Products > Test Equipment
Which DMMs can reasonably measure RMS?
bdunham7:
What is the source of your signal? One possible explanation for the discrepancies that I see (discrepancies that seem fairly consistent, so perhaps I've maligned your meters unnecessarily) is that you have about a 135mVDC offset on the signal, so it is 0.135 to 4.235V.
bdunham7:
--- Quote from: shakalnokturn on April 25, 2023, 11:28:00 pm ---Sorry for my ignorance, I fail to see where the theoretical 2.9V RMS or the 3.035V RMS measured on the DSO come from.
--- End quote ---
50% duty cycle, so half the time it is zero, the other half 4.10V. So (4.1)2 is 16.81 (the squares), half of that is 8.405 (the mean), the square root of that is 2.90V (the root).
--- Quote ---How far off zero was the real low level?
--- End quote ---
I think that's the issue with the readings being off from what I'd expect, but I don't think it was the OPs primary question. As I wrote, it looks like a ~135mVDC offset would account for all of the errors except the AC readings of the Tenma and the OWON scope-thingy.
W6EL:
--- Quote from: shakalnokturn on April 25, 2023, 11:28:00 pm ---Interesting topic... It got me a little puzzled.
Sorry for my ignorance, I fail to see where the theoretical 2.9V RMS or the 3.035V RMS measured on the DSO come from.
Is this about a "0" to 4.1V amplitude 50% duty cycle square wave?
How far off zero was the real low level?
--- End quote ---
The source was single-ended and grounded at the scope's input, and zero was definitely zero. It's a test generator with a TTL output (among others).
And yes, it's 0 to about 4.1 volts.
The real RMS value is Vmax * sqrt(D) for a square wave, where D is the duty cycle, so basically 4.1 * sqrt(1/2) = 4.1*0.707 = 2.90 volts.
However, the DMM has a capacitor in series when it does AC readings. So the DMM is showing the RMS voltage for an AC coupled version of the input signal. This means it is measuring a square wave with a total amplitude peak to peak of 4.1 volts, centered at zero volts, with vmin = -4.1/2 = -2.05 and vmax = 4.1/2 = +2.05 volts. It's easy to visualize how the RMS is calculated from this data, and it's no wonder that the AC-coupled meters all reported values around 2 volts.
Placing the scope in ac-coupled mode put it at the same vantage point, and read about 2 volts as well.
The lesson here is to do tests just like this and learn more about your test equipment. Who would have thought a DMM from RadioShack purchased in 2002 would be that good, even at 200 KHz?
joeqsmith:
Welcome to the forum. Assuming that is your ham call sign, your photo of your lab looks nice. I had one of those 141Ts many years ago with the 1.3ish GHz plug in, and a couple others. I had the tracking generator for it as well.
Anyway, I setup my arb with a 50ohm load. Peak was measured with my HP34401A at 4.9155 or roughly 3.476 VRMS.
I measured seven different meters at 100Hz, 2kHz and 200kHz. Attached showing the measured values and their error relative to the 34401A.
The CEM is the lowest cost out of the group at $120 on sale. The Gossen Ultra was by far the most expensive, now rebranded as Prime after my review of it. The BM789 is an early pre-release and I have done a some rework to bring it up to the latest revision (using factory parts). The BM689s shown is the first one I purchased several years back. It was damaged during my testing and I did repair it. None of these meter have been realigned.
Once the UT181A with it's odd ball rechargeable battery gets a charge, I will measure it. Bad design but still one of my favorite products from UNIT.
***
The UT181A was allowed to charge to 40% and I retook the first measurement plus the others.
Because you seem to like old hardware, I tried an old Fluke 97 scope meter but like the Gossen, it could not read the value at 200kHz in DMM mode.
I saved an old Fluke 8506A Thermal RMS meter from the recycle bin that needed repairs. I aligned the DC stages using my HP34401A as a reference. The AC stages are still factory set as I don't have anything near this accurate and thought I would do more harm than good. For fun, I show the signal at 2MHz compared with the UT181A.
I'm not sure what accuracy you need but my personal pick of these meters is still the BM869s. I would take the Fluke 189 if they still offered them new. The 789 is a nice meter as well. Has a few things on the BM869s but I like the multi displays.
Also note the previous percentage was off 100X and has been corrected.
GigaJoe:
btw .. speaking of Tenma 72-410A and Owon B35T , such devices doesn't have dedicated RMS\DC converter like popular AD637.
so true RMS in some narrow brackets of frequency and AC waveform.
I'm guessin HDS272S , and RadioShack same story.
most basic dmm AC up 5kHz , if something add it would be 20K , maybe 50K, who claim 1% accuracy up to 100K usually dedicated RMC-DC converter.
like owon bt41+ has such dedicated chip
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