无线电接收机英文资料及中文翻译

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, loudspeaker, and power supply.

Fig.2-4 A Block Diagram For Modern Radio Receiver

Any 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 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 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 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 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 energy 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 be 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 transmitted 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 current 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 amplitude 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 circuit 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 Waves

Radio Waves are a member of the electromagnetic of waves. They are energy-carriers which trave l at the speed of light (ν), their frequency(?) and wavelength(λ) being related , as for any wave motion, by the equation

ν=?* λ

where ν=c=3.0*108 m/s in a vacuum (or air). If λ=300m, then ?=ν/λ=3.0*108 /(3.0*10 2)=106Hz=1MHz. The smaller λis, the larger ?.

Radio Waves can be described either by their frequency or their wavelength. But the former is more fundamental since, unlike λ (and ν ), 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.. This travels along a ground, the curvature of the earth’s 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 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 form about 80km above the earth to 50km, which have become positively charged through the remova l of electrons by the sun’s 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 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 hills, buildings or trees.

Oscillators

Electrical 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 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 LC 21, i.e. its resonant frequency f0. Electrical resonance thus 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. opposes 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 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 obtained. In practice , energy is gradually dissipated by resistance as heat and a damped oscillation is produced.

Oscillator

As 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 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 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 .

AMPLIFIER

Introduction

The 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 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 audio 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 stereo 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 signal 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 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" 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 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 necessarily 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 amplifiers 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 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 biggest 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 transformers 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 not 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 "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 circuitry 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 inefficient 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 "signal" 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 coupling.

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 where 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 similar 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 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 amplifiers 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.

Class 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 person. 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 stereo 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 class 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

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

ν=?* λ

这里，ν=c=3.0*108 m/s（在空气中），如果λ=300m,则?=ν/λ=3.0*108 /(3.0*10 2)=106Hz.=1MHz。波长λ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LC 21，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，这个感应电动势与电流相反。当电容放电完成以后，两金属片之间电场中储存的电场能全部转变成了电感中的电磁能。

然后，电感对电容反充电，电场能重新储存在电容中。当电容再次放电，回路电流反相，再电感中产生极性相反的电磁场。然后电感再次放电，电容再次充电，电流再次反相，这个过程不断重复。

若电路中无电阻，则可获得无衰减的正弦交流电流。实际上，由于回路损耗电阻的存在，能量会逐渐以热能的形式消耗掉，产生的 是一个逐渐衰减的振荡波形。

振荡器

当振荡电路的电阻增加，振荡迅速衰减。为了获得不衰减的振荡信号，必须对LC 回路馈入能量，以补充因回路电阻而损耗的能量，这在实用振荡电路中是通过晶体管完成的。

一个简单的调谐振荡器如图2-2（b ）所示，LC 回路接在晶体管的集电极回路（作为负载），当开关合上时，振荡开始，振荡频率由LC 回路的谐振频率决定。晶体管

只是将电源的能量馈给振荡回路以保证正常的状态，晶体管的基极偏置电流由电阻R 获得。

放大器

这种放大器是非常常见的，通常来说，这个放大器的目的是用来接收一个信号并使之放大，放大器常常应用在各种电子设备中，用来执行各种功能。各种不同类型的放大器有不同的用途。例如，音频传送使用RF放大器，（RF表示音频）；这样的放大器用来放大从天线接收来的信号，这篇文章将着重介绍音频放大器，这种音频放大器来驱动扬声器。特殊的，下面讨论的是为DJ和音频放大，大部分的放大器应用于家庭立体声系统。

简单来说，放大器的目的是从一个信号源得到一个信号并放大能使它去驱动扬声器，唯一不同的是：输出信号是输入信号的放大。从数学角度来说，如果输入信号定义为S，则输出信号为X*S，X为放大系数常量。*代表乘运算。

在现实当中，没有一种放大器是理想的，在先进的设备中它们能更好的工作。放大器的输出包含一些输入信号没有的讯杂信号，这种讯杂信号被集中到一块现象统称为失真。------

失真的类型很多，最常见的两种失真是调谐失真和交调失真，额外的无用信号通常被成为失真，所有的放大器都能产生一定量的噪音，更详细的内容将在以后介绍。

放大器

所有放大器都有一定放大系数，它的基本单位被称为瓦特。放大器的系数决定了它的负载阻抗，它的基本单位被称为欧姆。常用的阻抗有8欧姆、4欧姆、2欧姆，当低阻抗负载被使用时放大器的放大系数会更高。

在早期，放大器设备是电子管来实现的，电子管很少被使用在DJ中，现代放大器都使用的是晶体管，晶体管和真空放大器在信号传输过程中经历了相似的过程，但他们工作环境有一些区别，电子管一般是高压低流，而晶体管则相反，电子管的效率不是很高，并且会发热，电子管放大器与晶体管放大器的一个最大的区别是真空放大器在音频输出上要求更高，高品质的音频传输器是很难设计的，设计起来更困难，更昂贵。晶体管放大器比电子管放大器更有实用价值，因为它具有高效、小而耐用，没有音频的传输是必须的，晶体管不需要周期性替换，与人们所通常认为不同的是，好的电子管放大器能产生高品质的声音，有些人认为电子管有他们独特的“声音”。这个“声音”是当他们达到极限时产生的结果。晶体管放大器代替了电子管放大器是有很大益处的。

放大器的种类

放大器的种类通常是按照放大电路来设计的，在音频放大有许多种类，下面简要介绍常用放大器的种类

A类A类具有非常低的失真，尽管他们效率非常低，很少用在高效率放大器中。它的失真很低是因为当空载时放大器工作在放大区的中点，结果，许多能量在无信号输入时已经被消耗掉了，因为A类放大器具有低失真的特性，所以常常当在放大器的前级，作为在电路中信号到达电压摆动的极限时，A类放大器的失真产生了消波，A类放大器是通过电容偶合连接扬声器的。

B类B类放大器通常被使用在廉价、低品质设计中，比起A类放大器，B类放大器有更高的效率，然而，当信号弱时会产生交迭失真，B类常常用在经济型电路中。在IC放大器出现之前，B类放大器一般用在收音机电路，便携式收音机以及对音质要求不高的电路中。

AB类AB类在家庭立体声和相似放大器最常用的一种放大器，AB类放大器综合了A类B类放大器的优点。它具有B类放大器的高效率和A类放大器的低失真。克服了B类弱信号失真和A类的削波失真。AB类放大器使用的是一副晶体管，它都处于低导通状态，这样更大消除了交迭失真。

C类C类放大器常常不用在音频电路中。她们通常用在RF电路中。C类放大器工作在具有大失真的晶体管电路中，C类放大器使用在带滤波的RF电路从而使信号在最后完整的接收，C类放大器的效率是很高的。

D类D类放大器的概念已经出现很长时间了，尽管到目前才被普遍使用。在快速发展以后，随着高容量、高效率现代半导体设备的出现，普通人使用D类放大器也成为了现实。D类放大器可应用与把音频信号调制到高频信号，这种放大器通常用在辅助低音频信号，这种D类放大器具有很好的效率。使高频信号夹带在音频信号中，在立体声应用D类放大器常常去限制辅助低频，尽管设计总是不停的发展。全波段D类放大器的普遍使用已经为期不远了。

其他类有许多其他类放大器，像G，H，S等。它们大部分与AB类设计区别不大，尽管它们设计有更高的效率和更高的输出级别，因为我也没有对它们更深入的了解，在这里我就不详细的介绍了，我们只对它有了初步的了解。