The Bell System Technical Journal 



January, 1935 



Wide Band Transmission Over Balanced Circuits * 



By A. B. CLARK 



In a recent paper ^ amplifiers capable of handling frequency band widths 

 of the order of 1,000 kc. or more are described, together with terminal 

 apparatus for efifectively utilizing these wide bands for telephone, telegraph 

 and television purposes. The paper confines itself to the coaxial line 

 structure for transmitting these wide frequency ranges but points out that 

 broad-band transmission is also applicable to balanced conductor systems. 

 The present paper discusses briefly some of the possibilities of the more 

 familiar balanced circuits, circuits more or less as they now exist in the 

 present plant being first considered, following which are circuits obtained 

 by new construction. Wide-band transmission over balanced circuits offers 

 interesting possibilities both for circuits in the present plant and for new 

 construction. 



A HIGH degree of electrical balance^ has been for a long time a 

 ^ ^ fundamental requirement of telephone transmission lines. This 

 has been required not only to prevent interference entering into tele- 

 phone circuits from other types of electrical circuits but also to prevent 

 mutual interference between the closely adjacent telephone circuits on 

 open-wire lines or in cables. 



In the central offices, to be sure, unbalance in apparatus has been 

 frequently employed for simplicity and convenience. In toll circuits, 

 however, such office unbalance has been electrically separated from the 

 outside plant by the use of repeating coils or otherwise. In local cir- 

 cuits the high standard of balance required for toll circuits has not 

 generally been necessary since the exposures to interfering fields are less 

 severe and the range of speech levels is much smaller. However, when 

 local circuits are connected to toll circuits the unbalances are kept 

 electrically separated from the toll circuits by the use of repeating coils. 

 In recent years the tendency to the use of higher frequencies in com- 



* Published in January, 1935 issue of Electrical Engineering. Scheduled for pres- 

 entation at Winter Convention of A. I. E. E., New York City, January 22-25, 1935. 



^ "Systems for Wide- Band Transmission Over Coaxial Lines" by L. Espenschied 

 and M. E. Strieby, Elec. Engg., October, 1934; Bell Sys. Tech. Jour., October, 1934. 



^ As used here, the term electrical balance refers to the two sides of a telephone 

 circuit. To secure such balance, the aim has been to construct the go and return 

 conductors of each circuit of the same gauge and material and to locate them sym- 

 metrically with respect to earth and to surrounding conductors. The aim has also 

 been to apply to each circuit terminal apparatus symmetrical with respect to its 

 series impedances and shunt admittances to ground. 



1 



