Electronics > Metrology

Solartron 7061 noise

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grizewald:
Dear metrologists, maybe you can give me some insight into something I discovered recently...

I'm a big fan of Solartron's multimeters, what with them being an excellent example of good British engineering and the fact that I hail from the UK, despite the Swedish flag under my user name. I recently found a 7061 meter, complete with all the original measurement cables, on eBay and bought it without a second thought. (Just the three Fischer connectors on the measurement cables are probably worth what I paid for the meter.)

When it arrived, I started doing some testing to compare the 7061 to my superbly stable 7075 meter so that I could get a feel for what kind of condition it was in. One of the first tests I did was to connect the 7061 to my Muirhead Weston cell to get a feel for initial accuracy and see how stable it was. The result of that test was both good and bad. The good part was that the meter read to within 20μV of what the temperature adjusted voltage of the Weston cell should be. The bad part was that I observed a +/- 3μV variation while measuring the cell.

My 7075 just sits there with even the last digit completely stable when measuring the Weston cell, so I was quite surprised to see that a more modern meter was so noisy by comparison.

Looking at the warmed up 7061 with my thermal camera gave me this image:





Most of the heat is coming from the transformer which uses the case as a heat sink.





One big difference between the 7075 and the 7061 is that the 7075 has its reference diode in an oven to keep it and its leads at a constant temperature. In contrast, the reference diode in the 7061 is left naked and raised up from the PCB at the rear right of the analogue board. (D301A in the next picture.)





Looking at the heat distribution, it's more than likely that there are air currents from convection rolling around inside the case. Surely those currents will also mean that the reference diode is subjected to local thermocouple effects due to these air currents creating temperature gradients on the body and legs of the diode? I know that the Zener current is hand trimmed on each meter to coincide with the zero TC point of the diode, but that's not going to help against local temperature gradients is it?

Although I don't have my GPIB interface completed yet so that I can log readings from the 7061, it occurred to me that the 7061 has built in statistics functions that I could use to look at how unstable the meter actually is. I therefore enabled the MaxMin and Statistics programs to get some more scientific results than just observing the meter's readings over an hour or so.

I also decided to use some cotton to shield the reference diode from any eventual draughts and see what effect that had.





I made two runs, one with my little cotton tea cosy and one without. The meter had been on for more than 48 hours when I ran the two tests, one for 1.5 hours and the other for 1.75 hours. During the test, the measurement leads were shorted and I also turned off the filter function to better expose the meter's noise. I left auto-ranging on as the meter is going to stay at the lowest range anyway with shorted inputs.

The results were very interesting!



              With cosy        Without cosy
MAX      3.166496e-7     2.495944e-7
MIN      -3.241002e-7    -5.178153e-7
PP         6.407499e-7      7.674098e-7
MEAN  -1.678598e-8    -1.631235e-7
SD         6.158931e-8     1.010704e-7
VAR       3.793245e-15   1.021523e-14
RMS      6.383583e-8     1.918972e-7


There would seem to be an improvement in noise of a whole order of magnitude with my cotton draught shield.

Far be it from me, as a simple hobbyist, to question the Solartron designers, but surely there's something wrong here? Having the reference diode in the rear right of the case with a roasting hot transformer creating constant air currents inside without any protection from those air currents would seem to be guaranteed to cause noise in the reference voltage.

Am I missing something here? The figures would seem to speak for themselves as to how effective my little cotton pad is. Any input from the knowledgeable and experienced volt-nuts here would be much appreciated!

(edit: I got the column heading swapped and it looked like my cotton pad made things worse rather than better!)

Kleinstein:
When measuring a short, the reference should have very little effect. Noise from the reference is multiplicative and thus not important near zero.

At the highest gain the limiting factor is usually the input amplifier. From the pictures this looks like a LDR base chopper stabilized amplifier.  If this i still using old style neon tubes, this could be a source of trouble.

The first part would be to check if the variations are more with the input amplifier, the ADC or the reference. All 3 are possible in case of the 1 V reference (I assume a 1-2 V range).

The values from the short test look similar and if the units are volts, the noise looks OK. There are some details no given for the test, but as far as I see it the ADC and amplifier look kind of OK so far. So it would point to a problem with the reference (or maybe the clock, of input current noise from the amplifier). However the test with a short says nothing about the reference stability.

grizewald:

--- Quote from: Kleinstein on February 11, 2020, 06:12:55 pm ---When measuring a short, the reference should have very little effect. Noise from the reference is multiplicative and thus not important near zero.

At the highest gain the limiting factor is usually the input amplifier. From the pictures this looks like a LDR base chopper stabilized amplifier.  If this i still using old style neon tubes, this could be a source of trouble.

--- End quote ---

The 7061 uses the classic ICL7650 chopper amp. You're thinking of the 7075 when it comes to the LDR/LED chopper.


--- Quote from: Kleinstein on February 11, 2020, 06:12:55 pm ---The first part would be to check if the variations are more with the input amplifier, the ADC or the reference. All 3 are possible in case of the 1 V reference (I assume a 1-2 V range).

The values from the short test look similar and if the units are volts, the noise looks OK. There are some details no given for the test, but as far as I see it the ADC and amplifier look kind of OK so far. So it would point to a problem with the reference (or maybe the clock, of input current noise from the amplifier). However the test with a short says nothing about the reference stability.

--- End quote ---

The units are indeed volts, but my question is really about why the Solartron designers put the reference diode flapping around in the breeze inside a case with obvious thermally induced air currents without giving it some kind of protection against those air currents. I'm sure I'm not smarter than the people who designed the meter, so I'm at a loss to understand why my simple cotton draught protector lowers the mean of the zero volt measurement from -1.631235e-7 to -1.678598e-8?  (I've edited my original post as I had the column headings the wrong way around).

Kleinstein:
I can understand that the unshielded zener looks odd. Some thermal shielding could help to get a little less noise at the reference. However the zener itself already has some noise, so the thermal effect may not be so bad if the current is adjusted well and thus the TC low.
The zener diodes run at a relatively high current and thus get quite some power. So much thermal insulation alone may not be a good solution.

I would consider it some random effect that padding the zener effects the average offset seen. There normally would not be much effect expected and the actual offset very much depends on the last adjustment / zero cal. So not sure which value is better. The offset value has not much to do with noise / variations seen, when measuring the 1 V ref.  0.1 µV difference in the offset has not much in common with 3 µV_pp variation.

The measured noise with a short looks not too high (some 0.7 µV_pp) for the lowest range. So the amplifier is likely not the culprit.  The ADC noise is relatively easy to test, with a short in the higher range ((10/20V).
One could also check if the noise is more white noise of has significant 1/f parts.

martinr33:
It isn't quite a whole order of magnitude - more like 30%. It just happened to cross the decade line. 

I would check out those few electrolytics. I have a similar vintage (Datron 1281) machine that got a lot quieter when I replaced the electrolytics.

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