Unit16SuperheterodyneRadioReceiverTextKeywordsTechnicaltermsExpressionsNotestothetextText

Inabroadcastingsystem,irrespectiveofwhetheritisbasedonamplitudemodulationorfrequencymodulation,thereceivernotonlyhasthetaskofdemodulatingtheincomingmodulatedsignal,butitisalsorequiredtoperformsomeothersystemfunctions:

(1)?Carrier-frequencytuning,thepurposeofwhichistoselectthedesiredsignal(i.e.,desiredradioorTVstation).

(2)?Filtering,whichisrequiredtoseparatethedesiredsignalfromothermodulatedsignalsthatmaybepickedupalongtheway.

(3)?Amplification,whichisintendedtocompensateforthelossofsignalpowerincurredinthecourseoftransmission.

Thesuperheterodynereceiver,orsuperhetasitisoftenreferredto,isaspecialtypeofreceiverthatfulfillsallthreefunctions,particularlythefirsttwo,inanelegantandpracticalfashion.Specifically,itovercomesthedifficultyofhavingtobuildatunablehighlyselectiveandvariablefilter.Indeed,practicallyallradioandTVreceiversnowbeingmadeareofthesuperheterodynetype.

Basically,thereceiverconsistsofaradio-frequency(RF)section,amixerandlocaloscillator,anintermediate-frequency(IF)section,demodulator,andpoweramplifier.TypicalfrequencyparametersofcommercialAMandFMradioreceiversarelistedinTable16.1.Figure16-1showstheblockdiagramofasuperheterodynereceiverforamplitudemodulationusinganenvelopedetectorfordemodulation.Table16.1TypicalfrequencyparametersofAMandFMradioreceiversFigure16-1BasicelementsofanAMradioreceiverof thesuperheterodynetype

Theincomingamplitude-modulatedwaveispickedupbythereceivingantennaandamplifiedintheRFsectionthatistunedtothecarrierfrequencyoftheincomingwave.Thecombinationofmixerandlocaloscillator(ofadjustablefrequency)providesaheterodyningfunction,wherebytheincomingsignalisconvertedtoapredeterminedfixedintermediatefrequency,usuallylowerthantheincomingcarrierfrequency.Thisfrequencytranslationisachievedwithoutdisturbingtherelationofthesidebandstothecarrier.Theresultoftheheterodyningistoproduceanintermediate-frequencycarrierdefinedby fIF?=fLO－fRF

wherefLOisthefrequencyofthelocaloscillatorandfRFisthecarrierfrequencyoftheincomingRFsignal.WerefertofIFastheintermediatefrequency(IF),becausethesignalisneitherattheoriginalinputfrequencynoratthefinalbasebandfrequency.Themixer-localoscillatorcombinationissometimesreferredtoasthefirstdetector,inwhichcasethedemodulatoriscalledtheseconddetector.

TheIFsectionconsistsofoneormorestagesoftunedamplification,withabandwidthcorrespondingtothatrequiredfortheparticulartypeofmodulationthatthereceiverisintendedtohandle.TheIFsectionprovidesmostoftheamplificationandselectivityinthereceiver.TheoutputoftheIFsectionisappliedtoademodulator,thepurposeofwhichistorecoverthebasebandsignal.Ifcoherentdetectionisused,thenacoherentsignalsourcemustbeprovidedinthereceiver.Thefinaloperationinthereceiveristhepoweramplificationoftherecoveredmessagesignal.Keywords

modulation n.調(diào)制

demodulate vt.?[訊]使解調(diào)，使檢波

carrier n.?[電]載波(信號(hào))

tuning n.調(diào)諧，調(diào)整，調(diào)音

incur v.招致

superheterodyne n.?[電子]超外差式收音機(jī)；adj.超外差的

superhet adj.&n.<口>=superheterodyne

mixer n.?[電]混頻器

antenna n.天線

predetermine v.預(yù)定，預(yù)先確定

disturb vt.弄亂，打亂，擾亂

sideband n.邊(頻)帶

baseband n.基帶

bandwidth n.帶寬

recover vt.重新獲得，恢復(fù)

coherent adj.一致的，連貫的Technicalterms

AMandFMradioreceivers 調(diào)幅與調(diào)頻接收機(jī)

carrier-frequencytuning 載頻調(diào)諧

superheterodynereceiver 超外差式接收機(jī)

aradio-frequency(RF)section 射頻部分

localoscillator 本地振蕩器

anintermediate-frequency(IF)section中頻部分

poweramplifier 功率放大器

anenvelopedetector 包絡(luò)檢波器Expressions

irrespectiveof… 無論…

separate…from… 把…從…中分離，分開

pickup 獲得

intendto… 打算…

bereferredtoas… 指的是…

correspondingto… 與…相應(yīng)Notestothetext

1.Thesuperheterodynereceiver,orsuperhetasitisoftenreferredto,isaspecialtypeofreceiverthatfulfillsallthreefunctions,particularlythefirsttwo,inanelegantandpracticalfashion.

超外差接收機(jī)是具備上述三種功能的一種特殊類型的接收機(jī)，它能夠簡(jiǎn)單方便地完成前兩種功能。句中，“orsuperhetasitisoftenreferredto”是插入語，與主語并列；“thatfulfillsallthreefunctions…”引導(dǎo)定語從句，修飾先行詞“receiver”；“particularlythefirsttwo”為插入語，起強(qiáng)調(diào)作用；“inanelegantandpracticalfashion”為方式狀語。

2.TheIFsectionconsistsofoneormorestagesoftunedamplification,withabandwidthcorrespondingtothatrequiredfortheparticulartype