專業(yè)英語考試復(fù)習(xí)總結(jié).doc

1.1Electronic communication can be summarized as the transmission , reception , and processing of information between two or more locations using electronic circuits .電子通信就是利用電路在兩地或多地之間進(jìn)行信息的發(fā)送、接收和處理。1.2Regardless of the particular application, all communications systems involve three main subsystems: the transmitter, the channel, and the receiver.無論何種具體應(yīng)用，通信系統(tǒng)都包括三個(gè)主要的子系統(tǒng)：發(fā)送機(jī)、信道和接收機(jī)。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 greater 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.信道的帶寬必須足夠大(寬),以便所有重要的信息頻率都能通過。換句話說,信道的帶寬必須等于或大于信息的帶寬。例如,話音頻率包含300Hz到3000Hz的信號(hào)。因此,話音頻率的信道必須具有等于或大于2700Hz的帶寬。Information capacity is a measure of how much information can be transferred through a communications system in a given period of time信息容量是在給定時(shí)間內(nèi),能通過通信系統(tǒng)傳送多少信息的量度。2.1The signal can be one-dimensional, as in the case of speech, music, or computer data; two-dimensional, as in the case of pictures; three dimensional, as in the case of video data; and four-dimensional, as in the case of volume data over time.信息可以是一維的，例如語音、音樂和計(jì)算機(jī)數(shù)據(jù)；也可以是二維的，例如圖像；也可以是三維的，例如視頻數(shù)據(jù)；還可以是四維的，例如隨時(shí)間變化的立體空間數(shù)據(jù)2.2The type of scanning used in television is a form of a spatial sampling called raster scanning, which converts 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.電視用的掃描是一種稱為柵格掃描的空間取樣形式，它把二維的圖像亮度轉(zhuǎn)換為一維波形，類似于我們閱讀印刷品時(shí)的逐行從左到右的掃描。2.3Suppose, for example, 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 retransmission.假設(shè)，例如，通信同意使用奇偶校驗(yàn)位；然后將在數(shù)據(jù)位的1的數(shù)目甚至和1時(shí)，奇怪的是0。因此，如果在一個(gè)字節(jié)一個(gè)單點(diǎn)接收誤差，從而違反奇偶校驗(yàn)規(guī)則，它可以檢測(cè)并更正通過重傳。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 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, however, capable of achieving a compression ratio radio higher than that attainable with lossless methods.基本上，有兩種形式：數(shù)據(jù)壓縮的無損壓縮通過去除冗余信息包含在感興趣的數(shù)據(jù)。壓縮是無損的，因?yàn)樗峭耆赡娴脑紨?shù)據(jù)可精確重構(gòu)。無損壓縮也被稱為數(shù)據(jù)壓縮。有損壓縮包括以受控的方式的信息損失；壓縮可能因此不完全可逆。有損壓縮，然而，能夠?qū)崿F(xiàn)壓縮比無線電高于無損方法實(shí)現(xiàn)。3.1In communication systems, the received waveform is usually 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.通信系統(tǒng)接收到的波形通常包括兩部分，所希望的部分包含信息，還有不希望的額外部分。希望的部分稱為信號(hào)，而不希望的部分成為噪聲。3.3Electrical noise may be defined as any undesired voltages 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. 電氣噪聲可以定義為最終出現(xiàn)在接收機(jī)輸出端的任何不希望的電壓或電流。對(duì)聽者來說，電氣噪聲常表現(xiàn)為靜電噪聲。它可能只是令人討厭的偶發(fā)靜電噪聲；也可能是連續(xù)的，幅度足夠大的，以致淹沒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 noise considerations in the study of communications systems cannot be overemphasized .接受信號(hào)中由傳輸媒介引入的噪聲稱為外部噪聲。由接收機(jī)自身引入的噪聲稱為內(nèi)部噪聲。在研究通信系統(tǒng)時(shí)，考慮噪聲的重要性，再怎么強(qiáng)調(diào)都不過分。4.1Practical inductors (coils) used at RF frequencies and above have an inductance rating in henrys and a maximum current rating .Similarly capacitors 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 component .用于射頻火更高頻率的實(shí)用電感（線圈）有一個(gè)標(biāo)稱電感值，以亨利（H）為單位；還有一個(gè)額定的最大電流。類似地，電容器有一個(gè)標(biāo)稱電容量，以法拉（F）為單位；還有一個(gè)額定的最大電壓。當(dāng)在射頻或更高頻率選用線圈和電容器時(shí)，還有另一個(gè)特性是必須考慮的，這就是元件的品質(zhì)因數(shù)（Q）。Q是元件儲(chǔ)能與耗能的比值。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 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: 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 application.振蕩器是通信系統(tǒng)中一種最基本的構(gòu)件。振蕩器是能把直流能量轉(zhuǎn)換成交流的一種電路。也就是說，振蕩器能產(chǎn)生波形。波形可以是任何形狀，但總有某一重復(fù)頻率。有各種不同的正弦波振蕩器用于各種電子電路。振蕩器類型的選擇基于如下的考慮：所需的輸出頻率。所需的頻率穩(wěn)定性頻率是可變的嗎？如果是，其變化范圍是什么？ 允許的波形失真。所需的輸出功率。通過對(duì)這些性能的考慮，再加上經(jīng)濟(jì)方面的因素，就可確定在一給定應(yīng)用中所用的振蕩器類型。 The effect of charging the capacitor in Figure 4-4(a) to some voltage potential and 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 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 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 .圖4-4（a）中的電容器先充電到某個(gè)點(diǎn)位，然后閉合開關(guān)就會(huì)產(chǎn)生圖4-4（b）所示的波形。開關(guān)閉合所引起的電流，使電容器開始通過電感放電。電感會(huì)阻止電荷的流通，從而形成一個(gè)以正弦規(guī)律漸變的電流。當(dāng)電容器完全放電時(shí)，該電流達(dá)到最大值。此時(shí)，電位能量等于零。但因電流最大，故電感周圍的磁場(chǎng)能量達(dá)到最大。該磁場(chǎng)能量因失去電容器電壓的維持而開始減小，但其反電動(dòng)勢(shì)將保持電流按相同的方向流動(dòng)。因此電容器又開始充電，但極性與原來的充電相反。這種反復(fù)的能量交換就是所謂的飛輪效應(yīng)。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 oscillator from the load and provide the necessary drive current .一個(gè)典型的波形發(fā)生器由四個(gè)基本部分組成: 振蕩器, 用來產(chǎn)生基本的周期波形。 波形成形器。 幅度調(diào)制器(選用)。 輸出緩沖放大器, 它把振蕩器與負(fù)載隔離開來, 并提供必要的驅(qū)動(dòng)電流。6.1However, the original information signals are seldom in a form that is suitable for transmission. Therefore, they must be 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 original form.然而，原始的信息信號(hào)很少具有適于傳輸?shù)男问?。因此，必須把它們從原來的形式變換成適于傳輸?shù)男问?。把低頻的信息信號(hào)加載到高頻載波信號(hào)的過程稱為調(diào)制。解調(diào)則是相反的過程，它把接收到的信號(hào)變換回原來的形式。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 information signals which may be a single frequency or a complex waveform made up of many frequencies.幅度調(diào)制器是有兩個(gè)輸入和一個(gè)輸出的非線性器件。其中一個(gè)輸入是具有單一高頻的等幅載波信號(hào)；另一個(gè)輸入是頻率相對(duì)較低的信息信號(hào)，它可能是單頻的，也可能是有很多頻率組成的復(fù)雜波形。6.2The location in a transmitter where modulation occurs determines whether the circuit is a low or 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 FET transmitter, or prior to the place of the output tube in a vacuum-tube transmitter.調(diào)制器出現(xiàn)在發(fā)射機(jī)中的位置決定了該電路是低電平還是高電平發(fā)射機(jī)。低電平調(diào)制時(shí)，調(diào)制出現(xiàn)在發(fā)射機(jī)末級(jí)的輸出元件之前。換句話說，在晶體管發(fā)射機(jī)的輸出晶體管的集電極之前，或在場(chǎng)效應(yīng)管發(fā)射機(jī)的輸出場(chǎng)效應(yīng)管的漏極之前，或在真空管發(fā)射機(jī)的輸出管的板極之前With emitter modulation, the amplitude of the output signal depends on the amplitude of the 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 extremely inefficient. Emitter modulators are also incapable of producing high-power output waveforms.發(fā)射極調(diào)制的輸出信號(hào)幅度，取決于輸入載波的幅度，以及放大器的電壓增益。調(diào)制系數(shù)則完全取決于調(diào)制信號(hào)的幅度。發(fā)射極調(diào)制的主要缺點(diǎn)是放大器工作在A類，其放大率很低。發(fā)射極調(diào)制器也不適于產(chǎn)生大功率輸出波形。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 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 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. 1880，一個(gè)年輕的科學(xué)家已經(jīng)贏得了國(guó)際聲譽(yù)，用光束在無線電話發(fā)射的聲音。亞力山大格雷厄姆貝爾發(fā)明電話早了四年，但他認(rèn)為光影電話是他最偉大的發(fā)明。它再現(xiàn)聲音的檢測(cè)在日光或人工光源量集中在空中上一個(gè)接收器的變化。貝爾很開心聽到輕笑梁和唱歌，但從來沒有被證明是實(shí)用的光影電話的城市，那里有太多的東西可以在路上的光。線和無線電波傳播計(jì)算更符合實(shí)際。12.2Direct modulation is simple and inexpensive.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 output optical signal is proportional to the input electrical signal.直接調(diào)制是簡(jiǎn)單和便宜的。它的作品最好的發(fā)光二極管（LED）和半導(dǎo)體激光器，因?yàn)樗麄兊墓廨敵鲭S驅(qū)動(dòng)電流通過的半導(dǎo)體器件。輸入信號(hào)調(diào)制的驅(qū)動(dòng)電流，使輸出光信號(hào)的輸入電信號(hào)成比例。External modulation is more complex,and external modulators cost more than driving the light source directly.However it generally offers higher performance.外部調(diào)制是更復(fù)雜的，和外部調(diào)制器的成本比直接驅(qū)動(dòng)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 then amplify that signal and process it to decode the signal.光纖接收器檢測(cè)到從纖維的新興的光信號(hào)并將其轉(zhuǎn)換成電子形式。一個(gè)探測(cè)器（通常簡(jiǎn)稱檢測(cè)器）從光接收機(jī)產(chǎn)生的電流或電壓。在接收電子再放大，信號(hào)進(jìn)行解碼的信號(hào)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 and 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.接收機(jī)的工作取決于它的靈敏度，速度如何，和信號(hào)到達(dá)它的力量。這，反過來，取決于兩個(gè)檢測(cè)器和電子信號(hào)處理電路產(chǎn)生的性能。探測(cè)器相對(duì)較寬的波長(zhǎng)范圍內(nèi)的響應(yīng)，但不可見和紅外波長(zhǎng)的光纖系統(tǒng)可以傳輸?shù)恼麄€(gè)范圍。課后題目4.將下述句子譯成英文。（lesson1） (1)模擬信息源產(chǎn)生的消息是一個(gè)連續(xù)集合，而數(shù)字信息源產(chǎn)生的則是消息的有限集合。An analog information source produces messages that are defined on a continuum, whilea digital information source produces a finite set of possible messages. (2)我國(guó)古代的烽火臺(tái)也是一個(gè)通信系統(tǒng) The beacon-fire tower in ancient China was a communications system. (3)證明當(dāng)發(fā)出二進(jìn)制數(shù)1和0的概率相等時(shí)，熵（entropy）是最大的 Show that the entropy is a maximum when the probability of sending a binary 1 is