晶振和电容的匹配

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1、晶振和电容的匹配LoadCapacitanceTheCapacitanceexternaltothecrystalcontainedwithinthefeedbackloopoftheoscillatorcircuit.For"parallel"resonantcrystals(seebelowfordiscussiononparallelvs.seriesresonantcrystals),thevalueofloadcapacitanceneedstobespecifiedbythecustomertoinsureinitialFrequen

2、cyTolerance.For"series"resonantcrystals,loadcapacitanceisnotused.LoadcapacitancecanbemeasuredastheamountofcapacitanceacrossthecrystalterminalsonthePCB.Aparallelresonantmodecrystalneedstohavealoadcapacitance(CL)specifiedwhenordering(thiswasalreadysaidabove).Theapproximatecryst

3、alloadingforagivencircuitcanbedeterminedfromtheformula(SEEbelowformoreusefulequations):CSisthestraycapacitanceofthecircuitandtheinput/outputcapacitanceoftheinverterormicroprocessorchipattheCrystal1(C1)andCrystal2(C2)pins,plusanyparasiticcapacitances.CSmaybeassumedtoequal5pF.M

4、ostcrystalmanufacturerswillspecifystandardparallelresonantloadcapacitancesofeither18or20pF.ThesevalueshavebeenfoundtoprovideforgoodFrequencyToleranceinmostcircuits.Changesinloadcapacitancewillresultinchangesintheoutputfrequency..Seriesvs.Parallel:Parallelresonantcrystalsarein

5、tendedforcircuitswhichcontainreactivecomponents(capacitors)intheoscillatorfeedbackloop.Thesecircuitsdependonthereactivecomponentsandthecrystaltoachievethephaseshiftneededtostartandmaintainocsillationataspecifiedfrequency.Seriesresonantcrystalsareintendedforcircuitsthatcontain

6、noreactivecomponentsintheoscillatorfeedbackloop.PleaseseeFigureAforSeriesandFigureBforParallelresonantcrystals:FigureA-SeriesFigureB-ParallelUsefulEquations:EquationsDefinitionsfS=(Series)frequency=C0=StaticCapacitanceinfaradsCL=Loadcapacitance=C1=MotionalcapacitanceinfaradsC

7、o=Shuntcapacitance=CL=LoadcapacitanceinfaradsC1=Motionalcapacitance=f=NominalfrequencyinHzL1=Motionalinductance=fL=Anti-resonantfrequencyinHzR1=Seriesresistance=fS=SeriesresonantfrequencyinHzQ=Qualityfactor=L=InductanceintoHenrysfL-fS=f=PL=Pullability(ppm/pF)PL=Pullability=Q=

8、Qualityfactor..R1=SeriesresistanceinohmsX