Dumb Ass with a scope

[easilyconfused]

"Hmm, I'm a bit baffled too. I assume the probes are where they're supposed to be if you're getting an AC voltage reading, but just to be sure, you have one lead on scope ground and one lead on the cal bar?"

I put the red mm lead on the scope's output (cal) lug. I put the black mm lead on the scope's ground lug. The voltage displayed in the auto-ranging DC voltage mode was 1.48. I switched to AC voltage mode and got a value about 1.52v or so. Nobody ever told me that the calibration voltage would coincide with the multimeter. I just assumed it would. And I still don't understand why it doesn't. Maybe it's got something to do with the pulsations so that I'm only seeing half of it.

[alm]

A good DMM will read the RMS value, which is the root of the mean of the squares, this is derived from the equation for  power (V²/R for voltage over a resistor), Wikipedia (link above) has the proper derivation. But for a square wave, the RMS and average value are actually identical, so for this purpose, you might as well use the average voltage. For a square wave from 0 to Vpp with a 50% duty cycle, the average voltage is Vpp/2. So ~1.5V RMS/average corresponds with 3Vpp. I assume that you use a 10x scope probe, which attenuates the signal ten times (voltage divider, 9Mohm / 1Mohm). So the 3Vpp becomes 300mVpp.

An average responding meter will multiply the average with 1.1 to correct for the difference between RMS and average for sinusoidal signals, but many average responding meters won't have the bandwidth for 1kHz square wave anyway (something like 10kHz might be OK, many cheap ones are just 50-500Hz), so I wouldn't trust their answers, although the results from your Fluke 17B (which is not true RMS if I remember correctly) seem plausible in this case, maybe the frequency roll-off and +10% correction cancel each other out.

[easilyconfused]

A good DMM will read the RMS value, which is the root of the mean of the squares, this is derived from the equation for  power (V²/R for voltage over a resistor), Wikipedia (link above) has the proper derivation. But for a square wave, the RMS and average value are actually identical, so for this purpose, you might as well use the average voltage. For a square wave from 0 to Vpp with a 50% duty cycle, the average voltage is Vpp/2. So ~1.5V RMS/average corresponds with 3Vpp. I assume that you use a 10x scope probe, which attenuates the signal ten times (voltage divider, 9Mohm / 1Mohm). So the 3Vpp becomes 300mVpp.

An average responding meter will multiply the average with 1.1 to correct for the difference between RMS and average for sinusoidal signals, but many average responding meters won't have the bandwidth for 1kHz square wave anyway (something like 10kHz might be OK, many cheap ones are just 50-500Hz), so I wouldn't trust their answers, although the results from your Fluke 17B (which is not true RMS if I remember correctly) seem plausible in this case, maybe the frequency roll-off and +10% correction cancel each other out.

Even thought 99% of what you've said is over my head I think I get the gist of it. And I think the gist of it is that the 0.148 volts I'm reading on my Fluke non-RMS is one half of the pulsating signal. If that's true- there is some diagnostic value in doing it as one of several steps in verifying the cal on your newly acquired garage sale scope. PS: I'm not sure but I suspect I've given that value of 0.148 with the decimal in the wrong place in parts of this string. 0.148 is correct and the lug value is said to be 0.3v.

[EEVblog]

"Hmm, I'm a bit baffled too. I assume the probes are where they're supposed to be if you're getting an AC voltage reading, but just to be sure, you have one lead on scope ground and one lead on the cal bar?"

I put the red mm lead on the scope's output (cal) lug. I put the black mm lead on the scope's ground lug. The voltage displayed in the auto-ranging DC voltage mode was 1.48. I switched to AC voltage mode and got a value about 1.52v or so. Nobody ever told me that the calibration voltage would coincide with the multimeter. I just assumed it would. And I still don't understand why it doesn't. Maybe it's got something to do with the pulsations so that I'm only seeing half of it.

As I said, multimeters can ONLY read RMS or Average values for AC waveforms which his what you are looking at. That is how they are designed.
If you want to measure the peak-to-peak value of a signal, like you just did with the calibration signal, you have to use an oscilloscope so you can actually see the waveform.
i suggest you go and study the difference between RMS, average, and peak-to-peak waveforms.

So trying to use the multimeter to measure the calibration signal was wrong in this case, it can't do it.

Dave.

[alm]

Even thought 99% of what you've said is over my head I think I get the gist of it. And I think the gist of it is that the 0.148 volts I'm reading on my Fluke non-RMS is one half of the pulsating signal. If that's true- there is some diagnostic value in doing it as one of several steps in verifying the cal on your newly acquired garage sale scope. PS: I'm not sure but I suspect I've given that value of 0.148 with the decimal in the wrong place in parts of this string. 0.148 is correct and the lug value is said to be 0.3v.

My guess about the 10x probe was wrong, you're either using a 1x probe or a plain piece of wire. The manual says 0.3V, your picture shows 0.3V, and your meter shows half that. Everything looks fine to me.

Just forget all the RMS stuff for a second. Go draw a square wave from 0V to 0.3V (i.e. it's 0V for the same amount of time as it's 0.3V) on a piece of graph paper. Use one box for 50mV (so the square wave is six boxes high), and make each horizontal line two boxes wide. Now imagine what the average value would be (hint: a straight line exactly in the center between 0V and 0.3V). That's the 0.15V you would get if you would measure the average value, or would use a true-RMS meter. A non-true-RMS meter measures this value, and multiplies it by 1.1 so it will show the average value for sines. The actual math for RMS is more complex, but would give the same answer for a square wave.

Your meter has a specified frequency response of 50-200Hz, so measuring this kind of signal is way out of spec. So the fact that it's only 10% low is actually pretty good. You should only measure AC signals that are 50/60Hz and not heavily distorted with this type of meter, other signals may give inaccurate results.

[scrat]

Even thought 99% of what you've said is over my head I think I get the gist of it. And I think the gist of it is that the 0.148 volts I'm reading on my Fluke non-RMS is one half of the pulsating signal. If that's true- there is some diagnostic value in doing it as one of several steps in verifying the cal on your newly acquired garage sale scope. PS: I'm not sure but I suspect I've given that value of 0.148 with the decimal in the wrong place in parts of this string. 0.148 is correct and the lug value is said to be 0.3v.

My guess about the 10x probe was wrong, you're either using a 1x probe or a plain piece of wire. The manual says 0.3V, your picture shows 0.3V, and your meter shows half that. Everything looks fine to me.

Just forget all the RMS stuff for a second. Go draw a square wave from 0V to 0.3V (i.e. it's 0V for the same amount of time as it's 0.3V) on a piece of graph paper. Use one box for 50mV (so the square wave is six boxes high), and make each horizontal line two boxes wide. Now imagine what the average value would be (hint: a straight line exactly in the center between 0V and 0.3V). That's the 0.15V you would get if you would measure the average value, or would use a true-RMS meter. A non-true-RMS meter measures this value, and multiplies it by 1.1 so it will show the average value for sines. The actual math for RMS is more complex, but would give the same answer for a square wave.

Your meter has a specified frequency response of 50-200Hz, so measuring this kind of signal is way out of spec. So the fact that it's only 10% low is actually pretty good. You should only measure AC signals that are 50/60Hz and not heavily distorted with this type of meter, other signals may give inaccurate results.

I agree. I can't see the problem: on the scope, with your 10x probe, you measured 0.3 Vpp, which is as specified (and it means 3 Vpp at the cal output). When you measured it with the DMM, you got 1.48. If your DMM is a true-rms, you should measure 1.5 V (while you got 1.48 V), since this is the rms value for a 3 Vpp square wave. Isn't it OK for an old scope? How could it be anything different?

The first definition they gave me for rms value was "the value of a DC that will have on a resistance the same thermal effect as the considered signal". Even if I could complicate it by saying that "the rms value is the square root of the mean value of the square of the signal", and that you should calculate it with an integral, the old informal definition still works