CRYSTAL CHANNEL FILTERS 



127 



an impedance transformation internal to the filters and thereby 

 permit the use of crystals of practical thicknesses for these filters. 

 However, the equivalent circuit for each of the twelve filters is the 

 same. In the system the filters work in parallel at one end and 

 between terminating impedances of 600 ohms. The two condensers 

 appearing in the series arms at the left end of the filter schematic are 

 used in obtaining satisfactory operation of the filters in parallel and 

 otherwise might be omitted provided the inductance at this end was 

 made smaller at the same time. 



Figure 2 indicates the separate physical elements and the manner 

 in which these are connected in the filters. In considering the per- 

 formance of the filters the crystal elements are replaced by their 

 equivalents, an inductance and capacitance in series, shunted by a 

 second capacitance as shown in Fig. 3. Also, the condensers in 



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Fig. 3 — The schematic circuits of the filters are each equivalent to two lattice 

 sections with a resistance pad between them, resistances at each end and at the 

 paralleling end a coil and condenser which resonate at the mid-band frequency. 



shunt across the filter are shown inside the lattice combined with the 

 direct capacitance of the crystals and the inductances are relocated 

 in series in each lattice arm. In making this conversion, however, 

 the effective resistance of the inductance coils are, for reasons which 

 will appear later, shown remaining outside the lattice. Also the 

 capacitances and the portion of the inductance which are used solely 

 for purpose of paralleling are left outside the lattice. The basis for 

 the conversion from Fig. 2 to Fig. 3 is shown in Fig. 4. 



Before considering further the filter as a whole, the nature of the 

 crystal elements and the reason for using them will be considered. It 

 is common knowledge to those familiar with the performance of 

 electrical wave filters that the energy loss unavoidably associated 



