An FM Detector for Low S/N

Conventional frequency-modulation (FM) detectors show a marked decrease in output S/N below an input S/N of 5 to 10 dB. This threshold effect is caused by "clicks" or phase slips of multiples of π. This effect is related to a normalization (clipping) which is very beneficial at high S/N . A detector is described which has no normalization and is superior to the ideal detector below zero to 10 dB input S/N . The noise at the output of the detector is additive for all input S/N ratios and in this sense is comparable to synchronous detection of AM. Plots of output noise spectrum and output S/N with and without sine-wave modulation are given. Another detector is also proposed that is asymptotic to the ideal detector at high S/N and the aforementioned detector at low S/N . Comparison is made with conventional detectors. The results indicate that this detector may be especially useful in the detection of frequency-shift keying and phase-shift keying in telemetry systems.