Unit 2 Modulation Technology Text A Modulation Technology 调制技术 Unit 2 v原文 Formula 2 1 introduced the basic function of a sine wave which already indicates the three basic modulation schemes typically the cosine function is used for explanation v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 Unit 2 v Unit 2 v原文 Digital modulation is required if digital data has to be transmitted over a medium that only allows for analog transmission v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 Unit 2 v原文 One example for wired networks is the old analog telephone system to connect a computer to this system a modem is needed The modem then performs the translation of digital data into analog signals vice versa v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 Unit 2 v原文 Apart from the translation of digital data into analog signals wireless transmission requires an additional modulation an analog modulation that shifts the center frequency of the base band signal generated by the digital modulation up to the radio carrier v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 Unit 2 v原文 For example digital modulation translates a 1 Mbps bit steam into a base band signal with a bandwidth of 1MHz There are several reasons why this base band signal cannot be directly transmitted in a wireless system v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 Unit 2 v原文 Digital transmission is used for example in wired local area networks or within a computer In wireless networks however digital transmission cannot be used Here the binary bit steam has to be translated into an analog signal first v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 Unit 2 v原文 The three basic methods for this translation are amplitude shift keying ASK frequency shift keying FSK and phase shift keying PSK These are discussed in more detail in the following sections v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 Unit 2 v原文 Apart from the translation of digital data into analog signals wireless transmission requires an additional modulation an analog modulation that shifts the center frequency of the base band signal generated by the digital modulation up to the radio carrier v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 Unit 2 v原文 For example digital modulation translates a 1 Mbps bit steam into a base band signal with a bandwidth of 1MHz There are several reasons why this base band signal cannot be directly transmitted in a wireless system v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 Unit 2 v原文 Apart from the translation of digital data into analog signals wireless transmission requires an additional modulation an analog modulation that shifts the center frequency of the base band signal generated by the digital modulation up to the radio carrier v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 Unit 2 v原文 For example digital modulation translates a 1 Mbps bit steam into a base band signal with a bandwidth of 1MHz There are several reasons why this base band signal cannot be directly transmitted in a wireless system v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 Unit 2 v原文 Antennas An antenna must be the order of magnitude of the wavelength of the signal in size to be effective For the 1MHz signal in the example this would result in an antenna some hundred meter high which is obviously not very practical for handheld devices With 1 GHz antennas of a few centimeters in length can be used v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 Unit 2 v原文 Frequency division multiplexing Using only base band transmission FDM could not be applied Analog modulation shifts the base band signals to different carrier frequencies The higher the carrier frequency the more bandwidth that is available for many base band signals v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 Unit 2 v原文 Medium characteristics Path loss penetration of obstacles reflection scattering and diffraction depend heavily on the wavelength of the signal v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 Unit 2 v原文 Depending on the applications the right carrier frequency with the desired characteristics has to be chosen long waves for submarines short waves for handheld devices very short waves for directed microwave transmission etc v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 Unit 2 v原文 As for digital modulation three different basic schemes are known for analog modulation amplitude modulation AM frequency modulation FM and phase modulation PM vFigure 2 1 shows a simplified block diagram of a radio transmitter for digital data v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 v图 2 1 表示一个数字无线反射区的简化程序框图 Unit 2 v原文 The first step is the digital modulation of data into the analog base band signal according to one of the schemes presented in the following section v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