Hello,
I have had got the possibility to make some measurements on a Geller SVR-T, which HighVoltage lent me recently.
I'd like to share stability measurements, I've done in comparison with my 3458A, NPLC100 (2sec aperture).
The room temperature during this 29h run was exceptionally stable, <= 0.2°C change.
It turns out, that the Geller SVR-T competes even with the LTZ1000 references in terms of noise ~ 400nV, standard deviations for 10min. transfer stability sigma = 0.06ppm , 2h sigma = 0.07ppm and 24h sigma = 0.1ppm.
Also see stability figures, noise ~ 200nV, and Allan deviation from a LTZ1000 in comparison.
(The room temperature was less stable, about 0.7°C)
These Allan diagrams were normalized to ppm.
I also did a rough, estimate measurement on the T.C. of the Geller SVR-T, see my usual beer box setup.
I heated the whole box to about 31°C, from originally 20.4°C, measured by a thermo couple and a pyrometer, on the surface of the reference aluminium box, after temperature stabilization.
The voltage deviation vs. time graph reveals several effects concerning temperature change and lagging, but clearly demonstrates the very low T.C. of around -0.1ppm/K over a 10°C range .. that also competes very nicely with the LTZ reference.
Concerning the absolute long term stability:
I got a poor-mans-calibration from quarks at about September 2013.
His gear (actually his 8508A) had a brand new calibration, and my 3458A was 0.3ppm low compared to his instrument.
Since 5 years, I monitor the drift of my 3458A against two external LTZ1000, and one 5442A, and the overall drift between all these four references is <= +/- 1.5ppm over the whole time period.
Therefore, this recent measurement of HighVoltages SVR-T should be within 2ppm absolute uncertainty, as estimated.
This also is feasible, as it turns out, that the SVR-T is measured to 10.0000223V (+/-100nV) , only 2ppm off from the intended calibration value from Joe Geller, 2 years ago.
I measured no bigger differences than 1µV to that value, but I omitted to measure the warm-up behaviour.
HighVoltage promised to present comparative measurements with his new 34470A, which fall really very well within assumed limits. (Can't be accidentally)
As a resumée, the Geller SVR-T is really a very stable reference, very precise and also long term stable.. A pity that Joe Geller went out of business.
Frank