Abstract: In the frequency measuring system disclosed herein, a control signal is generated which is related to the value of function of the input signal for a selectable delay time. The average value of the control signal is employed to control the clock rate of a shift register which is used to obtain a delayed version of the input signal, the input signal being digitized and fed into the shift register. This feedback path forms a servo-loop which adjusts the clocking rate of the shift register in correspondence with the frequency of any coherent component in the input signal, even though the input signal may be a composite comprising many random components.