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BELL SYSTEM TECHNICAL JOURNAL 



modulation was left at its initial value. Harmonic levels soon reached 

 a point beyond which they could not be measured accurately. 



In a practical system the loss in signal level resulting from operation 

 in this manner could easily be overcome by the addition of a low- 

 distortion audio-frequency amplifier at the output of the receiver. 

 This amplifier might well embody negative feedback of the more usual 

 type. 



i 10 12 14 16 18 

 FEEDBACK IN DECIBELS 



20 22 24 26 



Fig. 9 — Effect of feedback upon receiver distortion. Fundamental level kept 

 constant by increasing transmitter modulation in proportion to the feedback. 

 Modulation with no feedback = zt 7 kilocycles at 1000 cycles per second. 



A composite of these distortion measurements is given in Fig. 12. 

 Harmonic levels are plotted in decibels below the fundamental and are 

 indicative of the improvements brought about by feedback. If it is 

 assumed that any loss in signal is compensated by additional audio- 

 frequency amplification, the fundamental level would be represented 

 in all cases by the axis of abscissae. 



Noise Measurements 

 In Fig. 13 are given the results of a series of observations of receiver 

 output noise versus amount of feedback for a number of high-frequency 



