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 <kb...@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-timen...@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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