Yes, the 3458A can be returned to Agilent to have the Option 002 high stability reference installed. However, in many cases the standard reference can obtain the same stability when it remains powered on continuously. Before ordering the option, contact Agilent at gem_support@agilent.com and provide the serial number of your instrument. Also if possible (but not required), include any measurement data that characterizes the performance of the standard reference
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Stability is the only advantage. The parts are the same burn in time and selection are the only differences between the parts. If you have an older meter that has been on all the time there is a good chance you have reached 4 ppm stability already. Track your cal var 2 value from calibration to calibration and see how much it is changing (cal? 2 enter) This will only take a few years to get a good idea of how much it drifts.
*if you are really interested in low drift, keep your meter on 24/7/365*
*'if you are really interested in low drift, keep your meter on 24/7/365'*
Sorry, that statement is not necessarily correct!
First, the environmental conditions for the 8/4ppm/yr. drift of the 3458A is not specified at all!
It's not said, whether this drift will be achieved in 24/7/365 mode only, or if it's also allowed to switch it off. Only in Service Note 18B, there is a hint, that the voltage reference might show hysteresis or might forget its conditioning, if left unpowered for an extended period of time.
The LTZ1000A inside the 3458A is running on about 90-95°C. Linear Technology intended it to be run on 65°C, giving about 1-2ppm/yr. drift typically.
This 30°C higher temperature has two effects: First, the ageing is about doubled every 10°C. (Ref.: Spreadbury, Pickering) This leads to 8 times higher drift rates (=>8 - 16ppm/yr.!) than intended. Spreadbury also showed, that unpowered LTZ1000 references do not drift at all, nearly.
Therefore, a lower temperature of the LTZ1000A would improve the drift rate, by reducing the oven temperature. Powering down for most of the time will definitely decrease the annual drift.
Second, the LTZ1000 shows a hysteretic effect, if it is operated at about > 40°C above room temperature. See also Pickering patent for removing that hysteresis by thermal cycling.
In this case, an LTZ1000A, which runs on 95°C and has been pre-conditioned at that temperature @ HP / Keysight factory, will show a creeping effect. The hysteresis will slowly,e.g. over months, recover towards the room temperature value of the LTZ. Some references will do that quickly, other more slowly. That's the background of SN 18B!
After powering up again, the LTZ might need more time to stabilize to the original value. That can be either a few hours only, or 6 weeks, as described in the SN18B. Obviously, agilent had a very sensitive batch of references at that time.
But that hysteresis / recovery effect should normally be on sub ppm level.
I have measured the warm-up time of several LTZs, and all returned to their initial value within 0.2ppm in 3-6 minutes.
My own 3458A (65°C oven) obviously shows neither hysteretic effects, nor excessive drift, (its drift is nearly zero), although I always power it down after measurements, to save energy, and also to avoid consuming unnecessarily its MTBF.
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