792 



BELL SYSTEM TECHNICAL JOURNAL 



quencies remote from the cutoff the attenuation of the network consid- 

 erably exceeds that of the filter equivalent. The attenuation produced 

 by the auxiliary (susceptance) networks used in conjunction with paral- 

 lel filters is not so easily evaluated in terms of a standard filter equiva- 

 lent. Since these networks produce peaks of attenuation just beyond 

 the filter cutoff, thus enhancing the selectivity of the systems, they are 

 however, in some respects particularly valuable. We can summarize 

 the economic aspects of impedance correction in the statements that a 

 severe impedance requirement will increase the number of elements 

 (coils and condensers) required for an average filter used in carrier 

 circuits by about 15 per cent or 20 per cent, and that the corresponding 

 increase in the cost of the filter as a whole will be about 10 per cent or 

 15 per cent. 



Practical Limitation to Impedance Correction 



The fundamental limitation on the correction of wave filter imped- 

 ances is practical rather than analytical. In other words it depends 

 upon the accuracy with which it is possible to manufacture filters. 

 All the curves so far exhibited have been based on the assumption 

 that the filter elements, coils, condensers, and resistances, have the 



25 



Z 20 



LU 

 (J 



a. 



UJ 



Q. 



15 



I- 

 z 



UJ 



G 



ii. 

 u. 



UJ 



O 

 u 



z 

 o 



I- 

 o 



LU 



10 



8 9 



FREQUENCY IN KILOCYCLES PER SECOND 



10 



Fig. 23 — Effect of element variations on a filter reflection coefficient. 

 I — Reflection coefficient when all elements have their exact design values, 

 II — Envelope of reflection coefficients of the best 99 per cent of the filters. 

 Ill — Envelope of worst possible reflection coefficients. 



