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

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Problem with capture playout WBX daughterboard USRP N210
Date: Tue, 23 Aug 2011 15:12:26 -0400
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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.

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