Re: [Discuss-gnuradio] Carrier leakage on transmit

[Matt Ettus]

On 05/25/2010 07:40 AM, Charles Brain wrote:
> Well I have had another look at the offset stuff.
>
> What seems to be happening is that when you apply an offset you have to
> scale back the max IQ values being sent to the USRP2.
>
> I get this situation.
>
> No LO offset, great raised cosine shaped QPSK signal
> Add LO offset, QPSK signal looks awful.
>
> Reduce IQ signal amplitude QPSK signal again looks good and I can see
> the LO offset on the spectrum analyser.
>
> I have also noticed that when I try OFDM the unwanted rubbish on either
> side of my OFDM signal changes with LO offset.
>
> There seems to be an interaction in there somewhere probably
> to do with the complex mixer in the FPGA. I have not ruled out my own
> code yet but it seems unlikely.


When there is no LO offset, your I signals passes straight through, as 
does the Q.  When there is LO offset, the cordic is rotating them, and 
when both are high, and the rotation is 45/135/225/315 degrees, the peak 
of the signal is now sqrt(2) times as big.

To look at it another way, your QPSK signal is being generated on the 
corners of a square in the IQ plane.  When you rotate it (which only 
happens with LO offset), the corners can now go to higher values, like 
when it is rotated 45 degrees and the points align with an axis.

Matt