On 09/13/2018 08:52 PM, Jack Yang via USRP-users wrote:
Hi,
I am trying to set up a 8 channels coherent receiver where I am using
two X310 and each X310 is equipped with two TwinRx. However, the phase
alignment across two USRP are always failed. The two X310 are using
the OctoClock-G to have 1M PPS and 10MHz clock. Then, I am using a
individual transmitter which can generate single tone with RF splitter
to connect to the 8 channels via RF coaxial cable. My GNURadio
configuration is shown as below the link (
https://www.dropbox.com/s/du15rakg1nacmql/DualX310_8Rec_v1.py?dl=0 ).
Alternatively, one can also see the below summary configuration. I
can have constant phase difference within the same USRP when I power
on and off for different trials, while the phase difference across two
USRP are always varying for each trial (i.e The phase difference
between Ch4 in first USRP and Ch5 in second USRP is not constant among
different runs). The below links refer to the results for two
different trials. In first trials, all the phase difference is shifted
based on itself mean value (First run,
https://www.dropbox.com/s/z8so39bejzdofi8/Run1.png?dl=0 ). Then, using
the first-run average phase difference to compensate the phase
difference in the second run (Second run,
https://www.dropbox.com/s/xhkmp6cuxzwylvx/Run2.png?dl=0 ). As the
result shown in the second run, the phase difference across Rx5 and
Rx3 has been shifted to -214 degrees while all the other phase
difference within itself USRP are still the same as the previous
measurement. Could someone let me know how I can configure two USRP
X310 with TwinRx for the coherent receivers?
Many thanks for the help!
All Best,
Jack
So in the first USRP, your TwinRx is configured to share the LO from the
first channel to the other 3 channels. The only phase-offset compensation
you'll need to do is fixed, and is dominated just by the length of
the cables within the enclosure that are used to share LOs.
However, in the second USRP, THAT LO is independent from the one on the
first X310, even though it uses a common phase reference
(the 10MHz shared between the two). What this means is that the two
USRPs won't drift relative to one another, but there will be
some unknown phase offset between them, due to the way that RF
synthesizers work--you have two synthesizers (one in the first USRP
on one of the TwinRX cards), and the other in the second. They won't
agree on startup phase, except "by accident". You'll have to resolve
this using a calibration procedure. There's really no way around
that with this type of architecture.
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