无线电接收机英文及中文翻译

1、英文资料及中文翻译Radio Receiver A block diagram for a modern radio receiver is shown in Fig.2-4.The input signals to this radio are amplitude-modulated radio waves. The basic electronic circuits include: antenna ,tuner, mixer, local oscillator ,IF amplifier, audio detector, AF amplifier, loudspeaker, and po

2、wer supply.Fig.2-4 A Block Diagram For Modern Radio ReceiverAny antenna system capable of radiating electrical energy is also able to abstract energy from a passing radio wave. Since every wave passing the receiving antenna. Induces its own voltage in the antenna conductor, it is necessary that the

3、receiving equipment be capable of separating the desired signal from the unwanted signals that are also inducing voltages in the antenna. This separation is made on the basis of the difference in frequency between transmitting stations and is carried out by the use of resonant circuits, which can be

4、 made to discriminate very strongly in favor of a particular frequency. It has already been pointed that, by making antenna circuit resonant to a particular frequency, the energy abstracted from radio waves of that frequency will be much greater than the energy from waves of other frequencies; this

5、alone gives a certain amount of separation between signals. Still greater selective action can be obtained by the use of additional suitably adjusted resonant circuits located somewhere in the receiver in such a way as to reject all but the desired signal. The ability to discriminate between radio w

6、aves of different frequencies is called selectivity and the process of adjusting circuits to resonance with the frequency of a desired signal is spoken of as tuning.Although intelligible radio signals have been received from the stations thousands of miles distant, using only the energy abstracted f

7、rom the radio wave by the receiving antenna much more satisfactory reception can be obtained if the received energy is amplified. This amplification may be applied to the radio-frequency currents before detection, in which case it is called radio-frequency amplification or it may be applied to the r

8、ectified currents after detection, in which case it is called audio-frequency amplification. The use of amplification makes possible the satisfactory reception of signals from waves that would otherwise be too weak to give an audible response.The process by which the signal being transmitted is repr

9、oduced from the radio-frequency currents present at the receiver is called detection, or sometimes demodulation. Where the intelligence is transmitted by varying the amplitude of the radiated wave, detection is accomplished by rectifying the radio frequency current. The rectified current thus produc

10、ed varies in accordance with the signal originally modulated on the wave irradiated at the transmitter and so reproduces the desired signal. Thus, when the modulated wave is rectified, the resulting current is seen to have an average value that varies in accordance with the amplitude of the original

11、 signal.Receiver circuit are made up a of a number of stages. A stage is a single transistor connected to components which provide operating voltages and currents and also signal voltages and currents. Each stage has its input circuit from which the signal comes in and its output circuit from which

12、the signal, usually amplified, goes out. When one stage follows another, the output circuit of the first feeds the signal to the second. And so the signal is amplified, stage by stage, until it strong enough to operate the loudspeaker.Radio WavesRadio Waves are a member of the electromagnetic of wav

13、es. They are energy-carriers which travel at the speed of light (), their frequency() and wavelength() being related , as for any wave motion, by the equation =* where =c=3.0*108 m/s in a vacuum (or air). If =300m, then =/=3.0*108 /(3.0*10 2)=106Hz=1MHz. The smaller is, the larger . Radio Waves can

14、be described either by their frequency or their wavelength. But the former is more fundamental since, unlike (and ), f does not change when the waves travel form one medium to another. Radio Waves can travel form a transmitting aerial in one or more of three different ways.Surface or ground wave. Th

15、is travels along a ground, the curvature of the earths surface. Its range is limited mainly by the extent to which energy is absorbed form it by the ground. Poor conductors such as sand absorb more strongly that water, and the higher the frequency the greater the absorption. The range may be about 1

16、500km at low frequencies (long wave, but much less for v. h. f.).Sky wave. This travels skywards and, if it is below a certain critical frequency (typically 30MHz), is returned to earth by the ionosphere. This consists of layers of air molecules (the D,E and F layer), stretching form about 80km abov

17、e the earth to 50km, which have become positively charged through the removal of electrons by the suns ultraviolet radiation. On striking the earth the sky wave bounces back to the ionosphere where it is again gradually refracted and returned earthwards as if by 'reflection '. This continues

18、 until it is completely attenuated.Space wave. For v. h . f., u. h. f. and microwave signals, only the space wave, giving line-of sight transmission, is effective. A range of up to 150km is possible on earth if the transmitting aerial is on high ground and there are no intervening obstacles such as

19、hills, buildings or trees. OscillatorsElectrical oscillators are widely used in radio and television transmitters and receivers, in signal generators, oscilloscopes and computers, to produce A.C. with waveforms which may be sinusoidal, square, sawtooth etc. and with frequencies from a few hertz up t

20、o millions of hertz. Oscillatory circuit When a capacitor discharges through an inductor in a circuit of low resistance, an A.C. flows. The circuit is said to oscillate at its natural frequency which, as we will show shortly, equals, i.e. its resonant frequency f0. Electrical resonance thus occurs w

21、hen the applied frequency equals the natural frequency as it does in a mechanical system.In Fig,2-2(a) , a charged capacitor C is shown connected across a coil L.C immediately starts to discharge, current flows and a magnetic field is created which induces an e. m. f. in L. This e. m. f. opposes the

22、 current . When C is completely discharged the electrical energy originally stored in the electric field between its plates has been transferred to the magnetic field around L. By the time the magnetic field has collapsed, the energy is again stored in C. Once more C starts to discharge but current

23、now flows in the opposite direction, creating a magnetic field of opposite polarity. When this field has decayed, C is again charged with its upper plate positive and the same cycle is repeated. In the absence of resistance in any part of the circuit , an undamped sinusoidal A.C. would be obtained.

24、In practice , energy is gradually dissipated by resistance as heat and a damped oscillation is produced.OscillatorAs the resistance of an LC circuit increases, the oscillation decay more quickly. To obtain undamped oscillations, energy has to be fed into the LC circuit in phase with its natural osci

25、llations to compensate for the energy dissipated in the resistance of the circuit. This can be done with the help of a transistor in actual oscillators. A simple tuned oscillator is shown in Fig.2-2(b). The LC circuit is connected in the collector circuit (as the load) and oscillations start in it w

26、hen the supply is switched on . The frequency of the oscillations is given by, i.e. then natural frequency of the LC circuit. The transistor merely ensures that energy is fed back at the correct instant from the battery. The current bias for the base of the transistor is obtained through R .AMPLIFIE

27、RIntroductionThe term amplifier is very generic. In general, the purpose of an amplifier is to take an input signal and make it stronger (or in more technically correct terms, increase its amplitude). Amplifiers find application in all kinds of electronic devices designed to perform any number of fu

28、nctions. There are many different types of amplifiers, each with a specific purpose in mind. For example, a radio transmitter uses an RF Amplifier (RF stands for Radio Frequency); such an amplifier is designed to amplify a signal so that it may drive an antenna. This article will focus on audio powe

29、r amplifiers. Audio power amplifiers are those amplifiers which are designed to drive loudspeakers. Specifically, this discussion will focus on audio power amplifiers intended for DJ and sound reinforcement use. Much of the material presented also applies to amplifiers intended for home stereo syste

30、m use.The purpose of a power amplifier, in very simple terms, is to take a signal from a source device (in a DJ system the signal typically comes from a preamplifier or signal processor) and make it suitable for driving a loudspeaker. Ideally, the ONLY thing different between the input signal and th

31、e output signal is the strength of the signal. In mathematical terms, if the input signal is denoted as S, the output of a perfect amplifier is X*S, where X is a constant (a fixed number). The "*" symbol means? Multiplied by". This being the real world, no amplifier does exactly the i

32、deal, but many do a very good job if they are operated within their advertised power ratings. The output of all amplifiers contain additional signal components that are not present in the input signal; these additional (and unwanted)characteristics may be lumped together and are generally known as d

33、istortion. There are many types of distortion; however the two most common types are known as harmonic distortion and inter modulation distortion. In addition to the "garbage" traditionally known as distortion, all amplifiers generate a certain amount of noise (this can be heard as a backg

34、round "hiss" when no music is playing). More on these later.All power amplifiers have a power rating, the units of power are called watts. The power rating of an amplifier may be stated for various load impedances; the units for load impedance are ohms. The most common load impedances are

35、8 ohms, 4 ohms, and 2 ohms (if you have an old vacuum tube amplifier the load impedances are more likely to be32 ohms, 16 ohms, 8 ohms, and maybe 4 ohms). The power output of a modern amplifier is usually higher when lower impedance loads (speakers) are used (but as we shall see later this is not ne

36、cessarily better).In the early days, power amplifiers used devices called vacuum tubes (referred to simply as "tubes" from here on). Tubes are seldom used in amplifiers intended for DJ use (however tube amplifiers have a loyal following with musicians and hi-fi enthusiasts). Modern amplifi

37、ers almost always use transistors (instead of tubes); in the late 60's and early 70's, the term "solid state" was used (and often engraved on the front panel as a "buzz word"). The signal path in a tube amplifier undergoes similar processing as the signal in a transistor

38、amp, however the devices and voltages are quite different. Tubes are generally "high voltage low current" devices, where transistors are the opposite ("low voltage high current"). Tube amplifiers are generally not very efficient and tend to generate a lot of heat. One of the bigg

39、est differences between a tube amplifier and a transistor amplifier is that an audio output transformer is almost always required in a tube amplifier (this is because the output impedance of a tube circuit is far too high to properly interface directly to a loudspeaker). High quality audio output tr

40、ansformers are difficult to design, and tend to be large, heavy, and expensive. Transistor amplifiers have numerous practical advantages as compared with tube amplifiers: they tend to be more efficient, smaller, more rugged (physically), no audio output transformer is required, and transistors do no

41、t require periodic replacement (unless you continually abuse them). Contrary to what many people believe, a well designed tube amplifier can have excellent sound (many high end hi-fi enthusiasts swear by them). Some people claim that tube amplifiers have their own particular "sound". This

42、"sound" is a result of the way tubes behave when approaching their output limits (clipping). A few big advantages that tube amplifiers have were necessarily given up when amplifiers went to transistors. What are Amplifier Classes?The Class of an amplifier refers to the design of the circui

43、try within the amp. There are many classes used for audio amps. The following is brief description of some of the more common amplifier classes you may have heard of. Class A: Class A amplifiers have very low distortion (lowest distortion occurs when the volume is low) however they are very ineffici

44、ent and are rarely used for high power designs. The distortion is low because the transistors in the amp are biased such that they are half "on" when the amp is idling. As a result, a lot of power is dissipated even when the amp has no music playing! Class A amps are often used for "s

45、ignal" level circuits (where power is small) because they maintain low distortion. Distortion for class A amps increases as the signal approaches clipping, as the signal is reaching the limits of voltage swing for the circuit. Also, some class A amps have speakers connected via capacitive coupl

46、ing. Class B: Class B amplifiers are used in low cost, low quality designs. Class B amplifiers are a lot more efficient than class A amps, however they suffer from bad distortion when the signal level is low (the distortion is called "crossover distortion"). Class B is used most often wher

47、e economy of design is needed. Before the advent of IC amplifiers, class B amplifiers were common in clock radio circuits, pocket transistor radios, or other applications where quality of sound is not that critical. Class AB: Class AB is probably the most common amplifier class for home stereo and s

48、imilar amplifiers. Class AB amps combine the good points of class A and B amps. They have the good efficiency of class B amps and distortion that is a lot closer to a class A amp. With such amplifiers, distortion is worst when the signal is low, and lowest when the signal is just reaching the point

49、of clipping. Class AB amps (like class B) use pairs of transistors, both of them being biased slightly ON so that the crossover distortion (associated with Class B amps) is largely eliminated. Class C: Class C amps are never used for audio circuits. They are commonly used in RF circuits. Class C amp

50、lifiers operate the output transistor in a state that results in tremendous distortion (it would be totally unsuitable for audio reproduction). However, the RF circuits where Class C amps are used employ filtering so that the final signal is completely acceptable. Class C amps are quite efficient. C

51、lass D: The concept of a Class D amp has been around for a long time, however only fairly recently have they become commonly used. Due to improvements in the speed, power capacity and efficiency of modern semiconductor devices, applications using Class D amps have become affordable for the common pe

52、rson. Class D amplifiers use a very high frequency signal to modulate the incoming audio signal. Such amps are commonly used in car audio subwoofer amplifiers. Class D amplifiers have very good efficiency. Due to the high frequencies that are present in the audio signal, Class D amps used for car st

53、ereo applications are often limited to subwoofer frequencies, however designs are improving all the time. It will not be too long before a full band class D amp becomes commonplace. Other classes: There are many other classes of amplifiers, such as G, H, S, etc. Most of these are variations of the c

54、lass AB design, however they result in higher efficiency for designs that require very high output levels (500W and up for example). At this time I will not go into the details of all of these other classes as I have not studied them all in detail. Suffice to be aware that they exist for now. 无线电接收机

55、图2-4为无线电接收机的方框图,输入信号为调幅无线电波。它的基本组成包括天线、调谐回路、混频器、本振电路、中放放大器、检波器、音频放大器、喇叭、电源等。任何天线系统既能辐射无线电波又能接收无线电波。任何经过天线的无线电波均能在天线中感应电压，因此，接收机必须能够从天线所收到的所有信号中分离出有用信号。这个分离过程是根据发射端发射的信号频率不同，利用调谐回路完成的。调谐回路能够有效地从众多频率中选择出某一个特定频率。通过天线调谐回路对某一特定频率地谐振，可以使天线从这一特定频率中吸收的能量比从其他平频率中吸收的能量大得多，这样，就从某种程度上实现了信号的分离。进一步的选择作用可以通过接收机中的某

56、些经过适当调谐的谐振回路实现，以这种方式进一步去除了有用信号以外的其他信号。将不同频率的无线电波加以区别的能力称为选频，将谐振回路的频率调在有用信号频率上的过程称为调谐。尽管接收的有用信号来自几千里以外，但如果经过放大，通过天线获得的信号还是具有令人满意的效果。放大过程可能应用在对检波前的射频电流，这种情况称为射频放大；也可应于检波后，这种情况称为音频放大。放大器的应用使令人满意的接收成为可能，否则，有些太弱的信号不能获得好的收听效果。从射频信号中重视被传输的原始信号的过程称为检波或解调。如果有用信号在发射时是通过改变信号的振幅（即调幅），则检波就是通过对射频电流进行整流完成的。整流电流随着原

57、始调制信号而变化，从而冲县了原始的有用信号，这样，已调波被整流而产生的电流可以被看成随原始信号幅度变化的平均值电流。接收机的电路由多级组成。每级由晶体管与提供工作电压、电流和信号电压、电流的元件相连构成，每级都有输入回路，它让信号进入；有输出回路，它让通常是放大后的信号输出。当一级接一级时，第一级的输出回路将信号馈送给第二级，信号经过逐级放大，直到足以推动扬声器。无线电波无线电波是电磁波大家族中的一员，它们携带能量且以光速在空气中传播，它们的频率与波长相关，即任何电磁波传播时，有 =* 这里，=c=3.0*108 m/s（在空气中），如果=300m,则=/=3.0*108 /(3.0*10 2

58、)=106Hz.=1MHz。波长越小，频率越高。无线电波既能用频率又能用波长来描述。但前者更常用，因为频率不像速度，不会因传播媒介的改变而变化。从天线电波辐射出去的无线电波通常以三种形式传播。（a）地表波或地波。这种波按地球表面的曲度，沿地表面传播。它的传播范围有限，其能量易被地表面吸收。恶劣的地形条件如沙漠比水面更易吸收能量。频率越高，能量被吸收得越多。低频波（长波）的传播范围约为1500千米。高频波的查范围要小得多。（b）天波。沿天空传播，若低于某个关键频率（如30MHZ），会被电离层反射回地面。电离层由空气分子层组成（包括D、E、F层），位于地球上方80千米到500千米处，它由于太阳紫外

59、线的辐射而失去电子，因而带正电荷。反射回地面的天波又从地面反射回电离层，并再次被反射回地面，如此反复多次直到能量完成衰减。(c) 空间波。甚高频、超高频和微波只能以空间波的形式才能有效 传播，空间波也称视距传播。如果天线架设很高且没有障碍阻隔，如高山、建筑物、大树等，空间波的传播距离可达150千米振荡器 电子振荡器广泛用于广播、电视发射机、接收机、信号发生器、示波器及计算机中，它被用来产生几赫兹到几百万赫兹的各种波形，如正弦波、方波、锯齿波等。振荡电路在一个低阻的RC 回路中，电容通过电感放电，回路中有交流电流流过，则称回路发生了振荡，其振荡频率等于，正好等于它的谐振频率 f0 。当外加信号频率等于回路的固有振荡频率时，回路发生谐振，这也和机械振动系统相似。再图2-1（a）中，一个充电的电容和一个电感线圈相连，电容立刻开始放电，电流流过电感并在其中产生磁场和感应电动势