Impedance Correction of Wave Filters 



Development of Impedance Requirements 

 By E. B, PAYNE 



The present importance of wave filter impedance correction arises chiefly 

 from its relation to crosstalk in carrier systems. Briefly, it appears that line 

 transpositions, an effective remedy for many types of crosstalk, are less 

 satisfactory when directed against the so-called "reflected near-end cross- 

 talk" and "reflected far-end crosstalk" produced when waves reflected from 

 the junctions between lines and repeater equipment of carrier systems induce 

 currents in neighboring systems. The expense of the elaborate transposition 

 scheme necessary for a substantial reduction in these types of crosstalk 

 makes it desirable to diminish the amplitude of the reflected wave as far as 

 possible by the improvement of the impedance match between lines and 

 repeaters. A detailed study shows that this is most conveniently done by 

 terminating the filters in the repeater by sections whose image impedances 

 at one end match the main body of the filter, while at the other they ap- 

 proximate constant resistances, matching the terminal impedances. 



The development of appropriate filter terminating sections has passed 

 through a number of stages. The earliest filters gave reflection coefficients 

 as great as 50% to 60% in the useful transmission band. The invention of 

 "m-derived" and " .r-terminated " filters, plus a number of more or less 

 empirical schemes, made it possible to obtain reflection coefficients ranging 

 from 10% to 15% in the useful band. Recent progress has resulted 

 chiefly from the development of a series of sections, the simplest of which is 

 equivalent to the w-derived type, while the others, of progressively increas- 

 ing complexity, give progressively better approximations to the ideal 

 characteristic. The use of the more complicated sections has made it 

 possible to reduce filter reflection coefficients to the order of 2%, or even less. 

 At present the chief limitation appears to be the difficulty of manufacturing 

 filters with sufficient precision to allow the theoretical characteristics to be 

 realized. The paper is illustrated by figures showing the various stages of 

 this progress as they are exemplified in actual designs. 



THE rapid increase in the demand for long distance or toll telephone 

 service in recent years led to the introduction, about 1920, of 

 carrier systems as a means of securing more intensive use from long 

 telephone lines. The growth of these circuits has resulted, still more 

 recently, in the multiplication of the number of carrier systems in 

 use and in the close association of several similar or different carrier 

 systems on a single pole-line. This development raised a number of 

 totally new engineering problems and demanded careful reconsidera- 

 tion of many other questions of comparatively small importance in 

 earlier systems. 



Among the factors thus brought into prominence by carrier system 

 development, the chief, for the purpose of this paper, is the impedance 

 mismatch between telephone lines and repeaters or terminal apparatus. 

 The components of a complete transmission system, such as the line 



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