On Tue, Mar 27, 2007 at 09:40:38PM +0200, Trond Danielsen wrote: > 2007/3/27, Eric Blossom <[EMAIL PROTECTED]>: > >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? Eric _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
