420 



BELL SYSTEM TECHNICAL JOURNAL 



resistances, with the result that the dissipation of the coils needed to 

 broaden the band has only the effect of adding a constant loss to the 

 filter characteristic, this loss being independent of the frequency. 



ELECTRICAL 



STRUCTURE 



A 



PHYSICAL 



STRUCTURE 



B 



REACTANCE 



CURVES FOR 



EACH ARM 



C 



ATTENUATION 

 CHARACTERISTIC 

 D 



ITERATIVE 

 IMPEDANCE 



E 



BAND- PASS FILTER 



rw^^' 



FREQUENCY 



FREQUENCY 



y FREQUENCY 



Fig. 12— Lattice network band-pass filter employing series coils. 



To vary the width of the band below the 13.5 per cent band ob- 

 tained with crystals only, added capacitances can be placed in parallel 

 with the crystals increasing the ratio r. This results in a smaller separa- 

 tion in the resonant frequencies and hence a narrower band width. 

 By this means the band width can be decreased indefinitely, although 

 the dissipation caused by the coils introduces large losses for band 

 widths much less than 1/2 per cent. By this means, however, it is 

 possible to obtain band widths down to the widths which can be real- 

 ized with crystals alone. On the upper side electrical filters can be 

 built whose widths are as small as 13.5 per cent, hence this method fills 



