14. A frequency sensing circuit for sensing the presence of a frequency component F.sub.O in an applied analog signal comprising: means for limiting the applied analog signal to produce an amplitude limited signal; means for sampling the amplitude limited signal; means coupled to said means for sampling, for encoding each sample as a digital signal at an output thereof; an adder having a first input coupled to the output of said means for encoding and having a second input and an output; delay means having an input coupled to the output of said adder for delaying signals applied thereto by an amount that is an integral multiple of one-half the time period of the frequency to be sensed said delayed signals being provided at an output; a multiplier having an input coupled to the output of said delay means and having an output coupled to said second input of said adder; means for clocking said multiplier at a rate of two or more times the desired frequency to be sensed; and means coupled to said frequency sensing circuit for deriving an output signal therefrom having a magnitude indicative of the presence of the frequency to be sensed in said input signal.
21. A frequency sensing circuit according to claim 14 wherein said output deriving means is coupled to said adder.