Products > Test Equipment
Which DMMs can reasonably measure RMS?
W6EL:
--- Quote from: joeqsmith on April 26, 2023, 12:20:21 am ---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.
--- End quote ---
This is great, what fun!
Yes, there is something attractive about the older stuff, I don't even know what it is. At work we have scopes that cost more than my house, but I just don't find it as fun.
Was your square wave with or without DC bias? I am seeing that without DC bias my meters agree much better.
--E
rf-loop:
--- Quote from: W6EL on April 25, 2023, 05:36:08 pm ---I was surprised the other day when I tried measuring the RMS value of a rather standard TTL square wave on several "True RMS" meters.
The waveform was 0 to ~4.1 volts, 50% duty cycle. I ran the test at several frequencies (100Hz, 2 KHz, 200KHz) just to make sure. Every meter I had basically failed to even get close to the correct RMS value, which should be ~2.90 volts (Vpp * sqrt(D)). The only gear I have which measured it correctly was my HP 54645D scope (not really surprising).
I figured a TTL waveform has to be one of the more common waveforms people would measure using a handheld meter. After all, TTL appears in many low-speed digital circuits. I wouldn't expect to measure more complex waves accurately or higher-speed waveforms (especially with a handheld unit), but come on!
Of particular disappointment is the more modern Owon meters, which surely could take a few readings and do an actual calculation? One would think? Even the scope mode on the HDS272S (a great handheld scope) lacks an RMS readout, providing only Vpp, Vax, Vmin, and Vamp (which reads the same value as Vpp).
The HP 400EL is excused of course, since it is calibrated to read RMS only for a sine wave (like many analog meters of its time). Somewhat ironically it provided a closer measurement than most of the other equipment though.
I get that RMS requires a calculation. But come on. How much trouble is it to take a few consecutive samples at 5 bits resolution and do the calculation? Surely modern meters can do such a thing?
Does anyone know of a good meter that would pass the TTL test? Maybe one of the EEVBlog models? Agilent? Fluke?
Here are my results at 2 KHz. I have measured using both DC and AC since this is a fully-positive signal:
* HP 4645D scope: 3.035V RMS, 4.094Vpp
* HP-400EL: 2.22V (3V scale used, read 74% of full scale)
* Tenma 72-410A True RMS: 2.224 (AC), 2.289 (DC)
* Owon B35T TrueRMS meter: 2.076 (AC), 2.278 (DC)
* RadioShack TrueRMS meter (can't find the model): 2.064 (AC), 2.278 (DC)
* Owon HDS272S: 1.761 (AC), 2.281 (DC)
I'm quite disappointed. I had thought the so-called "True RMS" would be a bit closer than this. How do the expensive meters stack up against my hobby-lot?
--- End quote ---
Brymen BM859S (europa version)
Input
2kHz 4.10 / 0.00V square wave (no need tell 50% because if it is not 50% then it is not square wave).
Brymen display RMS 2.8932 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.
alm:
--- 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.
joeqsmith:
--- Quote from: W6EL on April 26, 2023, 04:30:45 am ---
--- Quote from: joeqsmith on April 26, 2023, 12:20:21 am ---...
Anyway, I setup my arb with a 50ohm load. Peak was measured with my HP34401A at 4.9155 or roughly 3.476 VRMS.
...
--- End quote ---
...
Was your square wave with or without DC bias? I am seeing that without DC bias my meters agree much better.
...
--- End quote ---
In the lower left of the spreadsheet is the reference. It was a squareish sort of waveform with a 50% dutycycle, 0 volt minimum and 4.9155 volt peak. The calculated RMS is last. The arb was loaded to 50 ohms and the peak level was measured with my old HP.
That old thermal RMS meter was set to normal mode when making these measurements. Nothing was allowed to warmup. The Arb isn't clean. Simple test wasn't meant to as a dive down the metrology hole.
bdunham7:
--- Quote from: rf-loop on April 26, 2023, 05:55:26 am ---Brymen BM859S (europa version)
Input 2kHz 4.10 / 0.00V square wave (no need tell 50% because if it is not 50% then it is not square wave).
Brymen display RMS 2.8932 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 ---
You seem to get a little hung up on semantics and while you have one interpretation of the terms 'square wave' and AC vs DC RMS, others may think differently. You can argue that they are wrong or you can take care to clarify exactly what is meant. I prefer the latter approach, the former somehow seems small minded.
So, since your 859s is 'correctly' designed and since you state that any meter that doesn't display what I would call TRMS AC+DC is demented, which of these ranges did you select to get that reading?
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