As the title says, I have a signal which is mostly DC but has some RF ripple on-top of it.
The signal comes from a pickup coil that receives a 13.56Mhz signal (RFID). That signal gets rectified by a high-speed diode bridge rectifier. This is followed by a leaky low-pass filter, e.g. a small capacitor and load resistor in parallel. (Note: I can't change any of this acquisition circuit).
Now I want to get an accurate measurement of the RMS voltage.
Currently I just hook it up to a digital oscilloscope and use the internal measurement functionality. This however gives slightly different readings depending on the time-base and voltage range selection.
Since I want to automate this measurement in my lab I thought about building a little box that does a more precise RMS to DC conversion in analog, so I can use a precision voltmeter to do the measurements. However, all the "true RMS to DC" chips that I've seen so far won't deal with signals of higher frequencies than a megaherz or so.
Any ideas how I could accomplish my goal?