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BELL SYSTEM TECHNICAL JOURNAL 



feedback are shown in Fig. 15. These are plotted against the ratio of 

 root-mean-square carrier and disturbance levels at the end of the 

 intermediate-frequency channel. Signal levels were measured in the 

 presence of the disturbance so as to take into account the depression of 

 the signal, while noise levels were observed in the absence of modulation. 



70 



60 



50 



< 



z 

 o 



m 30 

 <ri 



a. 



20 



10 



10 20 30 



R.M S. CARRIER-DISTURBANCE RATIO IN DB 



Fig. 15 — Output signal-to-noise ratio vs. carrier-disturbance ratio witii and 

 without feedback. Modulation ± 124.5 kilocycles for 25-decibel feedback and 

 ± 7 kilocycles for no feedback. 



The improvement resulting from the application of feedback is given 

 by the difference between the two curves and approaches the theoret- 

 ical improvement at the low noise levels. The curve obtained with 

 feedback exhibits a rather sharp break when the ratio of carrier to 

 effective disturbance is in the vicinity of 10 decibels. Experimental 



