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The 50Hz and harmonics you're seeing might be a stray hum field from a transformer affecting the toroid core permeability a little - does your test enclosure have good magnetic field screening, I wonder ?
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I've not done any further tests but I think you may be right. Last night I did try turning off lots of things that were nearby but it achieved nothing. I don't think the enclosure was designed for this type of screening. It's interesting that the 50Hz and 150Hz pickup is so bad. Maybe the enclosure actually makes things worse somehow.
In SSA mode below 40MHz the analyser feeds the test signal direct to the ADC used for its digital IF. So there is no up/down conversion and no LO noise in the analyser apart from the master clock. Therefore, below 40MHz, the analyser's internal phase noise and spurious are minimal. So I can rule out the analyser as the culprit. I think it can measure down below -120dBc/Hz at just a 10Hz offset. This is the cleanest test signal I can give it. By a few hundred Hz offset the limit is the ADC/DSP and it seems to max out at about -136dBc/Hz.