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ESG89001Electromagneticcompatibilityandprintedcircuitboard(PCB)constraintsJune1989INTEGRATEDCIRCUITSPhilipsSemiconductorsApplicationnoteESG89001Electromagneticcompatibilityandprintedcircuitboard(PCB)constraints2683June19891.INTRODUCTIONTheroutingofthetracesonaPrintedCircuitBoard(PCB)largelyeffecttheElectroMagneticCompatibility(EMC)performanceofthePCBwithrespecttobothElectroMagnetic(EM)radiationassusceptibilitytoEM-fields.ThePCBwillconnectelectroniccomponentssuchaspassivecomponents,transistorsandICs.Furthermore,cablestointerconnectthePCBwithothersystemparts,e.g.,anotherPCB,signalgenerator,CATVwall-outlet,DCpowersourceoranAC-mainsconnection,willlargelyinfluencethePCBwithrespecttoEMC[7].InordertogetaPCBonwhichthecircuitsfunctionproperly,thetracerouting,theplacementofcomponents/connectorsandthedecouplingusedwithcertainICswillhavetobeoptimizedaccordingtotheconstraintsgiveninthisreport.ToreachaneconomicandfunctionalPCBdesign,thefollowingitemshavetobekeptinmind:3.CorrectchoiceofthePCBformat(mono,bi-ormulti-layer)4.Takecarethat“every”signaltracehasitssignalreturnnearby5.ProperdecouplingforeachICorgroupofICs6.Allowedtracelengthsandallowedloopareas7.Placementoftheconnectors8.Rightcablechoicewithaproperconnector9.Properuseandplacementoffiltersandfilterparts.Theseitemswiththeappropriatemeasureswillbefurtherexplained.ThemaintargetistogetcontroloveryourPCBcurrents.2.GENERAL2.1.ConductorsSingleconductorshave,asaruleofthumb,aninductanceof1μH/m.Atlowfrequenciesonly,below1kHz,Rdcapplies.Theseimpedances,togetherwiththecurrentsthatwillflowthroughtheseimpedances,willberesponsibleforthevoltagedropbetweenpointsasOhmslawapplies.Thevoltagedropcanbediminishedbyeitherreducingtheimpedanceorloweringthecurrentthroughthatimpedance.Intypicaldigitaldesignsthevoltagedropwillbefrequencyindependent.Asquarewavecurrent,resultingfromasquarewaveoutputvoltagetoaresistiveload,canbedescribedasaseriesofsinewavesofwhichtheamplitudeoftheharmonicsdecreaseproportionalwiththefrequency(Fourierexpansions),seeFigure1b.Theimpedanceoftheinductorincreasesproportionalwithfrequency(seeFigure1a),thereforetheproduct;voltagedrop(Figure1c)remainsconstant.Whenthecurrenthasatriangularwaveshape,asfunctionoftime,duetocapacitiveloading,theamplitudeoftheharmonicsdecreaseswiththefrequencysquareandthevoltagedropacrosstheinductorreducesproportionalwithfrequency.2.2.TransmissionlinesByusingtheinductanceofasinglewire,Li,themutualcoupling,M,andthecapacitancebetweenthetraces,Ci,atransmissionline,showninFigure2,canbedefinedofwhichthecharacteristicimpedance,ZO,equals:ZO=√(Leff/C)where:Leff=L1+L2–2⋅M,k=√(L1+L2)/MandC=C1+C2.Whenthecoupling,k,betweenthetracesofthetransmissionlineishigh,theeffectiveinductancewilldecreaserapidly.SomecouplingfactorsaregiveninTable1.Anindifferentsignalpathdesign(Figure3a)canbechangedintoatransmissionlinedesign(Figure3b).Thischangewilllowertheeffectiveinductance,Leff,betweenthetwocircuitblocksandwillthereforelowerthevoltagedropbetweenthetworeferencesofthosecircuits.Table1.CouplingFactorsbetweentheConductorsofaTransmissionlineTRANSMISSIONLINETYPECOUPLINGParallelwires0.5–0.7Bi-layerPCB0.6–0.9Multi-layerPCB0.9–0.97Coaxialcable0.8–1.0RG-58coax0.996Z=f(freq)I=g(freq)V=h(freq)––f––f––f×=×=ZIVFigure1.TherelationbetweenvoltagedropasaresultofcurrentandimpedanceasfunctionoffrequencyPhilipsSemiconductorsApplicationnoteESG89001Electromagneticcompatibilityandprintedcircuitboard(PCB)constraintsJune19892684C1ML1L2R1R2C2Figure2.AsegmentofatransmissionlineanditsnetworkelementsFigure3.TypicalsignalpathdesignonaPCB(a)IndifferentsignalpathNOcouplingbetweenS↔VEE,VCC(b)TransmissionlinesignalpathGOODcouplingbetweenS↔VEE,VCCSignallineSignallinePCB#1PCB#2VEE,VCCVEE,VCCFigure4.TypicalapplicationsofthePCB-formatsinglelayer:d(S1↔GND)d(S2↔S1)(a)(b)S2S1GNDS1GNDS2bi-layer:(c)d(S1↔GND)d(S2↔S2)or(d)d(S1↔GND)andd(S2↔GND)d(S1↔S2)(c)(d)S1S2S1S2GNDS2GROUNDPLANEmulti-layer:d(Si↔VEE)ord(Si↔VCC)d(Si↔Sj)1≤i,j≤numberoftraces(e)5-layer(f)4-layerS1S2S1S2S4S5VEEVCCS3S3VEEVCC2.3.CapacitiveandInductiveCouplingSeparately,thecapacitiveandinductivevalues,derivedfromthedefinitionofthetransmissionline,canalsobeusedtocalculatethecrosstalkbetweenadjacenttraces,notbeingafunctionsignalpath.Thecapacitivecoupling,representingandinducedcurrent,isgivenby:ICk=1/Ck⋅dV/dt,where:Ck=couplingcapacitancebetweenadjacenttraces;inpractice:100pF/m(dependsuponthevicinityofothertraces,seeAppendixA),andtheinductivecoupling,representinganinducedvoltage,isgivenby:VMk=Mk⋅dI/dt,where:Mk=mutualcouplingbetweentwotraces(ForfurtherdetailseeChapter4.)Inbothcouplingmodes,thetransferfunctionwilltypicallyshowahighpassbehavior.3.CHOICEOFTHEPCB-MATERIALByaproperchoiceofthePCB-materialandtheroutingofthetraces,agoodtransmissionlinewithlowcouplingtoothertracescanbecreated.Lowcoupling,orlittlecrosstalk,canbeobtainedwhenthedistance,d,betweenthetransmissionlineconductorsislessthantheirdistancetootheradjacentconductors(seeFigure4).ByusingtheseexamplesofgeometryoftracesthedefinitionofthetransmissionlinebetweenS1,S2,Si,jand(S