Offset correction

Abstract

A radio receiver includes apparatus for correcting phase and frequency offsets in a digitally modulated carrier signal. The receiver includes processing circuitry which outputs a baseband signal at point. The circuitry includes a local oscillator generating a signal intended to have a phase and frequency related (e.g. substantially equal) to the carrier signal. The baseband signal from the circuitry is input to a demodulator which generates a signal in the form of a sequence of digits corresponding to the original carrier signal. The baseband signal is sampled by a sampler and the demodulated signal is sampled by a sampler. Processing circuitry calculates the correlation function of the two sample signals and determines the value of the phase .PHI..sub.x at a time when the correlation function is a maximum. A control signal V.sub.c indicative of the phase .PHI..sub.x is generated by a signal generator and applied to the oscillator whereby the phase of the oscillator output signal is adjusted by an amount corresponding to the phase offset. Preferably, sampling is effected on at least two occasions. The phase differences determined on each correlation are subtractively combined and the resultant phase value is divided by the time interval between the peaks of the two associated correlation functions to derive a value for the offset frequency. A control signal indicative of the frequency offset is then fed to the oscillator means to modify the oscillator output frequency accordingly.