the increasing susceptibility of systems is the increase in bandwidth of circuitry

which has arisen because of the requirement for faster processing speeds. As mentioned earlier, this producesunwanted Electromagnetic (EM) emissions but also causes systems to respond in an unwanted manner to a broader

range of frequencies.

In addition to magnetic components of electromagnetic fields coupling with loops contained in an electronic system to

produce induced voltages, any significant length of a conductor aligned with, or normal to the electric field component

will have current induced in it. Systems which operate on signal levels of up to a few volts and/or currents of a few

microamps, will be susceptible to electromagnetic fields unless steps are taken to prevent susceptibility. Many of the

steps taken to avoid problems with susceptibility will also reduce the electromagnetic emissions.


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Solutions to the EMI Problem

A full solution to the problems posed by electromagnetic interference will depend, as has been stated, on the nature of

both the equipment as well as the immunity specifications to be met.

Each and every aspect of the design needs to be considered, starting at the circuit board level and working outwards

through the connectors to the enclosure and then on to the power supply and interface cabling.

The following are all areas of design that can enhance the overall system performance, both for active electronics and

the cabling system:

• PCB design

• Cabinet or enclosure design

• Connector technology

• Power supply and interface cabling

It is the design of the interface cabling which is discussed in this document.

EMC Standards

The following are the four categories of EMC standards:

• Basic standards

• Generic standards

• Product family standards

• Product standards

The generic, product family and product standards should call upon basic standards for the methods of test. The followingare standards relevant to the testing and compliance of LANs and associated equipment:

Emissions Standards

• IEC CISPR 22 or EN 55022 - Limits and methods of measurement of radio interference characteristics of ITE

• IEC/EN 61000-6-3 - Generic Emissions Standard Part 1: Residential, Commercial and Light Industrial Environment

• IEC/EN 61000-6-4 - Generic Emission Standard Part 2: Industrial Environment

Immunity Standards

• IEC CISPR 24 or EN 55024 - Product Standard: Immunity of ITE

• IEC/EN 61000-6-1 - Generic Immunity Standard Part 1: Residential, Commercial and Light Industrial Environment• IEC/EN 61000-6-2 - Generic Immunity Standard, Part 2: Industrial Environment

• IEC 61000-4 - Series of Basic EMC Standards

Part 2: Electrostatic Discharge (ESD) Immunity Test

Part 3: Radiated Fields Immunity Test

Part 4: Electrical Fast Transient (EFT)/Burst Immunity Test

Part 5: Surges Immunity Test

Part 6: Conducted Fields Immunity Test

Part 8: Power Frequency Magnetic Fields Immunity Test

The most commonly quoted emission and immunity standards for LAN hardware and structured cabling are EN 55022

(IEC CISPR 22) and EN 55024 (IEC CISPR 24) respectively. Within EN 55022, requirements for both radiated and

conducted emissions are specified for what are defined as Class A and Class B environments. “Class A” corresponds

to a commercial or business environment. “Class B” is a residential environment.


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EMC and Customer Premises Cabling

It is problematic to test an installed cabling network for EMC as most cable installers install a passive system (i.e. cable,

connectors, patch panels, and outlets). Computers and communications devices are usually connected to the installed

cabling system by the owner of the network, and are not under the direct control of the cabling installer. EMC mustprimarily be the responsibility of the manufacturer of the computer and communication equipment that is to be

connected into the network. The network cabling is well defined (Category 5e and 6 for example) so manufacturers of

computer equipment have a well defined connection environment to work with.

EMC performance of the cabling system can be taken into account in the following ways:-

• Manufacturers of electronic equipment/systems take account of cabling specifications in EMC qualification.

Manufacturers then qualify their product descriptions for attachment to different types of cables, for example

Category 5e or 6 UTP

• Cabling System Suppliers who also provide a “systems integration” service (and guarantee the overall performance

of the system) assume responsibility

All the cabling standards committees continue to investigate the EM performance of cabling. New and future

versions of existing standards contain performance requirements and parameters for good EM performance.Four possible areas are:

• Differential to common-mode conversion limits to control RF emission from cabling (and vice versa for immunity control)

• Shielding effectiveness of shielded system (coupling attenuation and/or transfer impedance of cable plus connector

plus connecting hardware)

• Integrity of shielded systems

• Grounding practices for shielded cabling

In lieu of these standards, the user should ensure the following:

• Electromagnetic compatibility between active and passive products

• Compatibility between cable, connector and connecting hardware• Cabling should meet relevant design and installation standards

Methods of Overcoming EMC A particular problem for LANs is the interconnection with an extensive range of cabling systems. A cable, as far as

electromagnetic signals are concerned, is nothing more than an antenna that will effectively radiate any common-mode

signal appearing on it, so the pr imary aim in LAN design must be to effectively limit such signals.

There are two common approaches providing EMC performance for a communications link (excluding coaxial and fiber

optic cabling) - screened or shielded twisted pair (FTP/STP) cabling and unshielded twisted pair (UTP) cabling.

In principal it would seem that screened/shielded systems would offer the best EMC control method by enclosing the

entire link in metal. With this scheme, external noise causes currents to flow in the shield that induce equal and

opposite currents in the signal conductors. These currents flow through the impedance of the conductor and shield,

producing noise voltages. When summed around the loop including the ground connection of the shield, the noisevoltages on the conductors cancel the voltages on the shield. Unfortunately for screened cabling to be effective the

screens need to be properly earthed at each end and this is usually in conflict with safety or other issues. Also it is

difficult to ensure the continued maintenance of the screening integrity of a complex installation. Shielding utilizes the

‘Faraday cage’ approach where a conducting shield/screen is placed around the conductors and all the associated

interfaces. However, this approach relies heavily on the total integrity of the conducting shield plus how and where the

shield is grounded. One must remember the fact that a shield is also a conductor and if it is not treated properly as a

shield, it will carry a signal, and radiate or pick up noise like any other conductor. The following conditions must be met

when considering a shielded premises cabling system:

• Correct cable shield/screen design

• Correct connector design

• Correct termination of cable shield at connector

• Careful installation procedures• Availability of clean power supply

• Availability of good ground system

• Good grounding procedures

• Maintain low impedance to ground


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The effective use of UTP cables relies on the balance of the cable itself and more importantly on the balance of the

interface circuit. The differential signal is placed on a pair of wires at the transmitter, and the two wires in the pair are

twisted together and closely matched over the entire link. Using this technique, any external noise will affect both wires

in the pair identically (called common mode noise). At the receiver, the differential signal on the pair is interpreted as the

data, while the common mode noise can be largely cancelled or filtered and ignored. This is taken into account with the

design of the interface port circuit, as this is where the conversion from the wanted differential data signal to the

unwanted common-mode (radiating) signal tends to occur.

The purpose of balanced transmission is to ensure that equal and opposite signals are generated on the conductors of

a twisted pair. These signals, in turn, generate equal and opposite electromagnetic fields that cancel each other and the

net result is no emissions from the twisted pair. Balanced transmission also makes the noise pickup equal on both

conductors and so no net noise signal is delivered to the receiver interface. In general, the better the balance of the

system, the lesser is its emissions and the greater is its noise immunity. The ‘balancing’ approach is often overlooked as

a cost effective way of improving EMC performance. Common-mode termination techniques on the unused pairs of an

interface are often performed within the electronics equipment. Its purpose is to provide the pairs with a well defined

common-mode impedance to ground, to reduce the loop (antenna) area for the common-mode signal and to balance

the unused pairs. This method of termination has been shown to greatly improve the EMC performance of a system.It is recommended by LAN standard committees for all high data rate applications.

In reality, no system can be perfectly shielded or perfectly balanced. As with most technical issues, there is an

engineering trade-off that must be evaluated to produce an optimum answer for a given set of conditions. A balanced

UTP system puts most of the emphasis of achieving acceptable immunity on the design of the interface electronics and

the cabling components (cable, connector, and interconnect hardware), something that is already standard practice for

electronic designers to ensure the equipment’s best EMC performance. In return for this now common design effort, a

UTP system is easier and less costly to install and maintain. On the other hand, an FTP/STP system is reliant on the

signal path itself over the endpoints. The shielding of a link must be maintained from transmitter to receiver, including

the endpoint electronics; otherwise, the effectiveness of the shield is greatly reduced. System defects must be

assessed at each link by installers. Verifying the shield integrity is extremely difficult in an installation, and so defects are

not readily detected or corrected. Furthermore, the design and construction of the shield plays a role in the

performance, as well as cost, of the cabling components and installation.

EMC and CommScope Solutions

Properly engineered, manufactured and installed IT equipment will comply with EMC requirements when used with

CommScope cabling systems. While emission tests in a controlled laboratory environment do not necessarily take into

account all the variables encountered in the field, CommScope systems have been successfully installed in many worst

case commercial and industrial environments exhibiting a high immunity to external noise interference and not causing

any interference problems.

CommScope has undertaken a full range of independent tests of their cabling systems. The cables and connectors

have been independently verified to meet the current international cabling standards. Although most EMC standards do

not directly apply to cabling, the extensive testing undertaken by CommScope gives a benchmark for the

manufacturers of active electronics to work within stating conditions of compliance of their equipment.

An EMC testing program for SYSTIMAX SCS was initiated in 1990 with the intention to address concerns about EMC

and to ensure that SYSTIMAX SCS complied with current and future EMC requirements. Since then, testing has been

completed for ISO 8802-3 10BASE-T, 100BASE-T, 1000BASE-T, ISO 8802-5 16 Mb/s Token Ring, ISO 9314 (ANSI X3.263)

100 Mb/s TP-PMD LANs, and 155 Mb/s ATM Forum-compliant LANs. This testing shows that SYSTIMAX customers

can be assured of total peace of mind that all legislation is met by their cabling system.

CommScope has published guaranteed channel performance, published design and installation documentation, and

published field testing guidelines documents to back up its solutions. These documents are essential, especially for

compliance to the current European EMC Directive, (2004/108/EC) for fixed installations. In addition, CommScope has

included EMC compliance in its SYSTIMAX SCS Application Assurance Program.


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In summary, the recommendation to a user wanting to ensure EMC compliance of their network is as follows. The major

contributors to emissions are the equipment connected to the LAN cabling, for example transceivers, hubs, PCs, etc.

These should all be tested to the appropriate EMC standards, bear the CE mark (a European conformance mark), be

correctly installed and used for the purpose intended. An active system should be able to meet the EMC regulations

regardless of the cable type (UTP,FTP or STP) provided the equipment connected to the cabling system is designed

and approved for connection to that cable type. The associated cabling system should be installed in accordance with

manufacturer’s guidelines and/or International, US and European Cabling Standard’s recommendations. It is important

to note that the EMC performance of cabling systems is affected by the way it is installed, especially for an FTP/STPcabling system. Although every user’s requirements are unique, it is important to choose a system from a vendor that

can provide end-to-end performance guarantees. These guarantees should be backed by detailed design, installation,

testing and product documentation.

Further reference material is available in white papers from CommScope.

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© 2007 CommScope, Inc.

All rights reserved.

Visit our Web site at

www.commscope.com

or contact your local CommScope

representative or BusinessPartner

for more information. All

trademarks identified by ® or ™

are registered trademarks or

trademarks, respectively, of

CommScope.

This document is for planning

purposes only and is not intended

to modify or supplement any

specifications or warranties relating

to SYSTIMAX Solutions products or

services

C041 Electromagnetic Interference WP Oct 07

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