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Which DMMs can reasonably measure RMS?
gamalot:
Those multimeters that have AC+DC mode will do it.
vk6zgo:
--- Quote from: alm on April 26, 2023, 07:46:00 am ---
--- Quote from: rf-loop on April 26, 2023, 05:55:26 am ---And RMS is RMS and it naturally include also DC. 4.1V DC RMS is 4.1V. If some meter do not display 4.1V RMS for 4.1V DC then designer need doctor. Or some extra lesson for math.
--- End quote ---
Or maybe they had one lesson more than you and learnt that calculating the RMS of an AC coupled signal is still RMS. Would you say that if you enable a bandwidth limit on your scope, it's no longer measuring RMS? RMS just means the root of the mean of squared values. It can be for AC or AC+DC. Some meters are marked like that and can measure both. Other meters can only measure the AC part and you have to do the math yourself to add the DC part. And yet other meters can only measure AC+DC, and you need to do math to subtract DC to get the AC value. As long as meters are clearly marked as AC or AC+DC I don't see a problem.
--- End quote ---
AC coupling balances the signal either side of zero volts, so to all intents & purposes, an ac coupled 0-4.1v square wave becomes
a continuous voltage of 2.05 volts, as power doesn't care about polarity.
A "true RMS" meter should display it as 2.05 volts RMS.
A "non-true RMS" meter will display 0.7071 of peak, yielding 2.899 volts if DC coupled, or 1.496 volts if ac coupled------- if, of course, the correction factor does work for non-sinusoidal signals.
Kleinstein:
--- Quote from: shakalnokturn on April 27, 2023, 11:20:23 pm ---
--- Quote from: W6EL on April 27, 2023, 02:57:48 pm ---The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do.
--- End quote ---
Not having the vertical scale set as near as possible to ADC's full scale would add inaccuracy, I'm not sure what the optimal horizontal scaling would be for RMS measurement on a DSO although I'd suspect one full cycle and that it may vary from one scope to another.
--- End quote ---
Which data are used for the RMS calculation depends and cause some trouble. Some use all the data in the memory and some only the data on the screen. Multiple full periods may be less sensitive to a rest of a partial period.
Using averaging mode will remove some of the noise, but the noise in the signal is also part of the RMS value. So averaging (over multiple triggered sections) mode is more like a bad idea.
alm:
Many scopes (Tektronix and Lecroy scopes I have used, but also Siglent I believe) have a cycle RMS/cRMS option that just measures the RMS of a single cycle. This is less sensitive to partial cycles and horizontal scaling. For this you should adjust the horizontal scale for a bit more than one period of the signal.
2N3055:
--- Quote from: Kleinstein on April 28, 2023, 06:13:35 am ---
--- Quote from: shakalnokturn on April 27, 2023, 11:20:23 pm ---
--- Quote from: W6EL on April 27, 2023, 02:57:48 pm ---The scope has an RMS "measurement" you can bring up (among other things). I turned on 8x averaging as well. Yes, scopes are not exactly known for their vertical accuracy, but it's the best I can do.
--- End quote ---
Not having the vertical scale set as near as possible to ADC's full scale would add inaccuracy, I'm not sure what the optimal horizontal scaling would be for RMS measurement on a DSO although I'd suspect one full cycle and that it may vary from one scope to another.
--- End quote ---
Which data are used for the RMS calculation depends and cause some trouble. Some use all the data in the memory and some only the data on the screen. Multiple full periods may be less sensitive to a rest of a partial period.
Using averaging mode will remove some of the noise, but the noise in the signal is also part of the RMS value. So averaging (over multiple triggered sections) mode is more like a bad idea.
--- End quote ---
Many scopes calculate RMS on whole buffer or screen AND have Cycle RMS and Cycle Stdev (AC RMS), that will calculate for either one single full period OR each period on screen.
Problem with accuracy on RMS calculation on full screen is what gets into calculation and it is more complicated with nonperiodic signals.. If you set timebase so it chops off piece of period RMS is going to be wrong.. If you have signal with large crest factor (large pause and then short spike) you have to capture exactly from same point at beginning and end of one period so you have exactly one period.
This is where Cycle RMS is useful, or you use gating and select which part of the curve you are measuring. Also, if scope can do it, use measurement cursors connected to measurement so scope will show you what exactly it is calculating on...
Or you simply go with deep memory, and set timebase slower to put hundreds of periods on screen. That will push down errors caused by chopping the signal off at beginning and end of the screen. It's good when you pretty much can't see much of the signal and it looks like a continuous block... For nonperiodic signal (like switcher going into discontinuous mode) that is only way to get any real estimate...
As for statistics, some scopes calculate exactly one measurement per screen (usually leftmost on the screen) and calculate stats between triggers (repetitive captures) and some are capable to capture long capture and if you do Cycle RMS, it will calculate it for every cycle it detects, 10s of thousands at the time from single capture....
Both of those will give you good estimate in slightly different way...
Averaging the measurements is good idea, because it does average of measurements of full signal not the measurement of average of signal...
16bit and 12bit scopes will give you excellent RMS results, because they have wider BW than any multimeter and they show you what are you measuring... So you get data and understanding of signal.... Even decent 8bit scopes will give you good accuracy compared to average meters in RMS measurements...
That being said, all my meters do good job in their BW. AC+DC TRMS is reason why I like dual or triple display meters...
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