QUARTZ CRYSTAL APPLICATIONS 



195 



motion of an A T plate. If we were to increase the thickness dimension until 

 the plate was nearly square, the A T motion would correspond to a face shear 

 mode which should be controlled by the same elastic constants as the A T 

 motion. At the same time in order to drive the crystal efficiently we 

 could decrease the width until it became the thickness. This procedure 

 would be the same as cutting a crystal at right angles to the AT and would 

 suggest that by so doing we should obtain a low-frequency shear crystal 

 with a low coefficient. Actually, Willard and Hight found that a crystal 



ELECTRICAL AXIS 



Fig. 1.13 — Relation oi AT and DT low temperature coefficient crystals 



at —52° or 87° from the AT would give a low coefficient. This crystal was 

 called the DT. Similarly, a crystal cut at +38° or 87° from the BT would 

 also give a low coefficient and this has been called the CT. It can be shown 

 that a component of the voltage applied along the mechanical axis will 

 drive the shearing type of motion. The CT is larger for the same frequency 

 and more strongly driven than the DT. It is extensively used in controlling 

 oscillators in the frequency range from 200 to 500 kilocycles. 



Quite a few other crystal cuts have been discovered as shown by Fig. 1.9. 



