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Re: [Discuss-gnuradio] Tuning in uhd_fft.py

From: Josh Blum
Subject: Re: [Discuss-gnuradio] Tuning in uhd_fft.py
Date: Fri, 17 Aug 2012 10:44:41 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120714 Thunderbird/14.0

On 08/17/2012 05:09 AM, Sanat Gulvadi wrote:
> Greetings,
> How are the effects of DC mitigated in GNU radio? For example, if I use the
> uhd_fft.py to just scan the spectrum, I do not see any DC spike at the
> centre frequency. When I looked at the code, it has :
> r = self.u.set_center_freq(target_freq, 0)
> So I assume, it does not use the advanced tuning feature of UHD. Is this
> correct? How, in this case is there no DC spike seen at the centre
> frequency?
> I am trying to code a scanning system that goes through the whole frequency
> band of my RFX2400 dboard and at each center frequency, I grab a packet
> from the air and apply an FFT to check for possible interferers etc. But I
> am ending up with a spike at every centre frequency iteration. I have used
> the two stage advanced tuning to try and push the DC leakage out of my band
> of interest but I see no difference. I still get that single spike. (I am
> using the UHD library and not GNU Radio)

There is a DC removal in the USRP, but it needs some time to integrate.
Perhaps in the gnuradio case, you are looking at an FFT that has had
some time to integrate at the new frequency. But in your finite
acquisition case, there is no time.


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