ELECTRICAL WAVE FILTERS EMPLOYING CRYSTALS 



233 



T network as shown in Fig. 11. The shunt coil Lo is usually con- 

 siderably smaller than the series coil Li so that Li can be divided into 

 two coils, Lo and Lx — Lq. The transformation then becomes as 



A B 



Fig. 10 — Equivalence between bridge T and lattice networks. 



shown in Fig. 11 with the element values shown. Q/l indicates that 

 the impedance of the crystal in the shunt arm is half that in the 

 lattice arm. This transformation is applicable particularly to low 

 and high-pass filters and band elimination filters. 



Fig. 11 — A bridge T band elimination crystal filter. 



Another transformation which can be employed is that for a three- 

 winding transformer, for, as shown in a previous paper,^ a three- 

 winding transformer connected to two impedance arms as shown in 

 Fig. 12 is equivalent to a transformer and a lattice filter with small 



Fig. 12 — Lattice equivalent of a three-winding transformer. 



^ Loc. cit. 



