专业英语考试复习总结

1、. 1.1Electronic munication can be summarized as the transmission , reception , and processing of information between two or more locations using electronic circuits .电子通信就是利用电路在两地或多地之间进展信息的发送、接收和处理。1.2Regardless of the particular application, all munications systems involve three main subsystems: th

2、e transmitter, the channel, and the receiver.无论何种具体应用，通信系统都包括三个主要的子系统：发送机、信道和接收机。1.4The bandwidth of a munications channel must be large(wide) enough to pass all significant information frequencies. In other word, the bandwidth of the munications channel must be equal to or greater than the bandwidt

3、h of information. For e*ample, voice frequencies contain signals between 300Hz and 3000Hz. Therefore, a voice-frequency channel must have a bandwidth equal to or greater than 2700Hz.信道的带宽必须足够大(宽),以便所有重要的信息频率都能通过。换句话说,信道的带宽必须等于或大于信息的带宽。例如,话音频率包含300Hz到3000Hz的信号。因此,话音频率的信道必须具有等于或大于2700Hz的带宽。Information

4、 capacity is a measure of how much information can be transferred through a munications system in a given period of time信息容量是在给定时间,能通过通信系统传送多少信息的量度。2.1The signal can be one-dimensional, as in the case of speech, music, or puter data; two-dimensional, as in the case of pictures; three dimensional, as

5、 in the case of video data; and four-dimensional, as in the case of volume data over time.信息可以是一维的，例如语音、音乐和计算机数据；也可以是二维的，例如图像；也可以是三维的，例如视频数据；还可以是四维的，例如随时间变化的立体空间数据2.2The type of scanning used in television is a form of a spatial sampling called raster scanning, which converts a two-dimensional image

6、 intensity into one-dimensional waveform; it is somewhat analogous to the manner in which we read a printed paper in that the scanning is preformed form left to right on a line-by-line basis.电视用的扫描是一种称为栅格扫描的空间取样形式，它把二维的图像亮度转换为一维波形，类似于我们阅读印刷品时的逐行从左到右的扫描。2.3Suppose, for e*ample, the munications agree

7、to use even parity ; then the parity bit will be a 0 when the number of 1s in the data bit is even and a 1 when it is odd. Hence, if a single bit in a byte is received in error and thereby violates the even parity rule, it can be detected and then corrected through retransmission.假设，例如，通信同意使用奇偶校验位；然

8、后将在数据位的1的数目甚至和1时，奇怪的是0。因此，如果在一个字节一个单点接收误差，从而违反奇偶校验规则，它可以检测并更正通过重传。Basically, there are two forms of data pression:Lossless pression operates by removing the redundant information contained in the data of interest. The pression is said to be lossless because it is pletely reversible in that the origi

9、nal data can be reconstructed e*actly. Lossless pression is also referred to as data paction.Lossy pression involves the loss of information in a controlled manner; the pression may therefore not be pletely reversible. Lossy pression is, however, capable of achieving a pression ratio radio higher th

10、an that attainable with lossless methods.根本上，有两种形式：数据压缩的无损压缩通过去除冗余信息包含在感兴趣的数据。压缩是无损的，因为它是完全可逆的原始数据可准确重构。无损压缩也被称为数据压缩。有损压缩包括以受控的方式的信息损失；压缩可能因此不完全可逆。有损压缩，然而，能够实现压缩比无线电高于无损方法实现。3.1In munication systems, the received waveform is usually categorized into the desired part containing the information and th

11、e e*traneous or undesired part. The desired part is called the signal, and the undesired part is called noise.通信系统接收到的波形通常包括两局部，所希望的局部包含信息，还有不希望的额外局部。希望的局部称为信号，而不希望的局部成为噪声。3.3Electrical noise may be defined as any undesired voltages or currents that ultimately end up appearing in the receiver output

12、. To the listener this electrical noise often manifests itself as static. It may only be annoying, such as an occasional burst of static, or continuous and of such amplitude that the desired information is obliterated. 电气噪声可以定义为最终出现在接收机输出端的任何不希望的电压或电流。对听者来说，电气噪声常表现为静电噪声。它可能只是令人讨厌的偶发静电噪声；也可能是连续的，幅度足够

13、大的，以致淹没了所希望承受的信息。 The noise presentin a received radio signal has been introduced in thetransmitting medium and is termed e*ternal noise. The noise introduced by the receiver is termed internal noise. The important implications of noise considerations in the study of munications systems cannot beove

14、remphasized .承受信号中由传输媒介引入的噪声称为外部噪声。由接收机自身引入的噪声称为部噪声。在研究通信系统时，考虑噪声的重要性，再怎么强调都不过分。4.1Practical inductors (coils)used at RF frequencies and above have an inductance rating in henrys and a ma*imum current rating .Similarly capacitors have a capacitance rating in farads and a ma*imum voltage rating. When

15、 selecting coils and capacitors for use at radio frequencies and above, an additional characteristic must be considered-the quality(Q) of the ponent.The Q is a ratio of the energy stored to that which is lost in the ponent .用于射频火更高频率的实用电感线圈有一个标称电感值，以亨利H为单位；还有一个额定的最大电流。类似地，电容器有一个标称电容量，以法拉F为单位；还有一个额定的

16、最大电压。当在射频或更高频率选用线圈和电容器时，还有另一个特性是必须考虑的，这就是元件的品质因数Q。Q是元件储能与耗能的比值。4.2The most basic building block in a munication system is an oscillator . An oscillator is a circuit capable of converting energy from a DC form to AC. In other words, an oscillator generates a waveform. The waveform can be of any type

17、but occurs at some repetitive frequency. A number of different forms of sine-wave oscillators are available for use in electronic circuits . The choice of an oscillator type is based on the following criteria: Output frequency required. Frequency stability required. Is the frequency to be variable,

18、and if so, over what range“ = 4 * GB3 * MERGEFORMAT Allowable waveform distortion. = 5 * GB3 * MERGEFORMAT Power output required .These performance considerations, bined with economic factors, will dictate the form of oscillator to be used in a given application.振荡器是通信系统中一种最根本的构件。振荡器是能把直流能量转换成交流的一种电

19、路。也就是说，振荡器能产生波形。波形可以是任何形状，但总有*一重复频率。有各种不同的正弦波振荡器用于各种电子电路。振荡器类型的选择基于如下的考虑：所需的输出频率。所需的频率稳定性频率是可变的吗？如果是，其变化围是什么？ = 4 * GB3 * MERGEFORMAT 允许的波形失真。 = 5 * GB3 * MERGEFORMAT 所需的输出功率。通过对这些性能的考虑，再加上经济方面的因素，就可确定在一给定应用中所用的振荡器类型。The effect of charging the capacitor in Figure 4-4(a) to some voltage potential and

20、 then closing the switch results in the waveform shown in Figure 4-4(b) .The switch closure starts a current flow as the capacitor begins to discharge through the inductor. The inductor, which resists a charge in current flow, causes a gradualsinusoidal current buildup that reaches ma*imum when the

21、capacitor is fully discharged. At this point the potential energy is zero, but since current flow is ma*imum, the magnetic field energy around the inductor is ma*imum. The magnetic field no longer maintained by capacitor voltage then starts collapse, and its counter EMF will keep current flowing in

22、the same direction, thus charging the capacitor to opposite polarity of its original charge .This repetitive e*change of energy is known as the flywheel effect .图4-4a中的电容器先充电到*个点位，然后闭合开关就会产生图4-4b所示的波形。开关闭合所引起的电流，使电容器开场通过电感放电。电感会阻止电荷的流通，从而形成一个以正弦规律渐变的电流。当电容器完全放电时，该电流到达最大值。此时，电位能量等于零。但因电流最大，故电感周围的磁场能量

23、到达最大。该磁场能量因失去电容器电压的维持而开场减小，但其反电动势将保持电流按一样的方向流动。因此电容器又开场充电，但极性与原来的充电相反。这种反复的能量交换就是所谓的飞轮效应。4.3A typical waveform generator consists of four basic sections:an oscillator to generate the basic periodic waveform; a wave shaper; an optional AM modulator;and = 4 * GB3 * MERGEFORMAT an output buffer amplifi

24、er to isolate the oscillator from the load and provide the necessary drive current .一个典型的波形发生器由四个根本局部组成: 振荡器, 用来产生根本的周期波形。 波形成形器。 幅度调制器(选用)。 输出缓冲放大器, 它把振荡器与负载隔离开来, 并提供必要的驱动电流。6.1However, the original information signals are seldom in a form that is suitable for transmission. Therefore, they must be

25、transformed form their original form into a form that is more suitable for transmission. The process of impressing low-frequency information signals onto a high-frequency carrier signals is called modulation. Demodulation is the reverse process where the received signal are transformed back to their

26、 original form.然而，原始的信息信号很少具有适于传输的形式。因此，必须把它们从原来的形式变换成适于传输的形式。把低频的信息信号加载到高频载波信号的过程称为调制。解调则是相反的过程，它把接收到的信号变换回原来的形式。AM modulator are nonlinear devices with two inputs and one output. One input is a single, high-frequency carrier signal of constant amplitude and the second input is prised of relatively

27、 low-frequency information signals which may be a single frequency or a ple* waveform made up of many frequencies.幅度调制器是有两个输入和一个输出的非线性器件。其中一个输入是具有单一高频的等幅载波信号；另一个输入是频率相对较低的信息信号，它可能是单频的，也可能是有很多频率组成的复杂波形。6.2The location in a transmitter where modulation occurs determines whether the circuit is a low or

28、 high-level transmitter. With low-level modulation, the modulation takes place prior to the output element of the final stage of the transmitter, in other words, prior to the collector of the output transistor in a transistor in a transistorized transmitter, prior to the drain of the output FET in a

29、 FET transmitter, or prior to the place of the output tube in a vacuum-tube transmitter.调制器出现在发射机中的位置决定了该电路是低电平还是高电平发射机。低电平调制时，调制出现在发射机末级的输出元件之前。换句话说，在晶体管发射机的输出晶体管的集电极之前，或在场效应管发射机的输出场效应管的漏极之前，或在真空管发射机的输出管的板极之前With emitter modulation, the amplitude of the output signal depends on the amplitude of the

30、 input carrier and the voltage gain of the amplifier. The coefficient of modulation depends entirely on the amplitude of the modulating signal. The primary disadvantage of emitter modulation is the amplifier operates class A, which is e*tremely inefficient. Emitter modulators are also incapable of p

31、roducing high-power output waveforms.发射极调制的输出信号幅度，取决于输入载波的幅度，以及放大器的电压增益。调制系数则完全取决于调制信号的幅度。发射极调制的主要缺点是放大器工作在A类，其放大率很低。发射极调制器也不适于产生大功率输出波形。12.1In 1880,a young scientist who had already earned an international reputation used beams of light to transmit voice in the wireless telephone.Ale*ander Graham B

32、ell had invented the telephone four years earlier,but he considered the photophone his greatest invention .It reproduced voices by detecting variations in the amount of sunlight or artificial light focused through the open air onto a receiver.Bell was elated to hear beams of light laugh and sing,but

33、 the Photophone never proved practical in cities,where too many things could get in the way of the light .Wires and radio waves proved more practical for munication. 1880，一个年轻的科学家已经赢得了国际声誉，用光束在无线发射的声音。亚力山大格雷厄姆贝尔创造早了四年，但他认为光影是他最伟大的创造。它再现声音的检测在日光或人工光源量集中在空中上一个接收器的变化。贝尔很开心听到轻笑梁和唱歌，但从来没有被证明是实用的光影的城市，那里有

34、太多的东西可以在路上的光。线和无线电波传播计算更符合实际。12.2Direct modulation is simple and ine*pensive.It works best for light-emitting diodes(LEDs) and semiconductor lasers,because their light output increase with the drive current passing through the semiconductor device.The input signal modulates the drive current,so the

35、output optical signal is proportional to the input electrical signal.直接调制是简单和廉价的。它的作品最好的发光二极管LED和半导体激光器，因为他们的光输出随驱动电流通过的半导体器件。输入信号调制的驱动电流，使输出光信号的输入电信号成比例。E*ternal modulation is more ple*,and e*ternal modulators cost more than driving the light source directly.However it generally offers higher perfo

36、rmance.外部调制是更复杂的，和外部调制器的本钱比直接驱动光源。然而它通常提供更高的性能。Fiber-optic receivers detect the optical signal emerging from the fiber and convert it into electronic form.A photodetector(usually simply called a detector) generates an electric current or voltage from the light it receiver.Electronic in the receiver

37、then amplify that signal and process it to decode the signal.光纤接收器检测到从纤维的新兴的光信号并将其转换成电子形式。一个探测器通常简称检测器从光接收机产生的电流或电压。在接收电子再放大，信号进展解码的信号How well the receiver does its job depends on its sensitivity,speed,and the strength of signal reaching it.This,in turn ,depends on performance of both the detector a

38、nd circuits that process the electronic signal it generates.Detectors respond to a relatively broad range of wavelengths ,but not to entire range of visible and infrared wavelengths that fiber systems can transmit.接收机的工作取决于它的灵敏度，速度如何，和信号到达它的力量。这，反过来，取决于两个检测器和电子信号处理电路产生的性能。探测器相对较宽的波长围的响应，但不可见和红外波长的光纤

39、系统可以传输的整个围。课后题目4.将下述句子译成英文。lesson1 (1)模拟信息源产生的消息是一个连续集合，而数字信息源产生的则是消息的有限集合。An analog information source produces messages that are defined on a continuum, whilea digital information source produces a finite set of possible messages. (2)我国古代的烽火台也是一个通信系统The beacon-fire tower in ancient China was a munications system. (3)证明当发出二进制数1和0的概率相等时，熵entropy是最大的 Show that the entropy is a ma*imum when the probability of sending