[volt-nuts] AC calibration
acbern at gmx.de
acbern at gmx.de
Mon Aug 25 15:05:51 EDT 2014
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.
> Gesendet: Montag, 25. August 2014 um 18:38 Uhr
> Von: "Dave M" <dgminala at mediacombb.net>
> An: "Discussion of precise voltage measurement" <volt-nuts at 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.
> Dave M
> pa4tim at 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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