I have posted a screenshot showing HDSDR tuned to a signal that is measuring exactly 3 dB S+N/N as measured with an RMS voltmeter at the audio ouput.
It is pretty interesting that this "minimum detectable signal" is a full 20 dB above the displayed noise floor.
The filter is set to 500 Hz. Display is set at RBW 2.9 Hz with a 128 sample average. FFT windowing is set to Hann and using Amplitude Spectrum display (all settings under "Options").
I believer if someone were to duplicate these settings and display any signal of a know level on a calibrated HDSDR, and subtract 20 dB from the displayed value (which, of course should be exactly as you know it, otherwise re-calibrate), the result would tell you how much that signal was above the MDS of your radio.
For those are interested to know their MDS and don't have attenutors, RMS voltmeters, and the like, this should allow you to do it.
Warren Allgyer
9V1TD
Warren Allgyer