This is about the numbers reported by programs such as HRD and others. Let me see if I have this correct. 1. The S/N measures the difference between the signal power and the (average?) noise power with the noise power as the reference? 2. The IMD measures the difference between the signal power and harmonics power with the signal power as reference? 3. IF the harmonics power is, in fact, below the noise power, can the programs that report IMD mistakenly assume that the noise power is the harmonics power because the noise masks the harmonics? If so, when the IMD is really -40 dB but the S/N is 20 db, the IMD number reported by these programs should be about the same as the S/N give or take a few dB and the IMD number reported is, in fact, meaningless. Unless, of course, the S/N is 60 or 80 dB or more.