LINE FILTER FOR PROGRAM SYSTEM 



389 



o<^ 



or, 



-O0.8 



0.4 



I 2 3 4 5 6-7 8 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 5 — Loss of filter in program frequency band. 



Impedance Correction 



In the discussion of the lattice section it was stated that its phase 

 shift is given by equation (1) only when the section is terminated in 

 its image impedance. To facilitate the design and simplify the filter 

 structure, this section has been given an image impedance of the 

 simplest type. This impedance, Z/, varies with frequency according 

 to the following equation : 



^^ = -4=' (3) 



1 -^- 



where Zo is the "nominal impedance" of the filter, a constant equal 

 approximately to the average impedance of the open-wire lines in the 

 program band; and fc is the theoretical cut-off frequency. Thus the 

 image impedance rises with increasing frequency to a very high value 

 near the cut-ofif; and, since the line impedance is practically constant 

 except at very low frequencies, a large mismatch would result at the 

 upper edge of the transmitted band if the lattice section were con- 

 nected directly to the line. The impedance correcting sections at 

 the ends of the filter are employed to avoid this mismatch. The 

 properties of these sections are such that when they are inserted 

 between the lattice section and the line or the office terminating 

 apparatus, the impedance of the filter matches that of the line and 

 the office apparatus, and the lattice section faces its own image im- 

 pedance. In this manner, both internal and external reflections largely 

 are avoided ; and the phase shift of the lattice section has the proper 

 value. ^ 



The general theory on which the design of the impedance correcting 

 sections is based is discussed at length in a recently published article.'' 

 In brief, the sections consist of two parts: a 4-terminal network to 



^ "Impedance Correction of Wave Filters," by E. B. Payne, Bell. Sys. Tech. Jour., 

 October, 1930. 



^"A Method of Impedance Correction," by H. W. Bode, Bell. Sys. Tech. Jour., 

 October, 1930, 



