Frequency-Modulation Interference Rejection with Narrow-Band Limiters

Essential to the interference rejection ability of a frequency-modulation receiver is the use of the proper bandwidths in its nonlinear sections. The weaker of two competing signals (whose amplitude may approach the amplitude of the stronger signal within arbitrary limits) can be suppressed by a frequency-modulation receiver if, other requirements being met, the limiter and discriminator bandwidths exceed certain minimum necessary values. After presenting a summary of design requirements established experimentally by previous investigators, this paper presents a theoretical determination of the minimum necessary values in terms of an ideal bandpass filter that follows an ideal limiterand is followed byan amplitude-insensitive detector. The results reported here show that the bandwidth specifications prescribed, by other investigators can be cut by a factor of more than ten in the limiter bandwidth, and a factor of two in the discriminator bandwidth. The paper concludes with observations on the possibility of reducing the required discriminator bandwidth to that of the intermediate frequency by cascading enough stages of bandpass limiters.