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AC RMS DMM tests
mawyatt:
--- Quote from: alm on January 16, 2022, 12:17:31 am ---
--- Quote from: mawyatt on January 15, 2022, 10:34:42 pm ---A simple thought experiment will show this is thinking is flawed. If you consider slowing the edges down, the extreme case shows the eventual waveform will approach a triangle wave which we all know has an RMS value of 1/(sqrt(3)) not unity as the normalized squarewave. That's a significant reduction in RMS value and shows a strong relationship between RMS and edge slowing the edges down reduces the squarewave harmonic content which reduces the energy available from the waveform and thus reducing the RMS reading.
--- End quote ---
What's the problem? Say you built a circuit to produce a triangle wave with an amplitude that's very close to a DC level Vdc. You can compare the RMS reading of the triangle wave to Vdc/sqrt(3). Why is this worse than comparing the reading to Vdc/1?
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Sure you can use a triangle, however you'll need a very accurate bipolar reference, good low offset high gain comparator, precision integrator with good capacitor (low dielectric absorption, leakage), good switches, and so on, and you'll need to change R or C values for different frequencies.
Can you be sure your comparator is switching at the precise peak, equal switching delays going up and down, linearity in both directions, all guaranteed by design?? With the squarewave just measure the DC voltage, that's it, no precision parts required, just some common CMOS logic. No problems with drift, offsets, waveform linearity and so on. As an added benefit the AC RMS and DC Average are identical, no other waveform has this feature I'm aware of.
One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!!
Best,
SilverSolder:
Kind of cool! 8)
Just thinking out loud and imagining a little here: One could try measuring signals with a duty cycle other than 50% with this kind of precise amplitude and see how the meters deal with it... so, for example, the error at e.g. 1% PWM might tell you something about how a particular meter responds to fast edges, which could then possibly be subtracted out from the 50% PWM test later... hmmm...
mawyatt:
--- Quote from: alm on January 16, 2022, 12:17:31 am ---That makes a lot of sense when testing instruments made for wide-band signals like a scope. I'm not sure if it's the most realistic test for DMMs. Generally the configuration of a DMM, like filter settings and reading rates, are tuned to measure signals with a particular, known, frequency and quite limited bandwidth.
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Think you are missing the point of using the squarewave, never said it was the most realistic or best test, just simply the easiest to implement and verify. Doesn't cost much either :-+
Also mentioned much earlier it's not a replacement for a proper RMS source, but certainly has been handy for our use :-+
Regarding particular frequencies DMMs are "tuned" too, this would be the mains and maybe a few harmonics that come to mind for obvious reasons. Regarding reading rates, some of the newer DMMs like the KS34465A and DMM6500 seem to have higher digitizing rates, although the old HP3458 also had high digitizing rates I believe.
Best,
mawyatt:
--- Quote from: SilverSolder on January 16, 2022, 01:06:15 am ---
Kind of cool! 8)
Just thinking out loud and imagining a little here: One could try measuring signals with a duty cycle other than 50% with this kind of precise amplitude and see how the meters deal with it... so, for example, the error at e.g. 1% PWM might tell you something about how a particular meter responds to fast edges, which could then possibly be subtracted out from the 50% PWM test later... hmmm...
--- End quote ---
You could easily create waveforms with 25% and 75% duty cycles with some simple logic. A little math should show what the True RMS values and average DC values should be.
Edit: If you like this stuff here's another discussion about using this CMOS FF concept.
https://www.eevblog.com/forum/projects/fun-circuit-to-play-with/msg3109636/#msg3109636
Best,
David Hess:
I have gotten similarly good results using precision square waves for verifying the performance of and calibrating the average AC and RMS AC measurements of my various meters to their accuracy limit. A precision sine wave source would be better of course, but the coat has to be cut to fit the cloth.
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