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BELL SYSTEM TECHNICAL JOURNAL 



difference between their frequencies. One oscillator is controlled by 

 the crystal plate being tuned and operates at its resonance frequency; 

 the other is controlled by a standard crystal of the desired frequency. 

 Starting 100 to 200 cycles low, the plate is ground on the ends until its 

 frequency approaches that of the standard. 



The plate is then transferred from the fixture to its final holder, 

 shown in Fig. 5. This mounting normally accommodates two plates 



mm 

 7 ^ 51 CRYSTAL 



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Fig. 5 — Crystal plates mounted in holder. The four points at the center of each 

 plate provide electrical contact and mechanical support. 



of different frequencies, each supported at its nodal point by contacts 

 projecting from ceramic blocks. The entire assembly is held together 

 by a spring suspension in order to apply uniform pressure at all con- 

 tacts. To minimize damping, the contacts must be accurately aligned 

 and the quartz plates must be carefully centered upon them. 



A final adjustment of frequency is now performed, as shown in 

 Fig. 6. Permissible tolerances vary from ± 20 to ± 150 parts per 

 million for different types of crystals. Crystals having the broader 

 tolerances and substantial quantity requirements are adjusted by 

 comparison with a standard crystal, as in the case of preliminary 

 tuning. The test set shown at the left in Fig. 6 is being used for this 

 purpose. The upper and lower panels are the oscillators controlled by 



