RF_balun_transformer

IntroductionThisapplicationnoteisdesignedtohelpthereaderunderstandhowbaluntransformerscanbeusedintoday’sRF/Microwavecommunicationapplications.Thereisanincreasingdemandforbaluntransformersintoday’stelecommunicationmarketandM/A-COMisleadingthefieldwiththisdevice.Asmanyassixofthisdevicecanbeusedaroundanyparticularchipasillustratedinfigure4.M/A-COM’sbaluntransformercanbeclassifiedintotwogroupsA)ThefluxcoupledbaluntransformerandB)Thetransmissionlinebaluntransformer.DefinitionofTermsInsertionLoss(dB):Lossduetotransmissionfromprimarydotporttosecondarydotportandsecondaryport.Mostbaluntransformersaresymmetricalthroughtheircentralhorizontalaxis,therefore,aninputcanbeappliedattheprimarydotportortheprimaryportwithdifferentialoutputsatthesecondarydotandsecondaryports.AmplitudeBalance(dB):Measureofthedifferenceinamplitudebetweenthetwodifferentialoutputs.PhaseBalance(degrees):Measureofthedifferenceinphasebetweenthetwodifferentialoutputs,i.e.measuredfrom±180°.ReturnLoss(dB):Lossduetoreflectionatanyport.Figure2.4:1TransmissionLineBalunTransformerM/A-COMDivisionofAMPIncorporatednNorthAmerica:Tel.[phone],Fax[phone]nAsia/Pacific:Tel.[phone],Fax[phone]nEurope:Tel.[phone],Fax[phone]fluxcoupledbaluntransformercanprovideawiderangeofimpedanceratios;1:1,4:1,9:1and16:1aremostcommon.DCisolationfromprimarycoiltosecondarycoilisalsoafeatureofthisdevice.Ithasalowlossandgoodbalanceoverabroadfrequencybandwidth.TheETC4-1-2hasaninsertionlossofapproximately2.5dBat800MHzandisanexampleofoneofourmostpopularfluxcoupledbaluntransformers.B.TransmissionLineBalunTransformersThetransmissionlinebaluntransformercomesintwoimpedanceratiosasoutlinedabove,i.e.4:1and1:1.Itoperatesathigherfrequenciesandoveragreaterbandwidththanthefluxcoupledbaluntransformer.ItcannotprovideDCisolationfromprimarysidetosecondaryside.TheETC1.6-4-2-3isanexampleofoneofourmostpopular4:1transmissionlinebaluntransformerswithanoptimumperformanceof1dBinsertionlossat900MHz.ApplicationsTheapplicationsofthesedevicesare:1.ImpedancematchingSeefigure5.TheinputstotheLObufferamplifierandthequaddioderingareat100Wapproximately.2.DCisolationSeefigure5.AtDCthereisnomagneticcoupling,therefore,theLObaluntransformerandtheRFbaluntransformerareactingasopencircuitsatDC.3.MatchingbalancedportstoasingleendedportSeefigure6.Theinputstothediodepairare180°outofphase.Thefluxcoupledbaluntransformercanincorporatealloftheabove,whereasthetransmissionlinebaluntransformercannotprovideDCisolation.Matchingsingleendedportstobalanced(differential)portsonanFigure1.FluxCoupledBalunTransformerM/A-COMDivisionofAMPIncorporatednNorthAmerica:Tel.[phone],Fax[phone]nAsia/Pacific:Tel.[phone],Fax[phone]nEurope:Tel.[phone],Fax[phone]•Differentialoutputsfromsecondlocaloscillatorbufferamplifier1+LOB1–LOB1•Secondlocaloscillatordifferentialinputs+2LOIN–2LOIN•DifferentialIFinputs+IFIN–IFIN•Differentialoutputsfromfirstmixer+MXOUT–MXOUT•Differentialinputstofirstmixer+MXIN–MXINRFBalumTransformersM565V2.00•Differentialinputstofirstlocaloscillator+1LOIN–1LOIN•Differentialoutputsfromsecondlocaloscillatorbufferamplifier2+LOB2–LOB2“Passivematching”referstotheuseofaferritewoundtransformer“offchip”todothisjob,while“activematching”incorporatesthetransformer(ormatchingdevice)onthechip.Theadvantageofactivematchingisthatthereisacostreductionduetotheabsenceoftheferritewoundtransformers,butthechipthenbecomesveryspecific,alsothequalityofthismatchingwillnotbeasgoodasthematchingthatapassivetransformerwouldprovide.Passivematchingallowsthedesignengineertodesignachipthatcouldcovermorethanoneapplication,i.e.hisdesignismoreversatile.Also,matchingatIFfrequenciesforGSM,forDCS,PCSandWLAN,forexample,canbeextremelyawkwardwithoutusinglowloss,broadbandtransformers.Examplesofthedifferentialportsthatwouldneedtobematchedtoasingleendedportarethedifferentialoutputsofadiodequadring,aGilbertcelloraFETquad.AtRFfrequenciesforGSM,WLAN,DCSandPCSFigure4.FirstMixerStageandSecondMixerStageofaSingle-chipDouble-conversionHeterodyneReceiverCommonlyUsedForDigitalMobileTelephonesAccordingtotheGSMStandardandOtherDigitalSystems.ShowsWhereSixBalunTransformersCanBeUsedM/A-COMDivisionofAMPIncorporatednNorthAmerica:Tel.[phone],Fax[phone]nAsia/Pacific:Tel.[phone],Fax[phone]nEurope:Tel.[phone],Fax[phone]