PRACTICAL INSTRUCTIONS FOR THE RF AND MICROWAVE ME

PRACTICALINSTRUCTIONSFORTHERFANDMICROWAVEMEASUREMENTTUTORIALF.CaspersCERN,Geneva,SwitzerlandG.HutterGSI,Darmstadt,GermanyAbstractForthispracticaltutorialusing(vector)networkandspectrumanalysersdetailedinstructionsaregivenonhowtooperatethesesophisticatedinstruments,avoidingtheneedforreadingheavyinstructionmanuals.Thetutorialisbasicallysplitintotwoparts,namelynetworkanalysisandspectrumanalysis.Inthenetworksectionthereflectionandtransmissioncharacteristicsofpassiveandactiveone-andtwo-portsaremeasuredwithaparticularemphasisonresonatorsandcavitiesaswellascalibrationprocedures.Spectrumanalysersareusedtoexamineamplitude-andfrequency-modulated(AM,FM)signals,signalstrengthmeasurement,non-linearbehaviourofamplifiersviaintermodulationandtheevaluationofnoiseandnoise-figureproperties.1.INTRODUCTIONRadio-frequency(RF)spectrumanalysers(SPAorSA)canbefoundinvirtuallyeverycontrolroomofamodernparticleaccelerator.Theyareusedformanyaspectsofbeamdiagnostics,includingSchottkysignalacquisitionandRFobservation.Wediscusshereonlytheapplicationofclassicalsuper-heterodyneSPAsandnotsystemsbasedontheacquisitionoftime-domaintracesandsubsequentFouriertransform(FFTanalysers).Suchasuper-heterodyneSPAisverysimilar(inprinciple)toanyAMorFMradio-receiver.TheincomingRFsignalismoderatelyamplified(sometimeswithadjustablegain)andthensenttotheRFportofamixer.Amixerisanon-linearelement(containingoneormorefastdiodes)andactslikeananaloguemultiplier.Theoutputsignalofalocaloscillator(LO)isconnectedtotheLOinputofthemixerandsuperimposedontheRF.Asaconsequenceweobtain(non-linear)mixingproductsatthesumanddifferencefrequencybetweenRFandLOwhichappearattheintermediatefrequency(IF)portofthemixer.ThisIFsignalissubsequentlyband-filteredandsenttotheverticallydeflectingplatesofthecathoderaytube(CRT)intheSPA(orviaademodulatortotheloudspeakerinaradio-receiver).Theword‘super-heterodyne’isamixtureofLatinandGreekmeaningadding(super)adifferent(hetero)force(dyne),wherethedifferentforceisnootherthantheLOsignal,superimposed(added)inanon-linearfashionontheRFsignal.AnetworkanalysercanoftenbefoundinacontrolroombutitsrealhomeistheRFlaboratory.Therearetwokindsofinstrument,namelytheScalarandtheVectorNetworkAnalysers(SNAandVNA,respectively).TheSNAcanonlydisplaythemagnitudeofsomeproperty(e.g.reflectionsortransmissioncoefficient)versusfrequency.ItbasicallyconsistsofatunableRFgeneratorandapowerdetector.OftenSPAswithatrackinggenerator(trackingtotheactualfrequencyshowninthedisplay)areusedforscalarnetworkanalysis.However,withitsadditionalphasedisplaycapabilityandmuchhigherdynamicrangewithasimplepowerdetector,theVNAoffersamuchwiderrangeofapplicationsthantheSNA.Thehigherdynamicrange(around100dB)isalsolinkedtothefactthatVNAsapplythemixingconceptmentionedabove.WeareonlyusingVNAsinthistutorial,butifaspectrumanalyserwithtrackinggeneratorisavailableonemayalsotryusingitasascalaranalyser.AtypicalRFlaboratoryapplicationofaVNAisthemeasurementofthescatteringparameters(S-parameters)ofsomedeviceundertest(DUT).Inthistutorialwewillperformmostofthestandardmeasurementsonobjectssuchascavities,compleximpedancesincludingnon-idealresistors,421capacitorsandinductors,filtersandamplifiers.Itwillalsobepossibletodosome‘fun’experiments(notdescribedinthispaper),e.g.usingacoaxialwaveguidetransitionasanantennaandbuildingasimpleradarsystem.2.EXPERIMENTSWITHTHEVECTORNETWORKANALYSER2.1GeneralinformationabouttheVNAsusedhereWeusethreedifferentmodelsoftheHP8753VNA,namelytypesB,C,andE.Allhavesimilarfunctionsandfrequency,power,anddynamicrange,butusedifferentsoftwareandhaveslightlydifferentcontrolmenus.So,forexample,thedesiredoutputpoweroftheHP8753Cisobtainedbyapplyinganexternalattenuatortoanoutputpowerfrom–10dBto+20dB.FortheHP8753Danoutputpowerrangeisselected(whichinternallyswitchesattenuators)andthenthedesiredoutputpowerischosen.However,theinstructionsgivenherearealso‘inprinciple’applicabletocomparableinstrumentsfromothermanufacturers.TheHP8753hastwokindsofcontrolbuttons:hardkeysonthefrontpanel,whichareusedtosetthekindofmeasurement,thefrequencyrangeandotherfunctions,andsoftkeysontheright-handsideofthedisplay.Inthefollowingdescriptionsboldlettersarefrequentlyusedtodenotehardkeyswhilenormallettersdenotesoftkeys:MEAS,REFL.,FWDS11(A/R),etc.TheHP8753analysershavetwodisplaychannels,whichcanbedisplayedinparallelintwomodes:splitscreenandcommonscreen.Eachchannelcanbeprocessedusingmathematicalfunctions,memory,data/memorydisplayandotherfeatures.Onlyonechannelatatimerespondstocontrolinputs.Changestosomesettingsactonlyontheactivechannel,whereasfrequencyrangeandotherprincipalparametersremainunchangedforbothchannels.ForcriticalmeasurementsthenoiselevelcanbeimprovedeitherbyaveragingorreducingtheIFbandwidthorbyincreasingtheoutputpowertothemaximumallowedlevel.TheVNAshavepowerfulmarkerfunctionstosimplifysearchoperationssuchas3dBbandwidths,maximaandminimaanddesiredpowerlevels.ThesecanbecombinedwithspecialmathematicalfunctionstoallowthecalculationoftheQ-factorofaresonator,forexample.Allanalysersusedinthetutorialhaveatome-domainoption,whichcanbefoun