NEGATIVE FEEDBACK 



409 



mediate-frequency wave will be restored to its original value Aco and 

 the signal level will remain unchanged. Then the noise power becomes 



Pn = 



27VVi 



ao2 + 



~2W 



which can be written 



2iVVi 



ao' + 



ai^Ac 



+ 





(12) 



(12a) 



The noise-to-signal power ratio is improved by the factor F~, since 



qa. 



(13) 



Of the factors in (12) the first is the result of modifications of the 

 amplitude of the incoming wave by the disturbance. Although sub- 

 ject to reduction by feedback it can be balanced out completely by the 

 use of differentially connected frequency detectors having slope 

 factors fli and — ai. The second term is dependent upon the degree 

 of modulation of the intermediate wave. It is usually of less conse- 

 quence in its effect upon the listener. The remaining term is the result 

 of phase modulation of the signal wave by the disturbance. Under 

 the condition that the output signal is held constant by increasing the 

 transmitted band, all terms which contribute to the noise level in a 

 given case are reduced alike by feedback. 



If differential frequency-detection is employed (12a) becomes 



Pn = 



2Nhiar 



f2 



Aco 



Qa' 



2^3 



(14) 



During non-signaling periods the first term becomes zero. Hence 

 during periods of modulation the background noise power is increased 

 by the factor 



1 + 



3 Ai22 



2 F'qa' 



^ 2 2.2 



(15) 



If conditions are such that the maximum shift experienced by the 

 intermediate-frequency wave is numerically equal to q^ then the noise 

 level will be increased by 4 decibels during periods of full modulation. 

 In the experimental system to be described the ratio of Aw to qa was 

 allowed to attain a value of 1.75, resulting in a maximum increase of 

 7.5 decibels. 



