无线电接收机英文资料及中文翻译Word文档格式.docx

无线电接收机英文资料及中文翻译Word文档格式.docx

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Fig.2-4ABlockDiagramForModernRadioReceiver

Anyantennasystemcapableofradiatingelectricalenergyisalsoabletoabstractenergyfromapassingradiowave.Sinceeverywavepassingthereceivingantenna.Inducesitsownvoltageintheantennaconductor,itisnecessarythatthereceivingequipmentbecapableofseparatingthedesiredsignalfromtheunwantedsignalsthatarealsoinducingvoltagesintheantenna.Thisseparationismadeonthebasisofthedifferenceinfrequencybetweentransmittingstationsandiscarriedoutbytheuseofresonantcircuits,whichcanbemadetodiscriminateverystronglyinfavorofaparticularfrequency.Ithasalreadybeenpointedthat,bymakingantennacircuitresonanttoaparticularfrequency,theenergyabstractedfromradiowavesofthatfrequencywillbemuchgreaterthantheenergyfromwavesofotherfrequencies;

thisalonegivesacertainamountofseparationbetweensignals.Stillgreaterselectiveactioncanbeobtainedbytheuseofadditionalsuitablyadjustedresonantcircuitslocatedsomewhereinthereceiverinsuchawayastorejectallbutthedesiredsignal.Theabilitytodiscriminatebetweenradiowavesofdifferentfrequenciesiscalledselectivityandtheprocessofadjustingcircuitstoresonancewiththefrequencyofadesiredsignalisspokenofastuning.

Althoughintelligibleradiosignalshavebeenreceivedfromthestationsthousandsofmilesdistant,usingonlytheenergyabstractedfromtheradiowavebythereceivingantennamuchmoresatisfactoryreceptioncanbeobtainedifthereceivedenergyisamplified.Thisamplificationmaybeappliedtotheradio-frequencycurrentsbeforedetection,inwhichcaseitiscalledradio-frequencyamplificationoritmaybeappliedtotherectifiedcurrentsafterdetection,inwhichcaseitiscalledaudio-frequencyamplification.Theuseofamplificationmakespossiblethesatisfactoryreceptionofsignalsfromwavesthatwouldotherwisebetooweaktogiveanaudibleresponse.

Theprocessbywhichthesignalbeingtransmittedisreproducedfromtheradio-frequencycurrentspresentatthereceiveriscalleddetection,orsometimesdemodulation.Wheretheintelligenceistransmittedbyvaryingtheamplitudeoftheradiatedwave,detectionisaccomplishedbyrectifyingtheradiofrequencycurrent.Therectifiedcurrentthusproducedvariesinaccordancewiththesignaloriginallymodulatedonthewaveirradiatedatthetransmitterandsoreproducesthedesiredsignal.Thus,whenthemodulatedwaveisrectified,theresultingcurrentisseentohaveanaveragevaluethatvariesinaccordancewiththeamplitudeoftheoriginalsignal.

Receivercircuitaremadeupaofanumberofstages.Astageisasingletransistorconnectedtocomponentswhichprovideoperatingvoltagesandcurrentsandalsosignalvoltagesandcurrents.Eachstagehasitsinputcircuitfromwhichthesignalcomesinanditsoutputcircuitfromwhichthesignal,usuallyamplified,goesout.Whenonestagefollowsanother,theoutputcircuitofthefirstfeedsthesignaltothesecond.Andsothesignalisamplified,stagebystage,untilitstrongenoughtooperatetheloudspeaker.

RadioWaves

RadioWavesareamemberoftheelectromagneticofwaves.Theyareenergy-carrierswhichtravelatthespeedoflight（ν）,theirfrequency（ƒ）andwavelength（λ）beingrelated,asforanywavemotion,bytheequation

ν=ƒ*λ

whereν=c=3.0*108m/sinavacuum（orair）.Ifλ=300m,thenƒ=ν/λ=3.0*108/（3.0*102）=106Hz=1MHz.Thesmallerλis,thelargerƒ.

RadioWavescanbedescribedeitherbytheirfrequencyortheirwavelength.Buttheformerismorefundamentalsince,unlikeλ（andν）,fdoesnotchangewhenthewavestravelformonemediumtoanother.

RadioWavescantravelformatransmittingaerialinoneormoreofthreedifferentways.

Surfaceorgroundwave..Thistravelsalongaground,thecurvatureoftheearth’ssurface.Itsrangeislimitedmainlybytheextenttowhichenergyisabsorbedformitbytheground.Poorconductorssuchassandabsorbmorestronglythatwater,andthehigherthefrequencythegreatertheabsorption.Therangemaybeabout1500kmatlowfrequencies（longwave,butmuchlessforv.h.f.）.

Skywave.Thistravelsskywardsand,ifitisbelowacertaincriticalfrequency（typically30MHz）,isreturnedtoearthbytheionosphere.Thisconsistsoflayersofairmolecules（theD,EandFlayer）,stretchingformabout80kmabovetheearthto50km,whichhavebecomepositivelychargedthroughtheremovalofelectronsbythesun’sultravioletradiation.Onstrikingtheearththeskywavebouncesbacktotheionospherewhereitisagaingraduallyrefractedandreturnedearthwardsasifby'

reflection'

.Thiscontinuesuntilitiscompletelyattenuated.

Spacewave.Forv.h.f.,u.h.f.andmicrowavesignals,onlythespacewave,givingline-ofsighttransmission,iseffective.Arangeofupto150kmispossibleonearthifthetransmittingaerialisonhighgroundandtherearenointerveningobstaclessuchashills,buildingsortrees.

Oscillators

Electricaloscillatorsarewidelyusedinradioandtelevisiontransmittersandreceivers,insignalgenerators,oscilloscopesandcomputers,toproduceA.C.withwaveformswhichmaybesinusoidal,square,sawtoothetc.andwithfrequenciesfromafewhertzuptomillionsofhertz.

Oscillatorycircuit

Whenacapacitordischargesthroughaninductorinacircuitoflowresistance,anA.C.flows.Thecircuitissaidtooscillateatitsnaturalfrequencywhich,aswewillshowshortly,equals

i.e.itsresonantfrequencyf0.Electricalresonancethusoccurswhentheappliedfrequencyequalsthenaturalfrequencyasitdoesinamechanicalsystem..

InFig,2-2（a）,achargedcapacitorCisshownconnectedacrossacoilL.Cimmediatelystartstodischarge,currentflowsandamagneticfieldiscreatedwhichinducesane.m.f.inL.Thise.m.f.opposesthecurrent.WhenCiscompletelydischargedtheelectricalenergyoriginallystoredintheelectricfieldbetweenitsplateshasbeentransferredtothemagneticfieldaroundL.

Bythetimethemagneticfieldhascollapsed,theenergyisagainstoredinC.OncemoreCstartstodischargebutcurrentnowflowsintheoppositedirection,creatingamagneticfieldofoppositepolarity.Whenthisfieldhasdecayed,Cisagainchargedwithitsupperplatepositiveandthesamecycleisrepeated.

Intheabsenceofresistanceinanypartofthecircuit,anundampedsinusoidalA.C.wouldbeobtained.Inpractice,energyisgraduallydissipatedbyresistanceasheatandadampedoscillationisproduced.

Oscillator

AstheresistanceofanLCcircuitincreases,theoscillationdecaymorequickly.Toobtainundampedoscillations,energyhastobefedintotheLCcircuitinphasewithitsnaturaloscillationstocompensatefortheenergydissipatedintheresistanceofthecircuit.Thiscanbedonewiththehelpofatransistorinactualoscillators.

AsimpletunedoscillatorisshowninFig.2-2（b）.TheLCcircuitisconnectedinthecollectorcircuit（astheload）andoscillationsstartinitwhenthesupplyisswitchedon.Thefrequencyoftheoscillationsisgivenby,i.e.thennaturalfrequencyoftheLCcircuit.Thetransistormerelyensuresthatenergyisfedbackatthecorrectinstantfromthebattery.ThecurrentbiasforthebaseofthetransistorisobtainedthroughR.

AMPLIFIER

Introduction

Thetermamplifierisverygeneric.Ingeneral,thepurposeofanamplifieristotakeaninputsignalandmakeitstronger（orinmoretechnicallycorrectterms,increaseitsamplitude）.Amplifiersfindapplicationinallkindsofelectronicdevicesdesignedtoperformanynumberoffunctions.Therearemanydifferenttypesofamplifiers,eachwithaspecificpurposeinmind.Forexample,aradiotransmitterusesanRFAmplifier（RFstandsforRadioFrequency）;

suchanamplifierisdesignedtoamplifyasignalsothatitmaydriveanantenna.Thisarticlewillfocusonaudiopoweramplifiers.Audiopoweramplifiersarethoseamplifierswhicharedesignedtodriveloudspeakers.Specifically,thisdiscussionwillfocusonaudiopoweramplifiersintendedforDJandsoundreinforcementuse.Muchofthematerialpresentedalsoappliestoamplifiersintendedforhomestereosystemuse.

Thepurposeofapoweramplifier,inverysimpleterms,istotakeasignalfromasourcedevice（inaDJsystemthesignaltypicallycomesfromapreamplifierorsignalprocessor）andmakeitsuitablefordrivingaloudspeaker.Ideally,theONLYthingdifferentbetweentheinputsignalandtheoutputsignalisthestrengthofthesignal.Inmathematicalterms,iftheinputsignalisdenotedasS,theoutputofaperfectamplifierisX*S,whereXisaconstant（afixednumber）.The"

*"

symbolmeans?

Multipliedby"

.

Thisbeingtherealworld,noamplifierdoesexactlytheideal,butmanydoaverygoodjobiftheyareoperatedwithintheiradvertisedpowerratings.Theoutputofallamplifierscontainadditionalsignalcomponentsthatarenotpresentintheinputsignal;

theseadditional（andunwanted）characteristicsmaybelumpedtogetherandaregenerallyknownasdistortion.Therearemanytypesofdistortion;

howeverthetwomostcommontypesareknownasharmonicdistortionandintermodulationdistortion.Inadditiontothe"

garbage"

traditionallyknownasdistortion,allamplifiersgenerateacertainamountofnoise（thiscanbeheardasabackground"

hiss"

w