Advanced-PCB-design-and-layout-for-EMC---Part-6-1

AdvancedPCBdesignandlayoutforEMCPart6-Transmissionlines(andanytracescarryinghigh-speedsignalsornoises)ByEurIngKeithArmstrongC.EngMIEEMIEEE,CherryCloughConsultantsThisisthesixthinaseriesofeightarticlesongood-practicedesigntechniquesforelectromagneticcompatibility(EMC)forprintedcircuitboard(PCB)designandlayout.ThisseriesisintendedforthedesignersofanyelectroniccircuitsthataretobeconstructedonPCBs,andofcourseforthePCBdesignersthemselves.Allapplicationsareasarecovered,fromhouseholdappliances;commercial,medicalandindustrialequipment;throughautomotive,railandmarinetoaerospaceandmilitary.ThesePCBtechniquesarehelpfulwhenitisdesiredto…•Savecostbyreducing(oreliminating)enclosure-levelshielding•Reducetime-to-marketandcompliancecostsbyreducingthenumberofdesigniterations•Improvetherangeofco-locatedwirelessdatacomms(GSM,DECT,Bluetooth,IEEE802.11,etc.)•Useveryhigh-speeddevices,orhighpowerdigitalsignalprocessing(DSP)•UsethelatestICtechnologies(130nmor90nmchipprocesses,‘chipscale’packages,etc.)Thetopicstobecoveredinthisseriesare:1.Savingtimeandcostoverall2.egregationandinterfacesuppression3.PCB-chassisbonding4.Referenceplanesfor0Vandpower5.Decoupling,includingburiedcapacitancetechnology6.Transmissionlines7.Routingandlayerstacking,includingmicroviatechnology8.AnumberofmiscellaneousfinalissuesApreviousseriesbythesameauthorintheEMC&ComplianceJournalin1999“DesignTechniquesforEMC”[1]includedasectiononPCBdesignandlayout(“Part5–PCBDesignandLayout”,October1999,pages5–17),butonlysetouttocoverthemostbasicPCBtechniquesforEMC–theonesthatallPCBsshouldfollownomatterhowsimpletheircircuits.Thatseriesispostedonthewebandthewebversionshavebeensubstantiallyimprovedovertheinterveningyears[2].Otherarticlesandpublicationsbythisauthor(e.g.[3][4]andVolume3of[5])havealsoaddressedbasicPCBtechniquesforEMC.Liketheabovearticles,thisserieswillnotspendmuchtimeanalysingwhythesetechniqueswork,itwillfocusondescribingtheirpracticalapplicationsandwhentheyareappropriate.Butthesetechniquesarewell-proveninpracticebynumerousdesignersworld-wide,andthereasonswhytheyworkareunderstoodbyacademics,sotheycanbeusedwithconfidence.Therearefewtechniquesdescribedinthisseriesthatarerelativelyunproven,andthiswillbementionedwhereappropriate.TableofContents,forthisPartoftheseries1MatchedtransmissionlinesonPCBs1.1Introduction1.2Propagationvelocity,Vandcharacteristicimpedance,Z01.3Theeffectsofimpedancediscontinuities1.4TheeffectsofkeepingZ0constant1.5TimeDomainReflectometry(TDR)1.6Whentousematchedtransmissionlines1.7Increasingimportanceofmatchedtransmissionlinesformodernproducts1.8Itistherealrise/falltimesthatmatter1.9Noisesandimmunityshouldalsobetakenintoaccount1.10Calculatingthewaveformsateachendofatrace1.11Examplesoftwocommontypesoftransmissionlines1.12Coplanartransmissionlines1.13Correctingforcapacitiveloading1.14TheneedforPCBtesttraces1.15Therelationshipbetweenrisetimeandfrequency2Terminatingtransmissionlines2.1Arangeofterminationmethods2.2Difficultieswithdrivers2.3Compromisesinlinematching2.4ICswith‘smart’terminators2.5Non-linearterminationtechniques3Transmissionlineroutingconstraints3.1Generalroutingguidelines3.2Atransmissionlineexitingaproductviaacable3.3InterconnectionsbetweenPCBsinsideaproduct3.4ChangingplanelayerswithinonePCB3.5CrossingplanebreaksorgapswithinonePCB3.6Avoidsharpcornersintraces3.7Linkingreturncurrentplaneswithviasordecaps3.8Effectsofviastubs3.9Effectsofroutingaroundviafields3.10OthereffectsofthePCBstack-upandrouting3.11Someissueswithmicrostrip4Differentialmatchedtransmissionlines4.1Introductiontodifferentialsignalling4.2CMandDMcharacteristicimpedancesindifferentiallines4.3ExitingPCBs,orcrossingplanesplitswithdifferentiallines4.4Controllingimbalanceindifferentialsignalling5Choosingadielectric5.1Effectsofwovensubstrates(likeFR4andG-10)5.2OthertypesofPCBdielectrics6Matched-impedanceconnectors7ShieldedPCBtransmissionlines7.1‘Channelised’striplines7.2CreatingfullyshieldedtransmissionlinesinsideaPCB8Miscellaneousrelatedissues8.1A‘safetymargin’isagoodidea8.2Filtering8.3CMchokes8.4Replacingparallelbusseswithserial8.5ThelossinessofFR4andcopper8.6Problemswithcoatedmicrostrip9Simulatorsandsolvershelpdesignmatchedtransmissionlines10EMC-competentQA,changecontrol,cost-reduction11Compromises12References13SomeusefulsourcesoffurtherinformationonPCBtransmissionlines1MatchedtransmissionlinesonPCBs1.1IntroductionMatchedtransmissionlinesallowdigitalandanaloguesignalstobeinterconnectedoverlongdistanceswithoutexcessivelydistortingtheirwaveforms,closingtheireyepatterns,causingsignificantemissions,orsufferingfrompoorimmunity.Modernelectronicproducts,especiallythoseusingdigitalprocessingorwirelesstechnologies,wouldfinditimpossibletoevenfunctionwithouttheuseofmatchedtransmissionlines.Amatchedtransmissionlineisaconductorthatmaintainsafixed‘characteristicimpedance’–whichwecallZ0–fromitssourcetoitsload,andthenitssourceand/orloadendsareterminated(matched)inaresistancehavinganumericalvalueequaltoZ0.MorethanthirtydifferenttypesoftransmissionlinecaneasilybecreatedonaPCBsimplybycontrollingtr