86 



BELL SYSTEM TECHNICAL JOURNAL 



Using these values in equation (12) the expected temperature coeffi- 

 cients for the low-frequency vibration are as shown on Fig. 8 The 

 measured points are shown on the curve. The zero temperature coeffi- 

 cients occur at the angles + 38° and — 53°. These crystals have been 

 designated the CT and DT low-frequency shear crystals. These types 

 of crystals are useful for stabilizing low-frequency oscillators ranging 

 from 50 KC to 500 KC. 



Just as the AT and BT crystals have harmonics which can be used 

 to control oscillator frequencies, so also do over-tones of the low- 

 frequency shear crystals exist. They do not bear, however, the simple 



y < 



Z H 



z o 



-100 

 -90 



Fig. 8- 



50 -46 -30 -16 16 30 46 60 



ORIENTATION ANGLE IN DEGREES (.9) 



-Calculated and measured temperature coefficients 

 for low-frequency shear crystals. 



relation to the fundamental that the high-frequency harmonics do. 

 S. C. Right has found a mode of motion, which is probably related 

 to the second flexural vibration, of nearly twice the frequency of the 

 low-frequency shear mode and which has zero temperature coefficients 

 at angles of + 66°-30' and - 57°. These crystals have been desig- 

 nated respectively as the ET and FT crystal cuts. Figure 9 shows a 

 plot of this frequency versus orientation. It will be observed that the 

 frequency constant of this mode of motion is about twice that for the 

 low-frequency shear mode and hence these crystals can be obtained in 

 reasonable sizes for twice the frequencies that the CT and DT crystals 

 can be obtained. 



