Demodulation of Wideband, Low-Power FM Signals

This paper discusses some theoretical aspects of the demodulation of wideband, low-power frequency modulated signals. A wide trans* P a r t s of the m a t e r i a l of this article were discussed by t h e a u t h o r in lectures at t h e U n i v e r s i t y of California at Berkeley d u r i n g M a y , 1963. 339 340 TIIE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1964 mitted bandwidth permits a saving in power. Frequency modulation implies a constant power level, which makes peak power identical with average power. It is advantageous, for example, when the practical restrictions on peak power determine system power levels rather than restrictions on average power. More specifically, the paper is concerned with FM systems subject to the following external requirements: A band-limited, continuous, analog signal is supplied to the input of a coder or modulator, which produces the transmitted signal. A demodulator reproduces the original baseband signal to a fidelity suitable for a television channel, a telephone channel, or a carrier system combining a number of telephone channels. For such puiposes, for example, the average errors in the output must be more than 40 db below the baseband signal. It is assumed that a large FM index is used, to conserve signal power. These conditions are implicit in many of the conclusions. They will be referred to collectively as "the conditions assumed here." Several different techniques and circuit forms are compared. The comparisons are concerned primarily, but not exclusively, with sensitivities to noise.