Measurements in 75Ω Coaxial Transmission Line Using PLANAR VNAs with 50Ω Test Ports

1 © 2012 Copper Mountain Technologies

One application of a vector network analyzer (VNA) is the measurement of 75Ω coaxial

transmission lines. Inthisarticle,wewilldiscussmakingthesemeasurementsusingthemostcommon

typeofVNAs,whichhave50Ωtestports,inconjunctionwith50Ω‐to‐75ΩMinimumLossPads(MLP),i.e.

impedance‐matchingattenuatorswithinsertionlossof5.7dB.As50ΩVNAswesuggestusingtheVNAs

manufactured by Copper Mountain Technologies: PLANAR TR1300/1, PLANAR 304/1, and PLANAR

804/1 (as of 2012). The use of anMLP affects accuracy ofmeasurements by changing the calibration

errorand,dependingon the locationof theattenuator in themeasurement circuit, impactsstabilityof

measurementsrelatedtotestcablebending.

ToassesstheimpactofMLPontheaccuracyofthemeasurements,wewillrefertothecalibration

comparisonmethod[1,2].WewillalsocomparetheadditionalerrorswiththetypicalerrorsoftheVNAs

with75ΩcoaxiallinesafterthemostwidelyusedandaffordableSOLTcalibration(usingSHORT,OPEN,

LOAD, and THRU standards). To assess the errors dependent on the location of the attenuator in the

measurementcircuit,itispossibletousetheresultsofpracticalmeasurements.

AssessingtheadditionalcalibrationerrorcausedbytheMLP,wewilluseanumberofcalibration

schemesforameasurementsystembasedonPLANARVNAsandwillapplythecalibrationcomparison

method.LetusperformcalibrationsofthemeasurementsystemwiththeMLPsasshowninFigure1.

TRLMcalibration is one of themost accurate calibrations in the industry today. It isperformed

using the following standards: THRU, REFLECT, LINE, and MATCH. This calibration technique is

recognizedasthebenchmarkcalibrationallovertheworld,andaTRLMcalibratedmeasurementsystem

achieves theeffectiveparametersofdirectivitymuchbetter than50dBandsourcematchmuchbetter

than‐50dB[3].

Measurementsin75ΩCoaxialTransmissionLines

UsingCMTPLANARVectorNetworkAnalyzers(VNAs)

with50ΩTestPorts

Measurements in 75Ω Coaxial Transmission Line Using PLANAR VNAs with 50Ω Test Ports

2 © 2012 Copper Mountain Technologies

Figure1

SOLTcalibrationisthemostwidelyusedandaffordablecalibrationtechnique.Inourexampleweuse a high‐quality Agilent 85036B calibration kit,which provides the calibratedmeasurement systemwith the following effective parameters: directivity no less than 46dB at frequencies up to 2GHz,directivity no less than 40dB at frequencies up to 3GHz, source match no more than ‐36dB, andreflectiontrackingnomorethan0.03dB.

To assess the measurement system from Figure 1 we will compare two successive TRLMcalibrations. Results fromFigure2 confirm that theproposedmeasurement systemallows comparingtwo calibrationswith the following effective parameters: directivity at least 73dB, sourcematches atleast65dB,andreflectiontrackingbeinglessthan0.003dB.ThismeasurementprovesthatthequalityoftheTRLMcalibrationperformedonPLANARVNAspracticallydependsonqualityoftheLINEandSHORTcalibrationstandards..

ComparingTRLMandSOLTcalibrations,wecanseethattheuseofanMLPinsignificantlyimpairedthe SOLT calibration parameters compared to the parameters claimed by the calibration kitmanufacturer.Forexample,themeasuredeffectivedirectivityofthegivenmeasurementsystemisbetterthan46dBat the frequencyof2.8GHz,andthuscomplieswith thespecificationsof thecalibrationkitmanufacturer,i.e.betterthan41.2dB.Therefore,theadditionalerrorsintroducedintothemeasurementsystem due to the use of MLPs are comparable to the range of calibration error when using acommerciallyavailableSOLTcalibrationkit.

Measurements in 75Ω Coaxial Transmission Line Using PLANAR VNAs with 50Ω Test Ports

3 © 2012 Copper Mountain Technologies

Figure2

Correspondingly, using MLP in a measurement system has a low impact on the measurementerrors. That allows recommending the use of PLANAR VNAs (PLANAR TR1300/1, PLANAR 304/1,PLANAR804/1)orother50ΩVNAsofsufficientdynamicrange(over120dB)andmeasurementstabilitywithahigh‐qualityMLPforperformingmostofVNARFmeasurementsin75Ωtransmissionlines.

It is important to analyze two possible ways of connecting MLP into the measurement circuit(Figure3).

Measurements in 75Ω Coaxial Transmission Line Using PLANAR VNAs with 50Ω Test Ports

4 © 2012 Copper Mountain Technologies

Figure3

TheMLPscanbeeitherconnectedtotheVNAports,ortotheendoftestcables.Thesecondvariantallowsusingwidelyavailable50Ωtestcablesthatsurpasses75Ωtestcablesinalltheparameters.Eventhoughitlookslikethemeasuringcircuitwithahigh‐quality50Ωtestcablehasobviousadvantages,suchcircuitisatastrongdisadvantageduetothehighsensitivitytotestcablebending.Figure4shows|S11|parameteroftheLOADstandardbeingmeasuredaftercalibrationandthesubsequenttestcablebendingusingthecheapestandsimplest75Ωcableandaseveraltimesmoreexpensiveandstable50Ωcable.Thebendingofthe75Ωcablekeepsthe|S11|valueoftheLOADatabout‐50dB,whilethebendingofamuchhigherquality50Ωcableresultsindegradationofthe|S11|to‐30dB.Such|S11|changeactuallyresultsin

Measurements in 75Ω Coaxial Transmission Line Using PLANAR VNAs with 50Ω Test Ports

5 © 2012 Copper Mountain Technologies

deterioration of the effective directivity of themeasurement system to 30dB, and thus increases themeasurementerrortoanunacceptablevalue.Forexample,themeasurementof|S11|equalto‐16dBwillhaveanerrorof2.1dBcomparedtothepossibleerrorof0.35dB;andthemeasurementof|S21|equalto‐3dBwillhaveanerrorof0.16dBcomparedtothepossibleerrorof0.08dB.

ConnectionofMLPtotheVNAtestportsanduseofthe75Ωtestcablesallowsforminimizingtheimpactofthemeasuringcircuitonthemeasurementaccuracy.

Figure4

UsingCMTPLANARvectornetwork analyzerswith50Ω testports, 5.7dBMLPconnected to theVNA testports, andperformingSOLTcalibrationwithahigh‐quality calibrationkit,wecan expect themeasurementerrortobedependentonlyonthequalityofthecalibrationkit,keepingimpactofthe50Ωto75Ωadaptertoaminimum.

References:

[1]DylanWilliams,ArkadiuszLewandowski,DenisLeGolvan, andRonGinley “ElectronicVector‐Network‐AnalyzerVerification”,IEEEMicrowaveMagazine,October2009.

[2]D.F.Williams,R.B.Marks,andA.Davidson,“Comparisonofonwafercalibrations,”Automat.RFTech.GroupConf.Dig.,vol.38,pp.68–81,Dec.1991.

[3]Doug Rytting, “Advances inMicrowave Error Correction Techniques”, Hewlett Packard, RF &MicrowaveMeasurementSymposiumandexhibition,1987.