[time-nuts] Thunderbolt E failing

[Poul-Henning Kamp]

--------
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.

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
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Never attribute to malice what can adequately be explained by incompetence.