rigol DS1054Z wrong rms measurement on long waveform

[merlinb]

I just discovered this quirk when compensating my probes. When monitoring the Rigol's own square wave generator (0 to 3V) the RMS measurment reads 2.0V, not 1.5V! (It doesn't matter what the time base / scale is set to).

It is correct ..

I had to think about that for a while but I see you are right. So my complaint instead is that my 'true RMS' multimeters read 1.5V for the same waveform (on the DC volts setting). This bothers me. I suppose they are discarding the AC component of the waveform before measuring, although that's not really what I would expect from a notionally DC-RMS voltmeter measuring a unipolar waveform! 

[ebastler]

I had to think about that for a while but I see you are right. So my complaint instead is that my 'true RMS' multimeters read 1.5V for the same waveform (on the DC volts setting). This bothers me. I suppose they are discarding the AC component of the waveform before measuring, although that's not really what I would expect from a notionally DC-RMS voltmeter measuring a unipolar waveform! 

Well, the average DC voltage on that 0V-to-3V signal is 1.5V indeed. You can interpret it as an AC-only signal (with +- 1.5Vpp) on top of a 1.5V DC offset. For a multimeter, I would indeed expect it to give me the DC offset (i.e. average DC voltage) when I measure the signal in DC mode, and the RMS voltage of the AC-only component when I measure in AC mode. So your multimeter probably performs as it should.

The scope gives you more flexibility, as you can choose to measure either the average voltage or the RMS voltage of the "mixed" 0V-3V signal. The multimeter does not have these two separate settings, I assume, so it opts for the more commonly required readout, namely the average DC.

[2N3055]

I had to think about that for a while but I see you are right. So my complaint instead is that my 'true RMS' multimeters read 1.5V for the same waveform (on the DC volts setting). This bothers me. I suppose they are discarding the AC component of the waveform before measuring, although that's not really what I would expect from a notionally DC-RMS voltmeter measuring a unipolar waveform! 

Well, meters are usually made to show AC TRMS (^~_~), DC average(=) (to show dc component directly) and AC+DC TRMS (the real TRMS  (^~₌)). On Brymen 869 you have to cycle through the modes to see all the combinations (it has dual, not triple display). Larger graph screen meters (Uni-T UT181 for instance) can show all 3 at the same time, cutting down on confusion.

[Fungus]

I just discovered this quirk when compensating my probes. When monitoring the Rigol's own square wave generator (0 to 3V) the RMS measurment reads 2.0V, not 1.5V! (It doesn't matter what the time base / scale is set to).

It is correct ..

I had to think about that for a while but I see you are right. So my complaint instead is that my 'true RMS' multimeters read 1.5V for the same waveform (on the DC volts setting). This bothers me.

The Rigol can read that, too. Just switch to AC coupling for the channel.

hint: RMS isn't the same when a waveform is centered around zero as when it's offset.

[merlinb]

On Brymen 869 you have to cycle through the modes to see all the combinations (it has dual, not triple display). Larger graph screen meters (Uni-T UT181 for instance) can show all 3 at the same time, cutting down on confusion.

I did not know I could even do this, but you're right! My meter was DC (mean) by default, but there is a DC+AC option in the menu! I always expected the DC range on a true-RMS meter to do that by default but I guess that's not the case.