A Technique for Lowering the Noise Threshold of Conventional Frequency, Phase and Envelope Demodulators

Envelope, phase and frequency demodulators exhibit a stronger-signal capture phenomenon which enables the noise to suppress the signal when the signal is the weaker of the two, and the signal to suppress the noise when the reverse is true. The presence of the noise thus imposes a threshold that must be exceeded by the signal for proper envelope, phase or frequency demodulation. A general discussion is presented of the relative signal and noise content of the envelope, phase and frequency of the resultant of an amplitude- or exponent-modulated sinusoid plus random-fluctuation noise, when one or the other is heavily dominant. An analysis is then presented of the effect of regenerative feedback around an amplitude limiter upon a sinusoid plus random noise in order to define the conditions under which a CW-type signal will be enhanced over a strong random-fluctuation noise. It is shown that an oscillating limiter whose output maintains a high degree of coherence in the presence of a pure random-noise excitation will boost a wide class of relatively weak coherent signals into predominance over the noise when such a signal is added to the noise. Substantial threshold reductions which have been demonstrated in the laboratory are thereby made possible.