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1、RFandMicrowaveBasicsImpactPCBDesignItisagiventhatprintedcircuitboarddesignsareutilizinghigherfrequenciestomeetperformancedemands.Asdataratesincrease,theresultingbandwidthrequirementsaredrivingtheupperlimitofsignalfrequencyto1GHzandbeyond.Andwhilethisisafarshotfrommillimeterwavetechnology
2、(30GHz),itisindeedRFandlow-endmicrowave.RFrequiresadesignengineeringapproachthataddressestheassociatedstrongerelectromagneticfieldeffectswhichnaturallyoccuratthesehigherfrequencies.Thesefieldscaninducesignalsinadjacentsignallines,orPCBtraces,creatingundesirablecrosstalk(interferenceandov
3、erallnoise),underminingsystemperformance.Returnloss(signalreflectedbackintotheincidentoncomingsignal)isprimarilycausedbyimpedancemismatchandhasmuchthesameimpactofaddednoiseandinterferencetotheprimarysignal.Therearetwoeffectsofhighreturnloss,bothofwhicharebadnews.First,signalreflectionbac
4、ktowardsthesourceaddsnoisetothesystem,makingitmoredifficultforthereceivertodistinguishnoisefromthesignal.Second,anyreflectedsignalisfundamentallyadegradationofthesignalitselfsincethe“meaning”orshapeoftheinboundsignalcanbealtered.Whileadigitalsystemcanbefarmoreforgivingsinceitisonlyattemp
5、tingtorecognizeaoneorazero(onoroff),theuseofharmonicsforfasterpulserisetimesinvolvesweakersignalsathigherfrequency.And,whilewecanimplementforwarderrorcorrectiontechnologytofixsomeoftheseeffects,theresultissystemdegradationascapacitygetsconsumedinredundanttransmissions.Amuchbetteransweris
6、tounderstandandengineertheRFeffectstohelp,nothurt,yoursignalmanagementassignment.Overallrecommendedtargetvaluesforreturnlossareminus25dBatthehighestfrequencyofinterest(usuallytheworse-casedatapoint),whichconvertstoabout1.1VSWR.TraditionalPCBdesignhasbeendrivenby"smaller,fasterandcheaper"
7、.AtRFfrequenciesonaPCB,"faster"doesnotalwaysallowfor"smaller"duetosomerealitiesofRFsignalmanagementdesign:1.Theprimarywaytomanageunwantedcrosstalkisbygroundplanemanagement,trace-to-tracespacing,and/orreductionofstubinductance.2.Theprimarywaytoreducereturnlossistomatchimpe
