專業(yè)英語參考答案7-9章

PAGEPAGE3第七章單邊帶調(diào)制2．Fillintheblankswithproperwords,phrasesandclauses.AMvestigialsideband(VSB)isaformof_amplitudemodulation__(幅度調(diào)制)inwhichthe_carrier__(載波)andonecompletesidebandaretransmitted,butonlypartofthesecondsidebandistransmitted.InVSB,thelowermodulating-signalfrequenciesaretransmitteddoubleband__(雙邊帶)andthehighermodulating-signalfrequenciesaretransmitted_singleband___(單邊帶)。Consequently,thelow-frequencymodulatingsignalsare_emphasized____(加重)andproducelarger-amplitudesignalsinthe__demodulator_(解調(diào)器)thanthehighfrequencies.3.TranslatethefollowingsentencesintoEnglish.(1)Theamountoffrequencyshiftisequaltotheabsolutevalueofthedifferencebetweenthefrequenciesofthequadratureoscillators(2)ThistypeofdistortioncausedbyfrequencyerrorinthedemodulationprocessisuniquetoSSBmodulationsystem.(3)Anyerrorinthefrequencyorthephaseofthelocaloscillatorsignalinthereceiver,withrespecttothecarrierwave,givesrisetodistortioninthedemodulatedsignal.(4)Inordertoreducetheeffectoffrequencyerrordistortionintelephonesystems,wehavetolimitthefrequencyerrorto2-5Hz(5)Aphase-lock-loop(PLL)circuitcanbeimplementedthatwillgiveahigherfrequencyquadratureoscillator第八章角度調(diào)制Fillintheblankswithproperwords,phrasesandclauses.Whenthermalnoisewithaconstantspectraldensity,FM,deviation,carrierfrequency,relativeamplitudeofthenoise,demodulated,frequencycomponent,Thespectralshapeofthedemodulatednoisedependon,whereasthenoisevoltageattheoutputofanFMdemodulatorincreaseslinearlywithfrequency,commonlycalled.Inessence,frequencyandphasemodulators,modulatingsignal,thecarrieroscillator,indirect,Whenthephaseofthecarriersignalismodulatedbytheinformationsignal,directPM(indirectFM)result.2．TranslatethefollowingpassagesintoChineseorEnglish.直接調(diào)頻法主要的缺點(diǎn)是載波頻率必須使用穩(wěn)定性相對(duì)較差的LC振蕩器來產(chǎn)生，而不能用晶體震蕩器。因此，直接FM法需要自動(dòng)頻率控制電路來將載波頻率保持在FCC要求的頻率范圍內(nèi)。直接FM最顯著的優(yōu)點(diǎn)是由于震蕩器的頻率不穩(wěn)定，所以很容易獲得相對(duì)較高的頻率偏移和調(diào)制指數(shù)。直接PM法主要的優(yōu)點(diǎn)是載波振蕩器與實(shí)際的調(diào)制電路是分開的，因此可以采用穩(wěn)定性好的晶體震蕩器。直接PM的主要缺點(diǎn)是由于晶體震蕩器產(chǎn)生的頻率很穩(wěn)定，所以很難獲得較大的相移和調(diào)制指數(shù)。使用FM和PM調(diào)制器的時(shí)候，調(diào)制器的輸出載頻通常比傳輸所希望的頻率要低。與幅度調(diào)制傳輸相比，角度調(diào)制傳輸（FM和PM）最主要的優(yōu)點(diǎn)是抗噪聲性能好。Withtheuseoflimiters,FMandPMdemodulatorscanactuallyreducethenoiselevelandimprovethesignal-to-noiseratioduringthedemodulationprocess.High-qualityanglemodulationproducesmanysidefrequencies,thusnecessitatingamuchwiderbandwidththanisnecessaryforAMtransmission.。Whenevertheperiod(T)ofasinusoidalcarrierchanges,itsfrequencyandphasealsochange,andifthechangesarecontinuous,thewaveisnolongerasinglefrequency.第九章傳輸線Fillintheblankswithproperwords,phrasesandclauses.radiationlossesandinterference,aconductivemetalbraid,connectedtogroundandactsasashield,electromagneticinterference,Ashielded-parallelwirepairconsistsoftwoparallelwireconductorsseparatedbyasoliddielectricmaterial,enclosed,aprotectiveplasticcoatingbalancedmode,potential,coaxialcable,preventstaticinterferencefrompenetratingthecenterconductors.TranslatethefollowingpassagesintoChineseorEnglish.這種類型傳輸線唯一的優(yōu)點(diǎn)是結(jié)構(gòu)簡(jiǎn)單。但是由于這種傳輸線沒有屏蔽層，所以輻射損耗很高，也容易受到噪聲影響。這些都是明線主要的缺點(diǎn)。因此，明線通常用于平衡式傳輸中。剛性空氣填充型同軸電纜的制造成本相對(duì)較高，同時(shí)為了將損耗降到最低，空氣絕緣體應(yīng)該不受濕度影響。柔性同軸電纜比剛性同軸電纜的損耗低，而且制造更簡(jiǎn)單。兩種類型的同軸電纜都不容易受到外界輻射的影響，而且它們自己的輻射也很低，因此和結(jié)構(gòu)相似的平行傳輸線相比，同軸電纜可以應(yīng)用于更高的頻率。同軸電纜傳輸線的主要缺點(diǎn)是成本高，而且必須在非平衡式傳輸中使用。Thecharacteristicsofatransmissionlinearedeterminedbyitselectricalproperties,suchaswireconductivityandinsulatordielectricconstant,anditsphysicalproperties,suchaswirediameterandconductorspacing.Becausecurrentflowsthroughatransmissionlineandthetransmissionlinehasafiniteresistance,thereisaninherentandavoidablepowerloss.第十章電磁波傳播2．Fillintheblankswithproperwords,phrasesandclauses.Long-rangecommunicationinthe(高頻段)high-frequencybandispossiblebecauseof(折射)_refraction__inaregionoftheupper(大氣層)atmospherecalledthe(電離層)ionosphere,wheresomeoftheair(分子)_molecules_are(電離)_ionizedby(太陽輻射)sunradiant.InEarth'satmosphere,ray-wavefrontpropagationmaybealteredfrom(自由空間)free-spacebehaviorbyopticaleffectssuchas(折射)refraction,(反射)_reflection_,(衍射)diffraction,and(干涉)interference.3.TranslatethefollowingsentenceintoEnglish(1)Thevelocityatwhichanelectromagneticwavepropagatesisinverselyproportionaltothedensityofthemediuminwhichitispropagating.(2)Refractionoccurswheneveraradiowavepassesfromonemediumintoanothermediumofdifferentdensity.(3)Whenthereflectingsurfaceisnotplane(i.e.,itiscurved),thecurvatureofthereflectedwaveisdifferentfromthatoftheincidentwave.(4)Asanelectromagneticwavepr