Re: [Discuss-gnuradio] UHD example using USRP1

[Marcus D. Leech]

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
address@hidden
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