Mehdi,
I think this is a spur from the real-time mode. Sweep mode lets us choose optimal LO frequencies, but real-time mode does not. This can result in additional low-level spurs, like this -77 dBc spur you found.
If you suspect a spur in real-time mode, set your span to, say 20 MHz, and step your center frequency by 5 MHz. This will configure the LO for a new frequency and result in a different set of potential spurs. If the spur doesn’t move, it is much more likely to be real than if it does.
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