# [time-nuts] Excel logarithmic function (was Thermal impact on OCXO)

Richard (Rick) Karlquist richard at karlquist.com
Thu Nov 24 19:56:24 EST 2016

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On 11/24/2016 5:16 AM, Bob Camp wrote:

> The biggest challenge is to take out the “early stuff”. One approach is to fit
> the same equation twice with the time constant restricted to a range on each.
> For most OCXO’s (90%) the equation when fit early represents an upper limit
> to the drift. You might get a another element that comes in and is apparent
> after a year or two. It might be replaced by another element after five or ten years.
> They generally (~80%) represent a change in sign (negative drift vs positive).
>
> If you look at the “other 10%” some have really poor aging and are not shipped.
> Some are very erratic and simply can not be fit. Some of the 90% are fit with a
> “upper limit” because they exhibit no measurable aging over the 30 days (or whatever)
> of testing.
>
> If you take the bad aging (out of spec) parts out of the pile, those are the ones
> with the best fit. They have very pretty curves and they stick to those curves
> for a *long* time. They have a single dominant cause for their aging ( = the defect).
> The rest of the parts have all of the causes bashed down by the process so that
> over a 20 or 30 year span, there probably is no single dominant cause.
>
> Bob
>

This excellent response channels what Jack Kusters used to say.  The
idea that aging follows any predicable pattern might have been true
decades ago.  For example, I remember being told in 1974 that
everyone knew that metal crystals aged downward and glass crystals
aged upward.  It was true at the time, but those aging processes
have been beat down.  According to Jack, 10811/E1938A aging is
primarily "stress relaxation".  It could be either direction and
a given crystal can change direction over time.  On top of that,
crystals have frequency "jumps" at unpredictable intervals.  At
HP, we had an "aging system" that watched crystals to try to reject
bad actors and find the well behaved ones.  The problem was that the
longer you watched an oscillator, the better chance of catching
it in the act of jumping.  They didn't necessarily get better
over time (over many months).  No matter how many crystals we
looked at, we never found one that had atomic like aging.

My observation is that the systematic (therefore predictable)
aging processes have been eliminated by improved manufacturing
techniques, leaving the true random (unpredictable) aging
processes.

The one thing I can say is that it is good to keep the crystal
ovenized at all times.  Even a momentary oven outage tends to
reboot aging.

Rick
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