668 



BELL SYSTEM TECHNICAL JOURNAL 



Enough work has been done, however, to indicate fairly well what the 

 result of such addition will be. 



As to distortion in telephone transmission, the most serious problem 

 has been to limit the intermodulation between various signals which 

 are transmitted simultaneously through the repeater and appear as 

 noise in the telephone channel. The requirement for such noise is 

 similar to that for line and tube noise, and similarly it will add up in 

 successive repeater sections for a long line. With present types of 

 tubes operating with a moderate plate potential, the modulation re- 

 quirement can be met only at relatively low output levels. To im- 

 prove this situation and also to obtain advantages in amplifier stabil- 

 ity, the reversed feedback principle employed for cable carrier ampli- 

 fiers, as described in a paper by H. S. Black, ^■^ has been extended to 

 higher frequency ranges. It has been found that amplifiers of this 

 type having 30 db feedback reduce the distortion to such an extent 

 that each amplifier of a long system carrying several hundred telephone 

 channels will handle satisfactorily a channel output signal level about 

 5 db above that at the input of the toll line. 



The maximum gain which can be used in the repeater, therefore, is, 

 in the illustrative case given above of a long system carrying several 

 hundred telephone channels, the difference between the minimum and 

 maximum levels of 55 db below and 5 db above the point of reference, 

 respectively, or a total gain of 60 db. (With a .3-inch coaxial line of 

 the type shown in Fig. 2, this corresponds to a repeater spacing of 

 about 10 miles.) If a repeater is to have 60 db net gain and at the 



PRE- INPUT 



EQUALIZER TRANSFORMER 



/ 



INTERSTAGE 

 ^COUPLINGS N^ 



OUTPUT 

 TRANSFORMER 



Fig. 10 — Circuit of 1000-kilocycle three-stage feedback repeater. 



