What is Aging of a Crystal Oscillator?

A small change in the frequency of a quartz crystal over a period of time is called as aging. This effect is always kept in consideration whenever a crystal is used for an application.

Aging in crystal oscillators is caused by two main effects:

Mass-Transfer: Addition or removal of mass from the crystal due to any sort of contamination during device packaging, leads to alteration in the frequency of the crystal. So during the production process, the environment is kept clean, so that no aging effect takes place.

For example, the conductive epoxy used to mount the quartz blank can produce ‘out-gassing’ which can create oxidizing material within the otherwise inert atmosphere inside the sealed crystal package and so this production process must be well controlled.

Stress: Quartz crystals also change frequency due to stress. This stress can be caused due to various sources like package and mounts, epoxy connection, metal films, and the quartz. Heating and cooling also causes stress due to different expansion coefficients. Stress in quartz can come from the growing process of the bars or the machining process of cutting, lapping, and polishing into wafers.

How to measure aging in crystal oscillators?

To measure aging is not an easy task. Accelerated aging is used to measure true aging of a device by putting devices at higher temperatures and measuring frequency at regular intervals.

For example, putting a device at 85°C for 30 days and 105°C for 168 hours is the equivalent of it being at 25°C (room temperature) for one year. 

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