242 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



To assess the magnitude of intermodulation noise, all channels in one 

 direction were loaded simultaneously with white noise and measure- 

 ments taken on each channel in the opposite direction. From the talker 

 volume data assumed in the design of the sj'stem, the expected mean 

 talker power is —11.1 dbm at a point of zero relative level, with an ac- 

 tivity of 25 per cent. For an equivalent system loading, therefore, the 

 level of white noise applied to each chamiel under the above test condi- 

 tions should be —14.1 dbm. Since this loading gave no sensible increase 

 in the circuit noise, the test levels were raised until a reasonable increase 

 in the noise level was obtained. In order to raise the channel noise to the 

 specified maximum of 28 dba it was necessary to raise the channel 

 levels to about — 1 dbm and — 4 dbm in the lower and upper bands re- 

 spectively. These levels, some 13 db and 10 db above the assumed 

 maximum loading of the system, give noise levels at least 26 db and 20 

 db above normal, and it is seen that adequate margins exist for varia- 

 tions and deterioration of the link. 



Closely alUed to the problem of intermodulation is the overload 

 characteristic of the sj^stem. Table III shows the measured overload 

 point of the Hnk expressed as an equivalent level at the output of the 

 amplifier in the repeater nearest to the transmitting terminal. It also 

 shows the margm between the channel level at that point and the over- 

 load point of the sj^stem; according to Holbrook and Dixon^' the mini- 

 mum requirement in this respect is 18 db. 



1.0 



0.5 



ffl 

 u 



UJ 



O 



z 

 o 



< 



D 

 2 



lU 



t- 

 I- 

 < 



-0.5 



-1.0 

 1.0 



0.5 







■0.5 

 -1.0 



GROUPS 

 3 



312 360 408 456 504 



FREQUENCY IN KILOCYCLES PER SECOND 



552 



Fig. 14 — Attenuation versus frequency characteristics of supergroup, (a) 

 Sydnej^ Mines — Clarenville. (6) Clarenville — Sydney Mines. 



