Oscilent Corporation - Technical References
The Aging of Bulk Acoustic Wave Resonators, Filters and Oscillators

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The Impacts of Aging.

In most device applications, the frequency of the oscillator must remain within specified limits in order for the device to operate properly. When aging shifts the frequency beyond the limits, the oscillator must be recalibrated. Since crystal oscillators have a finite frequency adjustment range, oscillator aging can cause "end-of-life" when the aging rate is so high that it produces a frequency offset that exceeds the frequency adjustment range. (This rarely happens in properly designed oscillators.)

Soon after an oscillator's calibration, the frequency offset due to aging is usually small compared to the frequency offsets due to environmental (especially temperature) changes, however, eventually, the frequency offset due to aging becomes the dominant frequency offset. For example, a state-of-the-art oven controlled crystal oscillator is specified to have a frequency vs. temperature stability of 1 X 10-9, an aging rate of 1 X 10-10 per day, and a frequency adjustment range of 4 X 10-7. Assuming a worst-case linear aging rate at the specified rate, the frequency offset due to aging will equal the worst case temperature induced offset on the tenth day after calibration, and the oscillator will no longer be able to be calibrated after 4000 days (i.e., 11 years).

Similarly in clocks, aging eventually becomes the dominant time error source. Whereas the time error varies linearly with elapsed time due to the relatively constant clock rate errors (i.e., frequency offsets) caused by environmental effects, it varies as elapsed time squared when the clock error is due to (linear) aging. In filter applications, recalibration is usually not an option. Aging can gradually degrade performance until an end-of-life frequency shift is reached.