The task of radio-frequency amplifier

The radio-frequency (r.-f.) amplifier employed in a broadcast frequency receiver may differ from that used in a receiver serving other purposes of tuned to other frequencies. A broadcast receiver, for example, must be capable of amplifying at any frequency between 500 and 1500 kc. It must be easily changed from one frequency to another and its amplification at all frequencies within this band should be uniform. It selects and amplifiers too. Its task is much more difficult to perform.

The energy thrust upon the other form a given broadcasting station is a complex bit of wave motion. If the microphone is idle what comes from the antenna may be considered as a very narrow band called the carrier wave. If a single tone, say 1000 cycles, is put into the microphone, the antenna current has frequencies of not only 600kc in it but 599 and 601 as well. When music is broadcast the frequencies in the antenna may be varying between zero and 5000 cycles above and below the carrier from instant to instant. These frequencies on either side of the carrier are called the side bands. The characteristics of the transmitter must be such that each of these audio frequencies is given equal power compared to the others.

Receivers

The essential components of a radio receiver are an antenna, amplifiers, a speaker and oscillators. The first component is used for receiving the electromagnetic waves and converting them into electrical oscillations. Amplifiers are employed for increasing the intensity of these oscillations, detection equipment for demodulating. The antenna needs a speaker for converting the impulses into sound waves audible by the human ear (and in television a picture tube for converting the signal into visible light waves). Besides in most radio receivers, oscillators are used to generate radio-frequency waves that can be “mixed” with the incoming waves.

The incoming signal from the antenna, consisting of a radio-frequency carrier oscillation modulated by an audio-frequency or video-frequency signal containing the impulses, is generally very weak. The sensitivity of some modern radio receivers is so great that if the antenna signal can produce an alternating current involving the motion of only a few hundred electrons, this signal can be detected and amplified to produce an intelligible sound from the speaker. Most radio receivers can operate quite well with an input from the antenna of a few millionths of a volt. The dominant consideration in receiver design, however, is that very weak desired signals cannot be made useful by amplifying indiscriminately both the desired signal and undesired radio noise. Thus, the main task of the designer is to assure preferential reception of the desired signal.

Most modern radio receivers are of the superheterodyne type in which an oscillator generates a radio-frequency wave that is mixed with the incoming wave, thereby producing a radio-frequency wave of lower frequency; the latter is called intermediate frequency. To tune the receiver to different frequencies, the frequency of the oscillations is changed, but the intermediate frequency always remains the same (at 455 kHz for most AM receivers and at 10.7 MHz for most FM receivers). The oscillator is tuned by altering the capacity of the capacitor in its tank circuit; the antenna circuit is similarly tuned by a capacitor in its circuit. One or more stages of intermediate-frequency amplification are included in all receivers; in addition, one or more stages of radio-frequency amplification may be included. Auxiliary circuits such as automatic volume control (which operates by rectifying part of the output of one amplification circuit and feeding it back to the control element of the same circuit or of an earlier one) are usually included in the intermediate-frequency stage. The detector, often called the second detector, the mixer being called the first detector, is usually simply a diode acting as a rectifier, and produces an audio-frequency signal. FM waves are demodulated or detected by circuits known as discriminators or radio-detectors that translate the varying frequencies into varying signal amplitudes.

Amplifiers

Radio-frequency and intermediate-frequency amplifiers are voltage amplifiers, increasing the voltage of the signal. Radio receivers may also have one or more stages of audio-frequency voltage amplification. In addition, the last stage before the speaker must be a stage of power amplification. A high-fidelity receiver contains both the tuner and amplifier circuits of a radio. Alternatively, a high-fidelity radio may consist of a separate audio amplifier and a separate radio tuner.

The principal characteristics of a good radio receiver are high sensitivity, selectivity, fidelity, and low noise. Sensitivity is primarily achieved by having numerous stages of amplification and high amplification factors, but high amplification is useless unless reasonable fidelity and low noise can be obtained. The most sensitive receivers have one stage of tuned radio-frequency amplification. Selectivity is the ability of the receiver to obtain signals from one station and reject signals from another station operating on a nearby frequency. Excessive selectivity is not desirable, because a bandwidth of many kilohertz is necessary in order to receive the high-frequency components of the audio-frequency signals. A good broadcast-band receiver tuned to one station has a zero response to a station 20 kHz away. The selectivity depends principally on the circuits in the intermediate-frequency stage.

Noise

Noise is a serious problem in all radio receivers. Several different types of noise each characterized by a particular type of sound and by a particular cause, have been given names. Among these are hum, hiss and whistle. The first one is a steady low-frequency note (about two octaves below middle C) commonly produced by the frequency of the alternating-current power supply (usually 60 Hz) becoming impressed onto the signal because of improper filtering or shielding. Hiss is a steady high-frequency note. And whistle is a pure high-frequency note produced by unintentional audio-frequency oscillation, or by beats. These noises can be eliminated by proper design and construction. Certain types of noise, however, cannot be eliminated. The most important of these in ordinary AM low-frequency and medium-frequency sets is static, caused by electrical disturbances in the atmosphere. Static may be due to the operation of nearby electrical equipment (such as automobile and airplane engines), but is most often caused by lightning. Radio waves produced by such atmospheric disturbances can travel thousands of kilometers with comparatively little attenuation, and inasmuch as a thunderstorm is almost always occurring somewhere within a few thousand kilometers of any radio receiver, static is almost always present. Static affects FM receivers to a much smaller degree, because the amplitude of the intermediate waves is limited in special circuits before discrimination, and this limiting removes effects of static, which influences the signal only by superimposing a random amplitude modulation on the wave.

Another basic source of noise is thermal agitation of electrons. In any conductor at a temperature higher than absolute zero, electrons are moving about in a random manner. Because any motion of electrons constitutes an electric current, this thermal motion gives rise to noise when amplification is carried too far. Such noise can be avoided if the signal received from the antenna is considerably stronger than the current caused by thermal agitation; in any case, such noise can be minimized by suitable design. A theoretically perfect receiver at ordinary temperatures can receive speech intelligibly when the signal power in the antenna is only 4 Ч 10-18 W (40 attowatts); in ordinary radio receivers, however, considerably greater signal strength is required.

net charge - результирующий заряд

sheath - оболочка

carrier - авианосец

men on foot - пехотинцы

phase shifter - фазовращатель

digital latching phase shifter - фазовращатель с цифровым управлением

sputtering - распыление

deposition - наращивание

substrate - подложка

kc (kilocycle per second) - килогерц, кГц