仪表与量测ppt课件_1

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1、Chapter5TimeandFrequencyMeasurementP.PatakiIntroductionAccuaracyofFrequencyStandardsFrequencySourcesFrequencyandPeriodTimeMeasurementTime-IntervalMeasurementPhaseMeasurementIntroductionFrequencyisbydefinitionthenumberofoccurencesofagivenperiodicphenomeno

2、ninagiventime.Timehastwomeaningsinpractice:thepositionofaneventonaconventionaltimeaxis.astronomy,spaceresearch,navigation,etc,arediosignalemittedfromaveryaccuratefrequencystandard(theAtomaticClock)synchronizedtoawelldefinedstartingpoint(1958.1.100:00:00)t

3、hetimeelapsedbetweentwoevents(time-lengthortime-interval)Topics:time,frequency,andphase.TheoreticalLimitFrequencyStandards-AccuracyFrequencyaccuracyof10-13canbetakenasatheoreticallimitintheEarth-fixedcoordinatesystem.Therelativisticeffectsandthechangeofth

4、egravitationalpotentialcauseatimeshiftinthisrange.Inspace,itis10-10(Reinhard1974,Ramsey1987,...)FrequencyStandards-AccuracyInearly'60,differentmathematicaldescriptionswereusedtocharacterizetheaccuracyofhigh-stability.1964,NASAandIEEEhasaconferenceonthesta

5、ndardization-(1)fractionalfrequencyoffset(2)phase-timeFrequencyOffset:InstantaneousFrequency:Voltageofasinewave:FractionalFrequencyOffset:Phase-Time:y(t)andx(t)areunchangedbyfrequencymultiplicationanddivisionifthedevicesarenoiseless.Theyareusuallyfordirec

6、tcomparisonsofthestabilityofsourceswithverydifferentnominalfrequencies(Barnes1971)FrequencySources-AtomicFrequencyStandardThemostprecisewaytoproduceastablefrequencygeneratoristheatomicclock.Itisbasedontheprincipleofquantummechanicswherefrequencisdetermine

7、dbyanatomicresonator.Thefrequencyofthemicrowaveradiationisproportionaltotheenergydifferencebetweenthediscretepandqenergylevel.h=6.63x10-34JisPlanck'sconstant.TheatomicclockelementsareusedfromthefirstcolumnofMendelejev'speriodicalsystem(Alkalimetals)Themat

8、erialhaveonlyonevalenceelectronontheirouterenergylevel.Therediationfrequencydependsontheenergydifferencebetweentwoouterlevels.(Hayatt1973;Kartaschoff1978,Ramsey1991).FrequencySources-QuartzCrystalOscillatorThemostge