[time-nuts] Aging rate of crystals

[Richard (Rick) Karlquist]

The long term aging rate is due entirely to the crystal,
for all practical purposes, for any well designed oscillator
circuit (or even a mediocre design).  The aging of the
crystal is basically not predictable.  It's like the famous saying
by J P Morgan when asked what the stock market will do:
"It will fluctuate."  About the best you can do is test
the aging for a few months and hope it won't get worse
in the future.  You have to be careful about getting
an oscillator that simply got lucky during your aging
test and put in much better than typical numbers.  This
can happen if the direction of aging changes (not unusual)
in the middle of the aging test.  You
shouldn't overpromise aging compared to what you know
your process can support.  Other than holding the temperature
constant, there is nothing else you need to do to get
the best aging the crystal can do.  If you get a lucky
oscillator that has really good aging, it might continue
to be really good, but there is no guarantee.

Cheap crystals might have more predictable aging due to
outgassing processes.  However, this will be a large
amount of aging.  As you eliminate known causes of aging,
it gets less predictable.

Rick Karlquist N6RK

iovane at inwind.it wrote:
> I learn from this discussion that the aging rate claimed by manufacturers would refer to the 
> aging of the whole assembly, not the crystal alone. And for practical purposes that is correct. 
> And even in the case of sealed assemblies, components other than the crystal itself may affect 
> the overall measured drift.
> So my original question on this subject seems to lose any sense, because we will never be able 
> to measure the aging of the crystal alone (if any, at this point) and hence variations in the 
> aging rate either.
> Anyway some doubts of mine are not yet fully answered by this discussion, and I would appreciate 
> your opinions.
> Given a good quality sealed OCXO running in constant ambient temperature, what kind of aging 
> curve  should one expect, a fluctuating one? (I understand that this might be the case, due 
> to the interaction of known "intrinsic" aging factors having different timescales, as I've just 
> learnt on this list. A "regular" curve would be hard to get).
> May it happen that fluctuations in frequency due to "external" causes such as tides, geomagnetic 
> storms, or so, and not actually affecting the "aging rate", are interpreted as fluctuations 
> in the aging rate?
> 
> I'm running a simple test comparing an OCXO (option 04E on a military Racal 1992 counter) to 
> rubidium (LPRO), the counter being counting the LPRO. The test is running since about two weeks, 
> and I started recording three days after power up. In the first days the OCXO showed a decreasing 
> drift starting with some 3x10e-10 per day until it reached a stability within +/- 1x10e-10 in the 
> last 5 days (that is, since 5 days back, the counters reads always the same value +/- the occasional 
> uncertainty of the rightmost (11th) digit (10 seconds gate time). The OCXO specs are <= 5x10e-10 
> per day. I didn't notice whether it is sealed, and won't check right now. I don't expect that the 
> counter will always stay there, and I don't know what to think when the drift (aging rate?) will 
> change.
> 
> Thanks,
> Antonio I8IOV
> 
> 
> 
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