[time-nuts] Thunderbolt E failing

Mon Sep 14 07:59:46 UTC 2020

On Sonntag, 13. September 2020 15:05:36 CEST Bob kb8tq wrote:
> Hi
> 
> The point is that there is no need to compensate the part while it is
> locked.

It depends. For short time constants, yes, likely the control loop is able to 
follow the temperature-induced drift of the OCXO. But you might want the TC to 
be as long as possible. Naturally, the control loop becomes very slow to 
react. By following the temperature, you have an additional input that allows 
the controller to act more quickly to a changing environment. Effectively this 
will lead to higher stability of the output.

> Indeed, trying to compensate *and* learn at the same time generates even
> more issues to deal with. The temperature effect is not a simple linear /
> first order sort of thing ….

Maybe, but a linear approximation is probably better than nothing as it still 
reduces the amount of change that the control loop has to counteract.

If you look at the attached screenshot - there's roughly 5500 seconds of data 
from my GPSDO. At about 2500 seconds the temperature compensation was engaged. 
It's just a linear correction factor for the DAC output. The factor was 
computed using a linear regression fit to find the correlation between the 
temperature and the OCXO drift. In the DAC chart the blue trace is the actual, 
corrected DAC output, the orange trace is the uncorrected DAC computed from 
the TIC input. The green trace is the OCXO drift. Without the temperature 
compensation, the TIC takes a dive down to almost -15ns. While the temperature 
continues its decline at an approximately constant rate, after the 
compensation was engaged the TIC excursions are smaller and therefore the 
stability of the output higher.

Now, of course the picture gets somewhat muddy if you keep computing the 
compensation factor while the compensation is active, but what you still get 
is a _residual_ factor, which you can maybe use to refine the compensation 
subsequently. The correlation factor (r_xy) will tell if there's still 
something to be done.

> 
> Bob
> 
> > On Sep 13, 2020, at 1:41 AM, Matthias Welwarsky <time-nuts at welwarsky.de>
> > wrote:> 
> > On Samstag, 12. September 2020 23:53:39 CEST Bob kb8tq wrote:
> >> Hi
> >> 
> >> As far as anybody knows (which *is* a big qualifier indeed ….), the TBolt
> >> family only does it’s “temperature compensation” stuff when it’s in
> >> holdover. While it’s locked, there *appears* to be no compensation being
> >> done. If it’s doing anything while locked, it’s learning what the TC is,
> >> so
> >> it can use that information while in holdover.
> > 
> > That is interesting, why wouldn't they use the information if it's
> > available? Watching the temperature gives you a prediction of what is
> > going to happen, while watching the phase only shows what has already
> > happened. And knowing what's about to happen is always very favorable in
> > control theory.> 
> >> Bob
> >> 
> >>> On Sep 12, 2020, at 3:52 PM, Matthias Welwarsky <time-nuts at welwarsky.de>
> >>> wrote:
> >>> 
> >>> Kevin,
> >>> 
> >>> how long has it been in storage? I expect the GPSDO doing quite clever
> >>> things to compensate for temperature effects. There may be some
> >>> parameters stored on the device and the characteristics of the OCXO may
> >>> have changed after being turned off for an extended period.
> >>> 
> >>> Regards,
> >>> Matthias
> >>> 
> >>> On Samstag, 12. September 2020 18:13:35 CEST Kevin Schuchmann wrote:
> >>>> Matthias,
> >>>> 
> >>>>   You are correct, having spent hours looking at all the stable areas
> >>>> 
> >>>> and cooling and heating the gpsdo I find that when it reports that it
> >>>> is
> >>>> 116.25 F then it is stable.
> >>>> So now I guess I need to figure out why it is so picky. I will measure
> >>>> the current draw from a cold start and see if I see the oven warming up
> >>>> and then stabilizing and then heat and cool it and see how it reacts,
> >>>> and also look at the electronics and see if an area is overly sensitive
> >>>> to temperature.
> >>>> 
> >>>> Thanks
> >>>> Kevin
> >>>> 
> >>>> On 9/11/2020 2:26 AM, Matthias Welwarsky wrote:
> >>>>> On Freitag, 11. September 2020 01:08:09 CEST Kevin Schuchmann wrote:
> >>>>>> Guess my image didn't make it, I will add it as an attachment this
> >>>>>> time.
> >>>>> 
> >>>>> The temperature curve seems to show some correlation to what is
> >>>>> happening
> >>>>> with the DAC. Seems that the DAC and OSC jumps are mostly during
> >>>>> periods
> >>>>> of some thermal perturbation.
> >>>>> 
> >>>>> Regards,
> >>>>> Matthias
> >>>>> 
> >>>>> 
> >>>>> 
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