Ageing means the change of the crystal frequency due to time passing by. Ageing will result in the deviation of the frequency in two different directions. And ageing will cause the overall drift of the frequency of the wafer installed in the oscillator.

Aging is caused by two factors, namely pollution and stress. Experiments show that the pollution on the surface of the wafer results in the negative frequency drift and the overpressure results in the positive frequency drift.

When it is placed on a support (base), the wafer may be pushed, extended or twisted by the structure where it is located. This will cause pressure on the wafer, and the pressure will be released with time passing by and result in the positive frequency drift. The correct way to arrange the wafer and the appropriate base applied are instrumental in removing or reducing the unnecessary pressure. For a finished product, the heat cycle is instrumental in releasing the pressure of placement.

The pollution on the surface of the wafer exists in all production links. The stain on the surface of the wafer causes overload and the negative frequency drift as a result. The clean working procedures and the cleaning of the surface of the wafer are instrumental in minimizing the pollution.

The crystal can have presaging to a certain degree and therefore the impact of ageing may be reduced to a minimal level. Since the ageing characteristic curve is close to the logarithmic curve, the ageing of crystal usually happens in the first few years after the birth of the crystal. Relatively speaking, the frequency change of the crystal in the first year is more frequent and quicker than it is in the second year.