[time-nuts] Aging and Thermal Correction in Holdover

[Bob Stewart]

      From: Attila Kinali <attila at kinali.ch>
 To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Thursday, October 27, 2016 1:45 PM
 Subject: Re: [time-nuts] Aging and Thermal Correction in Holdover
   
On Thu, 27 Oct 2016 16:13:06 +0000 (UTC)
Bob Stewart <bob at evoria.net> wrote:


> Since I want to keep this as simple as possible, I'll probably stay with a
> simple division of the DAC delta over 3.5 days.  And since the thermal
> impact seems to be 1:1 to the DAC delta under most circumstances, I may
> experiment with modifying the beginning and end points of the calculation
> by the temperature.  But, hopefully with a long enough averaging period,
> it won't make much of a difference.

>I.e. you have a very simple, linear temperature dependence and aging model?
Not quite.  The aging model is linear, sure.  But as mentioned earlier, I'm experimenting with a sort of linear-nonlinear thermal model.  What this means is that over some set time period I take the difference between the beginning temperature and the end temperature, and then I step only one DAC step in the direction that the temperature changed.  Why?  Because I noticed a strong correlation between small temperature differences which broke down as the temperature change curve steepened.  And yet, it was only the value of the step that broke down, not the direction.  Putting a limit on it like this seems to have improved the result.

> The odd thing about the thermal impact is that if the thermal change
> delta is too high, it's no longer 1:1.  I suspect that may be because
> it's an easy thing for the OCXO's heater to add BTUs, but it can only
> subtract them passively.  And shedding BTUs into a rising temperature
> environment is problematic.

>I guess you are measuring the temperature of the OCXO outside of the can.
Keep in mind that the temperature outside is lower than inside. I.e. you
have a temperature gradient. If the temperature changes, then the gradient
changes as well. As long as the rate of temperature change is low, the
gradient will stay linear (ie the temperature inside the OCXO increases
linearly with the distance from the outer can). If the rate of temperature
change is fast, then the gradient will start to bend. Which in turn means
that the heat transport between inside and outside of the OCXO is not a
linear function of the outside temperature anymore. Or in other words:
your simplistic system model does not capture the dynamic behavior of
the OCXO correctly, which results in discrepances between expected and
observed behaviour.
Yes, and this is what I'm trying to capture; especially when the temperature increases.  Because that's when it will bend the most.

>And if I may, please talk about energy (or rather power in this
context), not BTU. The former is a physical term, while the latter
is an outdated unit that shouldn't be used anymore and has no clear
definition.
As I've mentioned many times, I'm not an engineer.  It may be that BTU has no clear definition, but you understood what I meant, which is all that's important.

Bob