专业英语参考答案7-9章

PAGEPAGE3第七章单边带调制2．Fillintheblankswithproperwords,phrasesandclauses.AMvestigialsideband(VSB)isaformof_amplitudemodulation__(幅度调制)inwhichthe_carrier__(载波)andonecompletesidebandaretransmitted,butonlypartofthesecondsidebandistransmitted.InVSB,thelowermodulating-signalfrequenciesaretransmitteddoubleband__(双边带)andthehighermodulating-signalfrequenciesaretransmitted_singleband___(单边带)。Consequently,thelow-frequencymodulatingsignalsare_emphasized____(加重)andproducelarger-amplitudesignalsinthe__demodulator_(解调器)thanthehighfrequencies.3.TranslatethefollowingsentencesintoEnglish.(1)Theamountoffrequencyshiftisequaltotheabsolutevalueofthedifferencebetweenthefrequenciesofthequadratureoscillators(2)ThistypeofdistortioncausedbyfrequencyerrorinthedemodulationprocessisuniquetoSSBmodulationsystem.(3)Anyerrorinthefrequencyorthephaseofthelocaloscillatorsignalinthereceiver,withrespecttothecarrierwave,givesrisetodistortioninthedemodulatedsignal.(4)Inordertoreducetheeffectoffrequencyerrordistortionintelephonesystems,wehavetolimitthefrequencyerrorto2-5Hz(5)Aphase-lock-loop(PLL)circuitcanbeimplementedthatwillgiveahigherfrequencyquadratureoscillator第八章角度调制Fillintheblankswithproperwords,phrasesandclauses.Whenthermalnoisewithaconstantspectraldensity,FM,deviation,carrierfrequency,relativeamplitudeofthenoise,demodulated,frequencycomponent,Thespectralshapeofthedemodulatednoisedependon,whereasthenoisevoltageattheoutputofanFMdemodulatorincreaseslinearlywithfrequency,commonlycalled.Inessence,frequencyandphasemodulators,modulatingsignal,thecarrieroscillator,indirect,Whenthephaseofthecarriersignalismodulatedbytheinformationsignal,directPM(indirectFM)result.2．TranslatethefollowingpassagesintoChineseorEnglish.直接调频法主要的缺点是载波频率必须使用稳定性相对较差的LC振荡器来产生，而不能用晶体震荡器。因此，直接FM法需要自动频率控制电路来将载波频率保持在FCC要求的频率范围内。直接FM最显著的优点是由于震荡器的频率不稳定，所以很容易获得相对较高的频率偏移和调制指数。直接PM法主要的优点是载波振荡器与实际的调制电路是分开的，因此可以采用稳定性好的晶体震荡器。直接PM的主要缺点是由于晶体震荡器产生的频率很稳定，所以很难获得较大的相移和调制指数。使用FM和PM调制器的时候，调制器的输出载频通常比传输所希望的频率要低。与幅度调制传输相比，角度调制传输（FM和PM）最主要的优点是抗噪声性能好。Withtheuseoflimiters,FMandPMdemodulatorscanactuallyreducethenoiselevelandimprovethesignal-to-noiseratioduringthedemodulationprocess.High-qualityanglemodulationproducesmanysidefrequencies,thusnecessitatingamuchwiderbandwidththanisnecessaryforAMtransmission.。Whenevertheperiod(T)ofasinusoidalcarrierchanges,itsfrequencyandphasealsochange,andifthechangesarecontinuous,thewaveisnolongerasinglefrequency.第九章传输线Fillintheblankswithproperwords,phrasesandclauses.radiationlossesandinterference,aconductivemetalbraid,connectedtogroundandactsasashield,electromagneticinterference,Ashielded-parallelwirepairconsistsoftwoparallelwireconductorsseparatedbyasoliddielectricmaterial,enclosed,aprotectiveplasticcoatingbalancedmode,potential,coaxialcable,preventstaticinterferencefrompenetratingthecenterconductors.TranslatethefollowingpassagesintoChineseorEnglish.这种类型传输线唯一的优点是结构简单。但是由于这种传输线没有屏蔽层，所以辐射损耗很高，也容易受到噪声影响。这些都是明线主要的缺点。因此，明线通常用于平衡式传输中。刚性空气填充型同轴电缆的制造成本相对较高，同时为了将损耗降到最低，空气绝缘体应该不受湿度影响。柔性同轴电缆比刚性同轴电缆的损耗低，而且制造更简单。两种类型的同轴电缆都不容易受到外界辐射的影响，而且它们自己的辐射也很低，因此和结构相似的平行传输线相比，同轴电缆可以应用于更高的频率。同轴电缆传输线的主要缺点是成本高，而且必须在非平衡式传输中使用。Thecharacteristicsofatransmissionlinearedeterminedbyitselectricalproperties,suchaswireconductivityandinsulatordielectricconstant,anditsphysicalproperties,suchaswirediameterandconductorspacing.Becausecurrentflowsthroughatransmissionlineandthetransmissionlinehasafiniteresistance,thereisaninherentandavoidablepowerloss.第十章电磁波传播2．Fillintheblankswithproperwords,phrasesandclauses.Long-rangecommunicationinthe(高频段)high-frequencybandispossiblebecauseof(折射)_refraction__inaregionoftheupper(大气层)atmospherecalledthe(电离层)ionosphere,wheresomeoftheair(分子)_molecules_are(电离)_ionizedby(太阳辐射)sunradiant.InEarth'satmosphere,ray-wavefrontpropagationmaybealteredfrom(自由空间)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