Theory of Stronger-Signal Capture in FM Reception

Abstract

The characteristics of the FM disturbance that is caused by two-signal interference are pointed out and compared with the characteristics of message modulation. The comparison suggests a new role for the amplitude limiter in FM receivers. The limiter spreads out the spectrum which carries the FM disturbance over a frequency range that often exceeds many times the bandwidth requirements of the message modulation. This makes it possible to reject important parts of the interference spectrum by proper filtering in the output of the limiter without substantially affecting the spectrum of the message modulation. A repeated cycle of amplitude limiting and spectrum filtering is found to be an effective scheme for suppressing the disturbance ahead of the discriminator stage, and for improving the capture capabilities of an FM receiver.