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1、ApplicationNoteTDRImpedanceMeasurements:AFoundationforSignalIntegrityIntroductionSignalintegrityisagrowingpriorityasdigitalsystemdesignerspursueever-higherclockanddataratesincomputer,communications,videoandnetworksystems.Attoday’shighoperatingfrequencie
2、s,anythingthataffectsasignal’srisetime,pulsewidth,timing,jitterornoisecontentcanimpactreliabilityatthesystemlevel.Toensuresignalintegrity,itisnecessarytounderstandandcontrolimpedanceinthetransmissionenvironmentthroughwhichthesignalstravel.Mismatchesandv
3、ariationscancausereflectionsthatdecreasesignalqualityasawhole.ImpedancetolerancesarepartoftheelectricalspecificationsforThemethodologyofchoiceformeasuringimpedancesisTimemanyoftoday’sdigitalsystemcomponents,includingRambus®,USBDomainReflectometry(TDR),c
4、arriedoutusinghigh-performance2.0(UniversalSerialBus),Firewire(IEEE1394standard)andmore.instrumentssuchastheTDS8000oscilloscopeequippedwiththeItisstandardpracticetousemodelingtoolstodesignhigh-speed80E04TDRsamplingmodule.TDRpermitsthesignaltransmissionc
5、ircuits.Modelinghastensthedesigncycleandminimizeserrors.environmenttobeanalyzedinthetimedomainjustasthesignalHowever,modeleddesignsmustbeverifiedwithhardwaremeasure-integrityofdatasignalsisanalyzedintimedomain.ments,includingimpedancemeasurements,aftert
6、heprototypeisbuilt.1www.tektronix.com/scopes/UsingTDRtoHelpSolveSignalIntegrityIssuesApplicationNoteWhatisTimeDomainReflectometry?ForafixedterminationZL,ρcanalsobeexpressedintermsoftheTimeDomainReflectometry(TDR)measuresthereflectionsthatresulttransmiss
7、ionlinecharacteristicimpedance,ZOandtheloadimpedanceZL.fromasignaltravellingthroughatransmissionenvironmentofsomeVreflected(ZL-Z0)kind—acircuitboardtrace,acable,aconnectorandsoon.TheTDRρ==Vincident(ZL+Z0)instrumentsendsapulsethroughthemediumandcomparest
8、hereflectionsfromthe“unknown”transmissionenvironmenttothoseproducedbyastandardimpedance.AsimplifiedTDRmeasurementNowthatwehavetheformulas,ifwepluginnumbersrepresentingablockdiagramisshowninFigure1.matchedload,ashortcircuitandanop
