The Demodulation of an F-M Carrier and Random Noise by a Limiter and Discriminator

Certain mathematical representations of a limiter and discriminator are postulated. On this basis, and on the basis of the statistics of narrow-band random noise, the output signal voltage and r.m.s. noise output voltage are calculated for a limiter and discriminator fed by an f-m carrier and narrow-band random noise. The output signal-to-noise ratio can thus be expressed as a function of the input signal-to-noise ratio. The former is plotted versus the latter in Fig. 5, for several different degrees of limiting. A sharp increase in the output signal-to-noise ratio from far below the input signal-to-noise ratio to only 0.9 db below it is found to occur as the latter increases through the range 0 db to 6 db. For the sake of comparison, Fig. 5 also shows the corresponding curve for square law detection of an a-m carrier, as well as that for f-m without a limiter. It is to be emphasized that these results pertain to the unfiltered discriminator output; consideration of filtering, which is generally present in a receiver to a considerable extent, requires a determination of the spectral distribution of the discriminator output noise.