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Hello Al,

I'm not completely satisfied with my last experiments :
1- I've tested the USB dongle alone (without the upconverter) at the same levels as before, and the positive result is the stray lines doesn't come from the converter.
2- Reducing the generator level makes disappear these parasitics lines.
3- Turning the generator OFF, and closing the input antenna of the dongle with a 50 ohms plug, and 0dB gain, but with 0KHz offset makes reappear the stray lines with a higher level and regularly at each 250KHz !
4- Putting 1KHz to 200KHz for the offset solve the problem and suppress these lines ...

I have also tested the dongle with SDR# and with a high level signal there is also parasitic lines like AM side bands.

I think you are right, the problem should come from an overloading of the USB tuner.

Could you explain what the offset does?
Has it an effect to the tuner bandwith?
I have read the manual, and honestly I didn't understood very well this function.