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1、中北大學(xué)2011屆畢業(yè)設(shè)計(jì)說(shuō)明書第1頁(yè)共16頁(yè)外文文獻(xiàn)原文外文文獻(xiàn)原文FundamentalsofTimeFrequencyIntroductionTimefrequencystardssupplythreebasictypesofinfmation：timeofday，timeinterval，frequency.Timeofdayinfmationisprovidedinhours，minutes，seconds，butoftena

2、lsoincludesthedate(month，day，year).Adevicethatdisplaysrecdstimeofdayinfmationiscalledaclock.Ifaclockisusedtolabelwhenaneventhappened，thislabelissometimescalledatimetagtimestamp.Datetimeofdaycanalsobeusedtoensurethatevent

3、saresynchronized，happenatthesametime.Timeintervalisthedurationelapsedtimebetweentwoevents.Thestardunitoftimeintervalisthesecond(s).However，manyengineeringapplicationsrequirethemeasurementofshtertimeintervals，suchasmillis

4、econds(1ms=103s)，microseconds(1μs=106s)，nanoseconds(1ns=109s)，picoseconds(1ps=1012s).Timeisoneofthesevenbasephysicalquantities，thesecondisoneofsevenbaseunitsdefinedintheInternationalSystemofUnits(SI).Thedefinitionsofmany

5、otherphysicalquantitiesrelyuponthedefinitionofthesecond.Thesecondwasoncedefinedbasedontheearth’srotationalrateasafractionofthetropicalyear.Thatchangedin1967whentheeraofatomictimekeepingfmallybegan.Thecurrentdefinitionoft

6、heSIsecondisthedurationof9，192，631，770periodsoftheradiationcrespondingtothetransitionbetweentwohyperfinelevelsofthegroundstateofthecesium133atom.Frequencyistherateofarepetitiveevent.IfTistheperiodofarepetitiveevent，thent

7、hefrequencyfisitsreciprocal，1T.Conversely，theperiodisthereciprocalofthefrequency，T=1f.Sincetheperiodisatimeintervalexpressedinseconds(s)，itiseasytoseethecloserelationshipbetweentimeintervalfrequency.Thestard中北大學(xué)2011屆畢業(yè)設(shè)計(jì)

8、說(shuō)明書第3頁(yè)共16頁(yè)necessary，leapsecondsareaddedtoUTConeitherJune30December31.Thepurposeofaddingleapsecondsistokeepatomictime(UTC)within0.9sofanoldertimescalecalledUT1，whichisbasedontherotationalrateoftheearth.Leapsecondshavebeen

9、addedtoUTCatarateofslightlylessthanonceperyear，beginningin1972.KeepinmindthattheBIPMmaintainsTAIUTCas‘‘paper’’timescales.ThemajmetrologylabatiesusethepublisheddatafromtheBIPMtosteertheirclocksoscillatsgeneraterealtimever

10、sionsofUTC.ManyoftheselabatiesdistributetheirversionsofUTCviaradiosignalswhichsection17.4arediscussedin.YoucanthinkofUTCastheultimatestardftimeofday，timeinterval，frequency.ClockssynchronizedtoUTCdisplaythesamehourminute，

11、secondalloverthewld(remainwithinonesecondofUT1).OscillatssimonizedtoUTCgeneratesignalsthatserveasreferencestardsftimeintervalfrequency.TimeFrequencyMeasurementTimefrequencymeasurementsfollowtheconventionsusedinotherareas

12、ofmetrology.Thefrequencystardclockbeingmeasurediscalledthedeviceundertest(DUT).AmeasurementcomparestheDUTtoastardreference.ThestardshouldoutperfmtheDUTbyaspecifiedratio，calledthetestuncertaintyratio(TUR).Ideally，theTURsh

13、ouldbe10：1higher.Thehighertheratio，thelessaveragingisrequiredtogetvalidmeasurementresults.Thetestsignalftimemeasurementsisusuallyapulsethatoccursoncepersecond(1ps).Thepulsewidthpolarityvariesfromdevicetodevice，butTTLleve

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