ELECTRIC WAVE FILTERS 



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design and construction which had not then been satisfactorily solved. 

 The highest frequency to be used in this system was determined, of 

 course, by the limitations of wire transmission as the frequency in- 

 creases. The lowest was set chiefly by the desire to avoid interference 

 from carrier telegraph systems on other wires of the same pole line. 

 Between these limits, the six bands (three two-way channels) had to 

 be located. At the same time it was desirable to provide a second 

 system, with a frequency allocation offset or "staggered" with respect 

 to the first, to permit operation of carrier systems on pairs in close 

 proximity where the normal crosstalk would not permit operation of 

 the same systems. This is shown graphically on Fig. 4. The cross- 



14 16 18 20 22 



FREQUENCY IN KILOCYCLES 



Fig. 4 — Frequency spectra of type "C" normal and staggered systems. 



talk advantage thus obtained is rather critical with respect to the 

 overlap between interfering channels. These two factors, first, inter- 

 ference between channels in the same system and, second, interference 

 between channels in adjacent staggered systems, emphasized the need 

 for a high degree of precision in the location of the individual bands. 



The theoretical requirements of the filters, although severe, were 

 met without any departure from conventional design practices. The 

 calculated attenuation characteristics satisfied the minimum loss 



