I have a number of meters but the best of which is only a pair of 3457A type. I also have a pair of 3456a meters which are pretty stable at least compared to those I have but they are only 6.5 digit compared to the 3457A at 7.5 when reading with HIRES.
When I short the leads on the 3457s, after as many as 3K samples, the stdev on either is around 70E-9th. With everything in the room shut down (lights, TV, etc), I've seen it drop to 40E-9th. This is with NPLC 100 and the filter off reading and adding the HIRES register.
When I log any of my voltage standards, and they aren't all that great, I see around 20 to 30uV of noise in the plots. Stdev on these types of tests are usually 6.5E-6. Watching others with 3458s and high-end voltage standards, I usually note that they are plotting less than 10uV of noise and most times even a lot less on a 10V source.
I hooked both my scope with FFT and my 8568b spectrum analyzer with 85865a preselector and preamp up to the voltmeter while it was reading from a battery operated Geller voltage standard and I note that yes, I see 20 to 30uv of high frequency noise consistent with the local broadcast bands with peaks even higher, up to 70uV at 7Mhz for some reason. That particular spike has plagued me for years by the way and I can't find the source.
My question is, when people are measuring voltage standards using for instance a 3458a, are they also filtering the input? Averaging in any way? It looks like their plots are better by a factor of 10, but if the external noise is the culprit, I would think I'm not the only person with broadcast band noise in the 30uv range, no>?
My voltage standards are generally pretty stable long term. If I plot a trend line, they are relatively flat since my office is partially built into the side of a sloping hill and the weather is pretty constant in San Francisco. I might see it go as low as 67 or as high as 69 in my office.
The noise is what is bothering me. I am using shielded 213 cable with BNC to banana plug adapters. Even still, I see a lot of noise, again like I stated above, as much as 70uV across a wide band from 100Khz to 103.5Mhz. I doubt the 3457A is picking up noise much higher than 100khz though it will read HZ to 3M, I believe.
And thinking back to the stdev with the terminals shorted at 70E-9, whatever it is, the noise is coming in on the leads. I put a shorted 213 cable on the meter and the stdev goes back up to 5E-9 compared to a short right at the meter again at 70E-9.
In the plot below, I measured a Dial-A-Souce KVD type standard over 4000 seconds. In this case, I had the 3457A filter on at 20th degree. The trend line in excel is also 20th degree average. You can see the samples in light blue which are coming from the 3457A after its filter is applied. The more recent samples are at zero in this plot. After hooking it to the meter, it dropped from around 9.999847V to 9.999835V and then back up to stabilize at 9.999855V. This unit is new to me since Monday and is reading about 15uV low. It took about 30 minutes to warm up and then ran for 30 minutes. By the way, from points 191 to 291 I was out of the room and you can see it was pretty flat.
Are others seeing noise like this in their long term plots?