通信与电子信息工程专业英语第2单元

Unit2ModulationTechnologyTextAModulationTechnology调制技术Unit2原文：Formula(2.1)introducedthebasicfunctionofasinewavewhichalreadyindicatesthethreebasicmodulationschemes(typically,thecosinefunctionisusedforexplanation):译文：公式（2.1）引入了一个基本的正弦函数，它可以表示三种基本调制方案（通常用余弦函数来说明）：Unit2Unit2原文：Digitalmodulationisrequiredifdigitaldatahastobetransmittedoveramediumthatonlyallowsforanalogtransmission.译文：如果数字信号必须在只允许模拟信号传输的媒介中传输，则要求进行数字调制。Unit2原文：Oneexampleforwirednetworksistheoldanalogtelephonesystem—toconnectacomputertothissystemamodemisneeded.Themodemthenperformsthetranslationofdigitaldataintoanalogsignalsviceversa.译文：模拟电话系统是有线网络的一个例子，它需要用一个调制解调器将计算机与系统相连。调制解调器完成调制解调模数转换的任务。数字传输在有线局域网或计算机内部是很有用的。Unit2原文：Apartfromthetranslationofdigitaldataintoanalogsignals,wirelesstransmissionrequiresanadditionalmodulation,ananalogmodulationthatshiftsthecenterfrequencyofthebase-bandsignalgeneratedbythedigitalmodulationuptotheradiocarrier.译文：除了数模转换外，无线传输还需要一个附加的模拟调制，将普通基带信号的中心频率通过数字调制提高形成无线电载波。Unit2原文：Forexampledigitalmodulationtranslatesa1Mbpsbit-steamintoabase-bandsignalwithabandwidthof1MHz.Thereareseveralreasonswhythisbase-bandsignalcannotbedirectlytransmittedinawirelesssystem:译文：例如，通过数字调制，将一个1Mbps比特流转换成1MHz带宽的基带信号。基带信号之所有以不能直接在无线系统中传输有如下几个原因：Unit2原文：Digitaltransmissionisused,forexample,inwiredlocalareanetworksorwithinacomputer.Inwirelessnetworks,however,digitaltransmissioncannotbeused.Here,thebinarybit-steamhastobetranslatedintoananalogsignalfirst.译文：然而，在无线网络中，二进制比特流首先必须被转换成模拟信号，因此，不一定必须用到数字传输。Unit2原文：Thethreebasicmethodsforthistranslationareamplitudeshiftkeying(ASK),frequencyshiftkeying(FSK),andphaseshiftkeying(PSK).Thesearediscussedinmoredetailinthefollowingsections.译文：三种基本的数字调制是幅移键控（ASK）、频移键控（FSK）和相移键控（PSK）。下面我们要分别详细讨论。Unit2原文：Apartfromthetranslationofdigitaldataintoanalogsignals,wirelesstransmissionrequiresanadditionalmodulation,ananalogmodulationthatshiftsthecenterfrequencyofthebase-bandsignalgeneratedbythedigitalmodulationuptotheradiocarrier.译文：除了数模转换外，无线传输还需要一个附加的模拟调制，将普通基带信号的中心频率通过数字调制提高形成无线电载波。Unit2原文：Forexampledigitalmodulationtranslatesa1Mbpsbit-steamintoabase-bandsignalwithabandwidthof1MHz.Thereareseveralreasonswhythisbase-bandsignalcannotbedirectlytransmittedinawirelesssystem:译文：例如，通过数字调制，将一个1Mbps比特流转换成1MHz带宽的基带信号。基带信号之所有以不能直接在无线系统中传输有如下几个原因：Unit2原文：Apartfromthetranslationofdigitaldataintoanalogsignals,wirelesstransmissionrequiresanadditionalmodulation,ananalogmodulationthatshiftsthecenterfrequencyofthebase-bandsignalgeneratedbythedigitalmodulationuptotheradiocarrier.译文：除了数模转换外，无线传输还需要一个附加的模拟调制，将普通基带信号的中心频率通过数字调制提高到无线电载波频段。Unit2原文：Forexampledigitalmodulationtranslatesa1Mbpsbit-steamintoabase-bandsignalwithabandwidthof1MHz.Thereareseveralreasonswhythisbase-bandsignalcannotbedirectlytransmittedinawirelesssystem:译文：例如，通过数字调制，将一个1Mbps比特流转换成1MHz带宽的基带信号。基带信号之所有以不能直接在无线系统中传输有如下几个原因：Unit2原文：Antennas:Anantennamustbetheorderofmagnitudeofthewavelengthofthesignalinsizetobeeffective.Forthe1MHzsignalintheexamplethiswouldresultinanantennasomehundredmeterhigh,whichisobviouslynotverypracticalforhandhelddevices.With1GHz,antennasofafewcentimetersinlengthcanbeused.译文：天线：天线必须由有效信号的波长大小来决定。例如，1MHz的信号，天线就有数百米高，这显然对便携设备来说很不实际。而1GHz，天线则只有几米高，这是很有用的。Unit2原文：Frequencydivisionmultiplexing:Usingonlybase-bandtransmission,FDMcouldnotbeapplied.Analogmodulationshiftsthebase-bandsignalstodifferentcarrierfrequencies.Thehigherthecarrierfrequency,themorebandwidththatisavailableformanybase-bandsignals.译文：频分复用：频分复用不用在单独的基带传输中。模拟调制将基带信号转换成不同的载波频率。载波频率越高，基带信号的可用带宽越宽。Unit2原文：Mediumcharacteristics:Path-losspenetrationofobstacles,reflection,scattering,anddiffractiondependheavilyonthewavelengthofthesignal.译文：介质特性：信道损耗、绕射、反射、散射和在信号波长紧密相关的衍射。Unit2原文：Dependingontheapplications,therightcarrierfrequencywiththedesiredcharacteristicshastobechosen:longwavesforsubmarines,shortwavesforhandhelddevices,veryshortwavesfordirectedmicrowavetransmissionetc.译文：根据实际应用选择所需合适的载波频率特性：潜艇采用长波，便携设备采用短波，定向微波传输采用超短波等等。Unit2原文：Asfordigitalmodulation,threedifferentbasicschemesareknownforanalogmodulation:amplitudemodulation(AM),frequencymodulation(FM),andphasemodulation(PM).Figure2.1showsa(simplified)blockdiagramofaradiotransmitterfordigitaldata.译文：至于数字调制，所知的三种不同的基本模拟调制类型：幅度调制（AM）、频率调制（FM）和相位调制（PM）。图（2.1）表示一个数字无线反射区的简化程序框图。Unit2原文：Thefirststepisthedigitalmodulationofdataintotheanalogbase-bandsignalaccordingtooneoftheschemespresentedinthefollowingsection.译文：第一步是将数字调制数据通过如下部分所示的形式转换成模拟基带信号，然后将模拟信号的中心频率提高到无线电载波频段，最后信号通过天线传输。Unit2原文：Thereceiver(Figure2.2)receivestheanalogradiosignalviaitsantennaanddemodulatesthesignalintotheanalogbase-bandsignalwiththehelpoftheknowncarrier.Thiswouldbeallthatisneededforananalogradiotunedintoaradiostation.(Theanalogbase-bandsignalwouldconstitutethemusic.)译文：接收机（图2.2）通过天线接收到模拟无线电信号，根据一定的载波对信号进行解调转换成模拟基带信号，全部应用在接收广播电台的模拟广播。（模拟基带信号承载音频。）Unit2原文：Fordigitaldata,anotherstepisneeded.Bitsorframeshavetobedetected,i.e.,thereceivermustsynchronizewiththesender.Howsynchronizationisachieved,dependsonthedigitalmodulationscheme.译文：对于数字信号来说，另一步是检测比特和帧是至