无线电接收机英文资料及中文翻外文翻译.docx

1、无线电接收机英文资料及中文翻外文翻译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, l

2、oudspeaker, and power 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 n

3、ecessary that the 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 circ

4、uits, which can be 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

5、frequencies; this 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 discrimina

6、te between radio waves 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 e

7、nergy abstracted from 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 b

8、e applied to the rectified 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 t

9、ransmitted is reproduced 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 c

10、urrent thus produced 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 amplitu

11、de of the original 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 c

12、ircuit from which 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 elec

13、tromagnetic of waves. 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

14、. Radio Waves can 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

15、or ground wave. This 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 ra

16、nge may be about 1500km 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 fo

17、rm about 80km above 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 c

18、ontinues 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

19、such as 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 he

20、rtz up to 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

21、occurs when 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. opp

22、oses the 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

23、current 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 ob

24、tained. 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 natu

25、ral oscillations 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

26、 in it when 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 .

27、AMPLIFIERIntroductionThe 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 numb

28、er of functions. 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 au

29、dio power 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 ster

30、eo system 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 signa

31、l and the 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 ideal, but

32、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 distortion. 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 background hiss