408 BELL SYSTEM TECHNICAL JOURNAL 



discrimination against interference can occur in a space diversity 

 system since it lacks the phasing which produces directional dis- 

 crimination. 



The Time Constant of the Automatic Gain Control 

 Thus far no comments have been made on the improvement figures 

 relating to "no automatic gain control" shown in the table of Fig. 39. 

 This table shows that the signal-to-noise ratio for one antenna {K = \) 

 is from 2.5 to 3.5 decibels higher when no automatic gain control is 

 used; i.e., perfect automatic gain control penalizes the signal-to-noise 

 ratio to that extent. ^^ The advantage of automatic gain control is a 

 constant output volume. In practice, a compromise is effected by 

 retarding the action of the control. A time constant of 0.5 or one 

 second is usually used. (This compromise is influenced by quality 

 considerations as well as noise considerations.) 



In the MUSA system signal-to-noise ratio measurements the time 

 constant of the automatic gain control circuit (0.06 second) was not 

 changed during the switchover from the MUSA to the single antenna. 

 If a time constant of 0.5 second had been used with the MUSA and a 

 one-second time constant with the reference receiver, the measured 

 improvement would probably have been reduced by a little less than 

 one decibel. 



Method of Averaging Noise 



In all of the signal-to-noise measurements and in the diversity 

 analysis noise voltage has been averaged arithmetically along the time 

 axis. Owing to a rather marked reduction of noise peaks with the 

 MUSA compared with a unit antenna different improvements would 

 result if different ways of measuring it had been adopted. To investi- 

 gate this, motion pictures were made of the signal meter and noise 

 meter variations for the MUSA and for the single antenna. The 

 transcribed records appear in Fig. 41. Some calculations have been 

 carried out for the noise distributions marked A, B, and C in Fig. 41. 

 If the noise ratio of B/A measured by arithmetically averaging noise 

 voltage is called decibels, it becomes -f 2.4 decibels by averaging 

 power arithmetically. The corresponding figures for BJC are and 

 -|- 2.7 decibels. Thus, if noise power is averaged instead of noise 

 voltage the measured primary diversity improvement is substantially 

 increased. 



'1 The action of the automatic gain control does not change the instantaneous 

 signal-to-noise ratio. Interpreting signal-to-noise ratio as average signal divided by 

 average noise rather than the average of the signal divided by the noise results in this 

 difference. 



