416 BELL SYSTEM TECHNICAL JOURNAL 



These levels, to an arbitrary decibel scale, have been plotted against 

 decibels of feedback, given by the expression 



10 log F^ = 20 log F. 



Balanced detection and the fulfillment of condition (28) have been 

 assumed. 



Over the region in which a constant signal output is maintained by 

 increasing the modulation level, noise and distortion levels decrease in 

 accordance with the feedback. The noise level during modulation 

 continues to exceed the background noise by 4 decibels, assuming an 

 initial frequency shift equal to the highest signal frequency to which 

 the system is responsive. 



Beyond the point at which the feedback factor has reached the value 

 Fi, the modulation level at the transmitter is held constant. A further 

 increase in feedback brings about a corresponding decrease in the 

 effective percentage of modulation for the system, causing the signal 

 level to fall in similar fashion. Distortion products now fall off still 

 more rapidly with respect to the signal, so that an increase in feedback 

 amounting to 1 decibel improves the second and third order distortion 

 ratios by 2 and 3 decibels, respectively. 



The ratio of signal to background or non-signaling noise remains 

 fixed in this region in spite of the reduction in effective modulation. 

 This ratio is that which would be obtained in a limiter system in which 

 the same high-frequency band is transmitted. The noise increment, 

 however, is diminished by the additional feedback and is made to ap- 

 proach zero. 



By suitable choice of the variables Fi and F^ it is possible to propor- 

 tion the benefits of feedback in the most advantageous manner. Thus 

 if noise is of more consequence than distortion, modulation would be 

 increased to the full extent of the feedback; if distortion is of primary 

 concern, as it might well be in a multiplex system, operation as indi- 

 cated in Fig. 3 would be preferable. 



Experimental Results 



Description of Equipment 

 Experimental confirmation of the principles which have been out- 

 lined has been obtained with the aid of a laboratory system shown 

 schematically in Fig. 4. This arrangement provided a transmitter, 

 receiver, and source of disturbance all under local control. The trans- 

 mitter operated at a carried frequency of 20 megacycles. This was 

 frequency-modulated by means of a circuit basically similar to that 



