Cable Carrier Telephone Terminals * 



By R. W. CHESNUT, L. M. ILGENFRITZ and A. KENNER 



This paper describes the circuits, performance and equipment 

 features of the terminals of a new 12-channel carrier system for 

 application to existing toll cables. The 12-channel group of ter- 

 minal apparatus has been designed also to form a basic part of the 

 terminals of other carrier systems now under development, such as 

 the type J system for open wire and the coaxial system. 



Introduction 



A BOUT twenty years ago the first commercial carrier telephone 

 ^ ^ system was installed between Baltimore and Pittsburgh. Until 

 recently, telephone circuits were obtained by carrier methods largely 

 on open-wire lines. The notable exceptions were on short deep sea 

 submarine cables.^- ^ Ten years ago, experiments were initiated 

 which have now resulted in the design of a carrier system which can be 

 applied with substantial economy to existing long distance toll cables 

 on land. Its general features are described in another paper.^ The 

 present paper describes in detail the circuits and performance of the 

 carrier terminals of this system. 



General Features 



The carrier system for existing cables, designated type "K," is de- 

 signed to provide twelve telephone channels in the frequency range 

 between 12 and 60 kilocycles, using one non-loaded 19-gauge paper 

 insulated cable pair in each direction. Previous carrier systems em- 

 ployed for open-wire lines used vacuum tubes for the modulating or 

 translating circuits and electrical filters composed of coil and condenser 

 networks for separating the frequency bands associated with the re- 

 spective channels. The terminals of the new type "K" system are 

 simpler and yet provide improved performance by using copper oxide 

 bridges for the modulation function and quartz crystal filters ^ for the 

 separation of the individual channel bands. 



The quartz crystal filter is economical only in a comparatively high- 

 frequency range, necessitating the use of high intermediate frequencies. 

 The high intermediate frequencies are reduced by a second stage of 

 modulation to the desired range of frequencies for transmission over the 



* Presented at Winter Convention of A. I. E. E., Jan. 24-28, 1938. 



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