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radio receiverSee receiver.
a device used with an antenna to receive radio signals or natural radiation and to convert the signals to a form that permits use of the transmitted information. Radio receivers are classified according to their purpose as broadcast, television, communications, radar, and other receivers.
The principal functions of a radio receiver are frequency selection, amplification, and detection. Frequency selection is the discrimination of the part of the radio-frequency spectrum that contains the desired information from the entire spectrum of electromagnetic oscillations acting on the antenna. Amplification is the magnification of the energy of the received oscillations, which are usually very weak, to a usable level. Detection is the conversion of modulated radio-frequency oscillations to electric oscillations corresponding to the modulation envelope that directly convey the transmitted information. These functions are fulfilled by frequency-selective resonant circuits (such as oscillatory circuits, cavity resonators, or electric filters) that are tuned to the required frequency or frequency band; by amplifiers of electric oscillations; and by detectors.
In addition, a radio receiver usually contains automatic-control circuits, most frequently for automatic gain control and automatic frequency control. The design of a radio receiver may also incorporate means for reproducing the received information, such as a loudspeaker or kinescope, and means for monitoring the operation of the receiver, such as dial gauges or various indicators. A radio receiver may be capable of operating on one or several fixed frequencies, or it may operate within a range of frequencies and have the capability of tuning to any frequency within that range. In the latter case, the operating frequency range is usually subdivided into bands.
In a radio receiver, oscillations are usually amplified prior to detection. A predetector amplifier is made selective by incorporating resonant circuits in the amplifier. For a postdetector amplifier, the spectrum of oscillations being amplified characterizes the received information, and the amplifier has a passband equal to the width of the spectrum of oscillations; the amplifier often includes means for correcting the gain-frequency characteristic at low and high frequencies. Depending on the type of predetector amplifier used, radio receivers can be classified as tuned radio-frequency, regenerative, superregenerative, reflex, or superheterodyne receivers.
In a tuned radio-frequency receiver, the received oscillations are amplified prior to detection without frequency conversion. In a regenerative receiver, a negative resistance is introduced into a resonant circuit tuned to the frequency of the signal being received; this is achieved by means of a positive feedback circuit or by connecting an appropriate electronic device into the circuit, such as a tunnel diode. In a superregenerative receiver, a circuit supplying intermittent positive feedback is connected to the oscillatory circuit of the radio-frequency-amplifier stage; the feedback periodically produces self-excited oscillations in the oscillatory circuit. In this case, the amplitude of the oscillations (or the average amplitude) is proportional to the amplitude of the received signal, but it is higher than the signal by a factor of 104 to 105. Although radio receivers of this type are of very simple design, they are not widely used because of relatively high distortion of the received signals. In a reflex receiver, the same amplifier is used simultaneously for predetector and postdetector amplification, thus simplifying the design of the receiver. The highest reception quality is obtained in superheterodyne receivers, which are the most widely used type.
Depending on the type of modulation used, a receiver may be classified as amplitude modulated, frequency modulated, phase modulated, or other type.
The basic performance characteristics of a radio receiver are sensitivity, selectivity, and stability. Sensitivity is the capability of receiving weak radio signals, where signal strength may be as low as 10–19 watt for a signal whose frequency bandwidth is approximately 1 kilohertz. Selectivity is the capability of separating the desired signal from extraneous radio-frequency oscillations (radio interference) by attenuating the interference by a factor of several thousand. Stability is the capability of providing reception of sufficient duration without the need for additional manual operations, such as tuning or switching. The usable sensitivity of a receiver depends on the interference encountered; if the interference occurs in the same frequency band as the radio signal and if the intensity level of the interference is higher than that of the signal, radio reception may become impossible. In order to ensure normal reception, radio receivers are equipped with devices for specially processing the radio signal with the aim of suppressing radio interference. The limiting sensitivity depends on the fluctuating intrinsic noise of the receiver itself. Such noise can be reduced by the use of low-noise input amplifiers, the simplest type of which is a regenerative amplifier with a tunnel diode. Much better results are achieved by parametric amplifiers and quantum mechanical amplifiers (masers).
REFERENCESRadiopriemnye ustroistva. Editor in chief, V. I. Siforov. Moscow, 1974.
Chistiakov, N. I., and V. M. Sidorov. Radiopriemnye ustroistva. Moscow, 1974.
N. I. CHISTIAKOV