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BELL SYSTEM TECHNICAL JOURNAL 



which the high-frequency shear mode had zero coupling with the trouble- 

 some low-frequency shear mode system of vibrations. Crystals cut at these 

 angles have a much cleaner frequency spectrum than Y cut crystals. Lack, 

 Willard, and Fair also found that the temperature coefficient of frequency 

 varied with angle as shown by Fig. 1.12. Starting from a high positive 

 value for the Y cut, the coefficient becomes zero at an angle of -|-35° — 15' 

 and —49°. The first angle cut is known as the AT cut and the second as 

 the BT cut. Since the A T angle is nearer the Y cut, the piezo-electric con- 

 stant is larger and it is more strongly driven than the BT. On the other 



-90 -75 -60 -46 -30 -15 15 30 45 60 75 90 



ANGLE OF ROTATION ABOUT X AXIS IN DEGREES (e) 



Fig. 1.12 — Temperature coefficients of oriented Y cut crystals 



hand, the BT has a higher frequency for the same thickness. Both crystals 

 are near enough to the AC and BC cuts so that the systems of low-frequency 

 shear modes are rather weakly driven. On the other hand, the shear mode 

 of both crystals is rather strongly coupled to flexure modes of motion, as 

 will be discussed by Mr. Sykes in a later chapter, and the crystal has to be 

 exactly dimensioned in order that the flexure frequencies and other disturb- 

 ing frequencies will not coincide with the desired shear mode. 



Other oriented shear crystals for lower frequency work are the CT and 

 DT crystals investigated by Willard and Hight. They are related to the A T 

 and BT crystals as shown by Fig. 1.13. The plate on the right shows the 



