Hi Stan,
the 720A can easily be replaced by an HP3458A, which is superior to the
KV divider in several aspects.
First, the 3458A has 3-10 times better linearity, 0.02ppm (typ.) of F.S.
compared to 0.1ppm of input for the 720A.
Therefore, a 10:1 transfer is accurate to 1ppm for the 720A only,
whereas the 3458A manages 0.1 to 0.3 ppm.
The self calibration is much easier on the 3458A, as are all these
calibration measurements, you mentioned.
(Fluke has published a good application note, how to replace their own
old style equipment as KV, Null VM, etc. by their own 8 1/2 digit 8508A).
And you may get a very reasonable and recent instruments for 3000$/€,
maybe much less for older ones.
A calibrator is limited in use, as it needs a Null VM at least.
Anyhow, I recommend the Fluke 5440A / 5442A DCV calibrators.
They are ultra stable, having 2 stacked SZ263A references inside, good
for 732A stability.
Their D/A is also extremely linear, I measured something like 0.2ppm INL
against my 3458A, and they are spec'd to 0.5ppm of output, also superior
to the 720A in some volt areas.
They also have this handy autocal function (like the 3458A) for the 4
higher volt ranges ( 11, 22, 220, 1000V), once that they are externally
calibrated, and because their internal component drift is low, after
these years.
That means, as their internal reference is very stable, you may bring it
near 24hr. specification, every time you do the autocal.
I could not measure any deviation to that, using my 3458A, plus a self
built precision Hammon divider for 1kV.
Their autocal feature is not explicitely promoted, but you will find
that description "between the lines" in the addendum of the user manual.
Only the 1V and 100mV range have to be externally calibrated quite often.
These instruments may cost about 2000$/€.
So you better have both, the 5440 and the 3458A.
Frank
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