[time-nuts] Thunderbolt E failing

[shouldbe q931]

On Mon, Sep 14, 2020 at 10:36 AM Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
>
> --------
> Matthias Welwarsky writes:
>
> > > 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.
>
> The word you are looking for here is not "Temperature Coefficient"
> but "Thermal Impedance" (more on this below).
>
> > 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.
>
> Only if you first spend months and years measuring all the parameters
> and time constants of the multiphysics model you use, well enough
> to make useful predictions with it.
>
> > Maybe, but a linear approximation is probably better than nothing [...]
>
> No, it is usually worse.
>
> A major part of the trouble is the complex hysteresis-effects when
> the temperture changes direction: The components which warm fast
> also cools fast.
>
> For example:  When the temp goes up you will likely find that your
> DAC warms faster than the XTAL, but when the temp goes down it also
> cools faster than the XTAL.
>
> That means the temperature difference between the DAC and XTAL depends
> on the temperature rising or dropping, so you have to model the tempco
> and temperature of them individually.
>
> > 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.
>
> This is nowhere near enough data to show anything.  Collect a full
> week with/without and we can talk.
>
> In the end, the proof is in your allan-variance, if it improves, you got
> something, if it does not, you wasted your time.
>
>
> The easiest and cheapest way for you to get better results, is to increase
> the thermal impedance between the surroundings and your GPSDO.
>
> That will make the temperature change slower, which also means it
> changes less and therefore your hysteresis effects also get smaller.
>
> Note that "thermal impedance" is not the same as "thermal insulation":
>
> Thermal insulation materials have high thermal resistance and low
> thermal mass, and wrapping your GPSDO in that would just make it
> run hot.
>
> Think of it is a thermal RC filter with a huge resistor and a small
> capacitor.
>
> We want the a low to moderate resistor, so the GPSDO can still dump
> its heat, with a huge capacitor to filter out the changes in
> temperature.
>
> Apart from the entire "get electronics wet" thing, water would have
> been near perfect.

There are alternatives to water...
https://www.3m.co.uk/3M/en_GB/novec-uk/applications/immersion-cooling/

Cheers

Arne

>
> Table-top granite (about 2cm thick) is really great, but not very accessible.
>
> Metals almost conduct heat too well, but a box of 1-5cm thick iron
> plates works great, but pay attention to the weight.
>
> For most of us, concrete is the way to go:
>
>         Get three cinderblocks of the kind that looks like a 'H' with two
>         horizontal bars.
>
>         Put the first cinderblock down on its side.
>
>         Put the next cinderblock on top of it in normal orientation.
>
>         Put your OCXO into the cavity.
>
>         Hack notches in the edge of the cinderblock for the cables.
>
>         Put the third cinderblock on top, also on its side.
>
> You have now increased the thermal impedance by almost two orders
> of magnitude, and your PLL will be boooooored.
>
> If need be, you can make the central cavity more air-tight by
> "sealing" between the cinderblocks with a layer of cloth or
> tissue-paper.
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
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