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

[Alex Zhang]

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.