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Re: [Discuss-gnuradio] Carrier offset on DBSRX.

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Carrier offset on DBSRX.
Date: Tue, 27 Mar 2007 12:55:35 -0700
User-agent: Mutt/1.5.9i

On Tue, Mar 27, 2007 at 09:40:38PM +0200, Trond Danielsen wrote:
> 2007/3/27, Eric Blossom <address@hidden>:
> >On Tue, Mar 27, 2007 at 10:36:05AM +0200, Trond Danielsen wrote:
> >> Hi,
> >>
> >> I have a problem with my DBSRX daughterboard. I tried to send in a
> >> single carrier on 1.57542 GHz and watch it with usrp_fft.py, and I
> >> observe that the received signal is offset by many KHz. This really
> >> does not come as a big surprise, since ack. to the usrp_fft.py GUI the
> >> actual BB frequency is 1.5755 GHz. Is it not possible to tune the
> >> DBSRX to 1.57542 GHz? BTW: How is the C++ version of the dbsrx driver
> >> coming along? Does it contain any improvements over the python
> >> version?
> >>
> >> --
> >> Trond Danielsen
> >
> >Trond, I would venture that the offset of a many KHz is not because of
> >anything having to do with the reported BB frequency, but rather
> >because of the offset between the Tx and Rx clocks.
> >They are spec'd at 50 ppm.  At 1.5 GHz each clock could be off by
> >75kHz and they'd still be within spec.
> The input signal is generated by a Spirent GPS simulator, not by the
> USRP, so the carrier should be _very_ stable.

Yes, but there's still the clock on the USRP.

> >Also, I think you may be worrying about the wrong thing, when you see
> >the baseband frequency reported.  On both the Tx and Rx paths, tuning
> >is a two step processes.  Part of it is handled on the daughterboard,
> >and part of it is handled with a DDC or DUC.  The value reported in
> >usrp_fft.py is the frequency that the front end is tuned to.  The DDC
> >is programmed such that the frequency you asked for is actually at DC
> >in the complex baseband signal (post DDC).
> Okay, I think I understand. But still, when inspecting the signal with
> ursp_fft.py, the signal is not at baseband, but offset by approx.
> 500kHz

500 kHz seems excessive.  Can you post a link to a screen shot?


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