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Re: [Discuss-gnuradio] DC offset and down conversion problems

From: Josh Blum
Subject: Re: [Discuss-gnuradio] DC offset and down conversion problems
Date: Tue, 11 Sep 2012 10:27:24 -0700

On Mon, Sep 10, 2012 at 11:30 PM, Shih Oliver <address@hidden> wrote:
> Hi,
> I'm using the lfrx board and try to turn off the DC offset correction but it
> seems no change in the data collected.
> I'm wondering if I did it wrong and here's part of my code:
>        self.uhd_usrp_source_0 = uhd_usrp_source()and provided some sliders
>        self.uhd_usrp_source_0.set_auto_dc_offset(0, 0)

That should be the right call. Technically the first param should be a
bool, False. But Im pretty sure python figures it out ok.
virtual void set_auto_dc_offset(const bool enb, size_t chan = 0) = 0;

If you tune the center frequency to zero; in an FFT, there should be a
DC level once offset correction is turned off. Especially if you
remove the signal, it will just be a noise floor and small DC level.

> Also I am trying to do a down-conversion on the USRP source to
> get the I Q. So I set the center frequency on USRP source, but the result  I
> get is really weird.
> Is there anything else I should do or I just get it wrong.

So your signal is on some kind of IF frequency? Perhaps the IQ are
swapped or the frequency inverted.

With these kind of things I find it helpful to send in a test tone and
monitor the spectrum with a live FFT. You can make sliders and stuff
in the GRC to tweak the parameters while the fg runs.


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