Re: [volt-nuts] Precision current source
Marvin,
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.
Bill....WB6BNQ
"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.
http://vmetrix.home.comcast.net/~vmetrix/ZenerP.pdfhttp://vmetrix.home.comcast.net/~vmetrix/ZenerP.pdf
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:
http://assets.fluke.com/appnotes/Calibration/deaver_msc01.pdf
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
Philadelphia
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Hi Bill for the clarification but I answered nearly all of the
current reply already in the previous reply. Nevertheless, thanks
for your input.
At 07:42 PM 8/23/2010, WB6BNQ wrote:
Marvin,
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.
Bill, I would choose the LTZ-1000 over the old
Fluke 731B - because I've typically found the LTZ-1000 to be approximately
12
times more stable than the Fluke 731B on a monthly basis, and 7 times more
stable on a yearly basis. (The 731B spec = 10ppm/month and
30ppm/year.)
Cheers,
Greg
----- Original Message -----
From: "WB6BNQ" wb6bnq@cox.net
To: "Discussion of precise voltage measurement" volt-nuts@febo.com
Sent: Monday, August 23, 2010 5:42 PM
Subject: Re: [volt-nuts] Back to voltage was Re: Precision current source
---clip---
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.
---clip---