BALUN基础介绍

1BALUNMar.01,2012Confidential2ContentsWhyDoWeNeedBalunBALUNBasicInformationBALUNParameterMeasurementCommonBALUNInformationandDesignBALUNApplicationsConfidential3DifferentialcircuitsarebecomingmorewidelyusedinRFcircuitsforthesamereasonthattheyhavebeenusedforyearsinlowerfrequencycircuits.Thebenefitsofusingdifferentialcircuitsincludeimmunitytoelectromagneticinterference,powersupplynoiseandgroundnoise,even-orderharmonicsuppression,andtolerancetolessthanperfectRFgrounds.Thus,balunsareemployedinmanymoderncircuitdesigns.1.WhyDoWeNeedBalunsWhydoweneedbaluns(1)Confidential4Balunprincipleisaccordingtotheantennatheory,dipoleantennaisbalanceantennas,andcoaxialcableisnotbalancedtransmissionlines,ifitsdirectconnection,thecoaxialcableskinshavehighfrequencycurrentthrough(Butbycoaxialcabletransmissionprinciple,highfrequencycurrentflowshouldbeincable,thecorticalisasshieldinglayer),andthus,itwillaffecttheantennaradiation.So,weneedaddanunbalancedtobalancedconvertorbetweenantennaandcoaxial,thehighfrequencycurrentflowwillbesuppressednicely.Whydoweneedbaluns(2)Confidential51.BALUNBasicInformationTheword“balun”isaportmanteautermformedfromthewords“balanced”and“unbalanced”inwhich“balanced”impliesadifferentialconfigurationand“un-balanced”representsasingle-endedconfiguration.AsshowninFigure1,abalunisatransformationblockbetweenasingle-endedstageandadifferentialpair,eitherfromthesingle-endedblocktothedifferentialpair.Sometimes,peoplerefertothebaluntypeinFigure1(a)asa“splitter”andthebaluntypeinFigure1(b)asa“combiner”.The“splitter”isareversed“combiner”,andviceversa.Fromnowon,wewillonlyfocusonthe“splitter”unlessotherwisenoted.Confidential6(a)Fromsingle-endedblocktodifferentialpair(b)Fromdifferentialpairtosingle-endedblockFigure1Thebalunisablockbetweenasingle-endedblockandadifferentialpair.Confidential7Thecharacteristicsofanidealbalunandcanbeoutlinedasfollows:TheCharacteristicsofaBalun(1)1)Therearethreeterminals:onesingle-endedandtwodifferential.However,thetwodifferentialterminals’impedancesmustbekeptequal.Confidential8TheCharacteristicsofaBalun(2)3)Totalinsertionlossfromthesingle-endedterminaltothedifferentialterminalsiszero.2)Atthetwodifferentialterminals,themagnitudesofthesignalareequalbuttheirphasesarekeptata180°difference.Confidential9Somebalunsprovideimpedancetransformationinadditiontoconversionbetweenbalancedandunbalancedsignalmodes;othersprovidenoimpedancetransformation.For1:1baluns(noimpedancetransformation),theinputandoutputareusuallyboth50ohmsor75ohms.Themostcommonimpedance-transformationratiois1:4(alternatively4:1).Somebalunsprovideotherimpedance-transformationratios,suchas1:9(and9:1),1:10(and10:1),or1:16(and16:1).Impedance-transformerbalunswithlargerratiosareusedtomatchhigh-impedancebalancedantennastolow-impedanceunbalancedwirelessreceivers,transmitters,ortransceivers.Thebalancedterminalsofsomebalunscanbeconnectedtoanunbalancedsystem.Confidential11BalancecircuitryarchitectureLCCLUIUO1UO2Rdiff11111IoIUjCCUUjLLjCC211IoIUjLLUUjLLjCCUI分别经过低通、高通得到反相的UO1、UO2由计算可知UO1、UO2始终有180度相差（Differential）适合在Balance系统上传输。Confidential12Inbalancedsystemsthesignalscanbedividedintoacommonmodeandadifferentialmode.Thebalancedoutputofanidealbalun(oranidealunbal./bal.SAWfilter)are180°phaseshiftedandarehalftheamplitudeoftheinputsignal:)(2121)()()()(,2,1tfeetftftfwtjjwtidealidealDM02121)()()()(,2,1wtjjwtidealidealCMeetftftfConfidential13Aswesee,anidealbaluncancelsthecommonmodecompletely.Therefore,thecommonmoderejectionisinfinite.Now,whathappensifwehavearealbalunwithnon-idealproperties?Thephasedifferenceisnotanylonger180°,representedbyaphaseerrorjinoneoftheoutputpaths.Furthermore,theenvelopeamplitudesofthetwooutputpathsarenotequal,representedbyanamplituderatior.Therefore,theamplitudeofoneoutputisrtimestheamplitudeoftheotheroutput,butthesumofbothhastostayequal.Theseconditionsareputintoformulas:(),1,2,1()()()11jwtjwtDMrealidealidealftftftee(),1,2,1()()()11jwtjwtCMrealidealidealftftfteeConfidential14Now,thepowercommonmoderejectionratio(CMRR)isbeingdefinedastheratiooftheinputsignalpowertothecommonmodesignalpowerforaloss-lessbalun:2222(),2222()CMRR1()1111111111CMRRjwtPowerjwtjwtCMrealjwtjwtjjwtjPowerjeftfteeeeeeeeConfidential15CMRRVSPhase&AmplitudeErrorConfidential163.BALUNParameterMeasurement•Itisdifficulttoprovideaccuratecharacterizationofbalunsfortwoprimaryreasons:itisa3-portdevice,andtheinputanoutputtypicallyhavedifferentimpedancesfromeachother,makingnetworkanalyzercalibrationmoredifficult.•Inordertocharacterizebaluns,wecommonlyusestwotypesoftestmethods.•The“A”typetestboardisdesignedtohavetwobalunsofthesamepartnumbermountedonitinaback-to-backconfiguration.Ithastwo50ohmI/OswherebyoneI/Oisconnectedtotheunbalancedportofoneofthebaluns,andtheotherI/Oisconnectedtotheunbalancedportoftheotherbalun.Thebalancedportsofoneofthebalunsaredirectlyconnectedtothebalancedportsoftheotherbalun.Thethroughinsertionlosscanbedeterminedbydividingbytwo.•The“B”typetestboardisdesignedt