[volt-nuts] Back to voltage

Marvin E. Gozum marvin.gozum at jefferson.edu
Tue Aug 24 23:52:37 UTC 2010

Hello Dick,

Thanks for your input.  Your posts on time-nuts have been invaluable 
as was the thread here and all its contributors:


The last I read on time and volt nuts, your HP 3458a had it share of 
trouble, but you describe here its working true now, did it take much 
to get back into factory shape?

At 02:18 AM 8/24/2010, Dick Moore wrote:
>I'll stick the oar of my opinions in here -- I've owned a couple of 
>Fluke 732As, both of which needed some serious TLC from me, and so I 
>got *very* familiar with them. Fluke used a Motorola monolithic 
>device with a transistor and a buried zener as references in the 
>732A, the 8505/6 meters, the 5440A and B DC standards, the 510A AC 
>standards, the 515, and a host of other products. In the critical 
>units, like the 732A and the 5440, the reference parts and the 
>critical resistors all cohabited inside a fairly massive assembly, 
>surrounded by resistance heaters controlled by a thermistor placed 
>inside the assembly near the reference.
>For the purposes of even a very good home lab or shop, the 732A is 
>overkill. Fluke did things we don't have to do, since we have 
>complete (or near enough) control over the way and place that a 
>reference will be used. I have one of Geller's AD587 10V references, 
>and if placed in a simple oven, it's good. But it isn't as good as 
>an LTZ1000 or 1000A running in a closed cabinet at room temp, +/- a 
>couple of deg C. This, anyway, is what my HP 3458 tells me, and 
>that's as good as I'm going to be able to have as a means of 
>verifying a reference's performance.

Given the precision of the 8458a, you can check the Geller and things 
like it, but few devices can check the 8458a other than a freshly 
NIST traceable calibrated 8458a with a high precision board or a 
calibrated 732B, at least.

>An LTZ1000A, a 723 regulator, a few decent 20ppm wirewound resistors 
>and a wirewound trimmer, an IC chopper amp like the LTC1150, plus a 
>box roomy enough to hold it all, will make a very good 10V reference 
>with sufficiently low output Z to be useful. All that's missing is a 
>way to measure the trimming of the output. If any of you make such a 
>simple device, I will happily set the output for you using my 3458, 
>if you pay the shipping of your box both ways. I can reliably 
>measure the ambient to about 0.2 deg C with a Pt100 probe and 
>provide that info as well. I think you can reasonably expect an 
>overall stability of 1ppm over several months to a year, providing 
>the unit is run for a month or so continuously before adjustment.

Thanks for making an offer, it may help in getting secondary 
confirmation on any measurements in the future.

Currently, I plan to amass several HP 3456a as deals allow [have 
several already], have officially calibrated one to set the others 
then spot checking critical points on all of them <= 90 days, then 
cross checked against a Geller, who provides his SVR traced to 
Fluke's calibration service.

>This isn't magic, so the only real question is: how good is good 
>enough? I sold my 732As and my 5440B. If I want to know what 
>something measures, I turn on the 3458. I'm one of the lucky few who 
>decided it was worth it and could at the time, anyway, afford it. 
>Other things come close at more reasonable prices, such as the 
>Datron 1081/2. I'll know better by October how well the 3458 did 
>over a year, since I will have it re-cal'ed in September. It's about 
>a 20 year-old dog that has some new parts, but its LTZ1000 is very well aged.

Please let us know.  I am contemplating getting a 3458a, but there 
seems to be problems with some model years, making eBay models, even 
if working, more perilous than necessary.

>And Bill is right -- the real bears are AC standards, which are much 
>flakier in every way, no matter how good they are. To that end, I'm 
>laying out a board for Bob Cordell's state-variable oscillator, 
>which has somewhat better distortion performance than the Fluke 510A 
>and which, thanks to a very tight AGC loop, has good amplitude 
>stability, providing the DC supply is adequately stable over temp 
>and metal film resistors and polypropylene caps are used. Again, a 
>simple oven would be helpful to hold the circuits to within a couple 
>degrees C. I don't know yet how good this can be -- much fun lies ahead.

That's it, eventually, fun!  But, per NIST, there isn't an AC 
standard in the US or the world, and its currently an active research field.


However, the research here really goes back further, to the early 2000s


AC is a complex bag, literally, since AC can be seen as 2 standards 
already in existence, that of frequency and of amplitude, whichever 
atomic clock that is, caesium or rubidium, and the JJ volt 
respectively.  However, what little I read of AC, a key need for 
standards related to reducing the source of AC per frequency compared 
to its current level, particularly compared to DC: here's the 
practical result in Relative Expanded
Uncertainty .


>Dick Moore
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Marv Gozum
Philadelphia, PA  

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