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1、TheEffectsofViaTransitionsonDifferentialSignalsChenWang,JunFan*,JamesL.Knighten*,NormanW.Smith*,RayAlexander*,JamesL.DrewniakElectromagneticCompatibilityLaboratoryUniversityofMissouri-Rolla,Rolla,MO65409Tel:(573)341-4969Fax:(573)341-4532cwang(n?umr.edu,drewniak@ece.umr.edu*NCRCorporation17095Vi
2、adelCampo,SanDiego,CA92127Tel:(858)485-3167Fax:(858)485-3788jun.fanO.ncr.comAbstractViasindifferentialtransmissionlineshavebeenmodeledusingthefinite-differencetime-domain(FDTD)method.Thevelocitythatthedifferentialsignalpropagatedthroughtheviaswasestimated.DifferentialS-parameterswerecalculatedu
3、pto50GHz.Below10GHz,thedifferentialsignalcanpropagatethroughviaswithoutmuchreflectionanddistortion.However,asfrequencyincreases,thereflectionfromtheviasandthelossofdifferentialpowerbecomesignificant.IntroductionTheeffectofsignaltransmissionthroughviascanbesignificantasoperatingfrequenciesonprin
4、tedcircuitboard(PCB)increaseintoGHzrange.Differentialsignalingisoftenemployedinthehighfrequencyrange.However,useofviasindifferentiallinesmayleadtodegradationofthedifferentialsignalduetoreflectionfromtheviasandexcitationofpowerbusnoise,etc.FDTDhasbeensuccessfullyappliedtoviainvestigations[1-41.M
5、oststudiesconsideredviasinsingle-endedsignalingconfigurations.Viaholesindifferentiallineweremodeledin[5]asacascadeofcapacitancesandinductancesusingaboundaryintegralequationtechniquetocalculatethecapacitance.Inthiswork,theFDTDmethodwasappliedtoinvestigatethebehaviorofviasindifferentialsignaling.
6、NumericalApproachTheFDTDmethodwasemployedtomodelviasindifferentialtransmissionline.Figure1showsa4-layerboardgeometry.Layers2and3arereferenceplanes,andlayers1and4aresignalplanes.Apairofmicrostriplineswasroutedonlayers1and4.Eachmicrostriplinewas12milswideandwas7milsaboveitsreferenceplane.Thediele
7、ctricconstantofthesubstratewas4.3.Dielectriclosseswerenotconsidered.Thespacingbetweenthetwolinesfromedgetoedgewas12mils.TheoddandevenmodecharacteristicimpedancescalculatedusingAnsoftXFXwereZoo=49RandZ,,=54R,respectively.Thelineson
