专业外语教案.doc

Communications 1This section serves to introduce the subject of “communications systems.” In its basic electrical sense, the term “communications”refers to the sending, reception and processing of information by electrical means,。

Radio communication was made possible by the invention of the triode. It has subsequently become even more widespread and refined through the invention and use of the transistor, integrated circuits and other semiconductor devices.A modern communications system is first concerned with the collation, processing and storage of information before its transmission. The actual transmission then follows. Finally we have reception.In order to become familiar with such a system, it is necessary first to know about amplifiers and oscillators. From time to time it is also well to consider some human factors as they influence a particular system.Before one studies a system, it is necessary to define and discuss some important terms. To correlate these concepts, a block diagram of a general communications system is shown in Fig.1.In long-distance communications, a transmitter is required to process the incoming information so as to make it suitable for transmission and subsequent reception.New Wordsequation 方程、等式basic 基本的、基础的communication 通讯、通信reception 接收invention 发明triode 三极管subsequently 随后；后来collation 整理；核对transmission 传输、传送；发射amplifier 放大器oscillator 振荡器diagram 图解；图示transmitter 发射机Phrases and Expressionsserved to (do) 适用于；用来refer to 提到；涉及；有关be concerned with 与…有关；涉及到know about 知道；了解from time to time 有时；不时block diagram 方框图so as to (do) 以至于；以便于be suitable for 适合于Radio Transmitters 2There are many kinds of radio transmitters, such as telecommunications transmitter, radar transmitter.A transmitter commonly consists of several parts. It is an equipment to send out radio waves. The use of a telecommunications transmitter is to transmit intelligence by radio. To transmit intelligence by radio, it is necessary to generate high-frequency signals, because radio waves can be sent out only if the frequency is high.If we want to detect object, the radar transmitter may be used. It transmits short signals, or pulses. When the radio waves meet objects, a part of them will be reflected. These reflected waves are called radio echoes. It is possible to detect objects by means of radio echoes.When we wish to obtain a good reflection of radio waves from an object, their wavelength must be less than the dimensions of the object. The shorter the wavelength, the better the reflection. The range of a radar set depends upon the output of its transmitter. It is possible to obtain high outputs at microwave bands. Only to detect the reflected signal is not enough. It is also necessary to know the distance and direction of the detected object.New wordstelecommunication 电讯，无线电通讯transmit 发射；发送；传送intelligence 信息；情报equipment 设备potential 势，位；潜力；势的，位的；frequency 频率pulse 脉冲echo 回波；回声wavelength 波长dimension 尺寸；维；量纲；大小microwave 微波band 带；波段；频带Phrases and Expressionssend out 发射by means of 利用only if 只有当….（才）；只有在….的时候；唯一的条件是Voltage, Resistance and Current 3Electricity is the main power used in industry. The engineering and technical personnel working in electronics have relations with voltage, resistance and current every day.Voltage is the potential difference in an electric circuit. The opposition given by a conductor or an insulator to the flow of electrons is called resistance.There are two kinds of current: direct current (DC) and alternating current (AC). A direct current is a current flowing in a conductor always in one direction. An alternating current is a current periodically changing its direction of flow.The electric current transmitted through electric power lines is alternating current. This is because AC is easy to control and more economical when we transmit it over long distances.Measuring with an ammeter and a voltmeter, and multiplying the amperes by the volts, we can get the power expended in a circuit. This is the most commonly used method when measure direct current. With alternating current, being unable to find the power expended in terms of the product of the amperes and volts, we have to use an instrument called “wattmeter.”From Ohm’s law we know that the current in an electric circuit is equal to the voltage divided by the resistance. In other words, we get the voltage when multiplying the current by the resistance.New Wordsengineering 工程personnel （全体）人员electronics 电子学periodically 周期性地voltmeter 电压计，伏特计multiply 乘expend 消耗wattmeter电表，瓦特计；功率表Three Functions of a Tuning Circuit 4The process of selecting the carrier wave of a desired station is referred to as tuning. This may be accomplished by adjusting one or more components of a series tuned circuit so that its resonant frequency will be equal to that of the desired carrier wave. The impedance of the series tuned circuit at resonance will be at its minimum value; therefore, the current in the series tuned circuit produced by the desired station adjustment will be at its maximum value.Selecting a desired signal is only one of three important functions performed by the tuning circuit. In addition, it must reject all undesired signals, and in many instances it also increases the voltage of the desired signal before passing it on to the following circuit. The ability of a radio receiver to accomplish each of its three functions is known as its sensitivity, selectivity, and fidelity. Although the terms and definitions presented here refer to radio receivers, they apply to tuning circuits in general as well as to all other forms of communication systems.Sensitivity is a measure of the ability of a radio receiver to reproduce, with satisfactory volume, weak signals received by the antenna.Selectivity is a measure of the ability of a radio receiver to reproduce the signal of one desired station and to exclude the signals from all others.Fidelity is a measure of the ability of a radio receiver to reproduce faithfully all the frequencies present in the original signal.New Wordscalculation 计算tune 调谐；语调carrier 载波；载体；载流子accomplish 完成，达到，实行resonant 谐振的，共鸣的impedance 阻抗resonance 谐振，共鸣adjustment 调节，调整receiver 接收机；接收者sensitivity 灵敏度selectivity 选择性fidelity 保真度，逼真度volume 体积；容量；音量antenna 天线original 最初的原先的有独创性的Phrases and Expressionso that以便于以至于in addition 此外on to 向，到….上apply to 适用于as well as 以及；除…..外（还）Binary-Coded-Decimal 5Since most of the electronic circuit elements used to construct digital computers are inherently binary in operation, the binary number system is the most natural number system for a computer. Also ,computers constructed using the binary number system require a smaller amount of circuitry and are therefore more efficient than machines operating in other number systems. On the other hand, the decimal system has been used for a long time, and there is a natural reaction to performing calculations in a binary number system. Also, since checks, bills, tax rates, prices, etc. are all figured in the decimal system, the values of most things must be converted from decimal to binary before computations can begin. For these and other reasons most of the early machines operated in binary-coded-decimal number systems. In such systems a coded group of binary bits is used to represent each of the 10 decimal digits. For instance, an obvious and natural code is a simple “weighted binary code,” as shown in Table 2-2.This is known as a binary-coded-decimal 8,4,2,1 code, or simply BCD. Notice that 4 binary bits are required for each decimal digit, and each bit is assigned a weight; for instance, the rightmost bit has a weight of 1 and the leftmost bit in each code group has a weight of 8. By adding the weights of the positions in which 1s appear, the decimal digit represented by a code group may be derived. This is somewhat uneconomical since 24=16, and therefore the 4 bits could actually represent 15 different values; but the next lesser choice, 3 bits, gives only 23, or 8, values, which are insufficient. If the decimal number 214 is to be represented in this type of code, 12 binary bits are required as follows: 0010 0001 0100. For the decimal number 1246 to be represented, 16 bits are required: 0001 0010 0100 0110.New WordsInherently 天性地，固有地circuitry电路, 线路Radio Receivers 6What must a receiver do? It has been known that the signal received is generally quite weak, powers of the order of picowatts being common. Therefore, the receiver must amplify the received signal. Since the signal is quite likely to be accompanied by lots of other (undesired) signals, it must be selected and then rejected. Finally, a process of demodulation must be performed in the receiver to recover the original modulation voltages.This chapter will deal with radio receivers in general, showing why their format has been to a certain extent standardized. Each block of the receiver will be discussed in detail. This will be done for receivers corresponding to all the modulation systems so far studied, whether they are for domestic or professional purposes. Although they follow the same basic pattern, television receivers will be dealt with separately. This is because they have a range of functions and a degree of complexity all their own.It is apparent that a receiver has the function of selecting the desired signal from all the other undesired signals, amplifying and demodulating it, and displaying it in the desired manner. This outline of functions to be performed shows that the major difference between receivers of various types is likely to be in the way they demodulate the received signal. In turn, this will depend on the type of modulation employed, whether it is AM, FM, SSB, or any of the forms treated later. However, it really appears that the same type of receiver should be capable of dealing with the basic requirements, and this will indeed be seen.New Wordspicowatt 皮瓦amplify 放大accompany 伴随demodulation 解调，反调制；检波standardize 使标准化correspond 符合，一致；相应，相当modulation 调制domestic 国内的；国产的；家庭的complexity 复杂性apparent 明显的outline 概要，提纲；略述requirement 要求，必要条件Phrases and Expressionsdeal with 处理；论述correspond to 相应的so far 迄今为止；就此范围来说 a range of 一系列的it is apparent that 显然；很明显in turn 依次，轮流Electromagnetic Waves 7It follows from electromagnetic induction that whenever there is a change in a magnetic field, an electric field is produced, and it follows from Maxwell’s hypothesis that whenever there is a change in an electric field a magnetic field is produced. It is evident that it is impossible to have either effect occur alone. The electric filed arising from a change in a magnetic field is in itself a change in the pre-existing electric field, and therefore causes another magnetic field. The latter magnetic field, too, represents a change, and from the change an electric field is in turn produced. The process continues indefinitely, with a definite coupling between the fluctuating electric and magnetic fields. On the basis of his hypothesis, together with the other principles of electricity and magnetism, Maxwell was able to develop a detailed picture of how these field fluctuations travel through space.The first idea emerging from Maxwell’s hypothesis was that the field fluctuations spread out in space from an initial disturbance in the same manner that waves spread out from an initial disturbance in a body of water: hence the name electromagnetic waves to describe them.New Wordshypothesis 假设evident 明显的indefinitely 无限的coupling 耦合fluctuate vi. 变动，起伏fluctuation n. 变动，起伏disturbance 扰动The Concept of Electric Field 8What can we say about the mechanism of electric force? The theory of action at a distance is sufficiently noncommittal to be accepted without much critical thought. Electric forces exist; theses forces act at a distance. Yet this is almost no theory at all. To describe what happens is often useful, but most of us wish to explain a phenomenon is terms of familiar laws and facts of physics. In other words, we intuitively seek a model for electric forces. Several models have been proposed as descriptions of electric force. A useful concept, that of electric filed, was introduced by Michael Faraday (1791-1867). We have set up a procedure to define what we mean by electric field.Nowadays, we consider that lines of force have no objective reality but are merely a convenient and useful representation of a field. Faraday went must farther; he made a model of electric forces. According to this model, the lines of force were considered to be the bounding edges of what he called tubes of force. Faraday, like most scientists of his time, imagined all space to be filled with an invisible weightless “ether” with certain elastic properties. The tubes of force, being made of ether, were like stretched rubber bands. In addition, Faraday advanced the hypothesis that adjacent tubes of force repelled each other. This ingenious model was refined and developed by Maxwell and others to include a description of electromagnetic forces as well as electrostatic forces.New Wordsmechanism 机构；结构；途径sufficiently 足够地，充分地noncommittal 不明朗的intuitively 直觉地；直观地procedure 步骤，过程；程序convenient 方便的weightless 无重量的ether 以太elastic 弹性的stretch 拉伸hypothesis 假设；前提adjacent 邻近的；毗邻的ingenious 机灵的，有独创性的；精致的electrostatic 静电的Phrases and Expressionsat a distance 隔开一段距离；远距离not …at all 毫不，一点也不，根本不in terms of 根据，依照set up 建立，设立Modulation 9Communication of intelligence from one point in space to another requires variations of some form in the signal used. A steady tone or a steady radio wave cannot convey intelligence. The variations produced in a radio wave for the purpose of including a message on this wave are produced by what is referred to as modulation. Television, for example, employs both AM and FM in the envelope.The elements of a communication system are the transmitter, the device to produce symbols in suitable form to be carried by the medium, and the element to transfer the symbols from the transmitter to the receiver. The receiver interprets the signal for thelistener.The telephone is a good example. The communication medium is a length of wire connecting the transmitter to the receiver. In the basic telephone system, the transmitter converts the voice sounds to variations in electric voltages and currents for transmission over the wire connecting one station to another.In radio communications, what the transmitter dose is generate electromagnetic waves to be transmitted through space to receive. The radio frequency spectrum is above the audio range, so that an unvarying electromagnetic radio wave cannot contain intelligence to be demodulated at the receiver to provide an audible signal. This is true only if the radio wave or carrier is of steady amplitude and frequency.New Wordsvariation 变化tone 音调convey 传送，传输envelope 包络medium 介质；媒介spectrum 谱audio 音频的，声音的amplitude 幅度，振幅Noise 10Noise is probably the only topic in electronics and telecommunications with which everyone must be familiar, no matter what his or her specialization. It is ever present, and limits the performance of virtually every system. Also, the measurement of it is very contentious, with almost everybody having a different pet method of quantifying it and its effects.Noise may be defined, in an electrical sense, as any unwanted form of energy tending to interfere with the proper and easy reception and reproduction of wanted signals. Many disturbances of an electrical nature produce noise in receivers, modifying the signal in an unwanted manner. In radio receivers, for example, noise may cause the loudspeaker output to sound fuzzy, whereas in television receivers “snow”or “confetti”(colored snow) become superimposed on the picture. In pulse communications systems, noise may masquerade as desired pulses or perhaps obliterate them; it may cause serious errors in this fashion. It is thus seen that noise must limit the range of systems (for a given transmitted power), and affect the sensitivity of receivers by placing a limit on the lowest signals that can be amplified. It may sometimes even force the reduction in the bandwidth of a system, as will be seen in radar.There are numerous ways of classifying noise. It may be subdivided according to type, source, effect, or relation to the receiver, depending on circumstances. It is most convenient here to divide noise into two broad groups: noise whose sources are external to the receiver, and noise created within the receiver itself. On the one hand, external noise is difficult to treat quantitatively, and furthermore there is often little that can be done about it, short of changing the geographical location of the system. Note how radiotelescopes are always located away from industry, whose processes create so much electrical noise. Satellite earth stations are also located in remote areas. On the other hand, internal noise is both more quantifiable and capable of being reduced by appropriate receiver design.Because noise has such a limiting effect, and also because it is often possible to reduce its effects through intelligent circuit use and design, it is most important for all those connected with communications to be well informed about noise.New Wordsspecialization 专业；专门化contentious 有争议的；好争吵的perception 感知；感觉confetti 五彩纸屑superimposed成阶层的，有层理的masquerade 化装obliterate 涂去，删除，使湮没classify 分类，分等circumstance 环境，情况quantitatively 数量上，定量Direct and Alternating Current 11The current which flows steadily in one direction is usually called a direct current. A direct current is, of course, useful. It is the kind of current which is always associated with batteries. We know that the electrical system in an automobile and an airplane, the telegraph, the telephone and the trolley-bus use the direct current. Direct current is also used to meet some of the industrial requirements.For industrial and many other purposes, however, most cities make use of another type of electric current which flows first in one direction and then in another. It was given the name of an alternating current. We know that the alternating current is the very current that makes radio possible.Although the direct current is useful, it has one great disadvantage, that is, there is no easy, economical way to increase or decrease its voltage. The alternating current has not this disadvantage. Using a special device, it is possible to transform a high voltage into a low voltage or a low voltage into a high voltage. The device that we use to transform the voltage is called a transformer. The alternating current can also be changed easily into the direct current. The device which can do this is spoken of as a rectifier.New Wordssteadily 稳定地associate 与…相关trolley-bus 无轨电车disadvantage 缺点transformer 转换，转变transformer 变压器、转换器rectifier 整流器The transformer 12One cannot call a transformer a machine, for it has no moving parts. We know the transformer to be an apparatus which is designed for changing the alternating voltages and alternating currents by means of magnetic induction without any change of frequency.One of the great advantages of the alternating current is the ease and efficiency with which power at low voltage may be transformed into an almost similar amount of power at high voltage, and vice versa. Using a transformer, it is possible to transmit the alternating current to very distant places at which the power is required.A two-winding transformer is known to consist of two coils which are so arranged that the magnetic lines of force of one coil pass through the other. The alternating current in one coil induces an emf in the other because of the alternations in the value of the coil current.In order to strengthen the magnetic field passing through the coils of a transformer, a closed core of iron is generally used. The coil on which the current is impressed on the input side of the transformer is called the primary, while the one from which the induced current is obtained on the output side is called the secondary.The power output of a transformer is necessarily less than the power input because of unavoidable losses. These losses include resistance losses in the primary and secondary windings, and losses in the core due to hysteresis and eddy currents.New Wordsapparatus 器械；仪器；设备ease 容易；不费力；自在efficiency 效率；效能distant 遥远的winding 绕组coil 线圈alternation 交替；交变core 芯primary 初级的secondary 次级的hysteresis 滞后；磁滞作用eddy 涡流Phrases and Expressionstransform A into B 把A转换成B vice versa 反过来（也是一样）pass through 通过due to 由于…(而引起的) ；应归于because of 由于Transformers 13One of the great advantages of AC over DC for electric power distribution is that it is much easier to step voltage levels up and down with AC than with DC. For long–distance power transmission it is desirable to use as high a voltage and as small a current as possible. The necessary voltage conversion is accomplished by use of transformers.A transformer is a component consisting of two or more coils that are coupled together by magnetic induction. Owing to this device the power may be transmitted at a high voltage, and reduced at the place at which it is required to a value suitable for electric motors and other machines.The coil to which the voltage is applied is known as the primary, while the one from which the induced voltage is obtained is known as the secondary. If the secondary has more turns than the primary, the transformer is called a step-up transformer in which the AC voltage of the secondary wingding is higher than that applied to the primary winding. If the secondary has fewer turns than the primary, the transformer is called a step-down transformer in which the AC voltage of the secondary winding is lower than that applied to the primary winding. In a well-built transformer the product of the secondary voltage multiplied by the secondary current is almost equal to the product of the primary voltage times the primary current.New Wordsdistribution 分配；配电desirable 所希望的couple 耦合step-up 升压的step-down 降压的multiply乘Electronic digital computers 14Modern science and engineering use mathematics as a language for expressing physical laws in precise terms. The electronic digital computer is a valuable tool for studying the consequences of these laws.An algebraic formula is an expression of a mathematical relationship. Many of the laws of physics, electronics, chemistry, etc., are expressed in this form, in which case digital computers may be easily used, because algebraic formulas may be directly changed to the basic steps they represent.In general there are two types of digital computers. The first is the special-purpose digital computer, which performs a fixed and preset sequence of calculations. This type of computer may be constructed more efficiently in that it can be lighter and smaller and may consume less power, etc. , than the general-purpose computer. Because of the advantages in construction, small special-purpose computers are used where such factors as weight, power consumption, etc., are critical.The second type of computer is defined as a general-purpose digital computer. The sequence of instructions the machine follows is generally read into this type of machine and stored in the memory of the machine. Since the sequence of operations performed by the general-purpose digital computer may be easily changed, the machine possesses great flexibility, and this is the type of the machine generally used in business and for scientific computations.New Wordsprecise 精确的consequence 后果，结果algebraic 代数的mathematical 数学的preset 预置，预定sequence 程序，指令程序；数列consume 消耗construction 结构，构造consumption 消耗instruction 指令；说明书flexibility 适应性，灵活性computation 计算Phrases and Expressionschange with 随…而变化in that 因为；在于read…into 把…读入Lasers 15To understand what a laser is, one has to understand how light is generated. Light comes from the electrons which surround the nucleus of every normal atom. When the atom, let us say, of sodium, is in its unexcited state the electrons that surround the sodium nucleus are in their normal energy levels. If this sodium atom absorbs some energy from outside, one or more of the electrons jumps from a low energy level to a higher energy level. The atom then remains in this excited state until it is able to release the energy it has temporarily stored. When this release occurs the electron in the high energy level falls back to the low energy level. But as it does so it emits the extra energy as a pulse we may see as light.The laser beam is made by exciting the atoms of a suitable material until most of the atoms have electrons orbiting in a higher energy level than usual. The excitation is then stopped and all the excited electrons fall back together to their normal orbits, each one emitting a pulse of light of the same energy, which generates an intense beam of light for a very short time. Every pulse in this beam is in step with every other pulse. In this way a beam of light is obtained which is both monochromatic and coherent.New Wordslaser 激光，激光器sodium 钠unexcited 未激活的release 释放beam 光束，射束orbit 绕轨道旋转（运行）；轨道excitation 激励intense 剧烈的monochromatic 单色的coherent 相干的；粘着的How Electricity Flows 16Electrons, as one knows, are minute negative charge of electricity. As these minute charges move along a wire, that wire is said to carry an electric current. In other words, an electric current is the flow of electrons through a metal conductor. That is why electrons play a very important role in the formation of a current. The electrons flow along a wire much the same way as water runs through a pipe.Though the electrons move from minus to plus, the electric current is conventionally considered to flow from the positive to the negative terminal.In order to have a steady current, we must have a completed circuit that is also called “closed circuit” and a continuous supply of electric charge. (The terms “closed circuit” and “completed circuit”being synonyms, we shall meet the one as well as the other when reading technical literature.) Another important factor to take into consideration is that current strength is equal at all points of a series circuit.As has just been stated, the electric current flows only when there is a completed circuit from the negative to the positive terminal. Thus, current starts flowing just at the moment we close the circuit.There are, as is already known to all, two kinds of current: DC and AC. Both of which are very useful in industry as well as in many other fields.New Wordsformation 形成，产生pipe 管子，管道minus 负号，减号，负数；负的plus 正数，正量；正号，加号；正的conventionally 习惯地，按照惯例synonym 同义词literature 文献，文学consideration 考虑，考虑因素Phrases and Expressionsthat is why 这就是…的理由；这就是为什么…much the same way as 与…差不多一样（地）take …into consideration 把…考虑进去；考虑到Electromotive Force (EMF) 17In order for a steady current to exist in a conducting path, that path must form a closed loop, or complete circuit. Otherwise charge would accumulate at the ends of the conductor, the resulting electric field would change with time, and the current could not be constant.However, such a path cannot consist entirely of resistance. Current in a resistor requires an electric field and an associated potential difference. The field always does positive work on the charge, which always moves in the direction of decreasing potential. But after a complete trip around the loop, the charge returns to its starting point, and the potential there must be the same as when it left that point. This is impossible if the trip around the loop involves only decreases in potential.We can compare this situation with that of a decorative water fountain. Water emerges from openings at the top, cascades down over terraces and spouts, and eventually reaches the basin in the bottom. It collects there and runs into a pump that lifts it back to the top for another trip. If there were no pump, the water would not be able to circulate continuously.Thus in the electric circuit there must be some part of the loop where a charge travels “uphill”, from lower to higher potential, despite the fact that the electrostatic force is trying to push it from higher to lower potential. The influence that makes charge move from lower to higher potential is called electromotive force.New Wordselectromotive 电动的loop 环，圈；回路accumulate 积聚；堆积decorative 装饰的fountain 喷泉opening 孔，空隙；开口cascade （使）瀑布似地落下；（使）级联terrace 台地；阶地spout 喷管；喷口；水落管；流道pump 泵circulate 循环uphill 往上坡Phrases and Expressionscompare A with B 把A与B相比较despite the fact that 尽管Magnetism 18Many scientists have worked at the theory of magnetism since its discovery. However, magnetism has long stopped being a problem. At present any of us knows that in magnetic materials, the molecules themselves are tiny magnets, each having a north and a south pole.。