CRYSTAL CHANNEL FILTERS 129 



dissipation in the filter elements is costly in that the channels must be 

 spaced farther apart than would otherwise be necessary, thereby 

 wasting frequency space. At the same time the loss to transmitted 

 frequencies must be made up for by amplification. The amount of 

 dissipation in a reactance element is measured by the ratio of the 

 efifective resistance component of its impedance to the reactance com- 

 ponent at any frequency. The reciprocal of this ratio is called the Q 

 of the reactance and hence is a measure of efficiency or freedom from 

 dissipation. In the design of inductances in the form of wire wound 

 coils, it is generally not practical to obtain Q's much in excess of 200 

 or 300 at any frequency. The quartz crystal element used in the 

 filters as previously stated is equivalent electrically to a two-terminal 

 reactance consisting of an inductance and capacitance in series shunted 

 by a second capacitance. For the Q of the inductance in the equivalent 

 circuit of the crystal element a value of 15,000 or more can readily be 

 obtained. It is for the purpose of utilizing this high Q inductance and 

 obtaining the benefits therefrom that crystal elements are used in 

 these filters. 



^i^S^ 



72 76 80 84 88 92 96 100 104. 108 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 5 — The length of the crystal elements used in the different filters varies about 

 inversely as the frequency of the filter band location. 



The filter schematic in Fig. 2 shows crystal elements in each filter 

 section ; the two in the lattice arms and the two in the series arms in 

 each case are identical. Electrically there are four crystals in each 

 section but for reasons of economy and for convenience in handling 

 and adjusting the crystals those in corresponding arms are physically 

 one. This is possible since the filter is a balanced structure and the 

 two like crystals vibrate in unison. Figure 5 is a photograph which 

 shows a representative double crystal element taken from each of the 

 twelve filters. The four crystals in the lowest frequency filter range 

 from 40.2 millimeters to 41.8 millimeters in length and those in the 

 highest frequency filter from 23.8 to 24.3 millimeters. The thickness 

 of the crystals in all four of the lowest frequency filters are 0.63 mi- 

 limeters, in the next four filters 0.82 millimeters, and in the highest 

 frequency filters 1.1 millimeters. Uniformity in thickness is main- 

 tained as far as practicable since it contributes to economy in manu- 



