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BELL SYSTEM TECHNICAL JOURNAL 



pass band of the filter is seriously distorted unless elements, such as 

 crystals, are used which have negligible dissipation. 



III. Band-Pass Resistance Compensated Crystal Filters 



All of the wide-band resistance compensated crystal filters proposed 

 so far can be shown to be equivalent to the two general types of lattice 

 crystal filters shown on Fig. 8. For example the crystal filter of Fig. 1 

 was shown to be equivalent to the lattice type filter of Fig. 8 (b) in which 

 the crystals in the lattice arms are left out. 







Fig. 8 — Wide-band lattice crystal filters. 



In the lattice filters of Fig. 8 the number of crystals employed can be 

 cut in half by employing in two similar arms a crystal with two pair of 

 equal plates. It can be shown that such a crystal used in similar arms 

 is equivalent to two identical crystals of twice the impedance of the 

 crystal used as a single plate and having the same resonance frequency. 

 Hence the lattice filters of Fig. 8 are as economical of elements — except 

 for two condensers — as an unbalanced type filter. For some purposes, 

 however, such as connecting together unbalanced tubes, it is desirable 

 to obtain a filter in an unbalanced form. Also, at high frequencies the 

 crystals become quite small and hence it becomes difficult to divide the 



