MODES OF MOTION IN QUARTZ CRYSTALS 



93 



respond to the dimensions resulting in low or no activity. This illustrates 

 quite clearly the necessity for grinding the edges of plates not dimensioned 

 for a specific frequency. Fig. 6.25 shows the same conditions when only 

 the Z' dimension is changed. In this case the dimensions shown at regular 

 intervals as Z, were derived from Fig. 6.25 as before and correspond to the 

 zero activity dimensions found experimentally. It will be noticed that low 

 activity regions are found halfway between the dimensions designated as 

 Zs. These correspond to even orders of the Zx shear and are the result of a 

 slight wedge in the airgap. This was intentional to show the existence of 

 this condition. 



Figures 6.24 and 6.25 show the necessity for avoiding certain dimensions 

 for oscillator plates at specific frequencies. This can be accomplished by 





Fig. 6.26 — Frequencies at which the Z'x shear along .Y, the Zi shear along Z' and the Xy 

 flexure along X coincide with the high frequency A'j, shear in ,4r-cut plates. 



individually adjusting the X and Z' dimensions by hand grinding of each 

 plate or by predetermining the proper dimensions and using mass production 

 methods of precise machine grinding. The advantages of predimensioned 

 crystal units is the insurance of proper operation over a wide temperature 

 range and uniformity of activity. The experience of most manufacturers 

 of low frequency crystal units in the broadcast range and high frequency 

 crystals requiring high activity over a wide temperature range has been 

 that it is necessary to use specific dimensions to insure low rejects in the final 

 tests. 



6.53 AT-Type Crystals 



The modes of motion encountered in the A T-cut crystal are the same as 

 that of the ^T-cut. The effects of coupling between most modes is greater 



