USRP synchronization

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.).

As mentioned above we are using 4 USRPs to receive a signal from a
source. We wish to retain the phase information of this signal between
due to the different positions of the antennas. We feed all 4 boxes with
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.
it correct, that this should also make all the internally generated
have zero phase difference between the boxes instead of locking to the
phases of the incoming signals (see Figure 2.)?

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
THEM, which is constant for a given power cycle. Does this imply that
phases of the internally generated carriers are locked to one another,
also with some NON 0 PHASE difference, i.e. that our actual system looks
like Figure 3?


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