Receivers

l. The essential components of a radio receiver are an antenna for receiving the electromagnetic waves and converting them into electrical oscillations; amplifiers for increasing of intensity of those oscillations; detection equipment for demodulating; 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); and, in most radio receivers, oscillators to generate radio-frequency waves that can be "mixed" with the incoming waves.

2. 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 hundreds 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.

3. Most modern radio receivers are of the superheterodyne type in which an oscillator generates 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 445 kHz for most AM receivers and at 107 MHz for most FM receivers). The oscillator is tuned by alternating 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 may be 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.

4. 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.

5. 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 receiver 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 circuit in the intermediate-frequency stage.

III. After-text exercises:

1. Agree or disagree with the next statements:

1) 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 strong.

2) Most radio receivers can operate quite well with an input from the antenna of a few millionths of a volt.

3) To tune the receiver to different frequencies, the frequency of the oscillations is not changed.

4) The detector is usually simply a diode acting as a rectifier, and produces an audio-frequency signal.

5) The most sensitive receivers have many stages of tuned radio-frequency amplification.

2. Find the information about:

1) the incoming signal from the antenna;

2) the type of the most modern radio receivers;

3) the tuning of the oscillator and the antenna circuit;

4) a high-fidelity receiver;

5) usual selectivity and excessive selectivity.