Fast frequency detection of sinusoidal signals

A method is proposed to detect the frequency of a sinusoidal signal within a fraction of a cycle. This is accomplished by carrying out an analogue computation on three successive samples of the input signal, yielding a DC output that is a measure of the frequency. The system has a constant settling time of three sampling intervals, and has a different sampling frequency for each range of measurement. System delay is shown to be equal to three times the ratio of the input frequency to the sampling frequency, and thus can be made to be a small fraction of a cycle.