# [time-nuts] Digital temperature compensation

Dan Drown dan-timenuts at drown.org
Sat Nov 1 17:09:58 EDT 2014

```Ok, I hadn't considered rate of change.  This data is currently 3
day's worth and seems to repeat itself on both days at the same
temperature point.  Attached is a time based graph to show that.  The
ppm axis (on the right) is inverted to make it easier to see the
relationship between the two.

Quoting Bob Camp <kb8tq at n1k.org>:
> Hi
>
> This sort of thing is normally done with a precisely controlled
> temperature chamber and multi day temperature ramp runs. Even then
> there is a bit of “wonder what that was, let’s try it again”.
>
> If you are looking at a crystal oscillator, what you have is a
> perturbation in the frequency / temperature curve. The response you
> get will be temperature rate of change dependent.
>
> Bob
>
>> On Nov 1, 2014, at 4:40 PM, Dan Drown <dan-timenuts at drown.org> wrote:
>>
>> I'm experimenting with using a temperature sensor to estimate local
>> oscillator frequency changes.  My goal is to have a decent holdover
>> clock for a NTP server with not so great GPS antenna placement.
>>
>> I've been sampling temperature every minute, measured by a DS18B20.
>>  I've been measuring local clock frequency differences, using
>> chronyd's logs from the GPS's PPS.  At 28C through 21C, I get a
>> pretty good result that fits a quadratic polynomial decently (0.117
>> ppm stddev).  I've attached the graph of that as
>> "temp-clock-warmer.png".
>>
>> With the colder temperatures, there's a sudden drop off in
>> frequency that I'm having a hard time finding a equation that fits
>> as nicely.  All the examples I can find on the web look like third
>> degree polynomials, while my data doesn't seem to fit that exactly.
>>  The best result I've had so far (0.198 ppm stddev) is attached as
>> "temp-clock.png" and uses the function:
>>
>> f(x) = -abs(a * (x - 20.88)) + b * ((x - 20.88)**2) + c * ((x - 20.88)**3)
>> a = 0.888582
>> b = 0.113806
>> c = -0.00445763
>>
>> Does anyone have any advice on how to better model this?  Has
>> anyone seen this behavior?
>>
>> I can provide the raw data if that would help any.
>> <temp-clock-warmer.png><temp-clock.png>_______________________________________________
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