82 



liLl.L SYSTEM lECllSlCAL JOURNAL 



and inductanci', are illustrated in Fig. 20, in which F"igs. 20A, B, and 

 C represent either individual sections or portions of composite filters 

 and Fig. 2()D represents a coini)osito filter. When equivalent re- 

 actance meshes occur entirely within a filter or within a section of a 

 filter, the filter or the section will have the same cut-off frequencies 

 and frequencies of infinite attenuation and the same attenuation, 

 phase, and image impedance characteristics, whiclu'\er cqui\alent 



Fig. 21 — Generalized Forms of Kquiwili-iU .Sorics-Sliunt, HridKcd-r, and'Lattice 

 Type Filter Structures 



form of mesh is substituted for an existing mesh. When cqtilvalent 

 meshes are interchange<l in either recurrent or composite filters the 

 substitution is generally made after the scrics-shunt structure is 

 designed an<l after it has been found that the substitution will effect 

 economies. The three terminal meshes referred to occur, in general, 

 in imbalanced filler structures. F'or balanced filter circuits, corre- 

 s|Min(ling meshes will be found for each t)f the efiuivalent networks by 

 the process of di\iiling e(|ually the series imi>edance between the two 

 series lines of the filter. 



While the discussion in tlii> jj.iper is based prin(ii).ill\- on the series- 

 shunt structure there are two other imjjortant t\pcs of structures 

 which will be mentionitl. These are the so-called lattice'' type strut- 



