Hello JM, The detected S/N is a non-linear function of the RF S/N in FM mode. The detected S/N is better than the RF S/N at high signal levels. The price one has to pay is a threshold below which detected S/N drops very fast. This is very useful for sensitivity measurement since the threshold is easily established. You migt for example look for the signal level that produces a detected S/N of 12 dB. When you look at the spectrum of the audio that might correspond to a level of the signal that would be 25 dB above the noise since you would measure S and N in a bandwidth of the bin resolution while one normally would measure N in the full bandwidth.
AM is also non-linear, but much less so while SSB is linear having the same S/N in the audio as in the RF signal. It is often better to talk about signal compared to the noise density. (dB vs dB/Hz or dBm vs dBm/Hz.) That is equivalent to normalizing the noise floor to 1 Hz bandwidth. Room temperature is -174 dBm/Hz. If you find that a signal level of for example -110 dBm gives S=-50dB while N=-100 dB/Hz you can conclude that a signal of -160 dBm would give S=-90 dB. An ideal (noise free receiver) would give S=N (in 1 Hz) at -174 dBm so in this case the receiver degrades by 14 dB. That means that the noise figure (NF) is 14 dB. Measuring NF has to be done on the RF spectrum or in SSB mode, but comparing the NF of different receivers is better done in FM mode. Regards Leif
