NEGATIVE FEEDBACK 



421 



cycles per second. Figure 9 shows the effect of increasing the modula- 

 tion in proportion to the feedback so as to maintain a constant output 

 level for the fundamental. Both second and third harmonics tend to 

 be reduced in proportion to the feedback, the improvement in third- 

 harmonic level being 23.5 decibels for 25-decibel feedback. Failure to 

 realize full reduction of the second harmonic is the result of distortion 

 beginning to manifest itself in one or the other of the modulated 

 oscillators. At the point of 25-decibel feedback the transmitter and 



270 

 240 

 210 

 180 



- 30 



- 60 



100 



500 1000 5000 10000 



FREQUENCY IN CYCLES PER SECOND 



25 



15 M 



40000 



Fig. 8 — Gain and phase characteristics of complete feedback loop including corrective 

 network. Measured between points A and C of Fig. 4 with switch S open. 



beating oscillator were being modulated to the extent of ± 124.5 and 

 ± 117.5 kilocycles, respectively. 



The curves of Fig. 10 were obtained by maintaining a constant funda- 

 mental level up to the point of 15-decibel feedback and then allowing 

 the modulation level at the transmitter to remain constant thereafter. 

 The results correspond rather closely with the theoretical curves of 

 Fig. 3 and show the very rapid decrease in distortion which takes place 

 when the modulation level remains unaltered; see (25). A more ex- 

 treme example of this method of operation is shown in Fig. 1 1 where 



