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A procedure involving autocorrelation is described which, for long averaging time and incoming signals with continuous spectrum, has a detection threshold equal to that of a corresponding conventional system consisting of band pass filter, rectifier, and low pass filter. The autocorrelation procedure is quite complex; a full autocorrelation function is computed and then subjected to a ``filtering'' operation. A simpler procedure, such as computing autocorrelation coefficients for only one or a few delay times, will in general be less effective. A third process, involving multiplication by a local sinusoidal signal, is also discussed and shown to be closely parallel to the conventional system.
Philip Rudnick (Sun,) studied this question.