156 



BELL SYSTEM TECHNICAL JOURNAL 



Noise 



Besides babble, many other sources of noise need to be considered in 

 cable carrier design. Figure 14, which shows the approximate mag- 

 nitude of several of these if no means are taken to suppress them, 

 indicates the noise at the end of a single 17-mile repeater section when 



50 



45 



40 



^'35 



30 



uj 25 



2 20 



If) 



[Ij 15 



15 20 25 30 35 40 45 



FREQUENCY IN KILOCYCLES PER SECOND 



55 



Fig. 14 — Noise, prior to suppression measures, per repeater section at output of 

 repeater whose gain equals line loss. 



A — Noise from thermal agitation. 



B — Thermal agitation plus tube noise. 



C — Noise from voice frequency telephone repeater office. 



D — Noise from telephone and telegraph repeater office. 



E — Noise from heavy static on open-wire tap close to carrier repeater input. 



amplified by a repeater whose gain equals the hot-weather line loss. 

 Curve A shows the unavoidable lower limit of noise, that produced 

 by thermal agitation of the electrons in the cable conductors and the 

 repeater.- This amounts to about 2 X 10~^^ watts per telephone 

 2 J. B. Johnson, Phys. Rev., 32, 97 (1928); H. Nyquist, Phys. Rev., 32, 110 (1928). 



