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1、RFandMicrowaveBasicsImpactPCBDesignItisagiventhatprintedcircuitboarddesignsareutilizinghigherfrequenciestomeetperformancedemands.Asdataratesincrease,theresultingbandwidthrequirementsaredrivingtheupperlimitofsignalfrequencyto1GHzandbeyond.Andwhilethisisafarshotfrommillimeterwavetechn
2、ology(30GHz),itisindeedRFandlow-endmicrowave.RFrequiresadesignengineeringapproachthataddressestheassociatedstrongerelectromagneticfieldeffectswhichnaturallyoccuratthesehigherfrequencies.Thesefieldscaninducesignalsinadjacentsignallines,orPCBtraces,creatingundesirablecrosstalk(interfe
3、renceandoverallnoise),underminingsystemperformance.Returnloss(signalreflectedbackintotheincidentoncomingsignal)isprimarilycausedbyimpedancemismatchandhasmuchthesameimpactofaddednoiseandinterferencetotheprimarysignal.Therearetwoeffectsofhighreturnloss,bothofwhicharebadnews.First,sign
4、alreflectionbacktowardsthesourceaddsnoisetothesystem,makingitmoredifficultforthereceivertodistinguishnoisefromthesignal.Second,anyreflectedsignalisfundamentallyadegradationofthesignalitselfsincethe“meaning”orshapeoftheinboundsignalcanbealtered.Whileadigitalsystemcanbefarmoreforgivin
5、gsinceitisonlyattemptingtorecognizeaoneorazero(onoroff),theuseofharmonicsforfasterpulserisetimesinvolvesweakersignalsathigherfrequency.And,whilewecanimplementforwarderrorcorrectiontechnologytofixsomeoftheseeffects,theresultissystemdegradationascapacitygetsconsumedinredundanttransmis
6、sions.AmuchbetteransweristounderstandandengineertheRFeffectstohelp,nothurt,yoursignalmanagementassignment.Overallrecommendedtargetvaluesforreturnlossareminus25dBatthehighestfrequencyofinterest(usuallytheworse-casedatapoint),whichconvertstoabout1.1VSWR.TraditionalPCBdesignhasbeendriv
7、enby"smaller,fasterandcheaper".AtRFfrequenciesonaPCB,"faster"doesnotalwaysallowfor"smaller"duetosomerealitiesofRFsignalmanagementdesign:1.Theprimarywaytomanageunwantedcrosstalkisbygroundplanemanagement,trace-to-tracespacing,and/orreductionofstubinductance.2.Theprimarywaytoreduceretu
8、rnlossistomatchimpe
