毕业设计说明书外文翻译时间和频率的基本原理Word下载.docx

毕业设计说明书外文翻译时间和频率的基本原理Word下载.docx

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毕业设计说明书

英文文献及中文翻译

学院：

信息与通信工程

专业：

电子信息科学与技术

2011年6月

外文文献原文

FundamentalsofTimeandFrequency

Introduction

Timeandfrequencystandardssupplythreebasictypesofinformation：

time-of-day，timeinterval，andfrequency.Time-of-dayinformationisprovidedinhours，minutes，andseconds，butoftenalsoincludesthedate（month，day，andyear）.Adevicethatdisplaysorrecordstime-of-dayinformationiscalledaclock.Ifaclockisusedtolabelwhenaneventhappened，thislabelissometimescalledatimetagortimestamp.Dateandtime-of-daycanalsobeusedtoensurethateventsaresynchronized，orhappenatthesametime.

Timeintervalisthedurationorelapsedtimebetweentwoevents.Thestandardunitoftimeintervalisthesecond（s）.However，manyengineeringapplicationsrequirethemeasurementofshortertimeintervals，suchasmilliseconds（1ms=10-3s），microseconds（1μs=10-6s），nanoseconds（1ns=10-9s），andpicoseconds（1ps=10-12s）.Timeisoneofthesevenbasephysicalquantities，andthesecondisoneofsevenbaseunitsdefinedintheInternationalSystemofUnits（SI）.Thedefinitionsofmanyotherphysicalquantitiesrelyuponthedefinitionofthesecond.Thesecondwasoncedefinedbasedontheearth’srotationalrateorasafractionofthetropicalyear.Thatchangedin1967whentheeraofatomictimekeepingformallybegan.ThecurrentdefinitionoftheSIsecondisthedurationof9，192，631，770periodsoftheradiationcorrespondingtothetransitionbetweentwohyperfinelevelsofthegroundstateofthecesium-133atom.

Frequencyistherateofarepetitiveevent.IfTistheperiodofarepetitiveevent，thenthefrequencyfisitsreciprocal，1/T.Conversely，theperiodisthereciprocalofthefrequency，T=1/f.Sincetheperiodisatimeintervalexpressedinseconds（s），itiseasytoseethecloserelationshipbetweentimeintervalandfrequency.Thestandardunitforfrequencyisthehertz（Hz），definedaseventsorcyclespersecond.Thefrequencyofelectricalsignalsisoftenmeasuredinmultiplesofhertz，includingkilohertz（kHz），megahertz（MHz），orgigahertz（GHz），where1kHzequalsonethousand（103）eventspersecond，1MHzequalsonemillion（106）eventspersecond，and1GHzequalsonebillion（109）eventspersecond.Adevicethatproducesfrequencyiscalledanoscillator.Theprocessofsettingmultipleoscillatorstothesamefrequencyiscalledsynchronization.

Ofcourse，thethreetypesoftimeandfrequencyinformationarecloselyrelated.Asmentioned，thestandardunitoftimeintervalisthesecond.Bycountingseconds，wecandeterminethedateandthetime-of-day.Andbycountingeventsorcyclespersecond，wecanmeasurefrequency.

Timeintervalandfrequencycannowbemeasuredwithlessuncertaintyandmoreresolutionthananyotherphysicalquantity.Today，thebesttimeandfrequencystandardscanrealizetheSIsecondwithuncertaintiesof≈1×

10-15.PhysicalrealizationsoftheotherbaseSIunitshavemuchlargeruncertainties.

CoordinatedUniversalTime（UTC）

Theworld’smajormetrologylaboratoriesroutinelymeasuretheirtimeandfrequencystandardsandsendthemeasurementdatatotheBureauInternationaldesPoidsetMeasures（BIPM）inSevres，France.TheBIPMaveragesdatacollectedfrommorethan200atomictimeandfrequencystandardslocatedatmorethan40laboratories，includingtheNationalInstituteofStandardsandTechnology（NIST）.Asaresultofthisaveraging，theBIPMgeneratestwotimescales，InternationalAtomicTime（TAI），andCoordinatedUniversalTime（UTC）.ThesetimescalesrealizetheSIsecondascloselyaspossible.

UTCrunsatthesamefrequencyasTAI.However，itdiffersfromTAIbyanintegralnumberofseconds.Thisdifferenceincreaseswhenleapsecondsoccur.Whennecessary，leapsecondsareaddedtoUTConeitherJune30orDecember31.Thepurposeofaddingleapsecondsistokeepatomictime（UTC）within±

0.9sofanoldertimescalecalledUT1，whichisbasedontherotationalrateoftheearth.LeapsecondshavebeenaddedtoUTCatarateofslightlylessthanonceperyear，beginningin1972.

KeepinmindthattheBIPMmaintainsTAIandUTCas‘‘paper’’timescales.ThemajormetrologylaboratoriesusethepublisheddatafromtheBIPMtosteertheirclocksandoscillatorsandgeneratereal-timeversionsofUTC.ManyoftheselaboratoriesdistributetheirversionsofUTCviaradiosignalswhichsection17.4arediscussedin.

YoucanthinkofUTCastheultimatestandardfortime-of-day，timeinterval，andfrequency.ClockssynchronizedtoUTCdisplaythesamehourminute，andsecondallovertheworld（andremainwithinonesecondofUT1）.OscillatorssimonizedtoUTCgeneratesignalsthatserveasreferencestandardsfortimeintervalandfrequency.

TimeandFrequencyMeasurement

Timeandfrequencymeasure