I 



Electrical Wave Filters Employing Crystals 

 with Normal and Divided Electrodes 



By W. p. MASON and R. A. SYKES 



I. Introduction 



N SEVERAL previous papers ^^ ^^ ^' "* the application pf piezo- 

 electric crystals to electric wave filters has been discussed. The 

 underlying principles and some of the design procedures were given. 

 These filters have received wide application in carrier telephone 

 systems and radio systems both in the United States and abroad.^ 

 It is the purpose of the present paper to discuss more completely all 

 the standard types of filters with crystals, and methods for deter- 

 mining their constants and attenuation characteristics. In addition 

 some of the newer results for simplifying such filters are given. 



The use of a divided plate crystal for filters resulted in cutting 

 the number of crystals in half as was pointed out in three former 

 papers. 2' ^' ^ The theory of the use of such crystals is discussed in 

 this paper and an equivalent circuit is given for a crystal with two 

 sets of plates. The application of this circuit to unbalanced filters 

 allows the results for balanced lattice filters to be realized for un- 

 balanced filters. For one connection of the two plates the resonance 

 of the crystal can be made to appear in one arm of the equivalent 

 lattice, while for the reverse connection the resonance appears in the 

 other arm of the lattice. 



II. Crystal Filter Sections Which Can Be Realized in 

 Lattice Networks 



As pointed out in a previous paper ^ the most general filter char- 

 acteristics for networks employing crystals can be realized in a lattice 

 network, since every known form of a network can be reduced to a 



1 "Electrical Wave Filters Employing Quartz Crystals as Elements," W. P. 

 Mason, B. S. T. J., July 1934, pp. 405-452. 



2 "Resistance Compensated Band Pass Crystal Filters for Unbalanced Circuits," 

 W. P. Mason, B. S. T. J., Oct. 1937, pp. 423-436. 



3 "The Evolution of the Crystal Wave Filter," O. E. Buckley, Jotir. of Applied 

 Physics, Oct. 1936. 



* "Crystal Channel Filters for the Cable Carrier Systems," C. E. Lane, B. S. T. J., 

 Vol. XVII, Jan. 1938, p. 125. 



^ "Channel Filters Employing Crystal Resonators," H. Stanesby and E. R. Broad, 

 P. O. E. E. Joiir., 31, pp. 254-264, Jan. 1939. 



' Loc. cit. 



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