Re: [Discuss-gnuradio] Problem with capture playout WBX daughterboard USRP N210

[Marcus D. Leech]

On 23/08/2011 3:49 PM, Bastien Auneau wrote:
> Thanks again for the answer
> Posting on both Discuss Gnuradio and USRP User. So far never got answer
> from USRP User list... So I stick on Gnu Radio
>
> Here is a screenshot of the the FFT. You can see some of the small
> bursty carriers on the right, but also this huge peak in the middle.
>
> We also transmitted some data on a different frequency that what we
> capture, to be sure this peak is not due to echo of the tx to the rx.
>
> Best Regards
My suspicion would be DC offset.  UHD has facilities for offsetting away 
from the LO so that your signal of interest is well away from DC.

This is an inherent "problem" in direct-conversion receivers.  There 
will always be *some* unwanted LO leakage, which always cause
  some amount of DC offset, not all of which can be cancelled.  For 
wide-band signals, you can often simply ignore it, since it constitutes
 such a small fraction of your overall signal.  Or you can high-pass 
your signals, or notch it out, or you can shift the LO.  It's typically very
  narrow, and whether it appears at all is dependent on a number of 
factors, including degree of LO leakage, and amplitude and
  phase balance of the analog mixers at the frequency of interest.

When you construct a tune_request in your UHD-only C++ code, you can 
specify the desired LO offset.  Consult the UHD Doxygen
  documents for more details.