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Measuring RMS voltage for PWM-like signals at 150 kHz
gnuarm:
--- Quote from: bdunham7 on August 09, 2023, 02:12:37 am ---
--- Quote from: gnuarm on August 09, 2023, 01:19:24 am ---None these measurements are close to the actual value. You can piecewise integrate this by hand to find a number that is much closer to 4V. The voltage is around -3V for more than half the time, then spends over 20% of the time at over 6V. Given the square involved, this 20% will be a large part of the result, pushing it to closer to 4V.
That's why all the measurements are wrong.
--- End quote ---
Are you referring to the OP's waveform or my example? Clearly I can't measure the OP's signal from here.... If you are using the displayed values of -3.04V and +6.70V, then you're not reading the scope properly--the top is less than 6.00V and the bottom is just a bit less than -2.00V. Those peak numbers are noise and overshoot. I think his scope display is about right but I didn't set out to prove that.
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Ok, you got me. I made a stupid assumption you were trying to approximate the OP's waveform, so I used his values. With -2V for the low end, I suppose you would get a much lower value. But to what end?
--- Quote ---Now if you doubt my measurements on my test signal, I have a simpler example with easy math. Instead of replicating the long fall time, I just used a top of 6.00V, a bottom of -2.00V, a duty cycle of 25% and rise/fall times of 8.4ns--the minimum for the AWG. So if you ignore the rise/fall altogether, you'll get an RMS value of √12, or about 3.464V.
--- End quote ---
Why -2 and 6, instead of values more like the OP's? What does your emulation tell us?
--- Quote ---In fact, the 8506A measured that as 3.438xxx volts and the scope measured it as 3.45xxxx volts. The scope seems to be about 0.4% higher than the meter on all signals including sine if I engage the 20MHz BW limiter, which seems reasonable to me. So I'm pretty sure the actual voltage in this case is 3.438 and the 0.7% discrepancy between that and the calculated value is about what you get from the seemingly miniscule 8.4ns rise/fall times.
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I don't know what "case" you are talking about.
bdunham7:
--- Quote from: gnuarm on August 09, 2023, 03:55:30 am ---Why -2 and 6, instead of values more like the OP's? What does your emulation tell us?
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I picked -2 and 6 because the OP's values are close to -2 and 6. My emulation tells us which multimeters are capable of accurately measuring such a signal and which ones don't do as well. That was the OP's actual question. To do that I generated a signal with a more or less known RMS value and compared.
--- Quote ---I don't know what "case" you are talking about.
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The case where I simplified the RMS calculations by eliminating the 1µs fall time. With this signal it can be easily calculated that the actual RMS value is very close to √12. It's still similar enough to the OP's waveform that it would be valid for determining what type of meters could accurately measure this type of signal.
sonpul:
On one of the forums I saw a discussion of this issue. In it, someone made a simulation in Micro-Cap by digitizing the graph. The calculation showed that the green RMS line tends to 3.34V RMS.
gnuarm:
--- Quote from: bdunham7 on August 09, 2023, 04:11:16 am ---
--- Quote from: gnuarm on August 09, 2023, 03:55:30 am ---Why -2 and 6, instead of values more like the OP's? What does your emulation tell us?
--- End quote ---
I picked -2 and 6 because the OP's values are close to -2 and 6.
--- End quote ---
Ok, looking at it more closely, I realize the scope reported values were pretty far off. So -2V and 6V are better estimates.
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