METHOD OF, AND A RECEIVER FOR, DEMODULATING A DOUBLE SIDEBAND AMPLITUDE MODULATED SIGNAL IN A QUASI- SYNCHRONOUS AREA COVERAGE SCHEME UTILISING SIDEBAND DIVERSITY

摘要

PHB. 32,785. 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 looking the local oscillator signal to the received carrier signal, even when the carrier has been completely suppressed. The demodulator disclosed comprises first and second mixers having first inputs to which a frequency converted input signal having sideband diversity is applied. Second inputs of the first and second mixers 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.degree.. The outputs of the first and second mixers are applied to respective audio bandpass filters the outputs of which are mixed in a further mixer to provide a sideband error signal (Es) which is low pass filtered in a filter. A carrier signal (Ec) is derived by low pass filtering the output of the first mixer. A composite error signal Ecs is produced from the two error signals (Es and Ec) and is applied to either the local oscillator to lock its frequency and phase to the input signal or to a local oscillator of a prededing frequency conversion stage. In operation when the radio frequency phase angle .PHI. approaches 180.degree. then the phase reference tracks across from a stable null at 0.degree. to another stable null at 90.degree.. The stable null tracks at 180.degree. for values .PHI. of 270.degree. and greater.