[USRP-users] Can not remove the receiving spike (DC component) by advanced tune request at USRP N210.

Alex Zhang cingular.alex at gmail.com
Mon Oct 1 02:59:27 EDT 2012


Hi,

I always get a strong spike in the attachment, when using uhd_fft to
measure the noise. Please note there is no any signal on the air, only
noise, but I got this spike.
I guess this is the so called DC component. Thus I tried to remove this
spike, as stated in
http://files.ettus.com/uhd_docs/manual/html/general.html#tuning-notes,
using the advanced tuning request.

My sample rate is 2Ms, so as my understanding, I need to set the lo_offset
as >= 4MHz, to get the spike out my band, like this:

        tune_req = uhd.tune_request(options.freq, 5e6)   # with lo_offset =
5MHz
        usrp.set_center_freq(tune_req, 0)

Then I get the tune result as:

Tune Result:
    Target RF  Freq: 2405.000000 (MHz)
    Actual RF  Freq: 2405.000000 (MHz)
    Target DSP Freq: 5.000000 (MHz)
    Actual DSP Freq: 5.000000 (MHz)

However, the spike is still there, almost the same, and the center
frequency is even still the same (this could be due to the some GUI usage)!
I really don't know if the advanced tuning works or not. It seems that the
incoming streaming still contains the DC component.

Spike of the received noise.


[image: Inline image 1]
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