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Aging can be positive or negative. Occasionally, a reversal in aging direction is observed. Typical (computer simulated) aging behaviors are illustrated in Fig. 1. The curve showing the reversal is the sum of the other two curves. So far, the simplest proposed aging model showing a reversal consists of two simultaneously acting aging mechanisms, as shown in Fig. 1.
The aging rate of an oscillator is highest when it is first turned on. Since the aging rate during the first few days to weeks is generally significantly higher than during subsequent intervals, the first part of the aging curve is sometimes referred to as "initial aging" or "stabilization
period." At a constant temperature, aging usually has an approximately logarithmic dependence on time. When the temperature of a crystal unit is changed, e.g., when an OCXO is turned off, and turned on at a later time, a new aging cycle may start, as illustrated in Fig. 2.
The aging rates of typical commercially available crystal oscillators (in 1991) range from 5 to 10 ppm per year for an inexpensive clock oscillator, to 0.5 to 2 ppm per year for a TCXO, to 0.05 to 0.1 ppm per year for an OCXO. The highest precision OCXOs can age less than 0.01 ppm per year.