Capture effect in a narrow-band fm system

Abstract

THIS investigation was made to study capture effect as it relates to narrow-band FM systems used in the power-line carrier field. Capture effect in FM systems is that phenomenon whereby the stronger of two signals increases its predominance over the weaker signal on the same frequency in a receiver, so that the weaker signal is eliminated from the receiver. The transition in control can be very sharp when one signal becomes even slightly stronger than the other signal. That is, a small differential in r-f input levels of the desired signal to the interfering signal results in a much higher ratio of signal to interference in the audio output. About 10 years ago, FM communication equipment was recommended for power-line carrier use with a major feature being that it possessed capture effect. This phenomenon supposedly would make it possible to work two channels back-to-back at the same station on the same frequency with less isolation between them than would be required for conventional AM channels. From the theoretical study and experimental investigation made by the author, it was concluded that very little if any capture effect was present in narrow-band equipment. This is also the conclusion reached by others in previously published literature. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,2</sup>