Radio receiver

(electronics) A device that converts radio waves into intelligible sounds or other perceptible signals. Also known as radio; radio set; receiving set.

The part of the radio communications system that extracts information from radio-frequency (rf) energy intercepted by the antenna. Radio receivers are the most common electronic equipment worldwide and a vital part of all radio, television, and radar systems. Since the 1960s, radio receiver performance has improved greatly, while size, weight, and cost have fallen dramatically. In the past, radio receivers were built from analog circuits, but increasingly they are realized by digital signal processing. See also Radio.

The antenna intercepts a band of energy in the radio frequencies containing many transmissions. These may have different modes of modulation; the two most common are amplitude modulation (AM) and frequency modulation (FM). Signals have a large size range, from a large fraction of a volt down to a small fraction of a microvolt. The receiver must be selective, responding to only one signal, must demodulate the signal, extracting the impressed information from the radio-frequency wave, and must raise it to an acceptable power level by amplification.

Single-sideband (SSB) transmissions are similar to amplitude modulation, but with one of the symmetrical pair of sidebands eliminated and the carrier suppressed. Single-sideband is significant because it conserves electromagnetic spectrum, introducing less spectrum pollution than any other modulation. Because of the growing prevalence of digital signals, digital modulation systems are increasingly important. See also Amplitude modulation; Frequency modulation; Modulation; Single sideband.

Amplitude modulation

Figure 1 shows a simple receiver for amplitude-modulated signals. A band-pass filter selects the required signal, which, after optional amplification, is passed to the demodulator (obsolete term: detector), which in this version consists of a limiting amplifier and a multiplier. The high-gain limiting amplifier has an output of the same sign as the input but constant magnitude (a square wave). When multiplied by the amplitude-modulated waveform, this inverts negative half-cycles. After low-pass filtering to remove residual radio-frequency ripple, the modulating waveform is obtained, shown as an audio waveform, which passes to the loudspeaker. There are other amplitude demodulator circuits, all of which either suppress or invert alternate half-cycles of the amplitude-modulated waveform. See also Amplifier; Amplitude-modulation detector; Electric filter.

Simple tuned-radio-frequency (TRF) receiver for amplitude-modulated (AM) signals.

Since the input signal may be on the order of microvolts whereas volts are required to operate a loudspeaker, the total receiver gain may range up to the order of a million or more. In this simple tuned-radio-frequency (TRF) receiver it is difficult to adjust the center frequency of a high-performance band-pass radio-frequency filter in order to receive other stations; also, the radio frequency may be too high for effective amplification. For this reason the superheterodyne (superhet) receiver (Fig. 2) superseded it.

Superheterodyne (superhet) receiver.

In superheterodyne receivers, after a radio-frequency filter and amplifier, the signal passes to a mixer, from which the output is formed by the product of the signal and a locally generated wave. If the signal carrier is at f_c and the local oscillator is at f_o, the result is a wave at a frequency ±(f_o − f_c) = f_i-f, known as the intermediate frequency (i-f), and this difference frequency can be kept constant, whatever the value of f_c, by a suitable choice of f_o. Subsequent to the mixer, the superheterodyne receiver becomes similar to the tuned-radio-frequency receiver except that it is now operating at a fixed frequency, so that, provided a low intermediate frequency is chosen, amplification is easy and high-performance filters can be used. Fixed-frequency intermediate-frequency filters, based on mechanical resonance in piezoelectric ceramics and crystals or on surface-acoustic waves, give extreme attenuation outside the passband. See also Heterodyne principle; Mixer; Oscillator; Piezoelectricity; Surface-acoustic-wave devices.

In amplitude-modulation systems, the amplitude of the signal carries the information, and so to keep the audio output from wide level changes it is desirable to adjust the strength of the signal at the demodulator to be roughly constant, despite variations at the antenna due to transmitter power or range. Thus, automatic gain control (AGC) is used in most amplitude-modulation receivers. See also Automatic gain control (AGC).

Frequency-modulation

For frequency modulation, the signal is of constant amplitude but varies in frequency, and so it is usual for the intermediate-frequency amplifier to be a limiter, giving a constant output. In other respects the receiver may be identical with one of those in Fig. 2. Automatic gain control is often omitted, but may be combined with limiting for the highest performance. The frequency demodulator is quite different, often utilizing a phase-sensitive detector. See also Frequency-modulation detector; Limiter circuit.