MANUFACTURE OF QUARTZ CRYSTAL FILTERS 519 



Fig. 2 — Inspection of quartz crystals. An arc light beam aids 

 in the detection of internal flaws. 



"twinning," appears as shown in Fig. 3 when observed with polarized 

 Hght. 



For use in filters, quartz must be cut into rectangular plates properly 

 oriented with respect to the electrical, mechanical and optical axes of the 

 crystal, as shown in Fig. 4. A polariscope and an X-ray spectroscope 

 are used in locating these axes to an accuracy of ± 0.25 degree. For 

 the majority of applications the plate is cut in the plane of the me- 

 chanical and optical axes, with the long dimension set at an angle of 

 18.5° from the mechanical axis. This orientation eliminates secondary 

 resonances in the completed crystal and makes the primary resonance 

 frequency relatively independent of slight errors in orientation. For 

 applications requiring a low coefficient of resonance frequency versus 



