MANUFACTURING DEVIATIONS IN CRYSTAL UNITS 281 



changes have been observed even when using GT-cut plates adjusted to zero 

 temperature coefficient as shown, for example, in the curves of Fig. 14.12. 

 Also, it has been observed that after a plate has been driven hard and the 

 transmitted current then reduced, the original resonant frequency is re- 

 stored only after a considerable time interval. The data of Fig. 14.12 pro- 

 vides a rough correlation between stabihty and current levels. For ex- 

 ample, if the stability desired for a crystal unit using a GT t}^e plate be in 

 the order of one part per million, the circuit design should be such as to 

 keep the current level in the plate below about 10 microamperes per milli- 

 meter of width. 



The parameter used in these paragraphs for measuring current levels is 

 the current per unit of width. This appears to be useful as a common basis 

 for comparing various plates of any one cut and mode of vibration. Theo- 

 retically, in the case of a plate vibrating longitudinally, the current, /, per 

 unit of width, w, is directly proportional to the elongation per unit of length, 

 yy, as shown by following equation: 



I/w = K yy 



where K is a constant which depends on the cut of plate and mode. 



Figure 14.12 also illustrates the importance of the surface condition of 

 the plates. Curve A is the average frequency-current characteristic for a 

 group of crystal units using plates etched for twenty minutes in 47% hydro- 

 fluoric acid and curve B the average characteristic for a group of crystal 

 units using plates etched for over forty minutes but less than ninety minutes. 

 Evidently, crystal units using plates which have been etched for a long 

 period exhibit a frequency-current characteristic which is appreciably more 

 constant than those using plates etched for a shorter period. 



