Abstract: A frequency synthesizer in which the frequency of a variable frequency oscillator is controlled by means of a control signal and wherein means are provided for determining whether any frequency variation results from a drift in the frequency of the variable frequency oscillator means being provided for generating and maintaining an additional drift control signal to compensate for such drift. The drift control signal is inhibited from change either when it is detected that the control signal is in excess of that which would result solely from drift or the control signal is representative only of a quantisation error.