Radio receiver with quadrature demodulation and digital processing

Claim 1

A radio receiver for receiving and demodulating at least single side band modulated signals wherein the center frequency of the single side band is offset by an offset frequency from the carrier frequency, comprising a first signal path including means for mixing the received signal with a local oscillator frequency running at the main transmission frequency which is the center frequency of the side band for the ssbm signals to obtain a fixed mixed signal and means for filtering said first mixed signal to give a first filtered signal defined as an inphase signal, a second signal path including means for mixing the received RF signal with the local oscillator frequency but with a relative phase shift to obtain a second mixed signal and means for filtering said second mixed signal to provide a second filtered signal defined as a quadrature signal Q, an analogue to digital converter stage to convert the I and Q signals into respective digitised I and Q signals, and a digital signal processor arranged to demodulate the digitised I and Q signals, including means to derive amplitude and phase of the original modulation from the digitised I and Q signals, means to subtract or add phase at the offset frequency to the phase angle θ to provide a new phase angle φ and means to derive R Sin φ or R Cos φ to produce digitised single side band demodulation signals for upper or lower single side band modulation depending respectively on whether the phase at the offset frequency is subtracted or added.