How to select different frequency out of the samples?

[0711]

Hello everyone,

      Recently I was doing some research with USRP2. I set the RX frequency to 2.441G and the IQ sample rate to 10M. According to the sample theorem, my USRP2 can sample signals with a bandwidth of 5M.  So How can I select the 2.442G, 2.440G signals out of the samples?

    Thanks in advance!

[ErikL]

USRP uses a direct conversion architecture. The center frequency you choose is mixed in quadrature thus moving it to baseband. Two ADC sample a total of 50MHz of bandwidth 25 MHz above and below the center frequency. The resulting signal is then down sampled to your specified IQ rate and passed to the PC over the Ethernet cable. In LabVIEW you see complex IQ waveforms as complex numbers that can be processed to recover the RF signals.

[0711]

ErikL, thanks for your kindl help.

      But how to set the center frequency that I want to process?  For example, when I want to process signals of 2.441G, I can  set the RX frequency to 2.441G, and IQ sample rate to 10M. But If I want to process signals of 2.442G or 2.440G, should I need to set the RX frequency and receive again?  Do the signals received with the RX frequency set to 2.441G  already contains the signals of 2.442G and 2.440G?

Thanks again!

[0711]

Thanks ErikL,

    I look up the spectrum of the samples I received. Yes, all the signals in the 10M bandwidth are received. If I want to do demodulation to the received signals, do I need to adjust the center frequency of the received samples to 0 Hz?

[sshearman]

The received complex valued signal samples are effectivly at 0 Hz, but since they have been downconverted, you can think of them as being signal samples from the original location of your center frequency.