Re: [Discuss-gnuradio] USRP synchronization

[Matt Ettus]

On 03/09/2010 09:20 AM, 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.


I assume you are using all USRP2s.  There are multiple sources of 
ambiguity here.

First, unless you are using a common PPS signal to all systems, using 
sync_on_pps, your time samples won't be aligned.  Think of it this way 
-- everyone's watch is running at the same speed, but everybody thinks 
the time is different because you haven't coordinated.  That's what the 
PPS is for

Second, and this is what Eric mentioned, the PLLs on the daughterboards 
will all be locked to the same reference, but they can have an arbitrary 
offset which will change every time you tune.  The important thing is 
that the relative phases don't change with time.  This means you can do 
MIMO operations, but phased-arrays will require you to calibrate every 
time.  You can use the TX side of the RFX-series as a signal generate to 
do this, but it will take some software.

You didn't tell us what daughterboards you are using.  The ones with the 
integer-N PLLs (DBSRX, RFX-series) will have a finite number of 
possibilities for that phase difference, usually less than 25, and you 
can tune in such a way that there is no ambiguity if you are careful. 
The fractional-N ones (XCVR2450, WBX) will have an effectively infinite 
number of possibilities, but you can still measure it and compensate.

If you are just using the BasicRX or LFRX then you don't have this 
problem, you just need to align their DDC oscillators using PPS.

Matt