An FM Detector for Low S/N

Abstract

Conventional frequency-modulation (FM) detectors show a marked decrease in output <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> below an input <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> . A detector is described which has no normalization and is superior to the ideal detector below zero to 10 dB input <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> . The noise at the output of the detector is additive for all input <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> ratios and in this sense is comparable to synchronous detection of AM. Plots of output noise spectrum and output <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> with and without sine-wave modulation are given. Another detector is also proposed that is asymptotic to the ideal detector at high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> and the aforementioned detector at low <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> . 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.