Frequency - Power Standard for Calibration

Measuring instruments such as fre-quency counters or wattmeters areamong the standard tools of manyactive radio amateurs. This equipmentis often required to monitor ones ownradio station or as an aid in circuitdesign and assembly of RF equipment.To be able to test that this equipmentis functioning correctly and to cali-brate it, a frequency / power standardis an important tool.The following article describes the ba-sic considerations for the circuit con-cept and assembly of such a standard.

1.Considerations regardingcircuit design

The aim of this article is to create afrequency / power standard for calibrat-ing simple frequency counters and watt-meters (mW meters) with a diode detec-tor or a bolometer head. Important con-siderations for this concept are, manage-able implementation cost and technicalcomprehensibility (not only for experi-enced technicians).In order to satisfy the requirements of theshortwave, ultra shortwave and UHF/VHF users at the same time, a compro-mise must be found with regard to the

output frequency. The range around50MHz is suitable here for both theshortwave amateur and the GHz compo-nents usually designed for broadbanduse. For example, commercial diode de-tectors may well be specified for up to18GHz (or beyond!), but can not be usedat low frequencies, i.e. <=10MHz.On the other hand detectors developed,e.g. as do-it yourself projects, supplyusable results only up to a few 100MHz.This is also true for resistance bridges inbolometer heads.From the description, “frequency / powerstandard for calibration purposes”, inaddition to the precise frequency a pre-cisely defined output level must also beavailable. Normal mW meters are de-signed for up to 100mW (+20dBm). Thisoutput should also be used for thisproject. Lower power levels can easily becreated without any problem by means ofa downstream attenuator. An attenuatorthat can be switched in 1dB steps is aninteresting option [1].Oven stabilised oscillators (OCXOs) arepractically tailor-made for generating ahigh precision frequency output. Thecharacteristics of such an OCXO are farbeyond everything the radio amateur isused to from the measurement equipmentavailable to him or her. For example, theshort-term frequency stability of anHP10544A is specified at 1 x 10-11/1s(1Hz at 100GHz), with a long-term sta-

Wolfgang Schneider, DJ8ES

Frequency / power standard forcalibration

VHF COMMUNICATIONS 3/2003

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bility of <5 x 10-10/day. Normal fre-quency counters, with a crystal as theinternal oscillator for the gate time con-trol offer only 10-6 Hz/1s here (1Hz at1MHz). The short-term stability, and thusthe accuracy of measurement of thefrequency counter, is thus worse by afactor of 100,000!Of course, the price of such oscillators isa deterrent to many people. Even in fleamarkets, you have to pay more than 100Euros for used OCXOs of this type. Fornew oscillators, the price would certainlyreach four figures.Alternatively, temperature compensatedoscillators (TCXOs) are also available. Itis true that their frequency stability islower than that of a good OCXO, but,with a short-term stability of approxi-mately 1 x 10-8 Hz/1s (1Hz at 100MHz),is still better by two powers of ten thanwhat many of us can reckon with today.If a higher frequency precision is re-quired, then the link to a high qualityfrequency standard can be made withoutany problem, using a PLL control circuit.A controlled amplifier must be used inorder to obtain the defined power level of

100mW. The precision of this powerlevel is only dependent on the referenceused.One consideration is the thermal behav-iour of the control circuit. However,because components are available at fa-vourable prices, this problem is mucheasier to solve than the one relating tofrequency stabilisation.Starting from the assumption that such afrequency / power standard is usuallyused at room temperature, this calibrationaid can be created at an acceptable price.

2.Approach to a solution

Highly stable oscillators are currentlyavailable based on GSM technology butan interesting product, manufactured byPhilips, is available at a favourable price.The TCXO (Fig. 1) oscillates at13.0000MHz. Various measurementshave given a short-term stability of ±2 x10-8 Hz/1s for the frequency.The precise oscillation frequency can befinely adjusted using a control input, bymeans of a tuning voltage (0.5V). In Fig.2, the frequency response is plottedagainst the voltage feed.The oscillator frequency of 13MHz is, atfirst glance, unusual. If the frequency isquadrupled to 52MHz, the nominal fre-quency for the frequency / power stand-ard is generated. The TCXO frequencydivided by 13 gives 1MHz. The oscilla-tor can be easily synchronised with ahigh quality frequency standard, for ex-ample, 10MHz by means of a PLL.The frequency of 52.0000MHz generatedin this way can achieve the definedoutput level of 100mW. The amplifiermust be able to deliver this output reli-ably and should still have a few dB inreserve. A small part of the output signalis rectified and fed back to a PIN diode

Fig 1: Picture of the TCXO in a15mm x 25mm housing.


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regulator connected in series to the am-plifier.

3.The TCXO assembly

The core of the TCXO assembly is thetemperature compensated quartz oscilla-tor (IC1) for 13MHz. At an operatingvoltage of just 6V, this TCXO suppliesan extremely stable output frequencywith a level of approximately 0.3Vss.The short-term frequency stability of the

oscillator is better than ±2 x 10-8 Hz/1s.The precise frequency is finely adjustedwith the R5 trimmer (2 kΩ). The possibletuning range is shown in Fig. 2.Only a small part of this wide tuningrange (±3 Hz) is used for this application.The resistance combination R2/R3 isused to pre-set the frequency. The proc-ess to determine the necessary resistancevalues, depending on the power supplyand, naturally, the specimen mean varia-tions of the TCXO module, is describedin detail in the following instructions forassembly and putting into operation.The TCXO is followed by the buffer

Fig 2: Frequency change of the TXCO with change of tuning voltage.

Fig 3: Block diagram showing the concept of the frequency / power standard.


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stage with a type BF247B FET (T1). Thelatter also acts as an impedance converterto 50Ω.The necessary amplification of approxi-mately 13dB is provided by theMSA1104 MMIC (IC3) with an outputof approximately 10mW. The latter isdivided by a Wilkinson splitter (L2, L3and C13 - C16). This gives +7dBm(5mW) at 13.0MHz available for thefollowing stages. The power splitterssecond output is required internally forthe PLL control circuit.Following conversion to TTL level usingIC4a (74HC00), the 13MHz is divideddown to 1MHz by means of IC5(74HC93). For any other possible usesfor example as a calibration mark setter,a 100kHz signal is generated in additionthrough IC7b (74LS490).The oscillator can be synchronised with a10MHz frequency standard, as initiallyrequired. This reference signal is dividedby 10 in IC7a (74LS490). Thus two1MHz signals are available to the IC8phase indicator (74HCT4046). The loopfilter at the PLL output is designed for ahigh recovery time constant due to thehigh value TCXO in the circuit, with100kΩ and 10µF.

3.1. Assembly instructions and puttinginto operation of TCXO

The TCXO is assembled on a doublesided copper coated circuit board - di-mensions 75mm x 100mm (Figs. 5 and6). Due to the many connections thatmay be necessary, it is not intended to bebuilt into a housing.Following the drilling of the circuit boardwith a 0.8mm or 1mm. drill, the compo-nents are mounted on the board in noparticular order (Figs 7 and 8). As far aspossible, connections to earth (resistors,terminal pins, etc.) are soldered on bothsides. This provides the necessarythrough plating for the earth areas.Before the assembly is put into operation,the frequency response of the TCXO isinitially recorded. All that is needed to dothis is a high precision frequency counteror a frequency standard. Irrespective ofwhether it is synchronised using a con-nected frequency counter or a PLL (IC8),the tuning voltage required for the nomi-nal frequency at the tuning input of the13MHz TXCO is important. In the speci-men assembly, this voltage required was2.490V.

Fig 5: Top side ofTXCO PCB.


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The voltage divider made from R2 andR3 must be calculated for the tuningvoltage in the next stage. This is done,following the appropriate conversion, us-ing the formulaVTune = V * R2 / (R2 + R3)The voltage, V is measured at the +Vconnection of the TCXO module. As aresult of the voltage drop through R1(100Ω), this is slightly less than the

output voltage of the IC2 fixed voltageregulator (78L06). In the specimen as-sembly, the TCXO operating voltage wasprecisely 5.740V.In the specimen assembly, the resistancesare calculated at R2 = 51kΩ and R3 =39kΩ. With a potentiometer of 2kΩ(R5), a tuning range of ±3Hz was ob-tained. For comparison: R5 = 1kΩ corre-sponds to ±1.5Hz and R5 = 5kΩ corre-

Fig 6: Bottomside of TXCOPCB.

Fig 7:Componentlayout for theTXCO PCBshowing the nonSMD parts.


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sponds to ±7.5Hz.Fig. 2 graphically represents the fre-quency response of the TCXO, plottedagainst the tuning voltage applied.The tuning potentiometer (2kΩ) shouldbe a 10 turn helical potentiometer and itshould be incorporated into the frontpanel with the frequency tuning change-over (internal/external). The external set-ting can be carried out using the PLLcircuit. To this end, K7 (VCO controlvoltage) is connected to the tuning inputof the TCXO (loop from R5).

3.2. TCXO assembly components list

T1 Transistor BF247BIC1 TCXO 13,000 MHz, PhilipsIC3 MMIC MSA1104IC4 74HC00IC5 74HC93IC7 74LS490IC8 74HCT4046IC2 78L06, 6V voltage regulatorIC9 78L05, 5V voltage regulatorL1 10 µH chokeL2, L3 BV5046 Neosid 0.9µH,C3 470µF / 16V, electrolytic

capacitor radialC1,C17 10µF / 25V, SMD tantalumC23 10µF / 16V, SMD tantalumC2, C18 1µF / 16V, SMD tantalumR5 Spindle carrier. 2kOhm,

Model 64WR8 Carbon film resistor.

120Ohm ½W, RM 10mmTR1 Transformer (trifilar), 7

turns 0.2mm Cu enamelledwire

10 x Terminal pin 1 mm1 x Circuit board DJ8ES 062

SMD components, model 1206:

14 x 47nF4 x 180pF1 x 5 Ohm3 x 100Ohm1 x 390Ohm2 x 22kOhm1 x 51kOhm1 x 39kOhm1 x 100kOhm1 x 1MOhm

Fig 8:Componentlayout for theTXCO PCBshowing the SMDparts.


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4.The quadrupler

One of the design considerations for thecircuit was an output frequency in thearea of 50MHz. To obtain this, theTCXO frequency of 13.0MHz must bequadrupled. This achieved with the tran-sistor T1 (BFR90a). It has a high resist-ance input using of a 1:9 transformer(TR1). Approximately 7 turns (trifilar)are wound on a ferrite core and wired upfor the transformer. The type and form of

this ferrite core are not at all critical.A 6dB attenuator is connected in seriesto match the level of this quadruplerstage to the output of the TCXO assem-bly.The 3 stage band filter following thetransistor filters out the desired52.0MHz. Fig. 23 shows the frequencyspectrum, measured at full power at theoutput of the amplifier stage describedbelow (Section 5). All harmonic andspurious signals are attenuated by morethan 35dB.

Fig 9: CompletedTXCO PCB.

Fig 10: Circuit diagram of quadrupler with three stage band pass filter.


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4.1 Assembly instructions and puttingthe quadrupler into operation

The quadrupler assembly is mounted ona double sided copper coated circuitboard, which has the dimensions 34mm x72mm ( Figs. 11 and 12). It thus fits intoa standard 37mm x 74mm x 30mmtinplate housing. In the specimen assem-bly, the input and output are SMAsockets. The operating voltage (+12V at40mA) is fed in through a 1nF

feedthrough capacitor.When the circuit board has been drilled,it is populated in accordance with Figs13 and 14. Special attention should bepaid to the 1:9 transformer, do not to mixup the two connecting wires!It makes sense to solder the circuit boardinto the tinplate housing before mountingthe components. The circuit board isinserted deep into the housing. The earthareas are soldered to the frame on bothsides. Both the input and output and the

Fig 11: Top sideof quadruplerPCB.

Fig 12: Bottomside ofquadrupler PCB.

Fig 13:Componentlayout ofquadrupler PCBshowing nonSMD parts.


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power supply are connected to the circuitboard using thin connecting wires.The assembly can be put into operationimmediately after a visual check. The foiltrimmers in the 3 stage band filter arepre-adjusted to the mean setting.With an input of +7dBm (5mW) from theTCXO assembly and a calibrated bandpass filter, the quadrupler should supplya frequency of 52MHz at the outputsocket, with a level of approximately+6dBm (4mW)

4.2. Quadrupler components list

T1 BFR90aL1-L3 BV5049 Neosid 0.33µHTR1 Transformer (trifilar), 7

turns with 0.2mm Cuenamelled wire

C1, C 45pF (violet), foil trimmerRM 7.5

C3 30pF (red), foil trimmer RM7.5

1 Circuit board DJ8ES 063

Fig 14:Componentlayout ofquadrupler PCBshowing SMDparts.

Fig 16: Bottomside of completedquadrupler PCB.

Fig 15: Top sideof completedquadrupler PCB.


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1 x 37 x 74 x 30 tinplatehousing

2 x SMA flanged bush1 x 1nF,

feedthrough capacitor

SMD components, model 1206:

C4,C5 1.2pFC6 10pFC7,C8 10nFR2 39OhmsR4 100OhmsR1,R3 180Ohm

5.The amplifier assembly

The amplifier is assembled using thetried and tested 50Ω broadband technol-ogy (Fig.17). The transistor T1 (BFQ34)

can adequately handle the output of+20dBm (100mW) plus a few dB inreserve. The stage supplies approxi-mately 17dB of amplification.Naturally, a cheaper type can be used inthe circuit. However, it must also be ableto produce the desired output!A PIN diode attenuator is connected infront of the amplifier. The circuit, whichhas 3 BA479 diodes (D1 - D3) is takenfrom an old Siemens application. Attenu-ation is adjusted using the diode current.A small part of the output is rectified,using a 1SS99 Schottky diode. The DCvoltage obtained is compared in theoperational amplifier, IC2a (LM358N)with the reference set using the trimmer,R14 (47kΩ). The output controls the PINdiode attenuator.The interesting design for stabilising thereference voltage using a light emittingdiode (D4) comes from Bernd Kaa(DG4RBF) [2].

Fig 17: Circuit diagram of amplifier.


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5.1. Assembly instructions and puttingthe amplifier into operation

The amplifier assembly is mounted on adouble sided copper coated circuit board,which has the dimensions 34mm x 72mm(Figs. 18 and 19). It fits into a standardtinplate housing with the dimensions37mm x 74mm x 30mm. In the specimenassembly, SMA sockets have been usedfor the input and output. The operationalvoltage of +12V (approximately 90mA)

is fed in through a 1nF feedthroughcapacitor.The fully drilled circuit board is inserteddeep into the housing and the earth areasare soldered to the housing frame allround on both sides. The circuit board ispopulated in accordance with the compo-nent plans in Figs 20 and 21 in noparticular order.The amplifier is designed to be forbroadband. There is thus no need forcalibration, as there normally would be.

Fig 18: Top sideof amplifier PCB.

Fig 19: Bottomside of amplifierPCB.

Fig 20:Componentlayout ofamplifier showingnon SMD parts.


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The 47kΩ trimmer is used to set theoutput at 100mW. The quadrupler de-scribed above is used for input.

5.2. Amplifier assembly componentslist

IC1 78L06, Voltage regulatorIC2 LM358N, Operational

amplifierT1 BFQ34D1-D3 BA479, PIN diodeD4 LEDD5 1SS99, Detector diodeL1-L3 10µH, Choke axial, RM 7.5

mmL4 10µH, SMD chokeC11 1µF / 16V, SMD tantalum

electrolytic capacitorC12 10µF / 16V, SMD tantalum

electrolytic capacitorR4 560Ohm, RM 10mm

TR1 Transformer (bifilar), 7turns 0.2mm Cu enamelledwire

1 x Circuit board DJ8ES 0641 x Tinplate housing 37 x 74 x

30 (mm)2 x SMA flanged socket1 x Feedthrough capacitor 1nF,

solderable

SMD components, models 1206:

16 x 10nF2 x 10Ohm2 x 47Ohm1 x 220Ohm1 x 270Ohm1 390Ohm1 x 470Ohm1 x 1kOhm1 x 1.5kOhm1 x 10kOhm2 x 47kOhm1 x 100kOhm

Fig 21:Componentlayout ofamplifier showingSMD parts.

Fig 22:Completedamplifier PCB init's tinplatehousing.


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6.Summary

The frequency / power standard de-scribed offers a favourably priced and yethigh quality aid for calibrating simplefrequency counters and diode detectorsor bolometer heads for power measure-ment.The modular construction, broken downinto oscillator, frequency multiplier andregulated power amplifier, makes indi-vidual tuning processes possible whilekeeping within the overall concept ini-tially discussed. Thus a modification toanother frequency range and/or anotheroutput level is possible.As an option, a switchable attenuator,like, for example, the one mentioned in[1], can be added at the output. Theoutput level can thus be adjusted in 1dBsteps from +20dBm to 107dBm. That ismore than adequate for calibration pur-poses.

7.Literature references

[1] Wolfgang Schneider, DJ8ES: Controllogic for a switchable attenuator; VHFCommunications 1/03, Pp. 8 - 13[2] Bernd Kaa, DG4RBF: KW-Synthe-sizer from 1 - 65 MHz with DDS andwobble function;UKW Berichte 4/99, Pp. 205 - 222

Fig 23: Picture ofthe outputspectrumshowing that anyunwanted signalsare at least 35dBbelow the wantedsignal.


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Frequency - Power Standard for Calibration

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