The Application of Negative Feedback to Frequency-Modulation Systems

Abstract

Negative feedback can be applied to a frequency-modulation receiver of superheterodyne type by causing a portion of the output voltage to frequency-modulate the local oscillator in such phase as to reduce the output signal. As a consequence of this arrangement the effective frequency modulation of the intermediate wave is diminished by the feedback factor. This reduction is accompanied by a decrease in noise and distortion. Restoration of the original signal level by increasing the degree of modulation at the transmitter brings about a corresponding increase in signal-to-noise ratio provided the disturbance is not too great, while distortion ratios are improved to about the same extent. These effects are treated analytically for the case where the disturbance level is sufficiently low to permit simplifying assumptions to be made. The results are in general agreement with observations made on an experimental laboratory system. Comparing the feedback system with a frequency-modulation system using amplitude limitation, the ratio of signal level to noise level in the absence of modulation is identical in two systems. During modulation the noise level increases in the feedback system by an amount depending upon the ratio of the effective frequency shift of the intermediate-frequency wave to the signal band width. By keeping this ratio small, the increase in noise during modulation can be made relatively unimportant. In cases where the disturbance level is high, phenomena have been observed which are very similar to those encountered when amplitude limitation is used.