ELECTRICAL WAVE FILTERS 



429 



constructed by using the circuit of Fig. 16Z), modified by using a paral- 

 lel condenser rather than a series condenser. The filter obtains a 

 65 db discrimination in less than a 0.12 per cent frequency separation. 

 Figure 23 shows a characteristic obtained by employing a filter of 



1193 1197 1201 1205 1209 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 23 — Measured insertion loss characteristic of a single crystal bridge T filter. 



the type shown by Fig. 17, together with a screen grid vacuum tube. 

 The result is plotted as the gain of the circuit since this gives the most 

 significant result for this type of circuit. 



APPENDIX V, 



The Modes of Vibration of a Perpendicularly Cut Quartz 



Crystal 



Introduction 

 Quartz crystals have been cut into two principal types of orienta- 

 tions with respect to the natural crystal faces. The first type is the so- 

 called Curie or perpendicular cut in which the crystal is so cut that its 

 major surfaces are perpendicular to an electrical axis and parallel to the 

 optical axis. Figure 1 shows such a cut. The second type is the so- 

 called parallel or 30-degree cut in which the major surfaces are parallel 



