CARRIER SYSTEMS ON TELEPHONE LINES 



623 



which, as has been noted, acts to separate the carrier currents from 

 the regular speech currents on the common line circuit. It consists 

 of a high-pass and a low-pass filter paralleled on the line side. Currents 

 entering these terminals from the line circuit pass through the high- 

 pass circuit to the carrier apparatus or through the low-pass circuit to 

 the circuit terminal or repeater. The transmission characteristics of 

 these filters are shown on Figure 41. It will be noted that frequencies 



100 1- 



CHAWNEL-J CtMNNEL-2 CHANNEL- 1 



CHANNEL-2 CHANNEL-I CHANNEL-J 



I £ 3 4 5 6 7 8 5 10 II le 13 14 15 16 17 18 19 20 21 £2 23 24 £5 26 27 28 29 30 



FREQUENCY KILOCYCLES 



Figure 41 — ^Typical line filter characteristics 



above approximately 3,300 cycles are transmitted in the high-pass 

 circuit and frequencies below about 2,800 cycles are transmitted 

 through the low-pass circuit. It is common to equip a few line circuits 

 with line filter sets, in addition to those which are normally in use for 

 carrier transmission. This makes it readily possible in case of an 

 emergency or for other reasons to use the spare wires thus equipped 

 for carrier transmission. 



Non-linear effects may be produced in the coils and condensers in 

 the circuit. The design of the filter parts must be made so that these 

 effects will be a minimum. This requires the use of non-magnetic 

 cores in the coils, and also that the containers be of non-magnetic 

 material. Condensers in magnetic containers must be located so 

 that they will not lie in the field of the coils and thus contribute to 

 the modulation products. The modulation in the line filters, telegraph 

 composite sets, and office and cable loading units, must also be con- 

 sidered. 



