SINGING ON TWO-WIRE CABLE CIRCUITS 



607 



over 80 per cent of these frequencies were between 2200 and 3100 

 cycles, while the nominal transmitted band on these facilities is from 

 250 to 3000 cycles. 



Consider the case of the return loss of one circuit in a group of cable 

 sections. A typical curve of the passive return loss of such a circuit 

 is shown on Fig. 5. It will be seen that it consists of a set of "wabbles" 



1400 1600 1800 2000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 5 — Typical return-loss-frequency characteristic of a 19-gauge H- 



circuit repeater section. 



-50 side 



around a general trend line. When a measurement of return loss is 

 made at a single pre-selected value, the return loss obtained is deter- 

 mined by what may be considered as two components, (1) the general 

 quality of the repeater section as determined by the trend line, and 

 (2) the particular part of the " wabble " on which the particular fre- 

 quency measurement happens to fall (i.e., the bottom or top, or in 

 between point of a given " wabble "). In measuring the return 

 losses of a large group of lines at, say, 2900 cycles, the lower values of 

 return loss will tend to be those which happened to be measured at 

 the bottom of a " wabble." The higher the return loss of a given 



