4wV has nothing to do with ratio. 4wV is to force the voltage on the "right" position for measuring.
Okay, I see what you mean now after reading a bit of manual. Enabled it now, restarted datalog. Don't see a difference yet.
He also told me that they use the 8508A as a transfer system between AC and DC, because it is that stable. Therefore, I'm not sure if the 3458A really beats the 8508A in AC.
For ACV we have more powerful contender - Wavetek 4920M. I'd like to see 8508A try to beat that one.
In DC, the 3458A might be more linear in the 10V range. But at least the 1000V range of the 8508A is much better. One have to keep in mind, that the 8508A don't rely on ACAL. Everything which needs to be stable in this meter is in a way stable, that it has (at least) the same specifications which the 3458A only reaches which ACAL.
3458A specs are very conservative. There are lot of speculations in sentences above, so I will surely be running comparison tests with all that to see actual data of this 8508 and four of my 8.5d meters (6581T does not deserve to be mentioned
), now that I have decent source.
If your 720A is still broken: Why not running some tests like the ones Dr. Frank showed here? Hook all meters on your calibrator, trigger all at the same time and run through the ranges.
I need 720A anyway for other purposes. Unlike 3458A, linearity of 720A I can at least calibrate out. I don't have friends (yet?) to get access for PJVS system to test linearity of 3458A directly.
I eagerly await the 8508A-teardown.
I'll await for that too, but it's not happening, xDevs labs ain't rich enough
1281 / 8508A are fully staffed with Vishay ultra stable resistors, where 3458A uses TaN arrays only.
After waiting gazillion weeks , I might just got some solution for that "problem":
Initial datalog.
3 hours from power on, voltage still climbing, +0.68ppm so far. Data before 9:50 was saved with lost precision due to snake bug, hence the steps.
Gap between 9:50 and 10.05pm is ACAL ALL on 3458B.