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BELL SYSTEM TECHNICAL JOURNAL 



percentage band width filters will have lower impedances than the 

 wider ones. For example, the filter whose characteristic is shown by- 

 Fig. 20, has an iterative impedance of 25 ohms. 



It is evident that a still wider band can be obtained with the sec- 

 tion discussed above by making the two resonances of Fig. 11 dissym- 

 metrical. If the lower one is brought in closer to the anti-resonant 

 frequency the top one extends farther out in such a manner that the 

 total percentage frequency separation is greater than 9 per cent. If 

 one element of this type is combined with one whose lower resonance is 

 brought farther away from the anti-resonance than is the upper reso- 

 nance, a filter whose pass band is greater than 13.5 per cent is readily 

 obtained. On the other hand as the band is widened by this means, 

 the cross-over points of the impedances of the two arms are of necessity 

 brought very close to the cut-off frequencies, so that such a filter would 

 introduce most of its loss very close to the cut-off frequencies. This 

 type of characteristic might be useful in supplementing the loss charac- 

 teristic possible with electrical elements, but by itself would not pro- 

 duce a very useful result. 



We have so far discussed the characteristics which can be obtained 

 by placing coils in series with crystals. An equally useful result is 

 obtained by placing coils in shunt with crystals as shown by Fig. 135. 

 This arrangement results in a band-pass filter capable of giving the 

 same band width as the first type discussed above. The only difference 



Fig. 14 — Band-pass filter used between vacuum tubes. 



occurs in the iterative impedance which will be as shown by Fig. \ZE. 

 For narrow band widths this type of filter has a very high iterative 

 impedance. For example, for a one per cent band width, using ordin- 

 ary sized coils and crystals, the iterative impedance may be as high as 

 400,000 ohms. Such filters can be used advantageously in coupling 

 together high impedance screen gird tubes without the use of trans- 

 formers. One such circuit is shown schematically by Fig. 14. 



Filters made by using either series or shunt coils in conjunction 



