On 09/13/2018 10:12 PM, Jack Yang wrote:
Hi Marcus,
Thanks for the prompt reply. I have two questions. First, I remember I
can align phase across multiple USRPs when using UBX-160
daughterboards (i.e 4 USRPs equipped with 8 UBX-160 for 8 phase-array
receivers.). Just to double confirm that I cannot use the UBX-160 time
configuration approach to align phase for TwinRx across two USRPs,
right? Since TwinRxs use superheterodyne structure, the time setting
configuration for phase alignment (which work for UBX-160) will not
work for the TwinRx due to the second stage down-conversion having
it's own LO and this LO is not able to access from the other USRP. Is
my understanding correct?
That is correct, as far as I understand the architecture of the TwinRX.
My second questions is that do you know any approach which can have
self calibration for each run? If I use two port RF splitter to
connect Rx4 in the first USRP and Rx5 in the second USRP for the same
antenna as input, ideally I can use the Rx4 & Rx5 phase difference to
compensate other three receivers in the second USRP. Then, I can still
set up a 7 receivers for coherent receivers, right?
The usual approach is "measure and correct". This works well if you have:
(A) A calibration source you can turn on that is visible to all inputs
and of equal phase delay
or
(B) You have some continuous, but distant, signal source that is "seen"
by all your receivers
or
(C) Other schemes that achieve the same thing
The key is that you have to measure the phase delays and compensate.
Really thanks for your time to answer my questions!
All Best,
Jack
On Thu, Sep 13, 2018 at 6:20 PM Marcus D. Leech via USRP-users
<[email protected] <mailto:[email protected]>> wrote:
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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