[volt-nuts] JJ standards and the 3458A

Tom Knox actast at hotmail.com
Mon Jan 20 13:08:31 EST 2014

As I said in an earlier post it does not require calibration, but set up and operation are beyond the difficulty of most equipments  calibration procedure. But great advances have been made as can be seen in Dave's quote below.
“The only problem with those (Past JJ's) is that they require an expert PhD 
physicist to operate,” Rudman says. “We wanted a standard that was 
considerably easier to use. If dissemination of the new, automated 
system proceeds as envisioned, then within our lifetimes there will no 
longer be a need for voltage transfer standards that have to be shipped 
off periodically for re-calibration. We can make primary standards, 
programmable from 0 to 10 volts, which are simple and cheap enough that 
every lab can have one. This is real now.” 
This "Now" you see in the photos is actually an AC JJ that will provide variable DC and AC signals.  



Thomas Knox

> Date: Mon, 20 Jan 2014 08:00:57 +0100
> From: frank.stellmach at freenet.de
> To: volt-nuts at febo.com
> Subject: [volt-nuts] JJ standards and the 3458A
> Hello,
> JJ standards are always primary ones, i.e. don't need to be calibrated.
> Preconditiion is only, that the setup is deigned correctly, mainly that 
> the JJ voltage on cryongenic level is transferred to room temperature 
> level with high accuracy.
> The JJ voltage is uncertain to 10e-17 on cryogenic level, i.e. on 
> comparing/transferring from one JJ to another by means of a SQUID.
> Transfer to RT is limited by thermal voltages to around 10e-9 uncertainty.
> That's also the practical limit in the real, macroscopic world.
> A 9.5 digit macroscopic or analogusly made DMM is unimaginable due to 
> following reasons:
> 1. Stability of Zener references are limited to around 0.1ppm in time, 
> temperature and noise
> 2. Same goes for the analogue range amplifiers, based on resistors.
> 3. The 3458A still has got the best linear A/D, around 0.02ppm of input.
> 4. Range transfer to the 5 Cardinal Points 1kV, 100V, 10V, 1V, 100mV is 
> also limited to around 0.2 - 0.5ppm (see Fluke 752A)
> A JJ array could not be designed into an analogue DMM, due to the fact, 
> that the setup needs cryogenic devices as dewars, and lHe4 or lN2. 
> Microwave generator, frequency and constant current source can be made 
> compact, as there already exist "portable" JJ array setups for 
> comparison between primary labs. (Those comparisons always were precise 
> and limited to 10e-9 uncertainty).
> Anyhow, as the Volt still is uncertain to about 2*10e-7 in the SI 
> system, a 9.5 digit DMM currently makes no sense at all.
> Let's see, if the BIPM will decide on the new SI by end of this year or 
> in 2 / 4 years only.
> Then, I would propose to build a 9 digit DMM, but based exclusively on 
> cryogenic electronics, i.e. a JJ array as an ultra linear D/A and as a 
> primary Volt standard in one.
> The only unsolved problem then would again be the range transfer, which 
> is not yet possible by cryogenic standards, afaik.
> Similar limitations apply also to an quatum Ohm standrad / DMM, as the 
> Hall effect standard requires several Teslas of magnetic field, and 
> cryogenics, which cannot be built into an analoguos DMM, and also the 
> Ohm transfer to analogue resistors is again limited to 10e-9 by thermal 
> voltages.
> Frank
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