2-12
• Load reactance change - adding a load capacitance to a crystal changes the frequency by
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• Example:  If C0 = 5 pF, C1 = 14fF and CL = 20pF, then a DCL = 10 fF
                    (= 5 X 10-4) causes »1 X 10-7 frequency change, and a CL aging of
                   10 ppm per day causes 2 X 10-9 per day of oscillator aging.
• Drive level changes:  Typically 10-8 per ma2 for a 10 MHz 3rd SC-cut.
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• DC bias on the crystal also contributes to oscillator aging.
Instabilities due to Sustaining Circuit
   See “Frequency vs. Drive Level” in Chapter 4.
   A DC voltage on the resonator can be a significant contributor to aging.  To minimize this aging mechanism, the sustaining circuit must be designed so as to not apply a DC voltage to the resonator.  This may be accomplished by, e.g., by placing a capacitor in series and a few M resistor in parallel with the resonator.


R.L. Filler, J A. Kosinski, V.J. Rosati and J.R. Vig, “Aging Studies On Quartz Crystal Resonators And Oscillators,” Proc. 38th Ann. Symp. On Frequency Control, pp. 225-231, 1984.