专业英语考试复习总结.docx

1、专业英语考试复习总结1.1Electronic communication 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行信息的发送、接收和处理。1.2Regardless of the particular application, all communications systems involve three main

2、subsystems: the 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 communications channel must be large (wide) enough to pass all significant information frequencies. In other word, the bandwidth of the communications channel must be equal to or grea

3、ter than the bandwidth of information. For example, 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

4、2700Hz的带宽。Information capacity is a measure of how much information can be transferred through a communications system in a given period of time信息容量是在给定时间内,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 computer data; two-dimensional, as in the case of pic

5、tures; three dimensional, as 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 conv

6、erts a two-dimensional image 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 exa

7、mple, the communications agree 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 ret

8、ransmission.假设，例如，通信同意使用奇偶校验位；然后将在数据位的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 compression:Lossless compression operates by removing the redundant information contained in the data of interest. The compression is said to be lossless because it

9、 is completely reversible in that the original data can be reconstructed exactly. Lossless compression is also referred to as data compaction.Lossy compression involves the loss of information in a controlled manner; the compression may therefore not be completely reversible. Lossy compression is, h

10、owever, capable of achieving a compression ratio radio higher than that attainable with lossless methods.基本上，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精确重构。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 communication systems, the received waveform is usually

11、categorized into the desired part containing the information and the extraneous 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部分，所希望的部分包含信息，还有不希望的额外部分。希望的部分称为信号，而不希望的部分成为噪声。3.3Electrical noise may be defined as any undesired voltages

12、or currents that ultimately end up appearing in the receiver output. 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

13、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，幅度足够大的，以致淹没了所希望接受的信息。 The noise present in a received radio signal has been introduced in the transmitting medium and is termed external noise. The noise introduced by the receiver is termed internal noise. The important implications of

14、noise considerations in the study of communications systems cannot be overemphasized .接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内部噪声。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 maximum current rating .Similarly capaci

15、tors have a capacitance rating in farads and a maximum voltage rating. When selecting coils and capacitors for use at radio frequencies and above, an additional characteristic must be considered-the quality(Q) of the component.The Q is a ratio of the energy stored to that which is lost in the compon

16、ent .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 communication system is an oscillator . An oscillator is a circuit capable of converting energy from a DC form

17、 to AC. In other words, an oscillator generates a waveform. The waveform can be of any type 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

18、: Output frequency required. Frequency stability required. Is the frequency to be variable, and if so, over what range? Allowable waveform distortion. Power output required .These performance considerations, combined with economic factors, will dictate the form of oscillator to be used in a given ap

19、plication.振荡器是通信系统中一种最基本的构件。振荡器是能把直流能量转换成交流的一种电路。也就是说，振荡器能产生波形。波形可以是任何形状，但总有某一重复频率。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，其变化范围是什么？允许的波形失真。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 gradual sinusoidal current buildup that reaches maximum when the

21、capacitor is fully discharged. At this point the potential energy is zero, but since current flow is maximum, the magnetic field energy around the inductor is maximum. 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 exchange of energy is known as the flywheel effect .图4-4（a）中的电容器先充电到某个点位，然后闭合开关就会产生图4-4（b）所示的波形。开关闭合所引起的电流，使电容器开始通过电感放电。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，该电流达到最大值。此时，电位能量等于零。但因电流最大，故电感周围的

23、磁场能量达到最大。该磁场能量因失去电容器电压的维持而开始减小，但其反电动势将保持电流按相同的方向流动。因此电容器又开始充电，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。4.3 A typical waveform generator consists of four basic sections:an oscillator to generate the basic periodic waveform; a wave shaper; an optional AM modulator; and an output buffer amplifier to isolate the

24、oscillator from the load and provide the necessary drive current .一个典型的波形发生器由四个基本部分组成: 振荡器, 用来产生基本的周期波形。 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 transformed form t

25、heir 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 original form.然而，

26、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 comprised of relatively low-frequency

27、information signals which may be a single frequency or a complex 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 high-level

28、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 FET transmi

29、tter, 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 input carri

30、er 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 extremely inefficient. Emitter modulators are also incapable of producing hig

31、h-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.Alexander Graham Bell had inve

32、nted 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 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 communication. 1880，一个年轻的科学家已经赢得了国际声誉，用光束在无线电话发射的声音。亚力山大格雷厄姆贝尔发明电话早了四年，但他认为光影电话是他最伟大的发明。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