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BELL SYSTEM TECHNICAL JOURNAL 



unity at the point of zero phase. This was accomplished by including 

 in the feedback path a network designed by R. L, Dietzold. The 

 gain-frequency characteristic of this network is shown in Fig, 7. The 

 modified loop characteristics as measured between points A and C 

 with switch S open, and with the attenuator set for an 8-decibel loss, 

 are given in Fig. 8. Full feedback is applied only over a band extend- 

 ing to 4 kilocycles, so that the range of frequencies applied to the trans- 

 mitter and delivered to the listener must be restricted to this figure. 



500 1000 5000 10000 



FREQUENCY IN CYCLES PER SECOND 



50000 100000 



Fig. 7- — Gain-frequency characteristic of corrective network inserted in 

 signal feedback path. 



The limit of stable feedback which can be realized is indicated by the 

 difference between the loop gain within the useful band and that at the 

 frequency corresponding to zero phase. 



Distortion Measurements 



The manner in which distortion levels at the output of the receiver 



were observed to vary with feedback is depicted in Figs. 9 to 12. In 



each case the modulation level for zero feedback was such as to shift 



the frequency of the transmitter ± 7 kilocycles at the rate of 1000 



