824 



BELL SYSTEM TECHNICAL JOURNAL 



The auxiliary network improves the susceptance of parallel band 

 pass filters even more than it does that of complementary filters. 

 Curve I of Fig. 14 represents the susceptance of a typical uncorrected 

 set of band pass filters. The first step in the improvement of this 

 characteristic is due to Mr. R. H, Mills, who suggested that networks 

 whose impedances resemble that of filters above and below the actual 

 set of bands be added to the system. This reduces the susceptance 



+ 1.0 



+0.8 - 



+0.6 



+0.4 



+ 0.2 



o 



U 



Z 



a. -0.2I- 



UJ 



o 



W -0.4 



-0.6 



-0.8 



-1.0 



-1.2 



-1.4 



.2 



10 



1.2 



•(■iSfi) 



Fig. 13 — Susceptance relations at the line terminals of a pair of parallel complementary 



x-terminated filters. 



to the level shown by Curve II. Curve III gives the completely 

 corrected characteristic. The auxiliary susceptance correcting net- 

 work consists of a number of tuned circuits in parallel, one resonating 

 between each pair of successive bands, together with one resonating 

 above the topmost band and one resonating below the lowest band. 

 The insertion of the auxiliary network has the further advantage 

 that it produces peaks of attenuation near the cutoffs of the filters, 

 thus enhancing their selectivity. 



