[time-nuts] Crystal Ageing

[Murray Greenman]

I support what Rick has said about crystal ageing. I also work for an
OCXO manufacturer, and will add my 2c worth.. Here are some points -

* Manufacturers do indeed have their own special techniques for
minimizing ageing and numerous other quality related parameters.
Cleanliness, quality and attention to process are very important. Many
techniques will be specific to their own manufacturing process, and of
course they are trade secrets.

* Because many different ageing processes are involved, prediction of
future ageing from past history is not in general reliable. A very good
analogy is prediction of weather - to say that tomorrow's weather will
be similar to today's is a fairly safe bet, but is no help in predicting
the weather for next weekend! The same applies to good crystal
oscillators - you can reasonably expect the rate of ageing next month to
be closely similar to last month's, but the slight differences build up
in extrapolation, and prevent long term prediction. Make your
measurements, predict the trend, then do the same next month and note
the differences.

* It is very important to recognise the difference between INITIAL
ageing (when a newly made crystal is first used) and long term ageing.
Most users will never see initial ageing, as it takes place in the
factory. Initial ageing is much more predictable, and the factory will
monitor this and from this behaviour, within a week or so can predict
when (if ever!) the device will be within specification and so ready for
final calibration and delivery.

I recommend reading the various papers on ageing published by the UFFC
and others. Start with this fairly comprehensive and authoritative one:

http://www.ieee-uffc.org/freqcontrol/vigaging91/aging.htm#CONTENTS

73,
Murray ZL1BPU