On Fri, Feb 17, 2012 at 10:22 AM, Tom Rondeau <t...@trondeau.com> wrote:
> On Fri, Feb 17, 2012 at 2:52 AM, guelord ingala <gueloring...@yahoo.fr>wrote: > >> Hi, >> I've got 2 units of USRP1 and DBSRX dboards. The two USRP are >> synchronized for master and slave. the uhd_fft.py is responding correctly. >> Now I want to use a signal generator to provide 900 MHz. signal level is >> -20 dBm. From the gnuradio-companion, I created a flow graph using UHD-USRP >> Source connected to WX-GUI-FFT Sink. The setup are as follow: >> For USRP source: >> ID: uhd_usrp_source_0 >> Device Addr: fpga=usrp1_fpga_4rx.rbf >> Sync: Don'y sink >> Clock rate: default >> Num Mboards:1 >> Mbo Clock source: default >> Mbo Time source:default >> Mbo Subdevice Spec: >> Num Channel: 1 >> Sample rate (sps): 32K >> Cho Center freq: 900M >> Cho Gain:30 dB >> Cho Antenna: RXA >> Cho Bandwidth:10M >> >> For FFT Sink: >> Sample rate:32k >> Baseband freq:900M >> >> With this setup, only noise is plotted in the FFT. I don't see the signal >> at 900MHz. And even if I remove the daughterboard from the usrp, the result >> is unchanged: Noise only. >> Can you please tell me what is wrong with the setup, and what to do if I >> want to use the side B. Also which USRP must I use: the master, the slave >> or both. Please help. >> > > I think your sample rate is way too low for the USRP. When you run the > program, you should see a warning being printed that you tried to set the > sample rate to 32 kHz but the actual sample rate was something else. This > could cause some confusion in your setup. > > Also, make sure that you are going into the right antenna port on the USRP. > > Finally, a -20 dBm signal and 30 dB of gain in the USRP is a really loud > signal. You might be swamping your front end. I'd put the input signal down > to -50 dBm to start with and lower the gain. Up the gain slowly as > necessary. > One other potential issue here: if you're tuning the LO to 900 MHz, and injecting a tone at exactly 900 MHz from a signal generator, the signal downconverted signal would end up sitting exactly at DC, and may be removed by DC offset correction if it is enabled. Once you've widened up your sample rate a bit per Tom's suggestion (try something on the order of a couple of MHz, or whatever decimation ratio would get you to something in this range), try offsetting your input tone to 900.1 MHz and see if it shows up. John -- John Orlando CEO/System Architect Epiq Solutions http://www.epiq-solutions.com
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