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BELL SYSTEM TECHNICAL JOURNAL 



utilized in any type of filter where two crystals occur in series or 

 lattice arms of a balanced lattice filter. 



The connections C and D of Fig. 16 can also be used to give wide 

 band filters but on account of the extra capacitance shunting each 

 crystal, as wide bands cannot be obtained. This is shown in Fig. 19 

 which shows two crystals connected as shown in Figs. 16C and D 

 used in a filter. Since each crystal is shunted by half the static 

 capacitance of the other, the ratio of capacitances will be about twice 

 that in the connection shown in Fig. 18 and the band width possible 

 will be about 70 per cent of that shown in Fig. 18. Hence the con- 

 nections of Fig. 18 are usually desirable. 



The connections of 16C and D can be duplicated in unbalanced 

 form as shown in Fig. 20. These equivalents are easily worked out 

 from the network of Fig. 15E by employing Bartlett's theorem. An 



to 



Ca= 



2 



^^^im^- -^i^h^^ 



LO 



LO 



^l + C.ri. 4-0,4-, +C 



Ca= 



' „ -^ ^^^-" LO Lo /Cop '^ 





Fig. 19 — Band pass crystal filter employing connections of Figs. 16C and D. 



unbalanced filter of the type shown in Fig. 19 can be obtained by 

 combining the two connections shown in Fig. 20 as shown in Fig. 21. 

 It will be noted that across the series arm we have a capacitance 

 Cb + 2Ci4i + 2Ci3j + Coj/2 while the lattice arm has only the 

 capacitance CoJ2. To get attenuation peaks which are separated from 

 the pass band by a large frequency range it is necessary to keep the 

 capacitances Cui and Cu^ small. This can be accomplished by using 

 shielding strips on the plating as shown in Fig. 22 for the two types 

 of connection. In the B connection, the grounding strip is effectively 

 obtained by making the grounded plates 2 and 3 slightly larger than 

 1 and 4. These grounding strips then act like a guard ring and reduce 

 the stray capacitances. 



The same process can be applied to any of the filters of Figs. 2 to 6 

 to obtain in unbalanced form the characteristics obtained in lattice 

 form. In general the characteristics are somewhat more limited since 



