the synchronization, in a receiver (for example, a radio receiver), of electrical oscillations, or signals, received from a transmitter and certain auxiliary oscillations with respect to frequency and phase. The auxiliary oscillations are known as reference oscillations and may be generated in the receiver.
Synchronous reception involves keeping the deviation of the carrier frequency f of the received signals and (or) the modulating frequency (in telegraphic communication, the keying frequency) F from the corresponding frequencies of the reference oscillations within the specified limits Δf and ΔF. In addition, the phase of the carrier oscillations Φ = 2 πft and (or) the phase of the modulating oscillations (or the telegraph pulses) Φ = 2 π Ft is kept within a certain deviation ΔΦ or ΔΦ from the corresponding phases of the reference oscillations; here, t is time. Accordingly, we speak of frequency and phase locking, that is, maintaining frequency and phase synchronism —high-frequency synchronism if the carrier frequency is under consideration and low-frequency synchronism if the modulating and (or) keying frequency is being considered.
Synchronization can be attained by a relative technique, wherein the generator of the reference-frequency oscillations in the receiver is tuned to the signals received from the transmitter. Alternatively, an absolute method can be used, wherein the generator is tuned to oscillations obtained from a high-stability local oscillator. The most common communications systems make use of phase synchronism, or phase locking, where the phasing of the oscillations is accomplished automatically in the receiver—for example, by using in the receiver a synchronous detector controlled by stable oscillations from a reference generator.
Instabilities of the frequency and phase (or of the local time t = Φ/2πf) occur in every communications system for a number of reasons, including frequency instability in the oscillations from the generators (both at the transmitting and receiving ends) and variations in the group propagating time of the signals. Synchronous reception makes it possible to suppress in the receiver any interference whose phase differs from the phase of the desired signal.
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A. A. PIROGOV