[volt-nuts] AC calibration

[Dave M]

Adrian,
Do you have a link or title for the NIST paper that you mentioned?

Dave M


acbern at 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" <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.
>> 
>> Cheers,
>> 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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Dave M