USA Cal Club: Round 2

[orin]

Hum on the 731A is not surprising - it depends entirely on the power supply rejection of the op-amp.  It sort of uses the battery as a smoothing capacitor by supplying it through a 510 ohm resistor while at the same time pulse-charging the battery with un-smoothed rectified AC.

If the battery has been removed from a 731A, it needs a capacitor in its place.  I guess an old tired battery with high internal resistance could be problematic too!

They improved things with the 731B, but not completely.  They still pulse charge the battery and when it is fully charged, depending on the component tolerances, the pulses can get through the diode switching they use from battery/AC supply.

[Conrad Hoffman]

I replaced my batteries with a capacitor and zener combination that imitates the NiCd it originally used. Seems to work well but I never noticed the difference in hum with the B version. In theory the hum averages out, but I wonder how perfect that is? Now I'm thinking some power supply mods are in order.

[hwj-d]

I didn't had the FX by me. So, i'm interested to the PX and the resistors, to compare them with my setup. Thanks Conrad

[Conrad Hoffman]

Now that I have several references calibrated, I'll make a quick check of the PX against the FX. Not as good as people with 7.5 or 8.5 digit meters, but just the 6.5 digit.

Looked at the schematics for both the 731A and B and wow. I just assumed they were the same, but far from it. Little time to post as work is busy; have patience!

[orin]

Now that I have several references calibrated, I'll make a quick check of the PX against the FX. Not as good as people with 7.5 or 8.5 digit meters, but just the 6.5 digit.

Looked at the schematics for both the 731A and B and wow. I just assumed they were the same, but far from it. Little time to post as work is busy; have patience!

Wow to be sure!

Do you still have NiCads in the 731B?

Orin.

[Conrad Hoffman]

No NiCads in any of them. They don't seem to care much about power-up/down, so I replaced the NiCads with capacitors in parallel with zeners, to imitate how a NiCad works. I'm thinking the A version needs a proper regulator. Seems an easy mod, but that's what I always say prior to actually doing it.

[Conrad Hoffman]

Bad road conditions today so hopefully the cal kit will see the post office tomorrow. Couldn't go to work, so I'm building a voltage regulator for my 731A. Actually, I could have gone to work, but the driveway was drifting in so fast I wouldn't be able to get back in.

[Conrad Hoffman]

OK, ref is on the way to *technogeeky. Now I just need time to post some results.

[Conrad Hoffman]

There isn't much of anything I can contribute concerning the FX reference accuracy or stability. It's an order of magnitude better than anything I own. Something that does interest me is noise. I know that that's been investigated too, and it's darn good. Still, it might be interesting to do a visual comparison with some other references. For my audio work I have a Tek 1A7A plug-in that goes down to 10 uV/div. Also have a similar one for my 7000 mainframe, but the 1A7A is my "daily driver." Warning- all my photos are hand-held and you'll see some intensity changes from the trace speed and shutter speed.

First, the 1A7A with a shorted input:


Next, the FX reference:


Next, my Fluke 731B reference:


The 1A7A is a 1 MHz plug-in and the 731B has some HF noise. Here it is with a 10 kHz lo-pass filter:


The real surprise is how bad my 731A is. That version had no voltage regulator and no internal floating guard:


Finally, the Mini-Metrology LT1027 based reference:


Pay attention to the vertical sensitivity labels, as I've used different scales as needed.

I've just completed a voltage regulator for the 731A. That helps a lot, but it still suffers from no internal guard. More on that later. I'll post some voltage comparisons shortly.

[Conrad Hoffman]

I did the Mini-Metrology articles 24 years ago! The original reference hasn't been touched since then AFAIK, and was 16.8 ppm higher than the FX. FX is about 4 ppm low, so we'll call it 12.8 ppm over ten volts. A friend of mine found an MM reference at a hamfest and bought it. It was in a plastic box and had my cal sticker from 24 years ago. I got the sign backwards on my previous posting of these. It was actually 39 ppm lower than the FX, so call it 43 ppm below ten volts. I readjusted both #1 and #2 and did some brief data logging. This was before I got temperature added, but I know my lab will typically change by 5 degrees F over a day or night- I heat with wood so control this time of year is iffy at best. FWIW, here's a screen grab of the first ref in Excel:

[Conrad Hoffman]

I didn't spend much time with the PX reference, other than to warm it up and compare it with the FX using my now-calibrated HP3455A antique steam powered voltmeter. I put the PX ref at 7.04565 volts.

[Conrad Hoffman]

Here's my 731A with its new voltage regulator. Note the vertical scale is 10 uV/div and 1 kHz bandwidth:

[SilverSolder]

Here's my 731A with its new voltage regulator. Note the vertical scale is 10 uV/div and 1 kHz bandwidth:

Is it almost beating the 731B now? 

Stunning improvement...

[Conrad Hoffman]

One of my B units is noisier than the other and one has just detectable popcorn noise. I need to see if it's the same unit, then decide if the problem is the reference or the opamp. You can guess which one I'm hoping for. The A unit is now excellent, save for the lack of an internal shield/guard to prevent hum pickup. It doesn't seem to affect accuracy but I can't believe it's a good thing. The "guard" connection on the front simply connects to a winding screen in the power transformer. I've no idea what that's supposed to accomplish as connecting it never seems to affect anything at all. I extended the transformer secondary wires and used my own bridge on the regulator board. That insures that the AC loop has no conductors in common with the DC loop. The reg is just an LM317 but most people don't realize how quiet they can be if you use a cap on the adjustment pin. Here's what the mod looks like- it occupies the space where the batteries went.

[hwj-d]

I didn't spend much time with the PX reference, other than to warm it up and compare it with the FX using my now-calibrated HP3455A antique steam powered voltmeter. I put the PX ref at 7.04565 volts.

At first, thanks Conrad.
But, hmm, thats a pitty. The PX reference is probably no longer taken very seriously besides the FX. But it would have been interesting to compare the noise behaviour. Also the resistors, no measurement?

[Conrad Hoffman]

I would have liked to do more but winter mostly has me carrying firewood and working. Time is short, and I didn't want to keep the kit for weeks on end. Data logging proved more troublesome than I thought it would be, so I lost a lot of time to that. Also lost time figuring out the best comparison setup as my 845 is noisier than I remember it being. Note to self- test and fix everything well before getting the kit. Note to others- see note to self! You're right, the FX has overshadowed the PX and I should have done more. In hindsight, there are also some different comparisons I should have made and more scope photos that would have been useful. I have a bit more data to ponder and maybe post, but it's more related to using the HP3478A as a null meter and the Mini-M null meter.

[technogeeky]

I am in possession of the USA Cal Club kit!

I won't have it long, my best multimeters are only 5.5 digits but it affords me the opportunity to test the calibration at all, so for that I'm very appreciative.

[bitseeker]

...Time is short, and I didn't want to keep the kit for weeks on end.  Data logging proved more troublesome than I thought it would be...Note to self- test and fix everything well before getting the kit...

Trying to do all the right things in a timely fashion is quite a challenge and we all learn a lot in the process. I'm certainly thankful we have the opportunity to try it.

[KK6IL]

> The "guard" connection on the front simply connects to a winding screen in the power transformer. I've no idea what that's
> supposed to accomplish as connecting it never seems to affect anything at all.

When we were trying to build a differential multimeter at Wavetek back around 1970, we had the transformer secondary for the voltmeter inside a foil shield. This was to prevent capacitive coupling from the primary, which could have put a common mode voltage between the supply and earth.

Maybe look at your 731 output with respect to earth with your 1A7 and see if the guard makes any difference.

Guess I should look at my LM399 standard for noise. Love the 7A22 plug-in for low level signals.

[Conrad Hoffman]

I understand the use of a guard in AC bridge work, as coupling to the primary can be an error source, but I don't see how it matters for DC metrology. I've got a couple 7A22 plug-ins and they're very good, though one of mine fell apart! Several of the many trimmer capacitors popped apart during storage in my unheated garage. I had to carefully desolder them and fix the retainer. Can't remember, but I think I soldered it to the post.

While using the FX standard and various null methods, I was always careful to note if third terminals were floating guards or grounded shields. With several pieces of equipment, some of which might not be floating (scope plug-in), it would be possible to short out the reference.

I'm not comfortable with it, but it seems that large amounts of hum still average to zero and don't affect the results much. IMO, everything should have floating guard systems.

[TiN]

hwj-d
I'll put PX thru paces, when kit make it's way to me.   

[CatalinaWOW]

Low noise is clearly good, but how good is good enough is slippery.  A low pass filter does not harm the measurments of something that in principal does not change.  The two principal constraints on the filter are settling time (you don't want to wait days for a stable reading) and drift ( you don't want the filter to mask real variations).  Neither constraint seems to prevent filters with fractional Hz bandwidth.  Now it is only component constraints.

[hwj-d]

hwj-d
I'll put PX thru paces, when kit make it's way to me.   

Yes, I expect so. You'll probably be the first with your elaborateness. But let the px still live, in your rating 

[MaxFrister]

After a bit of a wait, I am approaching the top of the cal-club queue.  I thought I would start to plan for its arrival.

Relevant available equipment:

• HPAK 34401A 6.5 digit meter
• HPAK 34970A with 34901A MUX, 6.5 digit meter, and thermocouple
• Datron 1081 7.5 digit meter
• HPAK E3631A Power supply
• HPAK 85357B GPID adapter
• Old laptop  for logging.
• Homebrew environmental logger (temperature, humidity, and barometric pressure).
• I also have a 3499B and various switch cards but do not plan to use them as I think they will degrade accuracy.

Pre-arrival plan:

• Rearrange gear to put the necessary equipment together and far from the every-day bench
• Wire up gear
• Review and update logging programs
• Set up and practice logging with 0-ohms/0-volts and stand-in (house) standards.

Upon arrival:

• Post a message to this list to keep other members informed
• Allow cal standards to warm up to room temperature
• Turn on test equipment for 12 hours
• Re-read standards kit contents and wiring documents

Data acquisition:

• Post periodic updates to this list
• 4 wire measurements of each resistor using the 3 meters sequentially (i.e., resistor hooked to only 1 meter at a time) for 2 hours
• Log Px for 24 hours with standard wired to 3 meters in parallel
• Log Fx for 7 days with standard wired to 3 meters in parallel

Bonus:

• Measure standards using various less precise meters
• Experiment with the Datron guarding signals
• Attempt some noise measurements

After acquisition:

• Package and return the standards.
• Try to produce some preliminary pretty graphs
• Post a message to this list
• Write a report and upload it and the raw data to the Xdevs site.

[Conrad Hoffman]

No battle plan survives first contact with the enemy, but that sounds pretty good!