If I understand correctly, the real signal picked up by the antenna is mixed with two sine waves, 90 degrees out of phase, to create the I and Q components at baseband (depending on the daughterboard, there may be an intermediate frequency involved). The I and Q signals are then sampled by the ADCs. I and Q are then interleaved and sent over USB. See page 9 of the USRP FAQ for more info. http://www.scribd.com/doc/9688095/USRP-Documentation
Jordan
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