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Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

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bdunham7:

--- Quote from: guenthert on January 27, 2021, 06:38:55 pm ---     Even if the ADC would be unable to report some codes, the averaging surely will.  I can only assume that what @bdunham7 sees is coincidence.

--- End quote ---

Without knowing exactly how the whole thing works, we can't be sure.  In any case, I shifted back to monitoring a 10V reference this morning and temperatures have been more stable, so I'm seeing numbers from 9.99999 6 to 10.00000 4, with the vast majority being 9.99999 9, 10.00000 0 and 10.00000 1.  I haven't been staring at it the whole morning, but check it out whenever I come into the room and I have yet to see a 9.99999 7, 10.00000 2 or 10.00000 3. 

It is almost scary to see it appear to be accurate and stable to 100ppb.  I moved the reference from on top of the meter to beside it and that seems to have stabilized it greatly.

SilverSolder:

--- Quote from: joeqsmith on January 27, 2021, 07:23:11 pm ---Shown is about 4.5 hours.   Increasing the histogram's bin size to look at the missing codes,  the meter has not put out any 1s, 2s, 6s and 7's in the extended code.   We see something similar with the HP not showing 3s & 8's.

To test it out, I changed the voltage standard to delta E (uV resolution) and slowly adjusted the output voltage.  No missing codes.

--- End quote ---

That is a little bizarre...   Some kind of rounding or precision issue with the calculation of the running average?

Floating point operations are not perfect, of course. 
https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html



SilverSolder:

--- Quote from: bdunham7 on January 27, 2021, 09:20:54 pm ---
--- Quote from: guenthert on January 27, 2021, 06:38:55 pm ---     Even if the ADC would be unable to report some codes, the averaging surely will.  I can only assume that what @bdunham7 sees is coincidence.

--- End quote ---

Without knowing exactly how the whole thing works, we can't be sure.  In any case, I shifted back to monitoring a 10V reference this morning and temperatures have been more stable, so I'm seeing numbers from 9.99999 6 to 10.00000 4, with the vast majority being 9.99999 9, 10.00000 0 and 10.00000 1.  I haven't been staring at it the whole morning, but check it out whenever I come into the room and I have yet to see a 9.99999 7, 10.00000 2 or 10.00000 3. 

It is almost scary to see it appear to be accurate and stable to 100ppb.  I moved the reference from on top of the meter to beside it and that seems to have stabilized it greatly.

--- End quote ---

Yes, it is amazing when two independent entities get that close, and stay that close.  Like two icebergs floating a cigarette paper's thickness apart, yet never touching!

bdunham7:

--- Quote from: SilverSolder on January 27, 2021, 09:32:32 pm ---That is a little bizarre...   Some kind of rounding or precision issue with the calculation of the running average?

Floating point operations are not perfect, of course. 
https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

--- End quote ---

I suppose the next thing is for someone else with one of these to test and see if they can get it to read exactly any of the voltages that I'm seeing as 'missing'.  I was unable to replicate it at a few other much lower voltages where I have 1uV adjustment capability. 

B/t/w, thanks for the post inspiring me to dig this thing out and finish the project.  I already completely dissembled it twice and replaced some power supply caps, two fuses and two resistors on the F/R input board.  Apparently someone used the rear connector to overload the Xref circuit--which I haven't tested yet.  Does anyone have a line on that strange connector they used on the back side?  Its an Amphenol low mating force, probably mil-spec, but I can't seem to find a part number or a mating connector.  Now I finally did a basic cal and tested it, all it needed beyond the DC cal was an adjustment to the AC zero and it all looks pretty good, including the current ranges.  Ohms may be a tad off, but now I'll have to buy or make a batch of resistance standards....

SilverSolder:


--- Quote from: bdunham7 on January 28, 2021, 12:47:08 am ---I suppose the next thing is for someone else with one of these to test and see if they can get it to read exactly any of the voltages that I'm seeing as 'missing'.

--- End quote ---

I'll have a play with this, once I get GPIB back up and running after my re-arrangement exercise.



--- Quote from: bdunham7 on January 28, 2021, 12:47:08 am ---B/t/w, thanks for the post inspiring me to dig this thing out and finish the project.  I already completely dissembled it twice and replaced some power supply caps, two fuses and two resistors on the F/R input board.  Apparently someone used the rear connector to overload the Xref circuit--which I haven't tested yet. 

--- End quote ---

It can get a little lonely playing with these, they are not super popular.

When you are looking at PPMs, drifty components that wouldn't be noticed in any other circuit suddenly begin to matter.  So they are like owning a classic car, where you have to spend some time taking care of it to get the best out of it - but when it is on song - what a ride!



--- Quote from: bdunham7 on January 28, 2021, 12:47:08 am ---Does anyone have a line on that strange connector they used on the back side?  Its an Amphenol low mating force, probably mil-spec, but I can't seem to find a part number or a mating connector.  [...]

--- End quote ---


The rear connector is an ELCO 8016- 20 Pin socket - you can find both sockets and connectors on eBay.....

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