624 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



watt per cycle of bandwidth; that is, the thermal noise power, in dbw 

 is —20-4 + 10 log (bandwidth). The first circuit or set noise is usually 

 higher than the theoretical minimum by a factor known as the noise 

 figure. For example, the set noise in a receiver with a 6-kc noise band- 

 width and an 8-db noise figure is 158 db below 1 watt, which is equiva- 

 lent to 0.12 microvolts across 100 ohms. Variations in thermal noise and 

 set noise follow the Rayleigh distribution, but the quantitative reference 

 is usually the rms value (63.2 per cent point), which is 1.6 db higher 

 than the median value shown on Figs. 4 and 10. Momentary thermal 

 noise peaks more than 10 to 12 db above the median xahie occur for a 

 small percentage of the time. 



Atmospheric static is caused by lightning and other natural electrical 

 disturbances, and is propagated over the earth by ionospheric trans- 

 mission. Static levels are generally stronger at night than in the day- 

 time. Atmospheric static is more noticeable in the warm tropical areas 

 where the storms are most frequent than it is in the colder northern 

 regions which are far removed from the lightning storms. 



Typical average values of noise in a 6-kc band are shown on Fig. 19. 

 The atmospheric static data are rough yearly averages for a latitude 

 of 40°. Typical summer averages are a few db higher than the value on 

 Fig. 19 and the corresponding winter values are a few db lower. The 

 average noise levels in the tropics may be as much as 15 db higher than 



2. 



> 



o 

 m 

 < 



o 



UJ 



o 



40 



30 



20 



to 



Z 



</> 



_l 



LU 



> -10 



<J2 -20 



o 



z 



< 



o 



CL 



I- 



■30 



•40 

 iO 



10 



10' 



10- 



10" 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 19 — Typical average noise level in u 6-kc band. 



