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1、1StepResponseTest——FundamentalsofSignalIntegrityOverviewLeCroyCorporation,inassociationwithSignalConsulting,Inc.,haspreparedaneight-partseriesonthefundamentalsofsignalintegrity.Authoredbytheworld'sforemostauthorityonsignalintegrity,Dr.HowardJohnson,theseries
2、isa“mustread”forengineerswhoneedaclearunderstandingofissuesessentialtohigh-speedperformance.Otherpapersintheseriesincludeconfirmingdiagnosis,uncoveringsourcesofdistortionscausedbyloading,DCloading,andunderstandingadequatebandwidth.Toreadotherpartsintheseries
3、,pleasevisithttp://www.lecroy.comStepResponseTestYourrecentEDNarticle"EyeDon'tLikeIt"pointsoutthataneyediagrammakesaterriblediagnostictool.Theeyeisgoodforcheckingmarginsinafinishedsystem(it'sagoodcompliancemeasurement)butapoordiagnostictool.Igotthatpart.Then
4、yougoontorecommenda"stepresponsetest"asabetterdiagnostictool,butdon'tsayanythingabouthowtouseit.HowdoIdothistest?HowdoIinterprettheresults?---------------------------------------------------------------------------Dr.Johnsonreplies:ThanksforyourinterestinHig
5、h-SpeedDigitalDesign.Thestepresponsetestisusedtocharacterizeadigitalcommunicationchannel.Inthelingoofchanneltesting,Iassumeyouhaveadriveratthesource,alongdigitalchannel(trace,backplane,connector,orwhatever)andareceiveratthefarend,possiblyincludingaterminatio
6、n.ForthisdiscussionIsetupanexamplechannelusingaNationalDS25BR100driver.ThisLVDS-styledriverhaswith100-psrise/falltimesandadjustabletransmitpre-emphasis.Forthistestthepre-emphasisisturnedOFF.Thedriverisconvenientlymountedonademoboard(National161⥟㗙ᔦᴹü∾䖯䖯䆆⼎⊶఼ⱘ
7、ᬙџDS25BR100EVK)withSMAconnectionsgoinginandoutofthedriverchipandlongmicrostriptracesthatareeasilyprobed.Theoutputsideofthedriver,showninFigure1,isour"channelundertest."Thelayoutworkslikethis:Thedriveroutputs(TX+andTX-)connectfirsttoaclosely-spaced,tightly-co
8、upled,100-ohmdifferentialpair.Thetracespacingisdictatedbythepinspacingonthedriverpackage.Thetightly-coupledpairrunsforadistanceof0.5inchesawayfromthedriver.Afterthatshortrunthetracesseparate,mak
