Copper Oxide Modulators in Carrier Telephone Systems * 



By R. S. CARUTHERS 



Copper oxide modulators are widely used in telephone systems 

 for translating either single speech channels or groups of speech 

 channels to carrier frequency locations on the lines. A number of 

 simple circuit arrangements have been developed that enable sup- 

 pression of certain undesired frequencies to a degree that is im- 

 practical in tube modulators. These modulators transmit equally 

 well in either direction and the modulating elements are more non- 

 linear than in tube modulators. As a result numerous effects are 

 found that ordinarily are not important in the tube arrangements. 

 Analytical studies have been considerably simplified by the use of a 

 small signal, and a large carrier controlling the impedance variation 

 of the copper oxide. It is found in this case that the superposition 

 and reciprocity theorems hold for all the circuits that it has been 

 possible to analyze even though the modulator is made up of non- 

 linear elements. Open and short-circuit impedance measurements 

 can be made use of as in four-terminal linear networks, and a 

 generalized reflection theory developed. Performance data are 

 given for an idealized modulator under a variety of operating 

 conditions. 



Introduction 



AT least as early as 1927, copper oxide rectifiers were being tried as 

 modulators for the speech channels of carrier telephone systems 

 in this country. At this time only a rather large type of rectifier was 

 available, better adapted for power use rather than in modulating the 

 few milliwatts of a speech signal. Largely because of instabilities these 

 early units were found to be unsatisfactory for modulator use. Further 

 developments in copper oxide rectifiers made in various laboratories 

 extended the variety and improved the quality of the product available, 

 so that by about 1931 they began to be promising as serious com- 

 petitors for vacuum tubes in modulators. Since 1931 continued im- 

 provements in copper oxide rectifiers have rapidly increased their 

 field of application until now they are employed in practically all 

 modulators of the latest types of carrier telephone systems. 



In the new systems a copper oxide modulator is used instead of the 

 previous push-pull arrangement of two vacuum tubes. In cable/ 



* Presented at Winter Convention of A.I.E.E., New York, N. Y., January 23-27, 

 1939. 



' The general features of carrier telephone terminals have been described in a 

 paper "Cable Carrier Telephone Terminals" by R. W. Chesnut, L. M. Ilgenfritz 

 and A. Kenner, Electrical Engineering, January 1938. 



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