Abstract: A method and apparatus for digitally demodulating a phase or frequency modulated signal using a quadrature mixing circuit to mix the phase or frequency modulated signal with a frequency equal to the carrier frequency of the received modulated signal to produce in-phase component signal (I) and a quadrature component signal (Q) of the received phase or frequency modulated signal which differ in phase by 90 degrees. The I and Q signals are provided via respective low pass filters to a phase to digital converter for generating a digital output signal. The digital output signal is provided to an interface circuit which generates a plurality of phase-change information signals which are provided to a signal processing circuit. The signal processing circuit uses interpolation circuitry to produce a demodulated signal. A further output of the signal processing circuit, i.e. a D.C. signal, is fed back to provide automatic frequency control to an oscillator in the quadrature mixing circuit.

Abstract: A system for digitally demodulating a frequency- or phase- modulated signal by mixing a carrier wave of the modulated signal and a local oscillation signal so as to produce first and second analog signals of quadrature-phase intermediate frequency having a phase difference of 90 degrees, the system comprising: a phase-digital converter for detecting the variation of a fixed phase period of the first and second analog signals to produce a signal for establishing the period of a quantization step; an interface device for producing variable phased information for the pertinent quantization step according to the phase relationship of the phase-digital signal; and signal processor for storing the quantized phase information of the interface, accumulating, by a sampling signal of a fixed period, the variable phase information stored during the sampling period, thereby demodulating the modulated signal.