PSPICE中的传输线模型介绍

APPLICATIONNOTETransmissionLineApplicationsinPSpiceBrianHirasuna,May20052IntroductionTransmissionlinesareusedtopropagatedigitalandanalogsignals,andasfrequencydomaincomponentsinmicrowavedesign.Transmissionlinesareusedforvariedapplications,including:•Powertransmissionline•Telephonelines•TracesonPrintedCircuitBoards•TracesonMulti-ChipModules•TracesonIntegratedCircuitPackagesCadence®PSpice®containsdistributedandlumpedlossytransmissionlinesthatcanhelptoimprovethereliabilityofmanyapplications.Foranaloganddigitalcircuits,thereisaneedtoexaminesignalqualityforaprintedcircuitboardandcablesinasystem.Foranalogcircuits,thefrequencyresponseofcircuitswithtransmissionlinescanbeanalyzed.ItisthepurposeofthisarticletoexaminethestepsandissuesinvolvedinmodelingandanalyzingtransmissionlinesinPSpice.ApplicationsFlowchartTheanalysisoftransmissionlinenetsrequiresmultiplesteps.Thesestepsaregiveninthefollowingflowchart:Figure1.Analysisflowchartfortransmissionlinenets.Thisarticleprovidesinformationforthetwocenterblocks,bydiscussingrelevantdevicesandmodelsinPSpice,alongwithspecificmodelingtechniquesandexamples.3ConceptsThissectionpresentsthebasicconceptsofcharacteristicimpedanceandpropagationdelay,andreflectionsandcrosstalk.•Characteristicimpedance,Z0Thecharacteristicimpedanceofatransmissionlineistheratioofthevoltagetothecurrent.Forauniformline,itisinvariantwithrespecttotimeandpositionontheline:IfRandGarezero,thecharacteristicimpedancewillnotdependonfrequency,andreducesto•AttenuationConstantTheattenuationconstantistherealpartofthepropagationconstantandisimportantwhenlossesmustbeconsidered.•PropagationDelayThepropagationdelayisthereciprocalofthephasevelocitymultipliedbythelengthofthetransmissionline:wherecisthespeedoflight,andεristherelativedielectricconstant.Forauniform,losslesstransmissionline.MediumDelay(ps/in.)DielecticConstantAir851.0Coaxcable(75%velocity)1131.8Coaxcable(66%velocity)1292.3FR4PCB,outertrace140-1802.8-4.5FR4PCB,innertrace1804.5AluminaPCB,innertrace240-2708-10Table1.DelayAndDielectricConstantsForSomeTransmissionLines.•ReflectionsWhenavoltagestepistravelingdownauniformimpedancetransmissionline,andthenencountersanabruptchangeinimpedance,aportionoftheincidentenergyis“reflected”back.Theamountofenergyreflecteddependsonthedegreeofimpedancemismatch.Thevoltagereflectioncoefficient,(Z1-Z0)/(Z1+Z0)isameasureofthismismatch:CjGLjRZtyityvωω++==0),(),(CLZ=0clengthtrdε*=LClengthtd*=incidentreflectedVtcoefficienreflectionV*_=4Figure2.Impedancereflectionacrossaboundary.•CrosstalkCrosstalkisundesiredenergyimpartedtoatransmissionlineduetosignalsinadjacentlines.Crosstalkmagnitudeisdependentonrisetime,signallinegeometryandnetconfiguration(typeofterminations,etc.).Quantitatively,thisenergyresultsfrommutualinductancesandcapacitancesintheTelegrapher’sEquations:Crosstalkisoftendiscussedintermsofforwardandbackwardcrosstalkcoefficients.Thesearedeterminedbytheratioofmutualcapacitancetoself-capacitance,andmutualinductancetoself-inductance.Ifadisturbinglinejiscoupledtoavictimlineithatisterminatedinitscharacteristicimpedance,thesecouplingcoefficientsaretVCtVCGVxItVCtVCGVxItILtILRIxVtILtILRIxVMMMM∂∂+∂∂--=∂∂∂∂+∂∂--=∂∂∂∂-∂∂--=∂∂∂∂-∂∂--=∂∂122221111222211141*)(jiijjjiiijijLjLLiCCCKB+=2*)(jjiijiijiijjjiiijijCCLjLLjLLiCCCKF-=5Theseexpressionsarevalidforloosecouplings(KB0.25).ItisclearthatcrosstalkcanbedecreasedbydecreasingmutualcouplingCijandLij,orbyincreasingthecouplingtoground.Figure3indicatestwosignallinesincloseproximitythatarecapacitivelycoupled(CM)andinductivelycoupled(LM).Bothlineshavethesamecharacteristicimpedance,Z0,andarefullyterminatedtoavoidreflections.Onelineis“active”andtransmitsapulse,whiletheotheris“passive”.Atthesourceendofthepassiveline,thecurrentduetoCMandthecurrentduetoLMareadditive.Thesesummedcurrentsproduceavoltagedropofthesamepolarityasthesourcevoltage,termed“NearEnd”or“Backward”crosstalk(ittravelstheoppositedirectiontothesourcepulse).Atthefarendofthepassiveline,thecurrentduetoCMandthecurrentduetoLMareofoppositepolarityproducing“FarEnd”or“Forward”crosstalk.Figure3.Twoparallelcoupledtransmissionlines.L=length.6Figure4.Nearandfarendcrosstalkresultingfromastepinputonanadjacentline.•“Long“vs.“Short”LinesDefiningthepointatwhichaninterconnectshouldbetreatedasatransmissionlineandhencereflectionanalysisappliedhasnoconsensusofopinion.Aruleofthumbiswhenthedelayfromoneendtotheotherisgreaterthanrisetime/2,thelineisconsideredelectricallylong.Ifthedelayislessthanrisetime/2,thelineiselectricallyshort.Hence,thefollowingguidelines:•Lumpedline:tr/Td=4•Shortline:4tr/Td2•Longline:tr/Td=2“Lumped”and“short”linesmayalwaysbemodeledbylumpedcircuits.Thetopicofthenextsectionistodecidehowtobestmodelanelectrically“long”line.7LumpedandDistributedTransmissionLinesIdealandLossyTransmissionLinesTransmissionlinesthatarelossless,thatisR=G=0,aretermedidealtransmissionlines.Thisisvalidifattenuationandskineffectareeithernegligibleornotofconcernforthesignalfrequenciesbeinganalyzed.Forreallines,theseriesresistanceisnotquitezero,andthephasevelocityisslightlydependento