418 



BELL SYSTEM TECHNICAL JOURNAL 



produce correction of the frequency of the local oscillator in case of slow 

 drifts in the frequency of either oscillator. 



The use of a conversion system having peaks separated by an 

 amount considerably exceeding the greatest frequency deviation is the 

 result of a compromise between the readily adjustable and high 

 impedance anti-resonant type of load circuit and others which, though 

 more linear in their characteristics, lead to much lower gain in the 

 conversion stage. While a peak separation of 14 kilocycles would 

 have sufficed in view of the limitations of the transmitter, a consider- 

 ably greater peak separation without corresponding increase in modu- 

 lation was used. As a result that portion of the circuit characteristic 

 actually embraced by the modulated intermediate-frequency wave 





BEATING 

 OSCILLATOR 

 (FREQ MOD.) 





-X- 



CORRECTI 

 NETWOR 



EH 



ATTENUATOR 



L.P. 

 fILTER 



Fig. 5 — Details of balanced frequency detector and feedback connections. 



presented a much better approximation of a straight line than would 

 have been possible with minimum peak separation. The penalty for 

 adjusting the circuits in this manner is merely a loss in detecting effi- 

 ciency and not an impairment of the signal-to-noise ratio. This can 

 readily be overcome by additional audio-frequency amplification. 



The signal-frequency feedback path includes an attenuator for ad- 

 justing the feedback and a corrective network for preventing singing 

 around the feedback loop. Frequency control and feedback paths are 

 finally combined at the modulation terminals of the local oscillator. 



The necessity for the inclusion of a corrective network to modify the 

 transmission characteristics of the feedback path is evident from Fig. 6. 

 This shows the measured gain and phase characteristics of the receiver 



