Negative Impedance Telephone Repeaters 



By J. L. MERRILL, JR., A. F. ROSE, and J. O. SMETHURST 



(Manuscript received June 8, 1954) 



In the exchange telephone plant, speech is transmitted largely at voice 

 frequencies over a single pair of wires that carries conversation in both direc- 

 tions. Until recently, adequate transmission was assured through a suitable 

 choice of coil loading and conductor size. The need for voice frequency ampli- 

 fiers had long been recognized, but wide use of the conventional hybrid type 

 of repeater with separate amplifiers for conversation in the two directions 

 had not been economic. In 1948, however, a new type of repeater was intro- 

 duced that used the same source of gain for transmission in both directions, 

 and did not require costly filters. This repeater is known as the El repeater. 

 It operates as a negative impedance in the line. 



This paper describes the wide field of usefulness of two new types of 

 negative impedance repeaters, the E2 and E3. Sufficient theory is given to 

 show the advantages of using two types of negative impedance in a line, the 

 series type and the shunt type, and how, by the addition of the shunt type 

 to the series type, it is possible to insert gain without serious reactions on 

 line impedance. The new equipment is described and maintenance fea- 

 tures, together with methods of testing this relatively new type of repeater, 

 are discussed. 



Application in the Bell System 



introduction 



The application of the telephone repeater, the development of which 

 made countr\'^vide telephone service practicable, had been confined 

 largel}^ to toll plant from the year 1915 when the transcontinental line 

 was first established, until a few years ago. About 1048 the negative 

 impedance repeater^ was developed and placed in production. This 

 repeater operates on the principle of inserting negative resistance (and, 

 if desired, negative inductance or capacitance^ in series with tho. line, 

 thus reducing the overall impedance and increasing the current in the 

 line. This results in transmission gain in the same sense as that resulting 

 from a repeater of the conventional type. This principle and the package 



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