Sensing behavior


Running I didn’t find some explications to the
algorithm behavior.
For example, I am using an usrp to transmit a 400Hz tone on 6 MHz.
With the other USRP I am sensing the band 4.5MHz to 13.5MHz.
I found the presence of signal at 6Mhz (the correct one) and also at
9MHz (a residual one).
Any idea what the 9MHz residual signal represents? It seems that the
9MHz signal
is not an aliasing problem. Any solution to remove it?

Thank you,