286 



BELL SYSTEM TECHNICAL JOURNAL 



requires no special shielding as its input voltage induced into the loop 

 is comparatively large. 



The selectivity of the set is determined by three separate units, 

 viz., the antenna circuits, the intermediate frequency filter and the 

 low frequency filter, each of which has a specific use. Carson and 

 Zobel ^ have made the following statement. 



"In filters designed to select a band of frequencies of width w, the 

 ratio of energy transmitted through the network by the signal and by 



r^ 



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Q z 



^1 OHM 



P.^ 



OuJ 



ol- 



o< 



up 



-CJ 



r€h 



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Fig. 2 — Schematic circuit diagram of receiving'set 



EL O 



random interference is inverselj' proportional to the band width and 

 increases inappreciably when the number of sections is increased 

 beyond two." 



The main purpose of the filters is therefore not to define the fre- 

 quency band of the set insofar as static is concerned, but to exclude 

 continuous wave interference. It is hoped that 500 cycles wide fre- 

 quency bands ^ can be maintained free of c.w. interference for static 

 measurements and the simplest way to obtain such a band in the 

 receiver is to make the low frequency filter an efificient low pass filter 

 that cuts off every frequency above 600 c\cles. More than two 

 coupled circuits are hardly required in the antenna circuits, but the 

 intermediate frequency filter ought to have sharper cut-off points 

 than two coupled circuits will give. The selection of filters naturally 

 depends upon the c.w. interference and it may in some cases be pos- 

 sible to reduce the number of filters and thereby make the recorder 

 cheaper. The records shown later correspond to a frequency band 

 of 2000 cj'cles — between 57.5 and 59.5 k.c, — but it will probably not 

 be long before c.w. interference makes it necessary to reduce this band 



^ "Transient Oscillators in Electric Wave-Filterb" — John R. Carson and Otto J. 

 Zobel, Bell System Technical Journal, Vol. II, \o. 3, p. 27. 



' Bands at 15, 30, 60, 120 . . . kilocycles would probably be satisfactory. 



