[volt-nuts] Back to voltage was Re: Precision current source
wb6bnq at cox.net
Mon Aug 23 23:42:36 UTC 2010
I think you have me confused with the other responder as to the links as I did not provide any. The current
sourcing/sinking that I was referring to was in relation to the output stages of the paralleled references, not
that of the measuring circuit.
The Fluke 732 only has one reference. So, you would still need to have several of them and hope that whatever TC
they had cancelled each other out. However, the way they do the ovens, it sort of negates the effort. The only
reason to have more then one is to cross check that one of them has not gone bad.
I am not negative to the interests of a group effort regarding the Volt Nuts list. All I was trying to point out
is the amount of effort needed to do the job right. The main problem is the costs involved in actually finding a
number of reference devices (like the LTZ-1000) that have TCs and drifts in opposite pairings to accomplish
the paralleled process.
Just having a single LTZ-1000 is not the whole answer, you still need additional circuitry to make a complete
item. The LTZ-1000 is around a $50 (I think) cost and the correct resistors from Vishay are not cheap either.
Proper packaging is another issue. Plus the additional circuitry and a PC board and I suspect the total cost for
one fully constructed is near $200 and I have not factored in the power source to run it. A clean power source
with battery backup would be at least another $50 if done right. Even with all that it would not equal a Fluke 731
or the newer 732.
Then you still have to characterize each one built. It is doable, obviously, and the only other thing left is to
find someone who could get us a calibrated reference to have as a comparison for the project.
To understand the complexity, it is very worthwhile to study what Fluke did with all their ancillary circuitry in
the 732. They put a lot of effort in the basic power supply before even getting to the actual reference device.
It would also be worth looking at the Fluke 731 which would be closer in style if a LTZ-1000 approach were used.
If you are going to play with DC then you really should spend the money getting a Fluke 845 Null meter. There is
no other to compare to it. The portable battery operated version is MUCH preferred to the rack mount one. The
portable has a much higher isolation resistance from either terminal to the chassis.
Many on the Volt and Time nut lists have 5 to 8 digit voltmeters. To fully appreciate that kind of resolution and
accuracy demands a high quality reference. The 5 digit and lower meters could be serviced with a Fluke 515 which
has quite a good reference itself (and includes an AC mode).
However, all this provides is a DC reference and says nothing of AC. Resistance can be had for a reasonable
amount, but the AC is quite a bit harder to achieve.
"Marv Gozum @ JHN" wrote:
> Hello Bill,
> Thank you so much for these pearls and the details! I love the way
> you reference links. A good idea!
> I think the gist of your post is making a reference is going to be
> futile, because we, as individuals, do not have the resources to
> build a 732 like device, nor access to calibrate it against a
> Josephson Junction, JJ.
> I think a reasonable facsimile or approximation can be done, its why
> I've joined this forum, to find others who have. Read on.
> But a short answer to the futility question is simple, buy a used
> 732, put in back into working order, and find a lab willing to
> calibrate it against their NIST traceable references, and now,
> maintain your 732. Now you have a reference well characterized by
> Fluke on the assumption, the rebuild has put it back into like
> original factory condition.
> On your anecdote, while temperature drift is a concern, its minimized
> by ovens in the 732 to keep a stable temperature for the reference,
> regardless of ambient. I do not know if more than one voltage
> reference chip is used inside each Fluke 732 or any equivalent solid
> state reference or others matching - or + tempco references, but if
> you can find out, I'd love to read of it. Its common to use
> resistors with such characteristics, where they are applied.
> What is typically done to most solid state references to improve
> their performance is apply a correction factor to the output every so
> many months to fix drift, and this can be done on any device based on
> a single reference once its characterized over time. Here is such a
> process. Its another freely available paper that was published in an
> IEEE proceeds but copied here, but there are others like it.
> Historical data from Fluke show aged mature 732 model A trends +/- 1
> 1ppm over a decade, so net over time its stable. Some of this data is
> abstracted in the 732 series user and maintenance manual available
> from Fluke's website, and it gives the correction equation. There
> may be more recent ones, but this one is free, for those interested:
> Regardless, a lot of circuit manipulation and other controls has not
> made solid state references as good as a JJ, over 40+ years of using
> zener based references. This conclusion led to the development of
> the portable JJ now being studied as improved transfer reference.
> A question for the user is really, how accurate is your need: e.g.
> 300, 30, 3, 0.3 ppm? For most professional production MetCal uses,
> the Fluke 732s [ or equivalent] suffices, and for others not. For
> most users, a NIST traceable cal is acceptable, for others one needs
> a _direct_ cal from NIST itself.
> Thus, if a less stringent 30 ppm or more suffices for one's need, why
> won't something like Geller's board work, and thus save the owner a
> lot of money? However, it must be characterized just like Fluke does
> for the 732. Empirically, such a home brew could rival a more
> expensive standard up to a ppm per time period, limited circumstances
> that a Fluke 732 cannot contend with given its intended use thus
> requiring the extra electronics, housings, redundancy and overall,
> cost. Such circumstances may not even be related to stabilizing the
> volt, such as passing UL, CSA, IEC safety requirements, which add to
> the cost of the device.
> Thus, what is missing in this noble and learned group's archived
> discussions [ and brought up in later post by Greg ] is collecting
> performance measurements of any device designed and used over time to
> ascertain stability, regardless of how the standard is constructed,
> be it as robust as the Fluke 732 or as spartan as a Geller
> SVR. Thus, one needs to consider basic statistics on the output of
> these devices if one is to maintain them as a local reference.
> I think electronic engineering uses the metric "reliability" to
> reflect stability, which is the combination of a devices' accuracy
> and precision _over_ time.
> Looking forward to a stimulating and enlightened discussion! Onward,
> to a home based volt reference on the cheap!
> Some minor comments below.
> At 07:26 PM 8/22/2010, WB6BNQ wrote:
> > Hi Marvin,
> > I have a friend, Art Rizzi, who was responsible for the Navys DC
> > voltage reference. He worked at the Navys version of NBS at the North
> > Island Naval Air Station, San Diego, CA. His lab was also responsible
> > for approval and acceptance of lesser standards used though out the
> > Navy.
> > In 1970, his lab received around 100 Fluke 731 voltage standards.
> > Though out 1970 and 1971 he spent considerable time characterizing
> > these devices. One aspect, performance over temperature, was quite
> > important because but for his environmentally controlled laboratory,
> > the rest of the Navy saw quite a variation in temperature and humidity.
> > Art was the original developer of the idea of paralleling multiple
> > Fluke 731 voltage standards to account for temperature coefficients.
> > The basic idea is to take multiple units that had the correct mixture
> > of temperature coefficients such that they reduce the effects of
> > temperature variation as much as possible (within limits of course).
> > Arts efforts, along with other improvements he suggested, were the
> > basis of, and incorporated into, the development of Flukes 732 voltage
> > standard.
> > Clearly, each of the voltage standards needs an output stage that can
> > sink and source a given amount of current (a few milliamps) without
> > affecting its internal reference or the temperature of an individual
> > standards internal environment. Ground paths within this ensemble are
> > very important.
> Yes, but generally when testing a voltage reference or standard, its
> vital the DUT draw near zero amps. A null voltmeter is preferred,
> but a very high impedance voltmeter can suffice. So such capacities
> to sink/source are not usually necessary of a reference, long term
> stability is.
> > Creating such an ensemble for an experimenter is not an easy task to
> > accomplish. Not withstanding needing expensive test equipment like
> > precise temperature and a very stable known reference to compare to, a
> > highly controlled lab environment and an environmental chamber are also
> > needed. Then you need a few hundred voltage references to, hopefully,
> > find the ones that will accomplish the right mixture. Finally you will
> > need 6 months to a year or more to do the work ! Did I mention money ?
> You certainly need time to gather data, but it needn't be expensive,
> or as involved as you suggest. For over 20 years, the references
> used inside high end DVM like HP 3458A or 3456A are typically just an
> LM399H or an LTZ1000 in a separate board very much like Geller's but,
> like wine, chosen, aged and characterized, will work without special
> environmental controls. It will work better with environmental
> controls, but its not necessary given such errors are already
> accounted for when calculating the frequency of a device's calibration cycle.
> > However, to answer your question, NONE of the above does anything for
> > stability. All it does is help to correct for temperature variation,
> > within limits of course.
> Reliability, as a synonym for stability, is subject to many
> variables, temp is just one of them. Humidity, and pressure also
> affect solid state references and have been studied too.
> Thus, the reliability calculation and variability takes all of these
> variables into account as a phenomenon, assuming that time exposes
> the DUT to all variations the variables can throw at it in the
> measurement cycle, say 90 days, to 1 year.
> The same can be done for something as cheap, and as exposed as a
> Geller reference.
> > Stability is an entirely different animal ! Many factors control
> > stability, some you can deal with and others you have no control over,
> > like the manufacturing processes. So, you buy the best diodes or
> > reference devices you can afford and hope for the best. You can take
> > it to the bank that Fluke and HP spend much time and money in producing
> > their top of the line products.
> > Bill....WB6BNQ
> What I see others perceive is that looking for a single absolute
> reference, that is solid as rock, 1.0180000000000 or 10.0000000000
> etc., with ~ zero variability over time. Even the JJ is not
> that. JJ is simply better than zener, but it is subject to
> uncertainty too, now ~ 1 pp billion, versus a good zener at 0.1 ppm.
> So, why not 1 ppm or 10 ppm, or 30 ppm, as you funds or need allow.
> Best Wishes,
> Marv Gozum
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