NEGATIVE FEEDBACK 



419 



I 



alone, viewed as a voice-frequency network between points A and B 

 in Fig. 4. This was obtained by applying signal frequencies to the 

 modulation terminals of the beating oscillator and making observations 

 at point B with switch 5 open, proper termination being provided at 

 both sides of the break. The unmodulated transmitter served, in 

 effect, as the beating oscillator during this measurement. At the 

 lower signal frequencies the phase is practically 180 degrees as indicated 

 by (4) with Xi = 0. As the signal frequency is increased the phase 



210 



180 



150 



120 



90 



60 



a 30 



a. 



o 



z 



- - 30 



u 



to 



< - 60 



I 



Q. 



- 90 

 -120 

 -150 

 - 180 



30 



25 



in 

 20 -I 



UJ 



in 

 1 5 '-> 



Q 

 10 Z 



O 

 



- 5 



500 1000 5000 10000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 6 — Phase and gain characteristics of receiver measured between points A and B 

 of Fig. 4 with switch 5 open. Transmitter in operation but not modulated. 



is progressively shifted from this value. Except for that produced by 

 the output transformer, the shift takes place within the intermediate- 

 frequency amplifier and conversion circuits. Its magnitude'is a meas- 

 ure of the slope of the phase-frequency characteristic of the inter- 

 mediate frequency system. 



The existence of positive gain at a point of zero phase shows that 

 singing would occur if feedback connections were made directly to the 

 beating oscillator. It was therefore necessary to reduce the gain below 



