Products > Test Equipment
Measuring RMS voltage for PWM-like signals at 150 kHz
siealex:
--- Quote from: gnuarm on August 08, 2023, 05:55:58 am ---
If the Vmax and Vpp values are correct, I can tell you the Vrms will be greater than 3.0V. Vpp - Vmax = ~3.0V. Ignoring the brief time in the transition and the duty cycle, the minimum possible Vrms would be 3.04V and the max possible would be 6.7V.
--- End quote ---
3.0 V would kill a VFD designed for 2.4 V +/- 10% in a couple of hours. And it operates perfectly. How?
bdunham7:
--- Quote from: siealex on August 08, 2023, 11:20:27 pm ---3.0 V would kill a VFD designed for 2.4 V +/- 10% in a couple of hours. And it operates perfectly. How?
--- End quote ---
Are you assuming that it would burn out that quickly or have you done actual tests? If you are convinced that there has to be a discrepancy somewhere, tell us exactly how you are measuring this voltage, which I presume and hope is directly across the VFD.
As for theories, perhaps the VFD filament is a bit inductive and would burn out on 3.3VDC, but at this frequency the current is a bit less. That's a stretch, but I can't think of anything else offhand.
bdunham7:
--- Quote from: Someone on August 08, 2023, 10:47:17 pm ---Quantifying the accuracy is the important part here, as the data sheets for multimeters put the limit somewhere (for example 1% for a 200kHz sine wave) but they may well produce acceptable results beyond those "limits" particularly if the user accepts reduced performance or manual calibration/comparison.
--- End quote ---
True, but that's a tricky area and requires specific characterization. For example you might find that a meter has better-than-advertised half-scale CF capability at half of specified BW and better than advertised sine wave BW, say 1% at twice the specified maximum. However, if you tried to measure something with both of those over spec you it might not work well due to slew rate limitations since slew rate is one possible limiting factor for both parameters. Also, meters that use the AD63x and AD73x TRMS converter chips seem to cover up incorporate its various nonlinearities, slew rate and other limitations into one single accuracy specification. It might seem better than advertised on some signals, but that spec is there because they know full well it won't be as good on others.
I think the best and most obvious solution for most users is simply a decent 10-bit or 12-bit DSO. If you calibrate it with a sine wave, you can be pretty sure it isn't going to suffer from any of the issues that a TRMS converter chip has and will give you a reasonable result as long as you keep the signal on screen and watch the gating, sample rate, etc.
gnuarm:
--- Quote from: bdunham7 on August 08, 2023, 08:36:44 pm ---
--- Quote from: gnuarm on August 08, 2023, 06:01:20 pm ---Nice experiment. You took seven measurements and got seven wrong answers.
--- End quote ---
How 'right' do you expect them to be? The meters that are specified to handle this signal got it right, the others took a swing with varying results. The ones with suboptimal results simply don't have the BW. The OP's question was "Can any "true RMS" multimeters measure..." I think I've shown two that can, some that sort-of almost can and some that can't.
You could calculate the RMS from what I specified but you'd still have some small error due to the AWG not being perfect. Or, if you like, I can pick a different waveform that will be easier to calculate. I think 1% is good enough for the OP's purpose and I'm confident the two large meters are within that. And the scope is right there as well, within reasonable expectations anyhow.
--- End quote ---
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.
bdunham7:
--- 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.
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.
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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