LOAD PULL­MEASUREMENT

OFLOAD IMPEDANCE

OFHIGH POWER DEVICES

DENNIS POULINNETWORK MEASUREMENTS DIVISION

1400 I=OUNTAIN GROVE PARKWAYSANTA ROSA, CALIFORNIA 95404

Flin- HEWLETT~~ PACKARD

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REAL TIMEMANUAL LOAD PULL

MEASUREMENT SYSTEM

• SYSTEM FEI.1\JRES

• ~C E\l£.Cl(~ OF 1\\E SYSreNI

• DISCUGSlC*! a' MEJ6UllEhlalT" AAD DESIGN 1Ec:}1!'lIQ.llE

• T\'PICAL DI\T,.LARE.E. Sl6NI.L OOTl'UT PLANE RlR A6M6 FErCU~ C 0Ul1'I.JT fUNE FOR A BIPOl./'J~

BIPOLI'.R ClSCILU1l)R OUfPUT PlANE

• ACCURl.C'I' CONSl~RAT1ONS

• CONCLU~ONS AIJD R.JTURf: WORK

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OPTIMUM LOADVS OUTPUT POWER

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INTRODUCTION

Most microwave designers are familiarwith the use of S parameters for designof small signal amplifiers. However, smallsignal S parameters are useless when adesigner is concerned about designingor troubleshooting an oscillator, largesignal amplifier or an amplifier operatingin a mode other than Class A. In thesecircumstances, it is necessary to measurethe load impedance required under theactual operating conditions. This typeof measurement has classically beenreferred to as a "load pull" measurement.

This paper describes a simple system toobtain this information. The measurementtechnique is described and a designtechnique is suggested. Some typicaldata is discussed for several largesignal circuits. Accuracy considerationsof the measurement system are also dis­cussed.

The problem a microwave designer encounterswhen dealing with a large signal amplifieror oscillator is that the load impedancerequired to match the device is a functionof output power. The fashion in which theoptimum load impedance shifts as a functionof output power is unpredictable. Thisrenders small signal S parameters uselessfor large signal design.

This is a general comparison of thecircumstances where S parameters areappropriate and when a load pull measure­ment is necessary. Load pull is mostuseful when a device is operated in anonlinear fashion.

LOAD PULL VS SMALL SIGNALS-PARAMETER MEASUREMENT

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S- PARAMETERS

• ME,.SURE!> 111M I\ND OOTM9oIW- SIGNAl. IMPEPMlCES~ FORW~ GNN N<!DRf:'iERSC ISOL,., ION

• DOES NOT MEASURE DE'lICErowER 0l!Tl'UT

LOAD PULL

• MEA5IJRES LOAD IMI'El».NCEREl¥JIRfO 10 OBTNW OP'TIWJMPOWER OUTPUT FROM ~ WICE

• IWM IMPEDANCE OF DEVCE FOR~ GIVEN LOM> COHOOlOH C"NBEM~SURED

• ONLY VAllO fOR LtlEAA ~ICE.S • FOI?WIJ<D G~ AND POWER OJTf'UTMEASURED

• OWlY VAliD FOR CLM6 "OPE~TlON

• OIlLY VAliD I'()R~~Al.LEVEL~

• DIFFICULr TO CBT"IN LJ!;ERlLOSttLL"TOR. DEVICE ClmI

• MOSURES NON I..lNEAA AS WELLAS LINEAR WICES

• MI:ASURES aoVICB3 0f'ER.Ii£El) ~CLI>.SS ~,B ORC

• MEASURES DEVICES AT ~REJ>LA~ SIG~ Ol'EAAl1~ f"OlllT

• OSCILLI>.lOR DeIlCES C~ BEMEMURED AND Ol"TlMlZED

.LOAD ~51TlVITY ~N 1£DETE~INEO

• PAAAMElERS ornER mMI roWER5UCII ~ HARMONIC f'ERFORWNCE00 BE MEI6U~D

• OPllMUM laD fOR /MXIMUMI'ONER 0L/l1'\IT ();l BE ME.'6JREl>FUR N<!'t INPUT FOM:R LEVEL

The result of a load pull measurement isa series of constant output power contoursplotted on a Smith Chart which representsall possible load impedances. A load pullmeasurement is required at each power leveland frequency of interest since the loadcontours are usually sensitive to thesevariables for most high power devices andoscillators. Hence, it is important thatthe load pull measurement is made quicklyand easily.

CONSTANT POWE~ OUTPUT CONTOURSva OUTPUT LOAD IMPEDANCE

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THE MEASUREMENT SYSTEM

A very straight forward, broadband, loadpull system can be assembled with theuse of a dual directional coupler reflec­tometer and parts of a 8409 Network Analyzer.The basic system will operate at any singlefrequency from 2 to 18 GHz and will makethe measurement in real time. The systemis rounded out by input and output powermeteTs, a frequency counter and a X-Yplotter to plot the data.

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OUTPUTTUNINGIMPEDANCE !>.ND POWER

ME/l.SUREMENTREFLECTOMETER

NETWORK II1P8410B 8ANALYZER IV

LINE • I1P MilASTRETCHER

""II

"" rA ~~ :>---t::======:::::::;Jf------j--jINPUT SOURCE II~AND TUNING ~

D.U.r.

• REFLECTOMETER = HP 116920 OUlL DIRECTIONAL COUPLERLINE Sf~TCHER= HP P!>.RT No. 08141-60004

REAL TIMEMANUAL LOAD PULLMEASUREMENT srSTEM

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A summary of the load pull measurementsystem features is shown. In particular,the system is inherently broadband and canthus be used as a general measurementsystem. Also, any type of impedance tunercan be used in conjunction with the measure­ment system. It is not necessary to have aspecial or calibrated tuner.

SYSTEM FEATURES

• TI1E S~sTE""IS INI1ERENTLY BROAOO).NO,OPEAATI~ ..1 ..NYSINGLE FREGUEIlCY IN TIlE l TO 186Hz RANGE.

• S~STEM C/I.l.lBAATlON CNII BE CIlECl<ED USING A.~T CIRCUITSfAND/l.RD.

• LO/I.D IMPEDANCE IS ME~REO SIMULTANEOO5LY WITHOUTPUT P"RMlETRICS SUCI1 "5 POWER-OUT IN REAL TIME.

• -mE SYSTEM Ct.N MEASURE TV«> PORT DE'lICE5 !>.NO ORCUITSAND ONE PORT SIGNI<L SOURCES.

• -mE Sl'STEM CAN BE USED WITI1 AN( TYPE OF OUTPUTTUNER INCWDING).N~ TUNER.

• SYSTEM !>'CCURM:( CAN BE ENH/l.NCEO USING C/l.LCUUTORCOK\'R)L ).NO ST!>.ND"RD NE'lWORK ANAL't'Z.ERCALIBR"TlON RDUTlNES

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IMPEDANCE &. POWERMEASUREMENT SYSTEM DETAILS

INPUT $I OUTPUT SYSTEMS IDENTICALIMPEDANCE ME~MENT.

INC

RECORDEROUTPUT

DIFFERENTIALMETER AAlPLlFIER

DIFFE~ENTIM... I'O>'IER METER

HP P~T No.1120-1513(O-Im/) DCMETER MOVEMENTWITII O-lmWLINEAR SCALECAUBRATION )

The RF power measurement is also accomplishedin the dual directional coupler. The incidentand reflected power are individually measured.The difference between the incident andreflected power in milliw~tts is then takenand displayed on a linear meter with alogarithmic scale. A differential powermeasurement is necessary to eliminate errorscaused by the insertion loss of the dualdirectional coupler. This error will bediscussed later on.

The impedance measurement itself is accom­plished with a dual directional couplerreflectometer. The "incident" signal isthe signal emerging from the output ofthe device under test. In the case ofan oscillator, the oscillator is its ownsignal source. In the case of an amplifier,a signal source and an appropriate inputtuner must be provided to supply therequired RF power. The "reflected" signalwill be the portion of the incident signalwhich has been reflected back from theoutput tuner. The network analyzer comparesthe "reflected" signal to the "incident"signal and outputs the information tothe polar display in the form of a reflec­tion coefficient. The reference plane isestablished at the output of the deviceunder test by adjusting the line lengthsin the "test" and "reference" lines of thenetwork analyzer.

CALIBRATION

The calibration procedure for the load pullmeasurement system is similar to that usedon a standard network analyzer. The deviceunder test is replaced by a good broadbandshort ciruit and connected to the "input"port. An RF source is connected to thereflectometer at the "tuner" port. Thesource is swept around the frequency ofinterest and the line stretcher and thephase and amplitude controls on the 8410B­are adjusted to indicate a broadbandshort circuit on the 8414A polar display.

For a two-port measurement, the inputreflectometer is calibrated in a similarfashion. The only difference is that theincident and reflected channels are re­versed on the network analyzer.

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SYSTEM CALIBRATION

El8 HP84108410B 84I4A NE1\\I0RK

kNkLYZ.ER

~It11J \tIC RlRl'J REFERENCE SPORT CIRCUIT~ )>----='---'------"-''----« AT REFERENCE PLIINE OF

BRlJADBo\ND SWEl'T_FRfQLENCY [;8/ICE UN~R TEST5lClNH.. SOU~ (flP 8I>2.OC/8"290)

i. CONNECT APPROPRIATE: SHQlq CIRCUIT REFERENCE 1'0 REFLECTOMETERINPUT PORT

2.. CONNE:CI SOURCE: 10 nE WilER I'Olrr Of T~E ~fLECTOMETER

~. ADJUST T~E LINE 51RETCIlER AND ~E t>.MPLfTUDE AND PI-lASE CONlRDLCONTROL ON TIlE 8'\106 10 INDICATE A B~ND SHOIU' ON~E MI4A FOLAR DISPLAY

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CALIBRATIONTECHNIQUE

A) CALIBRATION MODE

(~) (A + B+ lC)REFL INC

C\ " >--__~..L.X.:>....T--T~X=__._~_--«~REFERENCE~ ) ABC " J SHORT CIRCUIT

SOURCE TUNER INPUTro~ ro~

(A,B,C, ARE EITHER ATTENUAno~ IN dB OR ELECTRICAL LENGTHS)

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CALIBRATIONTECHNIQUE

8) MEASUREMENT MODE

(0) (B+2C)REFL INC

SHORT ~) Y ~ <~•CIRCUIT A 8 CTUNER INPUT SOURCE

PORT PORT

WITW ASHORT CIRCUIT COWNECTED IN PLba: OF TilE OUTPUT TUNER THE TE6fCIlANIlEL Sl«>ULD BE (ZA+ZB+2C) GREATER TWIN THE REFERENCE CIlANNEL.TIllS 15 TRUE SINCE TI4E REFERENCE = C AND TWE 1ES1 - lB+ 3C+ 2A.

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In calibration, the reflectometer isconnected backwards. The quantity measuredwill be the reciprocal of the reflectioncoefficient of the calibration shortconnected to the "input" port. However,since the reciprocal of a reflection co­efficient of 1 < + 180° is still 1 < + 180°,the calibration short will be displayed asa short circuit on the polar display. Cali­brating in this fashion automaticallyaccounts for the reference plane extensionto the D.ll.T. and for the amplitude adjust­ment required on the network analyzer toaccount for the insertion loss between theD.ll.T. and the output tuner.

OUTPUT TUNER CONSIDERATIONS

The load pull measurement system describedcan be operated with any type of outputtuner. The magnitude of the load reflectioncoefficient presented to the device undertest is limited by the maximum reflectioncoefticient of the tuner and the lossbetween the device under test and thetuner, Some examples of standard outputtuners include double or triple stubtuners, movable probe tuners and A/4 slugtuners.

An active tuner can be used if it isnecessary to tune to the outside of theSmith Chart. A basic active tuner consistsof an amplifier, an attenuator, a phaseshifter and either a circulator or a highdirectivity directional coupler. A signalinciden~ on the tuner is amplified, atten­uated, and phase shifted the desired amountand sent back towards the device under testas a "reflected" signal. One considerationto take into account with the active tuneris that it is potentially unstable if 180 0

of phase shift and a gain of 1 is achievedaround the path from the input to the outputof the amplifier in the tuner.

PASSIVE TUNERSDIELECTRV: OR METM.LIC SLLGS

SU';'UNER'~

LOAD PULLACTIVE TUNER

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12

VARI1SLEAnEHU"TOR

POWERREfLECTED--POWERINCIDENT

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Pl-\~ESl-\IFTER POWER

AMPLIFIER

DESIGN EXAMPLE: GaAs FET POWER AMPLIFIER

13 One application of a load pull measurementis the design of the output circuits for alarge signal GaAs FET power amplifier. Themeasurement system setup is shown. Bothan input and output reflectometer areused. The input reflectometer is a standardreflectometer and it measures the inputimpedance of the device as the loadimpedance is changed. The input power isalso measured differentially to obtainthe true power delivered to the input ofthe device.

son.1ERM.OUTPUT

IMPEr».NCElIND ()JTf'UT

POWER 17~,---MEASU.,..,...REt.IDIT---J COJIITER_SYSTEM

I--~----'~~>---{>7 >>--="---~----7

DUJ.

LOAD PULLTWO PO~T MEASUREMENT

..I

\

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The designer would start by mounting thedevice to be measured in a fixture with ason line output. The input circuit caneither be an appropriate input matchingcircuit derived from small signal Sparameters or a son line in combinationwith an input tuner. The output of thetest fixture is connected to the loadpull measurement system at the "RF Input"port, the device is powered up, and theappropriate input power is applied. Theimpedance that the output tuner is pre­senting to the device under test willbe ·displayed on the polar display andthe corresponding output power will bedisplayed on the output power meter. Theinput impedance of the device for thisload condition will be displayed on theinput reflectometer. Both the impedanceinformation and the power output informationis recorded on a Smith Chart via an X-Yrecorder.

14/15

w4] WJm ----c::::r- -a-

~ ~ r m&1}50 OHM TEST FlmJRE OLJeAATlON PIlCK.6GED

~ DEVICE

LARGESI&N..LINPUTIMl'EO~

16/17

OPTIMUMLO~ FORIMllIMUMOUTPUTPOWER

OPTIMIZED BEST FIT IMT~ING ell'CUITS

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LOAD PULLGAAS FfTAMPLIFtER DATA

CONTOURS OF CONST~NT OUTPUT POWER VS. LO"l>IMPED~E UNDER LARGE SIGNAL CONDITIONS

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The resultant data will be a set ofcontours of constant output power onthe Smith Chart for the given testcondition. An output matching circuitcan now be designed for a desired outputpower by matching to the appropriateimpedance point indicated by the loadpull data. The load pull data also indicatesto the designer how sensitive the outputpower of the device under test is tooutput load. An input match can also bedesigned using the appropriate inputimpedance information obtained from theinput reflectometer.

INPUT >>----S ) OUTPUT

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DESIGN EXAMPLE: BIPOLAR OSCILLATOR

The load pull measurement system canbe used to come up with an empiricaldesign of the output circuit of anoscillator. The measurement systemsetup is shown. The oscillator acts asthe RF power source for the system.

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LOAD PULLONE-PORT 05CtLlATO~ MfASU~EMENT

IMPED~NCE

ANDPOWER

MHSUREMENTSY5rEM(FIG 7 )

~ ~COUNTER

~~) X X)~TEE

nUl son.OUTPUT IFUNER I TERMINATIONTUf'lER SUf'!'l'(

The designer must first determine hisbasic oscillator topology, feedback circuitand resonator. The oscillator is then builtup with a 50~ line in place of the outputmatching network. The oscillator is attachedto the load pull measurement system at the"input" port of the reflectometer andpowered up. The impedance displayed onthe polar display will be the impedancepresented to the output of the oscillatordevice. Contours of constant output powercan then be drawn on a Smith Chart usingthe power readings from the output powermeter and a X-Y recorder connected to thepolar display. In a similar fashion, therange of impedance that the device willoscillate into can also be determined andrecorded. The designer can then use theresultant information to layout hisoscillator output circuit.

LOAD PULLOSCILLATOR DATA

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LOAD PULL DATAFOR BIPOLAR

AMPLIFIER OPERATING CLASS C

INPUT MATCH OPTIMIZED AT MAY. ~ER OUT

r-1\JNABLE OUTPUT

MATCHED INPUT >>-----<rr----1

HARMONIC LOAD PULL

~DUl. " ,.,

CQllNecre:O 7

HERE CI~t::oit 'Y~6 = WI16b7A POiHER SPliTTER

Krn =LaN MS ALTERIi!W = HIGH ~ss FILTER

ft = LINE ST"RETCHER

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DESIGN EXAMPLE: CLASS C AMPLIFIER

Devices operated in Class C can be measuredon the load pull measurement system. Thetwo port setup is used to make the measure­ment. Input and output impedance informationis obtained similar to the case of the GaAsFET power amplifier. This information isthen used to design the input and outputmatching networks for the amplifier. Anexample of some output load data is shownfor a bipolar device operated under ClassC conditions.

SPECIAL APPLICATIONS: HARMONIC LOAD PULL

It is sometimes important to know how muchsecond harmonic will be generated in anamplifier or an oscillator as a functionof the output load impedance presentedat the fundamental and second harmonic.The load pull measurement system can bemodified for this measurement with theaddition of a frequency sensitive tuner,an additional 8410B, 8411A combinationand some power splitters and filters.The second network analyzer is used tomeasure the impedance presented to thedevice under test at the second harmonic.The filters in the reference and testchannels of the harmonic converters separatethe fundamental frequency components ofthe output impedance from the secondharmonic frequency components so thatthey may be displayed separately. Thefrequency sensitive tuner is accomplishedwith an octave-band circulator, a highpass filter, and a low pass filter.

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ACCURACY CONSIDERATIONS

The accuracy of the load pull impedancemeasurement will be compar~ble to theaccuracy of the 8409 Network Analyzer usedin a manual mode at a single frequency. Thelargest source of error in the system willbe due to the connection of the deviceunder test to the "input" of the reflecto­meter test set. Any VSWR in the testfixture of the connection to the test setwill cause an ambiguity that is difficultto calibrate out.

The accuracy of the load pull impedancesystem can be enhanced by automating thesystem in a fashion similar to the 8409ANetwork Analyzer. The same type of errormodels can be used and applied to the loadpull system. There is one basic differencebetween the load pull system and the8409A Network Analyzer that must be takeninto account in the error models, however.This is the fact that during calibrationon the load pull system the quantitymeasured is not (f) of the calibrationstandards but (I/f). Once this is takeninto account the error correction can beaccomplished in the same fashion as itis on the 8409A used in a reflectioncoefficient measurement mode.

Another accuracy consideration that mustbe taken into account is the accuracy ofthe output power measurement. Ideally, therewould be no loss between the output tunerand the output of the device under test.In this case, the insertion loss of thetuner will be [10 log (1 - IfI 2 )] where fis the reflection coefficient between thetuner and the device under test. In theactual case, there will be some finite lossbetween the tuner and the device under test.The insertion loss of the tuner will now beboth a function of the finite loss andof the reflection coefficient betweenthe device under test and the tuner.This iosertion loss will be [1 _ Ifl 2

(IO)L/~)], where L is the loss in dBbetween the device under test and thetuner and f is the effective reflectioncoefficient between the device undertest and the loss-tuner combination.This loss will lead to an error both inthe output-power measurement and inthe load impedance measured if the outputpower is measured after the output tuner.The differential power measurementeliminates this source of error sinceit measures the actual power deliveredto the load.

LOAD PULLPOWER MEASUREMENT ACCURACY

IDEAL CASE

INSERTION LOSS IN dB= 10 L06(1-1'i'1 2)

L04D PULLPOWER MEASU~EMENT AcroRACV

ACTUAL CASE

lIN ORaoR TO ACHIEVE SAME Ii' AS BEFORE

fl = II (10 LIIO )

INSERTI().I LOSS =L + 10 LOG (H~12 [10 L/5J)THE INSERTION LOSS OF THE TUNER WILL BE A FUNCTION OF THEREFLECTION COEFFICENT PRESENTED 6'< THE TUNER. FOR LARGEL THERE WILL BE A SIGNIFICANT CONTRIBUTION OF INSERTIONLOSS [)JE TO THE REFLECTION COEFFICENT OF THE TUNER.

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BIBLIOGRAPH)'t CUSACK, J, ETAL "AUTOMATIC LOAD CONTOUR

MAPPING FOR MICROWAVE POWER TRA\IISISTORS"IEEE MTT,# 12, DEC 1974, PP. 1146-1152

2. TAKAYAMA,Y, "A NEW lOAD-PULL CHARACTERIZATIONMETHOD FOR MICROWAVE POWER TRANSISTORS"IEEE. MTT- 5 DIGEST, PP. 216 - 220, JUNE, 1976

3. KELLY, W. M., ETAL "DESIGN OF UNEAR 6A~ FErAMPLIFIERS·, CONFERENCE PROCEDINGS OF THE7TH EUROPEAN M'CROWlVE CONFERENCE, f'f.' 105-109,SEPT. 1977

4. STANCLIFF, R, POUUN, 0, "HARMONIC LOAD PULL~IEEE MT'T - 5 DIGEST P. 185, MAY 1~1~

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SEPTEMBER 1981

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PRINTED IN U.S.A.