ELECTRIC ir.l VE FILTERS 



265 



Theory of the Electric Wave Filter" by G. A. Campbell, which 

 appeared in the November, 1922 issue, and "Theory and Design of 

 Uniform and Composite Electric Wave Filters" by O. J. Zobel, 

 in the issue for January, 1923. It is not the purpose of the author to 

 discuss further the theory of wave filter design or operation, but rather 

 to discuss some of the problems that arise when an attempt is made to 

 construct filters which must meet certain requirements of a carrier 

 system with a very small margin of variation, and to describe the 

 methods of solution adopted. It is assumed that the reader is familiar 

 with the general principles of electric wave filter theory. 



MODULATOR 



BAND 

 FILTER 



pRHP — nnnn-J" demodulator 



BAND 

 FILTER 





TRANSMITTER 

 AMPLIFIER 



DIRECTIONAL 

 FILTER 



(TglP OW^J" 



RECEIVER 

 AMPLIFIER 



DIRECTIONAL 

 FILTER 



HIGH PASS 



LINE 



FILTER 



VOICE 

 FREQUENCY 



LOW PASS 



LINE 



FILTER 



J 



Fig. 1 — Type "C" system terminal arrangement. 



Different carrier systems require different numbers and designs of 

 filters. A carrier system designed to provide three two-way telephone 

 channels would have six band filters at each terminal. These filters 

 would be of a type that passes all frequencies between certain upper and 

 lower frequency limits, and provides a high attenuation or loss to all 

 frequencies below and above these limits. A simplified diagram of 

 such a terminal arrangement is shown in Fig. 1. 



In addition to the band filters, a pair of directional filters is required 

 at each terminal to separate the three incoming from the three outgoing 

 channels. This directional filter pair is composed of a low-pass and a 

 high-pass filter, which as their names imply, pass respectively all 

 frequencies from zero up to the predetermined cut-oft" point and 



