CARRIER SYSTEMS ON TELEPHONE LINES 



593 



Impedance. It is found desirable in connection with communication 

 circuits in general to match carefully the impedances of the various 

 circuit and apparatus components if for no other reason than to insure 

 the best transmission by keeping the reflection losses at a minimum. 

 In connection with carrier systems the matter of crosstalk constitutes 

 an additional important reason for doing this. As noted above, the 

 crosstalk situation is simplified by the standardization of frequency 

 arrangements by which only far-end crosstalk is normally received. 

 This not only reduces the level differences at which crosstalk takes 

 place as explained, but it simplifies the transposition design problem 

 because near-end crosstalk is normally greater in magnitude than the 

 far-end crosstalk. However, if the line circuit is irregular, i.e., if 

 there are abrupt impedance differences in the circuit as it passes from 

 point to point which bring about wave reflections, these may result 

 in near-end crosstalk being reflected and appearing as far-end crosstalk, 

 thus adding to the true far-end crosstalk and making it more difficult 

 to keep within desirable limits. For this reason every effort is made in 

 the layout of the carrier lines to avoid such reflection effects. This 

 makes it desirable to load even relatively short cables including office 

 cables and wiring. The apparatus terminal impedances are also 

 carefully designed, so that their values simulate the characteristic 

 impedance of the line circuits over which the systems are operated. 



Overall Line Circuit. A situation sometimes occurs in a long carrier 

 system where the line is made up of sections in which the wire pairs 



Figure 17 — Schematic of high-pass transfer line filter circuit 



occupy different pin positions in each section and the voice circuit on 

 the pair in which the carrier system operates is terminated at different 

 points or perhaps joins other lines. The use of line filter sets at the 

 intermediate points makes this arrangement possible. Special transfer 

 line filter sets have also been designed where it is desired to transfer 

 the carrier currents from one pair of wires to another without affecting 



