[time-nuts] Voltage standards

[Greg Burnett]

The Linear LTZ1000 is a pretty wonderful voltage reference, but they're not 
all created equal. It would be nice to pick the "cream of the crop" from a 
sample of 100 of them, but that would get expensive (and time consuming) in 
a hurry.

For use in their 3458A DMM, Agilent burns-in the LTZ1000 references and 
hand-selects the ones deemed good enough. The few, "cream of the crop" are 
set aside for use as high stability references in Option 02 3458A DMMs (spec 
= 4ppm/year) and the references not quite that good are set aside for the 
standard version 3458A DMMs (spec = 8ppm/year).

I've found that, after a few years use, the vast majority of standard 3458As 
easily meet the option 02's tighter 4ppm/year spec. In fact, I've seen a lot 
of 3458As drifting less than 2ppm/year, which is really good.

I'm guessing that the LTZ1000 can exhibit, perhaps, as much as 0.5 to 0.7 
ppm hysteresis when powered down and back up again, because I've 
occasionally observed shifts that large after power down/up and 
restabilization.

Greg