Marcus: Thank you for your reply. Let me ask you another question to make sure I'm understanding this correctly. Remember, I have a USRP2 board with a WBX daughterboard. If I construct a flow graph where a signal source block generates a 7 khz cosine at a sample rate of 200 khz (100e6/500), and if this cosine is fed into a USRP2 sink block with its "frequency" parameter set to "900M", then will I see a spike in the WBX output spectrum at 907 MHz, or at 893 MHz, or both? (assuming perfect precision of the internal reference clock on the USRP2) What if it were a sine instead of a cosine? Thanks again for your help. I really appreciate your time.
Steve McMahon --- On Wed, 11/3/10, Marcus D. Leech <[email protected]> wrote: > From: Marcus D. Leech <[email protected]> > Subject: Re: [Discuss-gnuradio] questions about USRP2 sink block and > upconversion > To: [email protected] > Date: Wednesday, November 3, 2010, 8:59 PM > On 11/03/2010 04:00 PM, Steve Mcmahon > wrote: > > Hello: > > > > I am still somewhat new to GNU Radio. I am running GNU > Radio 3.3.0 under openSUSE 11.2, and I have two USRP2 > boards, each with a WBX daughterboard. I need some help > understanding some fundamental things about GNU Radio and > the USRP2 and upconversion. > > > > I am trying generate a tone at 900.001 MHz > (900,001,000 Hz). I am using GRC to construct a simple flow > graph where I have a signal source block generating a 1 khz > cosine at a sample rate of 195.312 khz (=100e6/512), > connected to a USRP2 sink block with the "decimation" > parameter set to 512, and with the "frequency" parameter set > to 900M. I then look at the output on a spectrum analyzer. > My understanding was that I should see a clear spike at > 900.001 MHz, but I don't. Instead I see a peak at 899.99701 > MHz. What am I doing wrong? I'm using the internal USRP2 > clock. Is this happening because the internal clock is good > to only 7ppm? > > > There are two sources of error--one, as you've observed is > the precision > of the reference clock on > the USRP2. And the other is whatever residual > measurement error your > Spectrum Analyser has. > Synthesized LOs are only as good as the reference > clock, at least from > a frequency-precision > perspective. If you want to do better than > that, then you can use an > external 10MHz reference > clock, such as a GPS frequency standard. > > This is entirely normal for synthesized RF gear. > Measure just about any > commercial radio out there > with a precision measurement device, and there'll be > some residual > frequency error, unless you get > lucky. > > It's usual in telecom systems for there to be some kind of > AFC on the > receive side to compensate for > transmit-side frequency error. > > In general, how do I need to setup the frequency of a > USRP2 source if I want to place tones in the spectrum? I > thought it was simple upconversion. If I want to modulate a > multitone signal (say with sine components 1 KHz, 3 khz, and > 7 khz) to obtain an upconverted signal with tones at 901 > MHz, 903 MHz, and 907 MHz, then I simply set the "frequency" > parameter of the USRP2 sink to 900 MHz, right? How exactly > does the USRP2 do the upconversion? What exactly does the > "frequency" parameter do? > > > > > The USRP2 takes your quadrature-sampled baseband signal, > and > interpolates it up to the required > Tx-side sampling rate. It programs the Tx LO > on the daughtercard to > the desired frequency, > and sends it on to the Tx mixer. Sometimes, > due to LO frequency step > size limitations on specific > daughtercards, the USRP2 FPGA will use a DUC > (Digital Up-converter) > stage to get to > exactly the desired frequency. > > I'm not sure whether you meant 901,903 and 907Mhz, or > 900.001MHz, > 900.003Mhz, and > 900.007Mhz. > > For purposes of experiment, you can have 3 different signal > generators, > add their outputs, > and send the resulting multi-tone baseband stream on > to the USRP2. > > > -- > Principal Investigator > Shirleys Bay Radio Astronomy Consortium > http://www.sbrac.org > > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
