Large spurious in data obtained from USRP2+DBSRX

Hi all,

I am implementing a GPS receiver based on the USRP2 with a DBSRX
daughterboard equipped, whereas the software is only using the URSP2 C++
interface, the rest is done without GNURADIO.
The receiver works quite well, but I noticed that whatever I do I get
really huge spurious signals in the obtained spectra. In order to make
clear what I mean, I am attaching a plot of direct sampling at 1.0 GHz
with a decimation of 4 and gain=1. There is no signal going in the URSP2
so what you should see is something like instrumental noise.
Nevertheless, there are these huge spurious signals. These spurious
signals don’t change their amplitude nor their relative position w.r.t.
the center frequency independent of which RF I choose.

Is this a problem of the DBSRX or is it related to the FPGA, firmware,

Any comments highly appreciated.

Best regards,
Thomas H.

Dr. Thomas H.
Space-Time Measurement Project
Space-Time Standards Group
New Generation Network Research Center
National Institute of Information and Communications Technology

4-2-1 Nukui-Kitamachi, Koganei
184-8795 Tokyo