QUARTZ CRYSTAL APPLICATIONS 



191 



seconds to die down which corresponds to a <2 of 330,000, which is about 

 1000 times as great as that for a good electrical circuit. 



1.5. Modes of Motion and Crystal Orientation to Produce Low 

 Temperature Coefficient Crystals 



As mentioned previously the first crystal cut used in oscillators was a longi- 

 tudinal vibration along the I" or mechanical axis excited by a field applied 



ZERO TEMPERATURE COEFFICIENT 

 OSCILLATORS AND FILTERS 



HIGH FREQUENCY AT, BT 



LOW FREQUENCY CT. DT, ET.FT 



- 1 ZERO COUPLING (s'- 

 -18° FILTERS 

 1-3-5-7 HARMONICS 



ZERO TEMPERATURE 



COEFFICIENT 



0° OSCILLATORS 



FUNDAMENTAL AND 



SECOND HARMONIC 



ZERO TEMP 

 COEFFICIENT 

 GT OSCILLATORS 



AND FILTERS 



DOUGHNUT 



ZERO TEMPERATURE 



COEFFICIENT 



A MT LONGITUDINAL CRYSTAL, 

 B NT FLEXURE CRYSTAL 



LOW TEMP COEFFICIENT 

 + 5° FILTERS 



Fig. 1.9 — Oriented quartz crystal cuts in relation to the natural crystal 



along the electrical or X axis. This mode gives a good resonance free from 

 other modes and a modification of it is now used in crystal filters. This 

 modification, as shown by Fig. 1,9, (— 18.5° filter crystal) consists in rotating 

 the direction of the length by 18.5° from the Y or mechanical axis, about the 

 X or electrical axis. As described previously^", the effect of this rotation is 

 to eliminate the coupling between the desired longitudinal mode and the 

 undesired face shear mode, thus simplifying the motion and eliminating an 



1° "Electrical Wave Filters Employing Quartz Crystals as Elements," VV. P. Mason, 

 B. S. T. J., Vol. XIII, p. 405 July 1934 or patent 2,173,589. 



