136 BELL SYSTEM TECHNICAL JOURNAL 



crystals is considerably greater, however, since at resonance the full 

 voltage does not appear across the crystals. In normal use the 

 voltages across the filters will be very much less than twenty volts. 



Other filters forming part of the terminal apparatus are the group 

 modulator and group demodulator low-pass filters, the channel and 

 group carrier supply filters, and the pilot supply filters. The group 

 modulator and demodulator filters are of the low-pass type employing 

 only coils and condensers as elements. The group carrier supply 

 filter is the same in schematic and mechanical design as the crystal 

 channel filters described. The pilot supply filters and the channel 

 carrier supply filters are equivalent in schematic to one section of the 

 channel filters; but of course they are only about half the size and are 

 hermetically sealed in the same manner. 



References 



For a further discussion of crystal filters the reader is referred to "The Evolution 

 of the Crystal Wave Filter" by O. E. Buckley, Journal of Applied Physics, October 

 1936, and "Electrical Wave Filters Employing Quartz Crystals as Elements" by 

 W. P. Mason, Bell System Technical Journal, July 1934. 



