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?
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? 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 < usrp-users@lists.ettus.com> 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. > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
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