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.ABSTRACT:In order to reduce signal loss particularly whentransmitting data in a quasi-synchronous area coverage schemeutilizing sideband diversity it is necessary to effectcoherent demodulation by looking the local oscillator signalto the received carrier signal, even when the carrier hasbeen completely suppressed. The demodulator disclosed com-prises first and second mixers having first inputs to whicha frequency converted input signal having sideband diversityis applied. Second inputs of the first and second mixersreceive a local oscillator signal. Phase shifting is appliedto the input or local oscillator signal so that the outputsof the first and second mixers have a relative phase differ-ence of 90°. The outputs of the first and second mixers areapplied to respective audio bandpass filters the outputs ofwhich are mixed in a further mixer to provide a sidebanderror signal (Es) which is low pass filtered in a filter. Acarrier signal (Ec) is derived by low pass filteringthe output of the first mixer. A composite error signalEcs is produced from the two error signals (Es and Ec) andis applied to either the local oscillator to lock its fre-quency and phase to the input signal or to a local oscillatorof a prededing frequency conversion stage. In operation whenthe radio frequency phase angle .PHI. approaches 180° then thephase reference tracks across from a stable null at 0° toanother stable null at 90°. The stable null tracks at 180°for values .PHI. of 270° and greater.