title: relation between FFT and IQ data
body: <p>I have been studying how the USRP works and, particularly, how to sense the energy from a signal.   So far, I have understood that the USRP sense IQ data, and then, process it applying FFT. (I have been looking at usrp_spectrum_sense.py )
  What are the units of the IQ samples?  What are the units after the FFT is done? The only data that is needed to develop the FFT is the IQ samples?</p>
  
  <p>Thanks in advance :D</p>