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Re: [Discuss-gnuradio] USRP inserts a peak in the carrier frequency

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] USRP inserts a peak in the carrier frequency
Date: Sun, 15 Apr 2012 10:27:07 -0400
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On 15/04/12 09:27 AM, frankist wrote:
> Now I have another problem related to peaks in the fft plot, but in this
> case, with no transmitter.
> Why does a peak appears at the center of the fft plot for some frequencies
> and not for the others. I mean, I am using a USRP2 to receive just noise and
> plot its fft and when I center my receiver in the frequencies 5.0GHz, 5.5
> GHz and 6.0 GHz I have no peaks but for the other frequencies in the same
> band I do have them.
> In this case it is not related to LO leakage since I can't get rid of them
> and I have no transmitter. Also, the peak always appears at the center of
> the plot
Direct conversion receivers suffer from the so-called "DC anomaly" --
spectral features caused by imbalances and 
  LO leakage in the analog mixer.  LO leakage will contribute to a small
DC offset appearing in the signals coming
  out of the mixer, which leads to a "bogus" spectral feature around
0Hz.  The USRP-family FPGAs have a
  DC-offset removal algorithm in them, but it isn't perfect, and the
degree of imperfection will change with
  center frequency. This "feature" is usually very narrowband in nature,
and for many types of modulations,
  it is merely a distraction rather than than a disaster.  You can use
offset-tuning on reception to arrange for
  the DC-anomaly to appear outside your passband.

I've attached a small flow-graph that allows you to see the effects of
DC offset, and phase/amplitude imbalance
  in a complex-sampled system, using a 10Khz pure signal with additive


Principal Investigator
Shirleys Bay Radio Astronomy Consortium

Attachment: iq_balance.grc
Description: application/gnuradio-grc

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