On 02/03/2011 09:44 PM, Shiva ? wrote: > Hi All, > > I'm using 2 USRP1s to send and receive a simple sine wave using UHD. > The TX/RX frequencies are 2.45 GHz as I have RFX 2400 as daughterboards. > > I use the UHD example tx_waveforms to transmit a sine wave of 10 kHz, > sample rate 250ksps. I received the waveform at the other end using > the UHD example rx_samples_to_file to dump the output to a binary > file. I used a receiving sample rate of 250ksps. I used this file as a > file source in a grc flowgraph, hooked up its output to the Oscope > sink (after converting the interleaved short I,Q binary file to > complex in the chain). I'm seeing a clear sine wave, but its frequency > is 5kHz, rather than 10 kHz. > > Any idea what is actually happening here? > > Thanks, > > Shiva > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > So, let's keep in mind what's going on here. The Tx side generates a 10KHz signal sampled at 250Ksps (which is then up-sampled to the rate for the ADC). The resulting 10KHz signal is then mixed up to 2.45GHz(+/- inherent frequency error). Which produces a final signal frequency of 2.45GHz(+/-inherent frequency error)+10KHz.
On Rx, the incoming signal arrives at 2.45GHZ(+/-inherent frequency error)+10KHz, which is then down-sampled with a local oscillator set at 2.45GHZ(+/-inherent frequency error). Which means that your signal converts to baseband at 10KHz(+/- inherent frequency error). The inherent frequency error can be a few PPM, which at 2.45GHz means a few KHz frequency error. So your Rx signal may arrive up to a few KHz "off" from where you expect it. This is rather normal for synthesized transmitters/receivers with crystal oscillator references. The crystal oscillator on the USRP1 is "good", but it's not $100.00/apiece "good". But this frequency error is entirely normal in RF communications gear. -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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