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

From: frankist
Subject: Re: [Discuss-gnuradio] USRP inserts a peak in the carrier frequency
Date: Tue, 17 Apr 2012 00:55:29 -0700 (PDT)

Hi. I deleted my last message and created a new thread where I explain things
properly. I tried to do some offset tuning but the peaks didn't move so I
suppose it has another cause. Also, after using your file, it seems the iq
balance is something that appears in both sides of the spectrum (negative
and positive) for the same frequency which is not my case. But things are
better explained in my new thread. I've also put some plots there

Marcus D. Leech wrote:
> 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
> noise.
> -- 
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
> _______________________________________________
> Discuss-gnuradio mailing list
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> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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