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RMS measurement error with square wave
taydin:
Hello guys. I'm generating a square wave from my signal genarator that is 100 Hz and 2 Vpp. The square wave is bipolar and the positive and negative peaks have the same magnitude. Then I attach this signal to my scope and make the following three measurements:
1) Duty cycle is 10%
2) Duty cycle is 50%
3) Duty cycle is 90%
And I'm measuring 1 Vrms at the scope. No problems so far.
Then I do the same three measurements with my Keysight U1282A true rms multimeter. With 50% duty cycle, the measurement is bang on. But with both 10% and 90%, the measurement is about 0.6 Vrms. With 20% and 80%, the measurement is about 0.8 Vrms. So what gives?
PS: Please don't get hung up on the usage of the "duty cycle" here, I'm just trying to explain the relative duration of the positive part and the negative part of the square wave.
taydin:
I have tried this with other multimeters I have and all exhibit about the same measurement error.
Kleinstein:
A square wave with other than 50% duty cycle comtains a DC part. For the RMS readings there is sometime a confusion in showing only the AC part or the combined AC and DC value. So both readings can be correct just different things. 1 V für AC +DC and 0.6 V for the AC coupled part. For DMMs it is normal to show the AC coupled RMS, so without the DC.
macboy:
Your scope is set to DC coupled (which is fine) but the multimeters are all using AC coupling. Set the scope to AC coupled input and look at what the RMS measurements become in that case. Note that the waveform shifts with repect to ground because it isn't equal positive vs negative duration.
Some higher end multimeters can use DC coupled input when measuring AC (also sometimes called AC+DC mode). This should give more accurate results in this case.
taydin:
Thanks a lot for the responses guys. This makes perfect sense. I vaguely remember that the U1282A can handle AC+DC, but not sure in what context. Time to read up on the manual.
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