The Transmission Characteristics of Toll Telephone Cables 

 at Carrier Frequencies 



By C. M. HEBBERT 



IN THE design of a new telephone transmission system a knowledge of 

 the characteristics of the medium over which the waves are to pass is, 

 of course, a prerequisite. What painstaking experimentation is necessary 

 to accumulate such knowledge, however, what voluminous data are involved, 

 what minutiae of detail, and what extremes of accuracy, are things far 

 less obvious. 



Recent papers have described a new 12-channel carrier telephone system 

 for operation over cable pairs. For this system a knowledge of the maxi- 

 mum cable losses is needed in order to determine the necessary repeater 

 gains. Accurate data on the insertion loss slope versus frequency are 

 required so that compensating equalizers can be designed to give uniform 

 transmission over the frequency band. In order to design a regulating 

 system to compensate for the variations in attenuation which result from 

 changes in cable temperature, precise knowledge of these variations as a 

 function of frequency is essential. It is necessary to know the impedance 

 of the cable pairs in order that the amplifier impedance may be matched to 

 it, thereby avoiding reflections which would aggravate cross-talk effects. 

 For various purposes, e.g., testing the cables, designing the coils to balance 

 out crosstalk, etc., it is also necessary to know the fundamental parameters 

 (resistance, inductance, capacitance and conductance) or so-called primary 

 constants of the pairs. The velocity of transmission also plays a part in 

 determining the characteristics of the channels. In addition to all these 

 transmission characteristics, it is, of course, essential to know the cross- 

 talk couplings between different pairs. This subject has been treated 

 elsewhere , however, and is not considered herein. 



In order that the cable carrier systems may be applied in the plant with- 

 out requiring extensive transmission measurements on each individual 

 carrier pair in each repeater section, it is important that the differences in 

 the transmission characteristics between different pairs be known. The 

 problem therefore becomes one of statistical analysis. In most cases the 



1 "A Carrier Telephone System for Toll Cables," C. W. Green and E. I. Green, B.S.T.J., 

 Vol. 17, January 1938, page 80. "Experience in Applying Carrier Telephone Systems to 

 Toll Cables," W. B. Bedell, G. B. Ransom and W. A. Stevens, B.S.T.J., Vol. 18, October 

 1939, page 547. 



* "Crosstalk and Noise Features of Cable Carrier Telephone System," M. A. Weaver, 

 R. S. Tucker and P. S. Darnell, B.S.TJ., Vol. 17, January 1938, page 137. 



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