[Discuss-gnuradio] Aliasing on USRP N210

I am observing aliasing on the spectrum analyzer in the following simple
transmission scenario:
sampling freq=25M, carrier freq=12.5MHz, and a sine wave freq=-11MHz at
baseband.
I use the following command:
> uhd_siggen -s 25e6 -f 12.5e6 --sine -x -11e6

I expect to see a delta at freq=1.5MHz, but there is another one
freq=26.5MHz (see attached jpeg).

It occurs on LFTX or BasicTX doughterboards. When I move the sine wave
from -11MHz toward zero, the aliasing gradually disappears. It appears
again when moving up toward +10MHz.

Why does it happen? Can I avoid it?

Thanks in advance.