Why would my true RMS multimeter give me a different AC voltage reading than my scope at frequencies above 2 kHz?

Question

Here’s the circuit and my problem. My function generator is simply parallel connected to a multimeter and my scope. No load is connected and I am not using a scope probe, just a BNC connector with alligator clips. Although I did try it with an 8 ohm precision resistor load connected in parallel to the scope, multimeter and function generator as well, but with no difference.

I was just checking to see if the reading on my scope for AC voltage is the same as the AC voltage reading on my multimeter. My multimeter is true RMS. The scope is calibrated and set to read AC RMS voltage. The scope has a 1 megohm input impedance. The function generator has a 50 ohm output impedance. I would think that the scope’s input impedance is transparent to the function generator and multimeter because it’s so high - so I don't believe that is the problem. I don’t think it’s reactance either. What’s weird is I can get the multimeter and scope to agree on AC RMS voltages between 500 Hz and about 2 kHz. Above 2 kHz the AC RMS voltage on the scope goes way below the AC RMS voltage on the multimeter, which would lead me to think that reactance is playing a part of this. But the scope only has 13 pF of input capacitance - so I looked at the capacitive reactance based on the frequencies and voltage I am using and I don't see that the readings should differ, so I don't think that's the problem either.

I set the function generator to put out 2 V of AC (as read by my multimeter) at 500 Hz (as entered on the function generator). Then I read the AC VRMS voltage on the scope and it matches the multimeter up to about 2 kHz. If the multimeter reads 2 V AC RMS then the scope reads 2 V AC RMS. Above 2 kHz, the multimeter always reads higher voltages than does the scope but never the same or by the same amount. Same thing happens if I crank up the amplitude of the function generator. The readings are always different above 2 kHz but never by the same amount. So I tried a different multimeter and get the same results.

Shouldn’t the multimeter and scope give me the same AC RMS voltage readings (within 10%) regardless of frequency? What is going on?

Answer 1

If your signal isn't sinusoidal, you need to consider the harmonic content of the highest frequency component.

You might also have issue with either the RMS chip inside your DMM or your scope not able to find the period properly. I'm not enough into design of RMS chip to know when it would happen, but in scope it can happen if your amplitude is too low or if your timebase is too slow. It can also happen if the amplitude of your signal is small compared to the DC offset.

Providing your instruments model number could be useful. And has you may or may not know, the self cal of an oscilloscope should be done about every month to ensure accuracy. If you could also provide the signal shape, and the readings of both units with it's set point could be useful. Finally, a screen shot could tell a lot too.

Answer 2

Figured it out. The DMM only reads AC accurately to 1 kHz!