Forum: GNU Radio unexpected aliasing tx output

Hello All,
I am trying to transmit a random sequence from one USRP N210 using the
basic TX daughterboard at 40Mhz IF to an external rf frontend. When I 
look
at the output of the basic TX on a spectrum analyzer (before any 
external
filtering, etc), I see a very weird copy at 20MHz. I would expect to see
one at 60MHz, which you do, however the copy at 20MHz is very strange. 
This
doesnt happen when I use tx_waveforms.cpp, where I just see a spike at
whatever frequency I'm transmitting at. To transmit random numbers, I'm 
am
essentially using exactly the same tx_streamer setup as tx_waveforms, 
the
only major difference is here:
       for (size_t n = 0; n < buff_tx.size(); ++n){

buff_tx[n] = std::complex<float>((float)rand()/(float)RAND_MAX,
(float)rand()/(float)RAND_MAX);//pilot_source(n);
//fill buffer with random complex floats

        }
I just use a random generator to fill the tx buffer. Everything else is 
the
same as tx_waveforms. I use set_tx_rate(2e6), though I get similar 
results
with 4e6, 5e6 and 6.25e6.
Is there any reason that this copy is appearing?
Thank You,