Quartz crystal ageing
Quartz crystals used in filters and oscillators in electronic circuits are
renowned for their performance, stability, frequency tolerance and their high Q.
Yet they do change their frequency very slightly with time in a process known as
ageing. Although the frequency variations are small by many standards, they are
permanent and may have an effect in some applications where the frequency is of
great importance. As a result manufacturing techniques take account of this to
reduce the effects of ageing in these crystals as far as possible.
Ageing is caused by a number of interrelated factors. These include internal
contamination, excessive drive level, surface change of the crystal, various
thermal effects, wire fatigue and frictional wear. The level of ageing can be
minimised in a number of ways. 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.
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, and 2 ppm for a cold weld sealed encapsulation using an HC43/U
holder. 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 stabilse after some years of operation. However
if these are changed they may cause the ageing rate to change. It may even alter
direction.
Cleanliness of the environment around the crystal is one of the main ways of
reducing ageing. It is therefore esential 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.
While the effects of ageing may not be of importance in applications such as
clock oscillators for running many digital circuits, they are important where
high frequency stability is required. By choosing the right crystal, these
effects can be kept within reasonable limits so that they do not cause any
problems.
|
