James Hess

Makes sense. Thanks.

I've attached a screen shot from my scope that shows an example of what I'm seeing.

I am using a function generator to drive current through a helmholz coil. There is a receiver coil inside the helmholz coil, picking up the field. 
In the screenshot:
Channel 4 is the current into the helmholz coil
Channel 3 is the output of a field probe (1 mV / 1 mGauss)
Channel 1 is the output of the sense coil.

The sense coil signal has a lot of environmental noise on it. The channels are all bandwidth limited and the traces are averaged over several hundred waveforms. (Averaging is set to 512, but I am freezing the screen before that number is reached)

The scopes DVM says "Under Bandwidth". Interesting since the driving waveform is 10 kHz. I lower it to 9500 Hz, No change.

Notice that the DVM AC RMS reading and the channel 1 measurement reading do differ by a bit. Since they are using the exact same input signal, I would have expected them to be very close. But apparently the DVM and the MEASURE process the signal differently. I'm curios what the difference is.

I do have an external meter I added to the setup to be an arbitrator. It didn't help.

I'm using and MDO3024 scope. I am using the measurements function to make RMS measurements of several channels. I also am using the DVM AD RMS function to make measurements of one of the channels.

Under some conditions the DVM and the channel RMS mean agree. (Within 0.5%) Under other conditions they differ. (By 5% to 8%) I am also using an external DMM to make yet another measurement of that channel. It agrees with the DVM under all conditions.

What is the difference in how these measurements are made or calculated? What could account for the difference?