526 BELL SYSTEM TECHNICAL JOURNAL 



by "two-way" considerations. The important "two-way" considera- 

 tions are crosstalk on the one hand and proper control of circulating 

 currents which might cause singing or serious echoes on the other 

 hand. These considerations keep the repeater gains so low that the 

 limit of repeater and loading coil capacity is not important nor is the 

 lower limit at which noise would become serious. 



The most important crosstalk consideration on two-wire circuits is 

 "near-end" crosstalk. The arrangement of repeater gains which 

 gives lowest near-end crosstalk is one in which the output levels of 

 the repeaters are the same throughout the circuit until the receiving 

 end is reached, at which point the repeater gain is reduced or loss 

 inserted as necessary to give the desired overall net loss for the circuit. 



This arrangement is not best from the standpoint of the circulating 

 currents, however. From this standpoint the best setup is one in 

 which the repeater output levels "taper" considerably from the sending 

 end to the receiving end of the circuit. 



In the Bell System plant a compromise is made between these two 

 considerations which calls for layout of gains about as follows: At 

 the transmitting end the transmitting repeater gain is made such that 

 the outgoing level is + 3 db. As the transmission passes through 

 other repeaters the upper level is allowed to drop about >^ db per 

 repeater for average temperature conditions. Finally at the receiving 

 end of the circuit gain or loss is introduced to give the required net 

 loss for the circuit. Of course, the application of the above rules for 

 laying out repeater gains results in giving individual repeaters different 

 gains in the two directions. 



Smoothness of Impedance 



For two-wire circuits it is important that the cables have a "smooth " 

 impedance-frequency characteristic. To attain this result, loading 

 coil inductances are held within close manufacturing tolerances while 

 the cable capacitance variations are also carefully controlled. In the 

 field care is, of course, taken with the spacing of the loading coils. 

 Following are some representative figures for side circuits of fractional 

 deviation from the average values per loading section or per loading 

 coil : 



H Spacing B Spacing 



Representative deviation of cable capacitance * 013 .018 



Representative deviation of loading coil spacing * 005 .005 



Representative deviation of loading coil inductance * 007 .007 



Total deviation 016 .020 



* Representative deviation is the square root of the mean square of the individual 

 deviations. 



