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Which DMMs can reasonably measure RMS?

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nctnico:

--- Quote from: W6EL on April 25, 2023, 05:48:24 pm ---
--- Quote from: nctnico on April 25, 2023, 05:43:45 pm ---You have to be very carefull when selecting a DMM that can measure odd waveforms and other frequencies than 50Hz. Most handheld meters are 50Hz only.

But you don't have to buy extremely expensive meters. Vici VC8145 is one that can measure RMS in the audio frequency range because it has a dedicated RMS converter chip inside.

--- End quote ---

Have you actually tested this with something other than a sine wave?

--- End quote ---
Yes. I can set my generator to RMS and the VC8145 tracks the level nicely. But it measures only the AC part of the signal.

Fungus:

--- Quote from: W6EL on April 25, 2023, 05:36:08 pm ---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.

--- End quote ---

It's a common signal but nobody measures TRMS of it.  :-DD

Stick to your oscilloscope for looking at TTL signals - shape is more important.

bdunham7:

--- Quote from: W6EL on April 25, 2023, 06:33:16 pm ---While I can understand RMS being different for AC and DC coupled measurements, the RMS calculation should be conceptually a differential measurement from Vmin to Vmax at duty cycle D.

What would an HP-3400 (a meter that converts the energy to thermal energy) read for a square wave about zero versus one with DC bias?

--- End quote ---

I'm not sure what you are trying to get at in the first statement.  The RMS value of a 0 to 4.1V 50% duty cycle square wave would be 2.05V AC and 2.90V AC+DC, or DC coupled, RMS.  In addition, the integrated DC value should also be 2.05VDC, meters not reading close to that on a 2kHz signal as you've described probably have terrrible normal mode rejection.  If you have a meter that does AC and DC, but not AC+DC (or DC-coupled RMS if you prefer) then you can measure DC and AC separately, square the results and add them and the square root of that sum is your AC+DC result.

A thermal transfer measuring instrument would measure the latter if it had an DC-coupled input, but the HP 3400A has an AC-coupled input with a 10Hz lower cutoff (or spec anyway, the cutoff may be lower) so I'd presume it would read the former.  I don't have one here at the moment to confirm that.  I do have a different thermal transfer meter, but there's no room on the bench for it at the moment.  I don't think it is necessary to go to that extreme for such a simple example, but it is probably the only meter I have that is going to read the TRMS of a 200kHz square wave with any accuracy.

Every TRMS meter I have will perform as I've stated and the only other limitation would be the bandwidth, obviously there will not be very many meters that will read a 200kHz square wave accurately.


--- Quote ---How do the expensive meters stack up against my hobby-lot?

--- End quote ---

Frankly yours are looking like rubbish!  :)

Seriously, they're just wrong.  Or something is wrong in any case.  If your signal is actually 0.00 to 4.10V and there isn't a loading issue, then only the HP 400EL is actually showing what it ought to with any reasonable accuracy.  Every other instrument is off by enough to be considered a fail (IMO) except maybe the scope which would need further discussion to conclude anything.

W6EL:

--- Quote from: Kleinstein on April 25, 2023, 06:10:48 pm ---The TTL signal is a mixed AC+DC signal.
For the RMS reading there are 2 ways to handle this:
1) have AC coupling and thus only show the AC part. For the 50% square wave this would be the same AC as DC reading (50% of peak voltage if the low voltage is at zero).
2) a combined DC+AC RMS values and thus the AC part + DC part as the geometric sum. In this case 1.41 * the DC reading.

A few meters offer both version and many meter offer only the AC coupled case. Usually the manul will tell.

So the Tenma, Owon B35T and radioshack meters don't look that bad.

Many handheld DMMs don't work to very high frequencies. So 2 kHz square wave can already be a bit on the fast side and thus a lower than expeced reading.

--- End quote ---

I read and re-read what you said. You were exactly correct. The DMM in AC mode is ac-coupled, and thus the result is given for the AC-coupled version of the signal.

Placing the scope's input in AC-coupled mode shows 2.045V RMS, which means that my list was sorted in reverse! The Owon B35T and radioshack meters were almost dead-on, and all others within about 10%. 

This makes a lot of sense. So to sum it up:
1. There was not a problem with bandwidth or high frequency cutoff
2. There was no issue with slewrate

The bottom line is that the DMM places the input in AC coupled mode, and thus, any result must be considered from that point of view. There are still some discrepancies, but they are much less severe once you consider the point of view of the measurement.

shakalnokturn:
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?

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