Quartz Crystal Ageing: long term stability & accuracy
Quartz crystal resonators are used in filters and oscillators as high performance resonant circuits.These resonators are known for their high performance and stability, but they change their frequency very slightly with time in a process known as ageing.Although the frequency variations are small, they are permanent and may have an effect in some applications where the frequency accuracy is of great importance.It is possibly to minimise the changes that result from quartz crystal ageing bin the manufacturing process and in use.
Quartz crystal ageing basics
Quartz crystal ageing is caused by many different factors. Differnet crystals age at different rates and this can be attributed to a number of factors: some in the manufacturing process and some by the way they are used.Such as Crystal surface change; Crystal lattice contamination; Thermal effects; Wire fatigue; Frictional wear; Drive level.
The level of ageing can be minimised in a number of ways both in manufacture and in use:
Reducing ageing in manufacture: During manufacture they should be encapsulated in an inert gas environment, the ensuring should have a good seal so that other gases do not enter. Also the final stages of the preparation of the crystal blank must be prepared as finely as possible. Rather than lapping the blank to bring it to the right dimensions, chemical etching is used. In this way the minimum disruption is caused to the crystal lattice, and this reduces the ingress of contaminants over time that will cause ageing.
Reducing ageing in use: The design of the circuit in which the crystal will be used also has an effect. By keeping the drive levels low again the crystal ageing will be less.
As expected the rates of change of the crystal frequency vary with the time after manufacture. The maximum rate of change of frequency occurs immediately after manufacture and decays thereafter. As a guide it is found that it is fastest within the first 45 days of operation. Even so there is always some degree of ageing throughout the life of the crystal. In view of the fact that the greatest rate of change is immediately after manufacture, high tolerance items are run for some time before being shipped. In very high tolerance items this may extend to a few months of operation.
Once the ageing rate has settled it is found that typical figures can be quoted for many types. It is found that one of the main variations is the type of encapsulation that is used. The two most common methods of encapsulation for through-hole crystals are resistance weld and cold weld. These will typically give figures of around 5 parts per million (ppm) for a resistance weld sealed encapsulation such as 2*6mm DIP, and 2 ppm for a cold weld sealed encapsulation. These both move in a downward direction. Glass encapsulated crystals may also be found on some occasions. These tend to move in an upward direction, and may have a tolerance or slightly less than 5 ppm. Also there is a wide variety of surface mount crystals. A typical plastic package or a glass seam weld package may give around ±5 ppm while a metal seam weld package may give less than 3 ppm.
If the crystal is maintained in the same circuit and at the same temperature then the effects of ageing may stabilise after some years of operation. However if these are changed they may cause the ageing rate to change. It may even alter direction.
Minimising ageing effects
Cleanliness of the environment around the crystal is one of the main ways of reducing ageing. It is therefore essential to ensure that the crystal package or encapsulation is not damaged in anyway. The seal should not be damaged, nor the pins bent as this may break the seal.
Where very high performance is required, often the oscillators will be pre-aged i.e. run at temperature to ensure that the period of maximum aging rate has been passed before the unit is delivered.
The effects of quartz crystal resonator ageing will always be present. As a result it is necessary to provide methods of finely adjusting the frequency in circuits where frequency accuracy is of paramount importance. It is also necessary to have a periodic calibration where frequency accuracy is important. In many circuits, the issue may not be important. To minimise the effects of crystal ageing, the right crystals can be bought and then they any circuit can be designed to minimise the effects. Also maintaining the crystal at a steady temperature helps.
