On Tue, Mar 09, 2010 at 09:20:28AM -0800, ValentinG wrote: > > Hi All, > > We are trying to use 4 USRPs to extract the phase information due to > differences in positions of the antennas. > > In a standard communications system an internally generated carrier is > locked IN PHASE with the incoming signal to perform downconversion (see > Figure 1.). > http://old.nabble.com/file/p27838821/Fig%2B1%2BUSRP%2Bv2.jpg > > As mentioned above we are using 4 USRPs to receive a signal from a single > source. We wish to retain the phase information of this signal between boxes > due to the different positions of the antennas. We feed all 4 boxes with a > clock reference signal (10MHz, 1.5Vpk-pk), so that their internal clocks > should be locked in phase with respect to each other. Are the internally > generated carriers generated using this clock as a phase reference, i.e. is > it correct, that this should also make all the internally generated carriers > have zero phase difference between the boxes instead of locking to the > phases of the incoming signals (see Figure 2.)? > http://old.nabble.com/file/p27838821/Fig%2B2%2BUSRP%2Bv2.jpg > > However, when we observe the internal clocks of 4 USRPs they are phase > locked with respect to one another, but THERE IS A PHASE DIFFERENCE BETWEEN > THEM, which is constant for a given power cycle. Does this imply that the > phases of the internally generated carriers are locked to one another, but > also with some NON 0 PHASE difference, i.e. that our actual system looks > like Figure 3? > http://old.nabble.com/file/p27838821/Fig%2B3%2BUSRP%2Bv3.jpg > > Thanks, > Valentin.
Matt can provide more info when he gets a chance, but I'm pretty sure that what you're seeing is is normal and is due to the PLLs on the daugherboards. There's a finite number of phases that the PLL's will lock to depending on the ratios programmed into the PLLs. I don't recall which daughterboards use integer-N vs fractional-N PLL's, but they give different numbers of possible solutions. Eric _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
