HighPerformance Analog Applications ABSTRACT The optical module offers an effective highspeed solution for growing telecom market Data rates range from 155 Mbps to Gbps and even up to 10 Gbps Transmitter optical subassemblies TOSAs and laser drivers ID: 24888
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ApplicationReport SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatching DesigninOpticalModules DanielLong................................................................................High-PerformanceAnalogApplications ABSTRACT Theopticalmoduleoffersaneffectivehigh-speedsolutionforagrowingtelecommarket.Dataratesrange from driversmayhavedifferentresistancesinagivenapplication,sothereflectioncouldbeworseifthe designerdoesnotuseanimpedancetransfercircuittoabsorbit.Additionaluncertainnoiseandreflection couldalsocomefrompoorprintedcircuitboard(PCB)layoutaswell.Thisreportdiscusses impedancetransfercircuitwhenweconnectamismatchedtraceandnon-terminatedTOSA,aswellas whatweshouldtakeintoconsiderationwhenwelayoutthePCBforopticaldesign.Thisdocument primarilyreferstotheONET8501VVCSELdriverandtheONET1101LDFBdriverasthedevicesunder consideration. Contents 2ImpedanceTransferCircuitConsiderations.............................................................................3 3ExamplesforImpedanceTransferAnalysis.............................................................................5 4TestResults.................................................................................................................7 5PCBLayoutConsiderationsforOpticalModules.......................................................................9 6Summary 7References.................................................................................................................11 ListofFigures 1ReflectionPointsonanOpticalModule..................................................................................2 2InternalStructureofTerminatedTOSA.................................................................................3 3OpticalModuleDriveModes..............................................................................................4 5ImpedanceTransferforMismatchedSourceandLoad...............................................................5 6EyeDiagramAftertheImpedanceTransferCircuit....................................................................6 7ImpedanceTransferCircuitforTOSAandDriver......................................................................6 8ONET1101LTestResults:PoorPerformance 9ONET1101LTestResults:ImprovedPerformance.....................................................................8 10LossvsLengthandFrequency...........................................................................................9 11PhysicalGeometriesofDifferentialTraces..............................................................................9 12PCBTraceStubsandDiscontinuities..................................................................................10 13DifferentialSignalPairLengthsonPCBs 14PCBTraceLayoutwithRadialBends..................................................................................11 Alltrademarksarethepropertyoftheirrespectiveowners. 1 SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated Introductionwww.ti.com 1Introduction Theopticalmoduleoffersanattractivehigh-speedsolutionforagrowingtelecommarket.Datarates rangefrom155Mbpsto6Gbpsandarenowapproaching10Gbps.Insuchultrahigh-speedfrequency areas,closeattentionmustbepaidtothePCBlayoutandimpedancematchingbecausetheseissuescan seriouslyaffecttheoutputperformanceanddestroytheresults. Generally,impedancematchingismodeledbysoftwaresimulationormanualcomputations.However, opticalmodulesareanapplicationwithseveralconstrainingfactors:frequencyoverGbps;variationsinthe laserdrivermodel;theactualtransmissionlines;and,mostimportantly,thelaserTOSA.Thesefactors oftenmakeitdifficulttosimulateimpedancematchingprecisely.Asaresult,evenwithgoodmodelsto predicttheoperatingconditionswithrelativeaccuracy,designersoftencannoteasilyobtainresultsthat matchwellwithactualmeasurements. Intheopticaldesignapproachdiscussedinthisdocument,impedancemismatchresultsinreflection.We willusea10-Gdirectmodulaterlaser(DML)moduleboardasourexample.Figure1showsthefour reflectionpointsthatoccurwhenusingthistypeofDMLboard. Figure1.ReflectionPointsonanOpticalModule Thetechniquediscussedhereincludesananalysisofthesignalpaththatcangeneratereflectionsfrom thedrivertothelaser(TOSA)infourareas: 1.TheICpintothetrace 2.ThePCBtraceonboard 3.ThePCBtracetotheflexcable 4.TheflexcabletoTOSA(ROSA)elements Atpoint2,thereflectionisprimarilygeneratedbythePCBlayout.Fortheotherpoints,thereflectionsare aresultofimpedancemismatching.Atanimpedancemismatch,aportionofthetransmittedsignalis reflectedbacktowardsthesignalsource,anditcanpropagatebackandforthinthetraceuntilitis attenuated.Thereflectedsignalscaninterferewiththeprimarytransmittedsignal,increasingthedatajitter andreducingtheSNR.ThereflectedenergycanbeterminatedbyadriverbasedontheSDD22 specification.Ifthereflectedenergyistoohigh,weneedanadditionalcircuittooptimizetheimpedance matchingsothatthereflectedenergycanbeabsorbed. 2 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical SLLA311February2011 Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated www.ti.comImpedanceTransferCircuitConsiderations 2ImpedanceTransferCircuitConsiderations Itiswellknownthattheamountofreflectedsignalfromtheloaddependsonthedegreeofmismatch betweenthesourceimpedanceandtheloadimpedance.Thereflectioncoefficientexpressionisdefined asshowninEquation1: (1) Inthisequation,Z0isthetransmissionlineimpedance;thisfactorisusuallyaconstantwithanindustry normalizedvaluesuchas50\ror100\r.Inamatchedsystem,whentheloadimpedanceZLmatchesthe transmissionlineimpedance,+L=0andwehavezeroreflection.Thisobjectiveisthetargetofthecircuit design. Inthereceiverportionoftheopticaldesign,theinputandoutputofthelimitingamplifierandtheoutputof thetransimpedanceamplifierareall100-\rdifferentialimpedance;thus,thetransmissionlinesare100-\r differential,andmatchingisrelativelyeasy. However,thetransmitterpartisnotsosimpletodesign.Adistributedfeedback(DFB)orFabry-Perot(FP) laserisalow-resistancecomponent,typicallyrangingfrom7\rto10\r.TheflexiblePCBcableis commonlya25-\r,single-endedtrace.Thereisareflectionattheinterconnectionbetweentheflexcable andtheTOSAatpoint4;refertoFigure1.Forreducingsuchreflection,sometypesofTOSAsintegrate resistorstoincreasetheTOSAimpedance,asillustratedinFigure2. Figure2.InternalStructureofTerminatedTOSA SuchTOSAsareintendedforsingle-endedapplications.Thetotalinternalresistanceiscloseto25\rand thetransmissionlinecanbemoreeasilymatchedtotheTOSAwithaminimumamountofreflection. However,manyopticaldesignersprefertousedifferentialmoderatherthansingle-endedmodetoavoid issueswithcommon-modenoiseandelectromagneticinterference(EMI). 3 SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated ImpedanceTransferCircuitConsiderationswww.ti.com Figure3comparesanopticalmoduledriverinbothsingle-ended(a)anddifferential(b)modes. Figure3.OpticalModuleDriveModes Inordertomatchthe50-\rdifferentialtransmissionlinesindifferentialmode,afully-matchedTOSAwith two20-\rinternalresistors,togetherwiththelaserresistance,resultsinaloadresistancecloseto50\r. Onepotentialproblemisthattheresistorsconsumecurrentandheatthelaser,whichreducestheoverall efficiencyofthelaser.Thedrivermustsupplymoremodulationandbiascurrenttomaintaintheextinction ratio(ER)andaverageopticalpower(PAV)settingbecauseofthehighertemperatureofthelaser.The greaterpowerconsumptionmakessuchfully-matchedTOSAsverydifficulttouse,especiallyforSFP+ modulesthatrequireatotalpowerdissipationoflessthan1W.Becauseofthispowerconsumptioneffect, manycustomershaveswitchedtounmatchedTOSAswithoutinternalresistors.TheTOSAreflectionis inevitableandreliesonthedriverorimpedancetransfercircuittoabsorbthereflectedenergy.Inrecent years,mostopticalcustomershavetransitionedtheirdesignstouseunmatchedTOSAs. 4 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical SLLA311February2011 Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated www.ti.comExamplesforImpedanceTransferAnalysis 3ExamplesforImpedanceTransferAnalysis Thissectionreviewsseveralexamplesfortheimpedancetransferanalysis. 3.1Impedance-MatchedSystems Animpedance-matchedsystem,suchasthatillustratedinFigure4,doesnotneedanimpedancetransfer circuit. Figure4.MatchedSystem IntheONET8501Vapplicationpresentedhere,theoutputimpedanceis100-\rdifferential;the transmissionlineisalso100\r,andtheloadis100-\rVCSELTOSA.Theresistancematchesverywell andproducesminimumreflection. 3.2ImpedanceTransferforMismatchedSourceandLoad Figure5illustratestheimpedancetransferconfigurationforamismatchedsourceandload. Figure5.ImpedanceTransferforMismatchedSourceandLoad 5 SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated ExamplesforImpedanceTransferAnalysiswww.ti.com Thelaserdriveris50-\rdifferentialoutputandthetestloadimpedanceis100-\rdifferentialinorderto transfertheimpedancebetweenthesourceandload,usingtheresistornetworktobuildupthe impedancetransfercircuit.Groundingwith35-\rand220-\rresistorsanda50-\rseriesresistor,the greatestamountofreflectioncouldbeattenuatedbythiscircuit.Thetransferequationillustratedin Figure5isgiveninEquation2: (2) ZLOADis100-\rdifferential;therefore,wecalculateZOUTbyusinghalfofZLOAD.ThefinalZOUTvalueis27\r, whichisveryneartothefronttransmissionlineresistance.ThisZOUTcanevengeneratetheeyediagram showninFigure6. Figure6.EyeDiagramAftertheImpedanceTransferCircuit 3.3ImpedanceTransferforTOSAandLaserDriverMatching Figure7showstheONET1101LdrivinganunmatchedTOSAindifferentialmode. Figure7.ImpedanceTransferCircuitforTOSAandDriver TheONET1101Llaserdiodedriverisoptimizedtodrivea50-\rdifferentialoutputtransmission impedance.Foralow-powerdesign,ithasa500-\rdifferentialbacktermination.BecausetheLRDFB TOSAhasonly10-\rimpedance(approximately),thereflectionismuchworseifthereisnotransfercircuit added.Thepurposeofthetransfercircuitistodrivetheloadimpedancecloseto50\rfromthedriver side.FromFigure7,wecanderiveEquation3: ZOUT=R1//(R3+R4+R2//RTOSA)(3) Withtheresistorvaluesshown,ZOUTisapproximately35\r. 6 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical SLLA311February2011 Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated www.ti.comTestResults Wecanthenanalyzethetransfercircuitinthefollowingmanner: 1.R1combineswiththeinternaldriverdifferentialresistancetocreateanewbacktermination:ZS, highlightedintheblueoutlinedarea(1)inFigure7.ThenewZSwillbe90\r.Forbestresults,R1 shouldconnectascloselyaspossiblewiththeICpins. 2.R2combineswiththeTOSAimpedancetoformanewZL,shownintheblueoutlinedarea(2)in Figure7.ThenewZLwillbeverynear9\r.Withthetwoserial20-\rresistors,theimpedanceis approximately50\r,whichmatchesthetransmissionlineimpedance. 4TestResults Intheactualtest,thevaluesofresistorsR1andR2canbeadjustedtoachievethebesteyediagramwith thelowestpower.Smallerresistorvaluescansometimesimprovetheoutputperformance,but correspondinglyincreasethepowerconsumption. Figure8andFigure9showvarioustestresultswiththeONET1101LandNX8341TBTOSA(2×10-\r internalresistors). Figure8.ONET1101LTestResults:PoorPerformance 7 SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated TestResultswww.ti.com Figure9.ONET1101LTestResults:ImprovedPerformance Fromthesetests,wecanseethattheimpedancetransfercircuitworksquitewelltoimprovethemodule reflectionperformance,althoughthecircuitalsoincreasesthepowerconsumptionatthesametime.For differentTOSAsandpowerconsumptionrequirements,wecanadjusttheresistorsvaluestoachievethe bestperformance. 8 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical SLLA311February2011 Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated www.ti.comPCBLayoutConsiderationsforOpticalModules 5PCBLayoutConsiderationsforOpticalModules Inanopticalmoduledesign,PCBlayoutmustbedoneverycarefullybecauseofthehigh-speedsystem. Severaladditionalfactorsmayaffectthehigh-speedsignalintegrity. 5.1TraceDimensions Inhigh-speedPCBtraces,transmissionlinelossesdependonthefrequencyandtheabsorptionof electricalenergybythedielectric.Figure10showsthesignallosswhenoperatiingwithdifferenttrace lengthsandatdifferentsignalfrequencies. Figure10.LossvsLengthandFrequency 5.2TypesofTrace(MicrostripvsStripline) Figure11showsthecomparisonbetweenthephysicalgeometriesofthemicrostripandthestripline differentialtracelayouts. Figure11.PhysicalGeometriesofDifferentialTraces 9 SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated PCBLayoutConsiderationsforOpticalModuleswww.ti.com Equationscanalsobeusedtocalculatethetransmissionlineimpedanceforadifferentialsignalpair;note thatsoftwaretoolsaremoreaccurate,however.Wecanwecalculatethetwotypesoftransmissionline impedancebasedontheformulasshowninTable1. Table1.ImpedanceEquationsforMicrostripandStriplineTraceTechniques Microstrip Stripline (4) (5) (6) (7) (8) (9) 5.3PCBRouting TherearealsoseveralcriticalaspectsofPCBroutingthatcanaffecttheimpedancematching. Avoidstubsanddiscontinuities(similartothoseillustratedinFigure12)inthePCBtraces. Figure12.PCBTraceStubsandDiscontinuities Thedifferentialsignalpairsshouldbethesamelength.Figure13comparesdifferentialsignalpairsof thesamelength(a)anddifferentlengths(b).Ifthedifferentialsignalpairsarenotthesamelength,the logicswitchestozeroatdifferenttimes.Consequently,asshowninFigure13(b),anoisepulsewill occuronthereceiver. Figure13.DifferentialSignalPairLengthsonPCBs 10 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical SLLA311February2011 Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated www.ti.comSummary Useradialbends,showninFigure14,ifpossible.AradialbendisthebestoptionforPCBlayoutina module. Figure14.PCBTraceLayoutwithRadialBends R�5Wisrequiredtokeepthetraceimpedanceunchanged.45-degreechamferedcornershavesmaller discontinuitythanright-anglebends. 6Summary Foropticalmoduletransmitterapplications,somereflectionisinevitablebecauseofthesmalllaser impedance.AtransfercircuitcanbeaddedbetweenthelaserdriverandtheTOSAtooptimizethe matchingandimprovetheoverallmoduleperformance. ParticularattentionmustalsobepaidtothePCBlayoutforopticaldesigns.Improvementstothetrace designandlayoutcansignificantlyincreasethesystemsensitivityandeyediagramoutputquality. 7References Unlessotherwisenoted,thesedocumentsareavailablefordownloadfromtheTIwebsite(www.ti.com). ONET8501PBproductdatasheet,TexasInstrumentsliteraturenumberSLLS910. ONET8501Vproductdatasheet,TexasInstrumentsliteraturenumberSLLS837B. ONET1101Lproductdatasheet,TexasInstrumentsliteraturenumberSLLS883. OKI10-GDFBOL3356l_z25productdatasheet. NECNX8346TB,NX8341,ADN2526,ADN2525seriesproductdatasheet. Weiler,A.andPakosta,A.(1996).ApplicationandDesignConsiderationsfortheCDC5XXPlatformof Phase-LockLoopClockDrivers.TexasInstrumentsapplicationreport,literaturenumberSCAA028. PCBLayoutGuidelinesforDesigningwithAvagoSFP+Transceivers.Applicationreportfrom www.avago.com. ConsiderationsforHigh-SpeedPCBTrackDesignin10-Gb/sSerialDataTransmission.Application reportfromwww.agilent.com. High-speedPCBdesignconsiderations.Technicalnotefromwww.latticesemi.com. 11 SLLA311February2011 ConsiderationsforPCBLayoutandImpedanceMatchingDesigninOptical Modules SubmitDocumentationFeedback ©2011,TexasInstrumentsIncorporated IMPORTANTNOTICE TexasInstrumentsIncorporatedanditssubsidiaries(TI)reservetherighttomakecorrections,modifications,enhancements,improvements, andotherchangestoitsproductsandservicesatanytimeandtodiscontinueanyproductorservicewithoutnotice.Customersshould obtainthelatestrelevantinformationbeforeplacingordersandshouldverifythatsuchinformationiscurrentandcomplete.Allproductsare soldsubjecttoTIstermsandconditionsofsalesuppliedatthetimeoforderacknowledgment. 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