On the Threshold Effect in FM Data Systems

The threshold noise performance of a limiter-discriminator, with input signal uniformly distributed over the full scale frequency deviation range, is analyzed in terms of an evaluation criterion which specifies the system resolution and the probability ( ) with which output noise exceeds that resolution. This criterion, meaningful above and below threshold, forms a basis for a precise definition of threshold, the location of which is determined equally precisely and found to lie between 6 dB and 10 dB input signal-to-noise ratio (SNR) for the range of ( ) values considered. The channel efficiency of FM relative to Shannon\´s law is also determined approximately in light of this criterion, from which the ultimate FM threshold follows.