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Actually, I wanted to attempt to calibrate the dB/Hz spectrum scale in dBm/Hz with a sine generator, measuring the Vpp output signal on a 50 Ohms resistor load and calculating the dBm power, I found that the spectrum scale is about dBm =dBscale -28dB (with 0dB of gain to have a reference).

Playing with the generator's attenuator (with two levels -16dBm -36dBm), shows that the relative dBLog10 scale works well.

Because I don't have a HF wattmeter to compare, this measurment is not very accurate, I don't know exactly where the 0dBm could be (I suppose it depends on the dongle and the upconverter gains).

I don't know whether -36dBm overload the USB tuner dongle, I haven't found any datas about it; since the upconverter has clipping input diodes I suppose that a 0.01Vpp should not overload it too much, if I'm not wrong it can accept +1dBm before distortion.

One thing is shure is that, except the harmonics, the generator has only one main sine signal, I tested it with my Sony ICFSW7600GR radio in SSB mode, which is selective and accurate enough to hear a parasitic signal.

Thanks for your advices, I will try to play with the "offset" next time, and I'll come back with the results.