Adrian,
Do you have a link or title for the NIST paper that you mentioned?
Dave M
acb...@gmx.de wrote:
fred,
generally you raise a good point, I had the same issue of calibrating
an ac voltage to a high level of accuracy. you need this e.g. to
validate the self.cal of a 3458a or other precison stuff like the
8506a0.
what i would recommend to do if you want to keep costs down is:
in a nutshell, get a thermal converter in the lowest range you need
and a second one on range above. build a set of resistor range
extenders (rf type with appropriate connectors and housings) to
expand the range to where you need to be max. get one of the thermal
converter calibrated (the higher one usually, and you need to havr
good cal lab, should be <10ppm accuracy) and use it to calibrate the
rest. generally, up to 20khz, the accuracy is some 20 ppm anyway for
thermal converters! at higher frequencies, due to reflections and
stray capacitance/inductance influences, the accuracy decreases. the
resistor range extenders though, if build up correctly, only have a
few ppm impact (there is a paper from nist on that, but this is only
typical). you can calibrate all converters to the one you got
externally calibrated. do some research in the web, when you do the
calibration, you need to determine the so-called constant N. then do
an ac, dc+, ac, dc-, ac measurement between the the two and establish
the deviation, also establish the error propagation. the end result
will be a set of highly precise (low inaccuracies9 thermal converters
good enough to calibrate a 3458a an better devices. if you want to
spend the money, you could also buy a set of converters/range
resistors (with/without a 540), that typically is a few k altogether,
while a single device sometimes is available for below 100 bucks. you
need to have a stable 7.5 digit nanovoltmeter though for the
measurements of the tvcs (34420a or 2182 typically ) and precision
(stable) dc and ac sources. but in the end, all you need is a single
calibrated thermal converter.
adrian
Gesendet: Montag, 25. August 2014 um 18:38 Uhr
Von: "Dave M" <dgmin...@mediacombb.net>
An: "Discussion of precise voltage measurement" <volt-nuts@febo.com>
Betreff: Re: [volt-nuts] AC calibration
Well, you sort of answered your own question. The equipment is
called a Thermal Transfer Standard, but instead of thermistors, it
uses a thermocouple. Look at the manual for the Fluke 540B
(http://bama.edebris.com/manuals/fluke/540b/) and you'll see how
it's done. Basically, the AC source is input into the transfer
standard, and the standard's internal reference voltage is adjusted
for a null on the galvanometer. Leaving the reference voltage
setting alone, a DC voltage is input into the unit, and the DC
source is adjusted for a null on the galvanometer. At that point,
the AC voltage source is equal to that of the DC voltage source.
Ther are thermocouple-type thermal converters used for RF voltage
measurements with the transfer standard. They aren't cheap, and you
have to have a converter for each range of voltages that you need to
measure. The thermal converters used with this type of transfer
standard isn't great (50 MHz or so typical), but their accuracy far
surpasses that of the thermistor type sensors.
There are other brands and models of thermal transfer standards, but
I have a Fluke model 540 and a few thermal converters. That's why I
referred you to the manual for it.
Cheers,
Dave M
pa4...@gmail.com wrote:
Is there a way to link an AC voltage to a DC source for compare. I
can check my calibrators (like a Fluke 332, 760 , 731 and a Philips)
against standardcells. But for AC I can not do that. I have two
AC+DC TRMS 7,5 digit meters but the last calibration was 2 years
ago.
My idea is in theory simple. It is based on the thermal converters
used in RF powermeters. Two resistors, two high resolution
temperature meters. AC on the first en DC on the second. If both are
the same temperature the AC voltage is the same as the DC voltage.
But I'm sure some people here have done this in the past. I would
like to use it for 50 to 100 kHz (or less) and something like for
1V, 10V and 100V (and use several resistors/heaters.)
Or mabey there is an other way to convert AC (for RF it can be done
with lightbubs but I never tryed that) I do not mind if it is slow
etc, I like this sort of experiments. You can learn a lot from it.
Fred, pa4tim
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Dave M
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