MANUFACTURE OF QUARTZ CRYSTAL FILTERS 



523 



the standard and the test crystals, respectively, while the center panel 

 indicates the frequency difference between them. For very accurate 

 work and for periodic checks of the standard crystals it is necessary to 

 use a precision oscillator, shown at the right. 



Fig. 6 — Final tuning of a crystal plate using a standard crystal 

 (in small box at upper left) for comparison. 



Occasionally, in the course of adjustment, plates are carried too high 

 in frequency. In such instances, as a result of the standardization of 

 thicknesses mentioned previously, the plate normally can be salvaged 

 by grinding it to the dimensions of the next higher frequency plate of 

 the same thickness. 



Aging occurs in both resonance frequency and effective resistance, 

 as slight strains created in the quartz and in the contacts during 

 adjustment relieve themselves. The greater part of the aging takes 

 place during the first few hours after calibration and nearly all of it 

 during the first week. In general, the frequency rises a few cycles and 

 the resistance drops slightly. Crystals on which the frequency toler- 

 ance is approximately equal to the shift due to aging are stabilized by 

 one or more temperature cycles, prior to final measurement of frequency 

 and resistance. 



