The Capture Effect in FM Receivers

In this paper a theoretical explanation of the capture effect is given by calculating the instantaneous frequency of the output signal of a limiter when two frequency modulated (FM) signals are present at the limiter input. When this signal is applied to a demodulator with unlimited bandwidth, the output signal of the demodulator proves to have an extreme capture effect. When however the demodulator bandwidth is limited, the capture effect is shown not be be extreme. This phenomenon is explained and possibilities are given to minimize the capture effect. Some of the results of measurements on limiters and demodulators are given in this paper; they prove that a weak capture effect can be obtained. A method of calculating the degree of capturing is included.