ELECTRICAL WAVE FILTERS EMPLOYING CRYSTALS 



239 



four-terminal representation of a crystal becomes that shown in 

 Fig. 15E. Ordinarily the capacitances C13, Cn, C23, C24 are small 

 enough to be neglected. Figure 15E then is a complete equivalent 

 circuit for a two-plate piezo-electric crystal which is valid for any 

 kind of impedance or transmission measurements. 



Co "?■ 



HHWW' 



-wv 



'-W-^W-AV 



f Co 



<^vvvHh 



C24. 



Fig. 15 — Four-terminal equivalent network for divided plate crystal. 



There are four possible connections for a crystal with two sets of 

 plates used in a balanced filter. These connections and their equivalent 

 circuits for transmission through as used in the filter are shown in 

 Fig. 16. In order to prove these equivalences let us consider the 

 equivalence shown on Fig. 16A. The four-terminal network repre- 

 sentation for this case is shown in Fig. 17, which is obtained from 



