Method of, and a receiver for, demodulating a double sideband amplitude modulated signal in a quasi-synchronous area coverage scheme utilizing sideband diversity

摘要

In order to reduce signal loss particularly when transmitting data in a quasi-synchronous area coverage scheme utilizing sideband diversity, it is necessary to effect coherent demodulation by locking the local oscillator signal to the received carrier signal, even when the carrier has been completely suppressed. The demodulator includes first and second mixers (48, 50) having first inputs to which a frequency converted input signal having sideband diversity is applied. Second inputs of the first and second mixers (48, 50) receive a local oscillator signal. Phase shifting is applied to the input or local oscillator signal so that the outputs of the first and second mixers have a relative phase difference of 90°. The outputs of the first and second mixers are applied to respective audio bandpass filters (56, 58) the outputs of which are mixed in a further mixer (60) to provide a sideband error signal (E.sub.s) which is low-pass filtered in a filter (62). A carrier error signal (E. sub.c) is derived by low-pass filtering the output of the first mixer (48) . A composite error signal E.sub.cs is produced from the two error signals (E.sub.s and E.sub.c) and is applied to either the local oscillator (52) to lock its frequency and phase to the input signal or to a local oscillator of a preceding frequency conversion stage.