Why does an AC Voltage reading on a DMM decrease as the frequency is increased?

Question

I'm doing an assignment for a lab and I've found that frequency doesn't affect the voltage reading of an oscilloscope while it does for a DMM. Online, it says this is because an oscilloscope displays peak voltage instead of RMS voltage like the DMM. But even then I don't understand why RMS voltage reading would decrease with higher frequency on a DMM. From my understanding, finding the voltage RMS on a sine function would be to divide the peak voltage by root square of 2. So if the frequency increases, then theoretically the DMM reading should stay the same since the peak voltage doesn't change. Can someone explain to me how this works? Thank you.

Answer 1

The oscilliscope is designed to display a wide range of frequencies accurately - from DC to 50 MHz or more, while a common DMM will usually only cover a much smaller frequency range - mine shows the AC accuracy as +/-1.5% from 50 Hz to 500 Hz.

There are specialized meters that will cover wider frequency ranges.

Answer 2

On a cheap meter the signal is AC-coupled, then rectified using a precision rectifier and then low-pass filtered yielding an average voltage, which is then (for display) (effectively) multiplied by ~1.11 = (\$\frac{\pi}{2\sqrt{2}})\$.

The precision rectifier will have a bandwidth associated with it so you'd expect it to roll off at the high end. Yours appears to show some peaking, which is also not too surprising. It should also roll off at the low end for most meters (due to the AC coupling), well below mains frequency.

Most such meters are designed to measure mains frequency reasonably accurately (given sine wave input) and perhaps up to aircraft/military 400Hz (because it's easy) and not much else.

For example, a typical (and decent, but for low voltages/currents only due to low-cost fusing) hobby meter is the Unit-T M890G which has specified performance:

Frequency Response: 40Hz - 200Hz (40Hz-100Hz for the 200V and 700V range).

A more expensive meter such as the 34401A has true-RMS and is reasonably accurate up to 100kHz, which allows accurate measurement of lower-frequency periodic signals that are far from sinusoidal (such as audio) as well as high-frequency sinusoidal signals.