NEGATIVE FEEDBACK 



417 



described by Travis/ Tube noise voltage appearing at the output of 

 a high gain radio frequency ampHfier supplied the high-frequency 

 disturbance. 



At the receiver an oscillator similar to that at the transmitter served 

 to beat down the incoming wave to an intermediate frequency of 438 

 kilocycles. This was applied to a three-stage amplifier having sub- 

 stantially uniform gain over a band of 50 kilocycles, and thence de- 

 livered to a balanced frequency detector. In addition to signal voltage, 

 automatic-frequency-control potentials were derived from the detec- 

 tors. Both were carried back to the local oscillator, but in order to 

 permit independent control of the amount of feedback their respective 

 paths were kept separate. In this way full frequency control could be 

 had even with signal feedback reduced to zero. 



TRANSMITTER RECEIVER 



SIGNAL 

 INPUT 



SIGNAL FEEDBACK PATH 



Fig. 4 — ^Schematic of experimental feedback system. 



Details of the frequency detector and feedback connections are 

 shown in Fig. 5. The conversion system derives its characteristics 

 from anti-resonant circuits LiC\ and L2C2, double-winding coils being 

 used to isolate the rectifier anodes from the plate battery. One circuit 

 is tuned to a frequency 15.4 kilocycles above the intermediate carrier 

 frequency and the other to a corresponding point below, their character- 

 istics intersecting at a point where the gain is approximately one half 

 of the peak value. Detection takes place in linear rectifiers Di and -02- 

 By means of the arrangement shown, signal potentials are impressed 

 upon the grids of amplifiers Ax and A2, while frequency-control voltage 

 appears across condensers Cz, C4. This voltage becomes zero when 

 the receiver is correctly tuned and appears with proper polarity to 



'Charles Travis, "Automatic Frequency Control," Proc. I. R. E., vol. 23, pp. 

 1125-1141, October 1935. 



