132 BELL SYSTEM TECHNICAL JOURNAL 



the use of 20-cycle signaling on many toll circuits. It has accord- 

 ingly been expedient to use a higher frequency for the majority of 

 these circuits, and the one which has been most generally used for 

 this purpose has been 135 cycles. It is desired, therefore, to bring 

 out here some of the more important features of the 135-cycle signaling 

 arrangements which are being developed for cable systems. 



The signaling equipment for cable systems has been required to 

 meet more severe conditions than those ordinarily encountered on 

 open-wire lines. In order not to interfere with the direct current 

 telegraph system used on the cables, it has been necessary to limit 

 the signaling current to a few milliamperes. Furthermore, the char- 

 acteristics of the cable apparatus have been such as to attenuate 

 the signaling currents to a greater extent than in open-wire systems. 

 It has accordingly been necessary to undertake the design of signaling 

 equipment of greater sensitivity, as well as to provide a source of 

 supply of signaling current possessing a high degree of freedom from 

 harmonics and capable of being closely regulated. 



The desired increase in sensitivity of the signaling system is ex- 

 pected to be obtained through the use of a highly selective circuit in 

 conjunction with a very sensitive 135-cycle relay. Fig. 19 shows 

 the general scheme of the circuit which is being developed for the 

 purpose. 



It is seen that this includes a filter which may be inserted in the 

 line circuit between the signal-receiving apparatus and the switch- 

 board "drop" to give the desired terminal impedance at the signaling 

 frequency, thus preventing the low impedance shunt on the signaling 

 relay which would otherwise be caused by the "drop" circuit at a 

 frequency as low as 135 cycles. The filter is arranged so that it need 

 be inserted in the circuit only in such cases as may require the in- 

 creased signaling range obtained in this manner, thus, it may be 

 omitted on the shorter circuits where it is usually unnecessary. 



The general circuit arrangement shown in Fig. 19 will permit the 

 ready interchange of different types of signaling apparatus. With 

 this arrangement of the apparatus, any "ringer" of any desired fre- 

 quency combination such as 20-135 cycles, 20-20 cycles, 135-135 

 cycles, etc., may be connected temporarily to the system when desired, 

 by means of "patching" cords, without requiring any changes in the 

 permanent wiring. 



Much of the sensitivity and selectivity which may be obtained with 

 this signaling system are due to the design of the 135-cycle relay, 

 As shown in Fig. 20, the relay is designed to make it capable of close 



