Gary E. Miller writes: > Ntpviz now has a plot for frequency vs temp. The results are > interesting.
…wasn't it you who said just two weeks ago that they wouldn't be? > Attached is from a Pi3. It plots the Local Frequency Offset vs. the > Pi CPU temp. You really need to record the temperature aligned with the loopstat values if you want to correlate them. I actually read it out for each PPS timestamp, then average onto the next loopstat value (I'm running the loop at poll=4, so 16 seconds). The temperature measurement on the Pi is quite noisy, even though it appears they already do some sort of filtering on it. > By eyeball looks like a frequency shift of 0.250 ppm/°C The denominator is a temperature difference, which can't be reported in °C; so that should be ppm/K. The ballpark number is roughly what I'd determined earlier for my rasPi. I had initially assumed that they'd use AT cut crystals which would produce a third-order temperature dependency, but the characteristic is quite clearly parabolic. So it is probably a BT cut, but the parabolic constant extracted from the data is smaller than the theoretical value for this cut. Over a period of time longer than a week you need to take crystal aging into account also. Aging is a logarithmic function of time, but assuming you've already aged out the initial transient and are looking at a short time period w.r.t. the total aging time, you can use linear aging instead. That makes the fit function p2a(t,T)=pa*t+pb*(T-T0)**2+pc I'm using these initial values for the fit pa=100e-6 pb=-5-3 pc=-3.5 T0=64. then fit fit [t=*:*][T=*:*][ppm=*:*] (p2a(t,T)-ppm) "joined.txt" using … via pa,pb,pc,T0 I'm only using values where the loop has converged to better than 1µs for the fit. Depending on how much data you have and what temperature range it spans you might need to fix some of those parameters to make the fit for the others converge. In order to reduce the disturbances by fast ambient temperature transients now that the heating season has begun, I've bubblewrapped my rasPi (still in it's case) + GPS module and put it into a corrugated cardboard box with some slits cut to feed the cables through. That has raised the thermal resistance enough that I can now operate the XO near the cusp of the TC curve. Currently I'm loading a single core with sha512sum /dev/zero & to raise the temperature a bit and there's a few hours of operation with two cores loaded this way in the data. I'll have to see how to get some finer control over that load (and maybe use a second core for "heating") so that I can operate exactly on the zero-TC point. But first I need to collect some more days worth of data.
Regards, Achim. -- +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+ Samples for the Waldorf Blofeld: http://Synth.Stromeko.net/Downloads.html#BlofeldSamplesExtra
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