62 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



2.4 

 2.0 



1.6 



I/) 



m 1.2 



5 



o 



S 0.8 



Z 



0.4 



-0.4 

 1-0.8 

 -1.2 

 -1.6 

 -2.0 



20 40 60 80 100 120 140 



FREQUENCY IN KILOCYCLES PER SECOND 



160 



180 



Fig. 15 — Effect of cable current on system gain — No. 1 cable. 



Nel Loss versus DC Cable Current 



Changes in cable current affect the repeater gain to a shght extent, 

 the amount depending on the magnitude and phase of the feedback in the 

 repeater as a function of frequency. Measured changes in the loss of the 

 2,000-mile system, for currents of 5 and 10 milliamperes less than the 

 normal value of 225 milliamperes, are shown in Fig. 15. Under normal 

 conditions the automatic control will hold the cable current variation 

 within ±0.5 milliampere. 



The shape of the curve on Fig. 15 is almost the same as that computed 

 in advance from laboratory measurements on model repeaters. 



System Noise 



Shown in Fig. 16 are values of noise on the No. 1 cable system meas- 

 ured in the Fall of 1955 and again in the Spring of 1956. The noise increase 

 is compatible with the decrease in undersea system net loss during this 

 period. To prevent overload, the loss in the transmitting temperature 

 equalizer has to be increased as the undersea loss decreases; this lowers 

 the levels of the various parts of the undersea system by various amounts. 



The noise shown in the top channel exceeds the 36 dba objective by 

 a small amount. The excess can be recovered, if necessary, by certain 

 changes in the terminals without recourse to compandors. 



Fig. 16 shows also the noise on the No. 2 cable system shortly after 



