The Application of Negative Feedback to 

 Frequency-Modulation Systems * 



By J. G. CHAFFEE 



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 arrange- 

 ment the effective frequency modulation of the intermediate wave 

 is diminished by the feedback factor. This reduction is accom- 

 panied 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 suffi- 

 ciently 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. 



Introduction 



' I ''HIS paper describes a method for improving the performance of 

 -■■ receivers designed to receive frequency-modulated waves. In 

 its broader aspects this method can be described as the application of 

 the principle of negative feedback to a superheterodyne frequency- 

 modulation receiver. In its details the application of the feedback 

 principle necessitates the use of a rather unusual circuit arrangement. 

 This circuit differs from that of the simple feedback system in that 

 the voltages fed back are not of the same frequency as those applied 



* Presented before New York Section of I. R. E., May 3, 1939. Published in 

 Proceedings, I. R. E., May 1939. 



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