A Simple Fast Frequency Detector for a Constant Amplitude Sinusoid

The frequency of a constant-amplitude sinusoidal signal is measured within a small fraction of a cycle. This is accomplished by carrying out computations on two successive samples of the input signal. System delay expressed in a fraction of a cycle is given by two times the ratio of the input-to-sampling frequencies. An inverse sine function is computed using digital hardware, giving a digital output representing the frequency.