410 



BELL SYSTEM TECHNICAL JOURNAL 



In order to secure large noise reduction it is necessary to produce a 

 frequency shift in the transmitted wave much greater than the signal 

 band width. Thus, in common with frequency-modulation systems 

 employing amplitude limiters,^ this advantage is secured at the expense 

 of band width. In this connection it is of interest to compare ampli- 

 tude limitation and feedback systems on the basis of equal width of 

 transmitted band. Hence it will be assumed that in each case the 

 transmitted wave is modulated to the extent of ± Afi = ± FAco. In 

 Fig. 2 are shown idealized characteristics of conversion systems which 

 might be used in the two systems. The adjustment shown in Fig. 

 2(a) is suitable for use with the limiter system. With the feedback 

 system that shown in Fig. 2(b) would be necessary to secure the same 

 percentage of amplitude modulation after conversion. This represents 



(b) 



FREQUENCY 



|-«j(i;-*l 



FREQUENCY 



Fig. 2 — Idealized conversion-system characteristics for (a) limiter system, 

 (b) feedback system. 



the minimum band width which could be provided in the conversion 

 system with feedback, though several considerations make it desirable 

 to use an adjustment lying somewhere between the two shown. The 

 manner of tuning or the slope factors assumed in each case are im- 

 material to the present comparison provided that, in either system, the 

 linear portion of the characteristic is of sufficient extent to effect proper 

 conversion of the intermediate-frequency wave. 



The noise-to-signal power ratio obtainable with the feedback system 

 will be that given by (13) with the first term omitted since it is balanced 

 out by the push-pull arrangement. Thus 



Pn 

 P. 



4iVV 



2[2 



qa' 



3Ac 



qa. 



(16) 



* E. H. Armstrong, "A Method of Reducing Disturbances in Radio Signaling by 

 a System of Frequency Modulation," Proc. I. R. E., vol. 24, pp. 689-740, May 1936. 



