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Hello Al,

Thanks for your comments and clarifications, and spending time about it; today I have better results and perhaps what causes my problem :
Maybe overloading the USB stick (mine has a R820T) triggers the stray spikes but for me this is not the only cause, I've noticed that adjusting the offset to 0KHz causes lines each 250KHz on the whole swept band, and precisely snapped on the grid (125.000MHz, 125.250, 125.500, 125.750 etc...).

Adjusting the offset to about 200KHz, I've tested these following functions with good results, and reduced the spikes down up to the dongle's noise floor (-48dB) :
- Increasing the time Dwell >131ms
- Increasing the FFT >4000-8000
- Increasing the PSD Overlap (different to 0, about 33%)
- Changing the window function

That makes me think it could be a kind of resonance or oscillation, not electrical but "mathematical" like a repetition in a serie, but where, in the program, in one digital component? I can't explain that, I have not enough knowledge, but maybe a pinch of random or of negative feedback in the calculations could solve that.