Advanced-PCB-design-and-layout-for-EMC---Part-6-2

AdvancedPCBDesignandLayoutforEMCPart6-TransmissionLinesEurIngKeithArmstrongC.EngMIEEMIEEE,CherryCloughConsultantsDuetoitslengthwehavehadtosplitPart6betweenthreeissues.ThisissuecontainsSection2-TerminatingtransmissionlinesandSection3-Transmissionlineroutingconstraints.Thisisthesixthinaseriesofeightarticlesongood-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.Segregationandinterfacesuppression3.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,forthisPartoftheseriesSection1iscontainedinIssue57,March20051MatchedtransmissionlinesonPCBs1.1Introduction1.2Propagationvelocity,Vandcharacteristicimpedance,Z01.3Theeffectsofimpedancediscontinuities1.4TheeffectsofkeepingZ0constant1.5TimeDomainReflectometry(TDR)1.6Whentousematchedtransmissionlines1.7Increasingimportanceofmatchedtransmissionlinesformodernproducts1.8Itistherealrise/falltimesthatmatter1.9Noisesandimmunityshouldalsobetakenintoaccount1.10Calculatingthewaveformsateachendofatrace1.11Examplesoftwocommontypesoftransmissionlines1.12Coplanartransmissionlines1.13Theeffectsofcapacitiveloading1.14TheneedforPCBtesttraces1.15TherelationshipbetweenrisetimeandfrequencyInthisissue:2Terminatingtransmissionlines2.1Arangeofterminationmethods2.2Difficultieswithdrivers2.3Compromisesinlinematching2.4ICswith‘smart’terminators2.5Bi-directionalterminations2.6Non-linearterminationtechniques2.7‘Equalising’terminations3Transmissionlineroutingconstraints3.1Generalroutingguidelines3.2Atransmissionlineexitingaproductviaacable3.3InterconnectionsbetweenPCBsinsideaproduct3.4ChangingplanelayerswithinonePCB3.5CrossingplanebreaksorgapswithinonePCB3.6Avoidsharpcornersintraces3.7Linkingreturncurrentplaneswithviasordecaps3.8Effectsofviastubs3.9Effectsofroutingaroundviafields3.10OthereffectsofthePCBstack-upandrouting3.11Someissueswithmicrostrip2TerminatingtransmissionlinesAtransmissionlinemustbeterminatedatitsendswithresistorsthatmatchitscharacteristicimpedance.Onlythendoesitbecomea‘matchedtransmissionline’,andonlythendoalloftheSIandEMCbenefitsarise.Wheretherise/falltimeislongenough–orthehighestfrequencyofconcernlowenough–theentirelengthofatracewithallofitslowimpedancediscontinuities(e.g.duetocapacitiveloads)andhigh-impedancediscontinuities(e.g.duetoperforationofthereferenceplane)canbetreatedasoneaveragedimpedance.Thisisthentheimpedancethatmustbematchedbytheterminatingresistors.Butwheretherise/falltimeisshortenoughforonesectionofatracetohaveasignificantlydifferentcharacteristicimpedancefromanothersection(seethe300pscurveonFigure6E),itwillbeimpossibletomatchthetransmissionline.Thecharacteristicsofthetraceanditsloads,referenceplanesandotherphysicalfeaturesmustbemodifiedsothateachsectionhasthesamecharacteristicimpedanceasisdesired,thenthisisterminatedinmatchedresistorsattheendsoftheline.VariationsinimpedancethatareconsideredacceptableforSIpurposesmaybetoohighforgoodEMC.So,forgoodEMC,takemorecareinmatchingtheimpedancesalongthelengthofatrace,andatitsterminatingresistors.2.1ArangeofterminationmethodsRFandmicrowavedesignersuseverynarrowfrequencyranges,sotheycanterminatetheirtransmissionlinesincompleximpedancesthatarecarefullydesignedtodoexactlywhattheywantt