XEOATivK iMPEDAxrr. ti:lepitoxi; kepkaters 1057 



in the nature of intertoU links and are recniiicd to operate at very low 

 loss similar to an intertoU circuit." 



The most extensive use of negative impedance repeaters has been in 

 connection with the interoffice trunks. In this role they ha\'e been ex- 

 tremely useful and have contributed greatlj^ to transmission improve- 

 ment in the last few j^ears. It would also be desirable to utilize this new 

 and effective tool in reducing the losses of toll connecting and tandem 

 triniks. There have been many cases of this type where improvement 

 has been effected, but a very wide use has been prevented by their 

 effect on return loss. 



The insertion of a series negative impedance in an otherwise uniform 

 loaded circuit introduces an impedance discontinuity which means that 

 a substantial amount of energy is returned to the sending end of the 

 circuit as ''echo." As the introduction of negative impedance is the 

 method of obtaining gain from the series repeater it follows that the 

 magnitude of the impedance discontinuity and therefore the "echo," 

 is proportional to the gain of the repeater. The effect of echo on ease of 

 conversation depends both on its magnitude and on the amount of delay 

 before the echo reaches the talker's ear. When the delay is small as in 

 the case of an interoffice trunk a large amount can be tolerated, but in 

 the case of a toll connecting trunk which becomes a part of a toll con- 

 nection the delay may be large and only a small amount of echo can be 

 permitted. This limits the amount of gain from a series repeater when 

 it is used in toll connecting trunks. 



As a result of this feature of the series negative impedance repeater, 

 most cases recjuiring substantial gains in toll connecting or tributary 

 trunks usuall}^ had to be handled by the older and more expensive hybrid- 

 coil type repeater. With this latter repeater, it is practicable to introduce 

 gain without serious reflection effects if proper attention is given to 

 network design and to uniformity of construction of the line itself. 

 However, the cost of the more complicated repeater was relati\"ely 

 high so that the new development described herein was undertaken to 

 meet the indicated need, i.e., a repeater embodying the same desirable 

 features of simplicity of design but still approaching in performance 

 that of the hybrid-coil repeater in its effect on return loss. 



The new repeater (E23) consists essential^ of the earlier series re- 

 peater with the addition of a shunt negati\'e impedance element. This 

 combination has approximately as small an effect on return loss as the 

 hybrid-coil repeater and will give about the same gain when used under 

 similar line conditions. As a result, the field of use of the negative im- 

 pedance t^^pe repeater will be greatly extended, especially in the toll 



