308 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954 



signals from the incoming trunk circuit to which it is connected and 

 transmits them to the intertoll faciUties and vice-versa. 



Four-Wire Switching 



Hybrid coils are provided in the concentrating ecjuipment to (1) couple 

 the tAvo-wire transmission incoming trunk circuit to the four-wire trans- 

 mission outgoing trunk circuit and (2) match the impedance of the in- 

 coming trunk to that of the outgoing trunk. 



The impedance of each two- wire leg of the outgoing trunk is always 

 600 ohms. The impedance of an incoming trunk may be 600 ohms or 

 900 ohms or 1,500 ohms. Thus three hybrid coil ratios are required 

 namely 1 to 1, 1.5 to 1 and 2.5 to 1. 



Since the hybrid coil and the associated balancing network are deter- 

 mined by the particular incoming trunk with which they function they 



TO JACK CIRCUIT 

 OF TEST BOARD 



TO 



TRUNK 



SELECTOR' 



SWITCH 



TO TA OS 

 CONTROLLER » ^ — f 



Fig. 3 — Outgoing trunk circuit. 



