Electrical Wave Filters Employing Crystals with Normal and Divided Electrodes

I N SEVERAL previous p a p e r s 2 > 4 the application of piezoelectric 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. 5 It is the purpose of the present paper to discuss more completely all the standard types of filters with crystals, and methods for determining 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. 2,3 - 4 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 unbalanced 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.