measurement of rf peak-pulse power by a sampling-comparison method-owk

8/8/2019 Measurement of RF Peak-Pulse Power by a Sampling-Comparison Method-OWk

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<280 IRE TRANSACTIONSON INSTRUMENTATION December

found that a difference could not be detected between 3) There is no upper limiton the power levelwhichpulsesan d CW for widths greater than 0.35 ,sec. The ca n be measured.system repeatabilitywas found to be better than 0.2db. 4) Pulse power levelsto -10 dbm can be measured

with the crystalswitch and pulseor CW powerE. CorrelationBetween Systems levelsas low as -60 dbm with the pulsedreceiver.

A crosscheck was made between the crystalswitch 5) Accuracies of 3 an d 5 per cent for the crystaland pulsed receiver techniques.The objectof the cross switch an d pulsedreceiver,respectively,are ob-check wa s to determine whether the two systems truly tainable.This includes errors in the measurementsample an RF pulse the same as a CW signal.Results of CW power (0.2per cent),attenuation (1 perindicatethat when pulsepower is equal to CW power cent),bolometer mount efficiency(1 per cent),andas determined by one technique,the other technique the CW-pulse comparison.supports that conclusion. An oscilloscopean d a crystaldetectorwere alsoused to double check the above re- BIBLIOGRAPHYsults. Since the error related to the measurement of at - [1] J. K. Hunton and A. G. Ryals, "Microwave Variable Attenua-ors and Modulators UsingPIN Diodes,"presentedat Westerntenuation an d CW power is identicalfo rboth systems it ElectronicShow and Convention,San Francisco,Calif.,Augustcan be said that they measure peak pulsepower 2]2-25,1961.

[2] R. W. Damon, "SolidState Control of Microwaves," Microwaveidentically. Associates,Inc., Burlington,Mass.[3] "Analyses of Errors in Power Measurement," PRD Reports,vol.

IV. SUMMARY 5; July,1957.[4] R.

W.Beatty

and A.C. Macphersen,

"Mismatch errorsin micro-The measurement of the peak power of RF signals w-ave power measurements,"PROC. IRE, vol.41,pp.1112-1119;

usingeithrarysta swich o puled reeive hasbeen September,1953.using either a crystal switch or pulsed receiver has been [5] G. U. Sorger and B. 0. Weinschel,"Precise InsertionLoss Meas-shown to have the following advantages. urements Using ImperfectSquare Low Detectors and Accuracy

Limitations Due to Noise,"Weinschel Engineering Co., Inc.,1) The measurement is independent of pulse rise and Kensington,Md., EngineeringNotes 1; February,1955.

[6 ] "1. F. Substitution Attenuation Measuring Test Set,"Weinschelfall time forpulse widths greater than 200 nsec for EngineeringCo., Inc., Kensington,Md.,BulletinNo.102;the crystalswitch and 350 nsec for the pulsed re- October,1959.

c7]G. F. Engen, "A Bolometer Mount EfficiencyMeasurementceiver. Technique, NBS, location,Rept. No. 67-6;June 20, 1960.2) The pulse repetitionrate to be measured has no [8] R. W. Beattyand F. Reggia,"A n improvedmethod of measuring

effect on th e crystal.switch.technique. efficienciesof ultra-high-frequencyand microwave bolometereffecton the crystal switch technique. mounts,"J. Res.NBS, vol. 54 ,pp.321-327;June,1955.

Measurementof RF Peak-PulsePowerbyaSampling-ComparisonMethoc*

P. A. HUDSONt, MEMBER, IRE, W. L. ECKLUNDt, AND A. R. ONDREJKAt

Summary-A method is described whereby RF peak-pulse power INTRODUCTIONmay be measured by comparison with CW power at the same fre-quency.Comparisonis made byfirst samplingthepowerin a small )R ESENT measurement accuracies of commercialportionoftheRF pulsewidthandsubsequentlysamplingthesame laboratory type RF peak-pulse power meters are,portionof the CWsignalwhose powerlevelis accuratelyknown. according to manufacturer's specifications,withinSampling is accomplished with a fast SPDT coaxialsolid-state 1 per cen to 15 pe c . When t IswitchsynchronizedwiththeRF pulse.A timedelaynetworkallows use to cairt sr i enofedtypsepulse-powenrethesampleto betakenanywherealongtheRF pulsewidth.usdt abresrvc orfl-yp plepwrEstimatedmaximumerrorfor peakpower levelsto 10kw is meters the inaccuracy of the lattermay be of the order of3 percent.High CWlevelsare notnecessarysincethe switchis 20 per cent. In many cases such large inaccuraciesused in conjunctionwithdirectionalcouplersto coverthe power are not in keeping with present-day requirements as evi-

ranges o f i nt er es t. ~~~~dencedby the fact that the National Bureau of Stand-ards has received requests from industry an d the mili-

* ReceivedSeptember 24,1962. tary for assistance in meeting their needs for improvedt NationalBureau ofStandards,Boulder,Colo, accuracy in RE pulse-power measurements.

AlultICLDoJ0a1UfIX Ra



1962 Hudson,et al.: Measurement of RF Peak-PulsePower 281The work described here was undertaketnprimarily ias ary port of the coupler is a fastSPDT diode switch with

a resultof these requests and represents a first attempt one output biasedlnormally closed an d the other biasedat establishinga pulse-power measurement capability normally open. The normally closed output port isand calibrationserviceat NBS. Even though accuracies terminated in a 50-ohm diode detector for viewing theobtainable with the method represent a substantial ini- pulse on an oscilloscopewhen (lesired.The normiallyprovemenitover existingmethods it should nevertheless open (sample) output port is terminiate(din a 50-ohmbe regarded as an interimiistandard becatusegreater thermoelemiienittype average power detector. Theaccuracies willbe needed in the near future. diodlesin the switch atre arranged so that a negative

pulse from the switchingpulserreverses the open-closedl>RIN(CIPI.EOi"OPER.ATI()N coniditioniof the switch outputs. This pulseris syn-

The samipling-comparisonmethod is based oii the chroiiizedwith the main pulse generator which modu-well-establishedprincipleof comparing one quantity latesthe RF source. Thlus during the time the switchingagainst a second(whose value is kiiowmior canlbe mcias- pulse is impressedlupoIIthe switchlthe RF pulse powerured with the desire(ddegree of accuracy. In general, is switched from output 1 to output 2. By adjusting theallowance must be made fordifferencesin the nature of width of the samplinig pulse to a small fraction(i.e.,the tw o quanititiescompared as in the case of measurinigone-tenith)of the RF pumlsewidth,a time sample (100CW power by comiiparisoniwith d(lpower.This error is per cenitamplittude)of the RF pulse power is fedto theoftenreferredto as RF-dc substitution error and arises average power (detector.The variable delay networkbecause the comparisonl device stclhas a calorimeter or allows the time sample to be taken anywlhere along thebolometer mounit does not respond exactly the same to RF pulse

widithan d hence it may be taken at the peak of

RE power as to (Ic power. the pulse by simply varying the delav for maximumii dcIn the mealsuiremiienitsystem to be (lescribedhere, output from the average power detector. 'I'his output

comparison is made between peak-pulse power' at a value is measured with a precisionmillivoltpotentiom-given carrierfrequency and CW power at the same fre- eter and recordled.Typical examples of the signalsatquency. Thus comparisoni errors clueto fre(luencyeffects the switch ports are shown in Figs.2 an d 3.are for practicalpurposes eliminated,but there still re-mains the possibilityof atpulse-CW substitution errorwhich willbe examined latter. T1 T2T3 T4

As indicated in the simplifiedblock diagramnof Fig.1,the mlethodis designed foruse with directionalcouplersanidtlhusit becomiiesanl ini-linieor feed-througlhtype in- |SITCINGPULSERstrument. This arrangement allows convenieiit calibra- T, T4 ITi TPtioii of other pulse-power measuring e(luipmiienitwhiclhis TV 40UPUT | E C1I|our end(iobjective. 2 T3 T2 T3

Referring to Fig.1, a measuremiienitis made by feedingRE pulse power through the directionalcoupler to the T113 T4instrumenitto be calibrated.If this instrumiienitis the ab-sorptiontype it will terminiatethe line, otherwise a Fig.2-Input-outptitwaveformsofSPDT)dio(leswitch.dummy load is used.Conniectedto the incidlenitsecond-

SYNC. |NETWR FeI;_

RF PTSORC OUPT2 xDOE ( .

SOURCE DIRECTIONALCOUPLER ITO BEICALIBRATED

Fig.1-Block diagram of sarnpling-coniparisoniiimethodforRF peak-ptise power measurement.

1rheterm "Peak-pulsepower" as uisedin this paper refers to"peak-pulsepower, carrierfreqienicy."'1'he IRE (lefiniitioniof the ~latter term is "the power averaged over that carrier-frequencycycle;which occtursat the maximumtiof the ptilseof power." See "Stanidardson pulses:definitionisof terms,part 11, 1952," PROC. IRE, vol. 40,pp. 552-554; May, 1952. Fig.3--Typical detected waveformisat output 1.




282 IRE TRANSACTIONSON INSTRUMENTATION December

The switch is then disconnected from the first coupler whose average powers may differby as much as 33 dbsidearm and connected through a second calibrated fora pulse-dutycycleof 0.0005.In order to limitleakagecoupler to a source of CW power. Keeping all param- at the sample output of the switch to a value 30 db be-eters ofthe sampling pulsefixed, the CW power is low the pulseaverage power the isolationmust be at

varied until the dc output from the average power de- least63 db in the of f condition if the error due to leak-tector is equal to that previously recorded. Under these age is to be no greater than 0.1per cent.

conditions the switch samples the CW power at the same Initialexperiments on fast switches were carriedoutrate an d for the same time intervalas for the pulse using thermionic diodes in a coaxialconfiguration.Thispower. It is then only necessary to measure the CW work was quicklyabandoned when it was found that thepower levelby means of a bolometer mount and bridge highresistance ofthesediodes did no t allow the switchor a calibrated thermoelement mount2 connected to the input to be readilymatched to a 50-ohm impedancesidearm of the second directionalcoupler.Within the even though isolationwas quitegood.A designbased onerror limitsof the method the power at the peak of the the use of semiconductor diodes was finallyadoptedRF pulse may be equated to the measured CW levelat sinceit had been shown3 that thecharacteristicsof thesethe switch input.That is, the power fedto the average devices were suitable.Several types of diodes were triedpower detector during the sampling of RF pulsepower is and the 1N270 germanium pointcontact type was

Pd,= kP,r8f, (1) selected because of its low forward resistance,highbackresistance,and reasonably fast switching time. The

where switching time is of the order of20 nsec and a reasonably

k =switch insertionloss rectangularsampleis obtained when the minimum sam-

Pp= input pulsepower of that portion of the RF pulse plewidth is limited to 100 nsec.A rectangular sample ofbeing sampled the pulse is desired to insure complete sampling of the

rb=durationof sampling pulse amplitude, good stability,an d sufficientaverage powerf =d sampling pulse to the detector.Thus the minimum width of RF pulsessampling pulsefrequencyPRR. During the which thesystem willmeasure is fixedat 1 Asecif the

sample width is to be no more than one-tenth of the RFPd2= kPcwTsfs, (2 ) pulse width.

and sincePd2 is set equal to Pdl and k, TS,fs are assumed The diodes are mounted in a slablineas shown in Figs.constant for the tw o cases,then 4 and 5. Alldc leads are choked and bypassed while the

RF leadsincorporate dc blocking capacitors.SwitchingPp= PCw. (3) transients which may appear at the output can be

The measured CW power when multiplied by the cou- eliminated with high-pass filters.plingfactorofthe first directionalcoupler gives the peak- In order to match the switch to 50 ohms a seriesofpulse power incident on the input of the instrument small tuning screws are incorporated in the switch cover.under test. With these the capacity between the inner and outer

The requirement that fs an d rs remain constant conductors can be varied to effecta match with a band-(assumed above) is relativelyeasy to satisfybecause width of approximately 40 Mc centered on 1000 Mc .pulserswith the necessary stabilityare commercially Other characteristicsof the switch include 3-watt safeavailable.That the switch insertionloss k remains con- power handling capacity, isolationgreater than 70 dbstant when sampling eitherpulse or CW power is chiefly an d insertionloss1.5db.a requirement that k be independent of average power Designing the switch so that the insertionloss be-levelsincethe average power levelof the CW may be tween the input and sample arm output was constantgreater than that ofthe pulse by a factorof2X 103(0.0005 with average power levelrequired considerable effort.duty cycle).This willbe discussed further in the next In early designs the diode arrangement was such thatsection. RF power flow through the diodes was from anode to

cathode (i.e., whisker to wafer) and the insertionlossDESCRIPTIONOF TH ESYSTEM varied as much as 10 per cent forCW input power levels

The heart of the system is the coaxialSPDT diode from 0.2 to 3 watts. This variation was ascribed to a

switch.Design specificationsfor this component in- heating effectbecause the insertionlosswas observed tocluded: switching time in the nanosecond region,so be a function of time with constant power input.Re-ohm input impedance, power handling capacity of a few versing the orientation of the diodes in the switch re-watts, low lossin the on condition an d high isolationin duced the variation to ± 2 per cent over the same powerthe off condition. High isolationis important because range while from 0.2 to 2 watts it wa s no more thanthe switch must sample both CW an d pulse-power levels ± 1 per cent. Average power input to the switch wa s

2 p. A. Hudson, "A precisionRF power transferstandard," IRE 3R.V. Garver, "Theory of TEM diode switching," IRE TRANS.TRANS. ON INSTRUMENTATION, VOl. I-9, PP. 280-283; September, ON MICROWAVE THEORYAND TECHNIQUES, VOl. MTT-9, PP. 224-1960. 238; May, 1961.




1962 Hudson, et at.: Measurement of RF Peak-PulsePower 283

SWITCHING PULSER AVERAGEPOWERDETECTOR- 1000 Mc

TO -30V SIAS L t TO -2 5 mO BIAS + DC OUTPUT

-I---------- - XIPT-I ~~~~~~~~~~~~~~~~~330pf41 330 pf

RF

(NO) C p f 5pf 5pt

L--------------- _______ ] Fig.6-Average power detector-1000 Mc.RF INPUT

Fig.4-SPDT RF diodeswitchin slabline. power is knowni.Precision of 0.1per cent an d accuraciesof 1 per cent can be obtained with the calibrated direc-tionalcoupler-bolometer bridge system shown in Fig.1,or with a calibrated directionalcoupler-thermoelementmount combination.

The switching pulser used in the system deliversa 50-volt pulse intoa 50-ohm load with a variable pulse widthfrom 0.05 to 1 ,usecat repetitionrates from 2 cps to 2Mc . A 50-volt pulse overdrives the switch so that smallvariations in the amplitude of the switching pulse are notdetectable. The pulser is commercially availableas is thedelay network which has a range of0 to 10,000 ,usec.

Although not essentialto the measurement function,a fast riseoscilloscopeis a valuable assetin the system.It may be used in conjunction with a 50-ohm detectorforviewing the output at the normally closedport of theswitch.However, the detector must be capable of dis-

showingconstructiondetails. sipating2 watts of power. Thus,any irregularitiesineitherthe RF pulse or the switching pulse can be quicklydetected before proceeding with the measurement.

then limitedto the latter range.The improvement ob-tained was probably due to greater heat coniductionfrom SYSTEMPERFORMANCECHARACTERISTICSthe wafer to the center conductor in the first diode. Evaluation ofsystem performance wa s made from the

Th e need for a stable an d reliableaverage power de- standpoint of its use in the laboratory for calibrationtectordictated the use of a passive device.A thermo- and comparison with other peak-pulse power meters.element detector wa s chosen because priorexperience Under laboratory conditions certainfactorssuch as thewith these devices had shown that they have the neces- pulse shape may be at leastpartiallycontrolled formeas-sary qualities.The detector employs two 10-ma thermo- urement convenience. Resolution of the system is deter-elements with their outputs connected in series as mined almost entirelyby the resolution of the potentiom-shown in Fig.6. Sensitivity is 2-mv output per mw RF eter used to measure the dc output of the averageinput. The VSWR can be reduced to 1.03at 1000 Mc , power detector.With proper instruments it is relativelyincreasing to 1.20at frequencies 20 Mc eithersideof easy to resolve 0.1 ,uv. When sampling the power in a1000 Mc . Thus, the detector sensitivitydecreases by pulse whose duty cycle is 0.002 the dc output is approxi-one per cent at the end poinltsof the stated band. The mately 100 ,uv an d thus the resolution is 0.1 per cent.power range of the detector is 20 db with a maximum The minimum duty cycle was fixedat 0.002 to preservepower input of4 mw. The dc output can be resolved to at the 0.1per cent resolutionfigureand it is wellwithin theleast0.1per cent,depending on the potentiometer used. range of commercial pulse-power measuring instru-Over the range indicated, the RF-dc conversioniis not ments. Work is in progress to increase the sensitivitytolinear,as would be desired in a directreading system. allow measurement of pulses with duty cycles as low asThis however, is not a point of major concern in the 0.0005.comparison method. Another important factoris the system stabilitywith

The power output of the calibrating CW generator time when all other parameters are held fixed.An evalu-must be continuously variable over the power range 0.2 ation of this factorwa s carriedout by observing the dcwatt to 2 watts and should have good short-term output of the average power detector while the switchstability. The accuracy of the system depends quite sampled a level-stabilizedCW signal.Over a ten-minute

heavily on the accuracy to which the calibrating CW period variations in the output were no greater than


measurement of rf peak-pulse power by a sampling-comparison method-owk

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