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12/4/2002DifferentialSignalingIntroductionReadingChapter6212/4/2002DifferentialSignalingAgendaDifferentialSignalingDefinitionVoltageParametersCommonmodeparametersDifferentialmodeparametersCurrentmodelogic(CML)bufferRelatetoparametersModeling&simulationTimingparametersClockrecoveryEmbeddedclockACcouplingCommonmoderesponseIssueswithsimulation8B10BencodingDCbalancedcodesDutyCycledistortionCycle312/4/2002DifferentialSignalingSingleEndedSignalingAllelectricalsignalcircuitsrequirealooporreturnpath.Singleendedsignalsubjectseveralmeansofdistortionsandnoise.Groundorreferencemaymoveduetoswitchingcurrents(SSOnoise).Wetouchedonthisinthegroundconundrumclass.Asingleendedreceiveronlycaresaboutavoltagethatisreferencedtoitsownground.Electromagneticinterferencecanimposevoltageonasingleendedsignal.Signalpassingfromoneboardtoanotheraresubjecttothelocalgrounddisturbance.Wecancounteractmanyoftheseeffectbyaddingmoreground.Asfrequenciesincreasebeyond1GHz,80%ofthesignalwillbelost.412/4/2002DifferentialSignalingReviewofthresholdsensitivityThewaveisreferencedtoeitherVccorVss.ConsequentlytheeffectiveDCvalueofthewavewillbetiedtooneoftheserails.ThewaveisattenuatedaroundtheeffectiveDCcomponentofthewaveform,butthereferencedoesnotchangeaccordingly.Hencetheclocktriggerpointbetweenvariousclockloadpointsisverysensitivetodistortionandattenuation.TxVssVrefVssRx2VrefLonglineVssRx1VrefShortline512/4/2002DifferentialSignalingDifferentialSignalingAnysignalcanbeconsideredaloopiscompletedbytwowires.Oneofthe“wires”insingleendedsignalingisthe“groundplane”DifferentialsignalingusestwoconductorsThetransmittertranslatesthesingleinputsignalintoapairofoutputsthataredriven180°outofphase.Thereceiver,adifferentialamplifier,recoversthesignalasthedifferenceinthevoltagesonthetwolines.AdvantagesofdifferentialsignalingcanbesummedupasfollowsDifferentialSignalingisnotsensitivetoSSOnoise.Adifferentialreceiveristolerantofitsgroundmovingaround.Ifeach“wire”ofpairisoncloseproximityofoneandother.electromagneticinterferenceimposesthesamevoltageonbothsignals.Thedifferencecancelsouttheeffect.SincetheACcurrentsinthe“wires”areequalbutoppositeandproximal,radiatedEMIisreduced.Signalspassingfromoneboardtoanotherarenotsubjecttothelocalgrounddisturbances.Asfrequenciesincreasebeyond1GHz,upto80%ofthesignalmaybelost,butdifferencestillcrosses0volts.Therearestilllossissuesfordifferentialsignalingbutonlycomeintoplayinhighlosssystem.Mostsingleendedsystemsassumeapproximately15%channelloss.612/4/2002DifferentialSignalingDifferentialSignaling-ConsThecostisdoublingthesignalwires,butthismaynotbesobadascomparedtoaddinggroundstoimprovesingleendedsignaling.Routingconstraint:Pairsignalsneedtoberoutedtogether.Differentialsignalhavecertainsymmetryrequirementsthatmayposeroutingchallenges.712/4/2002DifferentialSignalingDifferentialSignalParametersVoltageonline1=aVoltageonline2=bDifferentialvoltaged=a-bCommonmodevoltagec=(a+b)/2Oddmodesignal,o=(a-b)/2Evenmodesignal,e=(a+b)/2Signalonline1a=e+oSignalonline2b=e-oUsefulrelations;o=b/2;e=cLine1Line2Reference812/4/2002DifferentialSignalingPropagationTermstoConsiderDifferentialmodepropagationCommonmodepropagationSingleendedmode(uncoupled)propagationThisiswhentheotherlineisnotdrivenbutterminatedtoabsorbedreflections.Transmissionlinematrixeswillreflectthesemodes.912/4/2002DifferentialSignalingDifferentialMicrostripExampleZ0ZseSse1.992nsftSseL11C11Zse39.744ZseL11C11Sc1.996nsftScL11L21()C11C21()Zcomm47.422ZcommL11L21C11C21Sd1.924nsftSdL11L21()C11C21()Zdiff66.185Zdiff2L11L21C11C21inductanceandcapacitancematrixesC11C12C21C22L11L12L21L22TransmissionlineC22C11C12C21C216.69000010-13faradinC114.17700010-12faradinL22L11L12L21L211.29100010-9henryinL116.59800010-9henryinSE:singleended=uncoupled1012/4/2002DifferentialSignalingDifferentialImpedanceCouplingbetweenlinesinapairalwaysdecreasesdifferentialimpedanceDifferentialimpedanceisalwayslessthat2timestheuncoupledimpedanceDifferentialimpedanceofuncoupledlinesis2timestheuncoupledimpedance.1112/4/2002DifferentialSignalingPropagationVelocitiesForTEMstructures,(striplines)Differentialmode,CommonMode,andsingleendedvelocitiesarethesameForNonTEMandQuasi-TEMstructures(microstrip)Differentialmode,CommonMode,andsingleendedvelocitiesandimpedancesarenotthesame.Commonmodecanbeconvertedtodifferentialmodeatareceiverandresultinadifferentialsignaldisturbance.1212/4/2002DifferentialSignalingExampleofCommonModeLine1andline2havethesameDCoffset.ThisisDCcommonmode.ItcanbedefinedasanaverageDCfortimedurationofmanyUIcyclesvalueaswell.Line1andline2havethesameACoffsetThisisACcommonmodeACcommonmodealsoresultfromtimedifferences(skew)betweensignalonline1andline2.ThiscanresultinACcommonmodeanddifferentialsignalloss.Thefollowingslidewillbeusedtoclarifytheabove1312/4/2002DifferentialSignalingDifferentialSignalingBasicsForlongchannels,atGHzfrequencies,signaltendlooklikesinewaves.Theartificialoffsetcommontoline1and2hasanaverageof1andvariesaroundthat