Electronics > Metrology

Ultra low-noise, short-term stable references

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branadic:
Hello everyone,

at and after MM2024, there is/was quite some discussion going on due to the Z10 presented by chuckb. While we have no clue what chuck is using as his short-term reference, I thought it is worth having a dedicated thread concerning that topic and maybe some of you have some ideas how to put together something similar.
Having a low-noise reference combined with a noisy long-term stable reference is something we have seen in some multimeters before (1N829 + LM399 IIRC).

So here is my first take on that. I remembered MM2022 and the SPC1550 I got from Rainer:

https://www.eevblog.com/forum/metrology/volt-nut-meeting-2019-in-stuttgartgermany/msg4409104/#msg4409104

While noise already looked good, it was time to test stability of two SPC1550 connected in series and measured with a 3458A. I started a first measurement and I quickly noticed a huge sensitivity to ambient temperature changes (AC in the lab turning on and off more or less frequently), so the measurement was stopped and the SPC1550s put into a styrofoam box with TEC-controlled, temperature-stabilized conditions inside.

Attached are the data for that measurement as well as a zoomed image of the temperature stable area (roughly 4 hours) at the very end of the first diagram. While the short-term noise looks rather good and most likely limited by the 3458A, there is a very low frequency change visible.
So the question arrises, where is this coming from? To me it seems like a sensitivity to ambient pressure, but with a certain time constant. Could be the SPC1550s or partially the 3458A, which would mean the cells have to go into a pressure tight box to surpress the effect on them.

What is your take on that experiment?

-branadic-

Kleinstein:
I would expect a super cap to behave a little similar to a battery. So there is a temperature dependence, depending on the chemistry and possibly charge history.

Another point is a possible effect of mechanical stress, e.g. from air pressure or via the leads and longer time humidity effecting plastic insulation.
AFAIR there are reports about the pressure effect of 10 V reference (more like LFLU based).

For the test one may have to measure the difference to a more stable 7 V reference, or use 2 such references and look at the difference.

dietert1:
I'd guess it is about making a constant temperature, constant humidity and constant pressure oven. I think that was the claim in the Z10 info.
Making a constant humidity oven is quite difficult. Most ovens have some tiny leakage. So that results in a low pass filter behavior. For the IP66 enclosures with rubber seals i am using i have seen time constants up to 5 years. Of course there are much better and much more expensive hermetic enclosures and feedthroughs.
After reading the Z10 info i started to prepare a study including pressure measurement, using a Weidmüller K41 enclosure that we had. Results can be expected after a year or so. Probably there will be a low pass behavior also for pressure.
I know it sounds strange to write about years in this thread, but at "short" time scales low pass filters work very well. During my LTFLU studies i learned that for precision references what you see as noise at time scales of minutes to days are ambient variations attenuated by some low pass.

Regards, Dieter

branadic:
The two SPC1550 I have today made their way into a sealed die cast aluminum case with sealed connector. Not fully pressure thight though, but the best I had at hand to see if this approach is worth to follow. The case went into the temperature stabilized styrofoam box. Tomorrow I have more data to show.

-branadic-

chekhov:
IMHO, whatever makes Z10 reference wonderful is not the ultra low noise but the way how this low noise ref is adjusted or kept long term stable.
Let's imagine we have low noise source already, short time stable. And we have our array of LTZ/whatever long-term reference.
How to compare them, quickly or during sufficient time period, keeping in mind all 1/f noise produced by it, different fluctuations, popcorn, ...
How we adjust our low noise ref ... so that this does not affect the process of watching it as well

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