Only now have I seen the noise and SNR information on Simon's website. I found the algorithm for noise measurement very interesting, in the fantastic SDRConsole software. Now I understand why we don't have SNR = 0 in the absence of a signal.
In SDR # it is necessary to have at least a part of the tuning band in which only noise appears to have a correct indication of the SNR, as shown in the following figures. It seems that this software searches within the tuned band for the largest signal (the signal S) and the smallest signal, which is the noise N, to calculate SNR.
73s de Roland PY4ZBZ