Could you help me to clarify what leads the interference?

Hi all,
Currently, I connect the Signal generator with the LimeSDR USB dev.kit, The signal generator sent a sine carrier to the board with 1MHz offset for frequency. when the carrier frequency is below 1GHz, found the spectrum was very good. when the carrier frequency is obove 1GHz, found there was a fixed interference to the demodulation signal( pls refer to the screemshot). When I changed the ADC sampling frequency in CGEN tab, found the distance between the interference with demodulated signal. could you pls help me to check what reason leads the interference?

Thanks a lot!

Best Regards!