AD9361 phase difference between 2 RX channels.

Hello,
I am interested to use Ettus B210 for DOA purposes.
According to[1], B210 is using AD9361 2X2 MIMO chipset.
According to AD9361 datasheet, both RX channels are being driven by the same LO[2].
However, according to[3][4], there is a phase difference between the two USRP-B210 channels.
According to[5], the RX channels are aligned in phase.
Would be glad to have a formal answer to the following 2 questions:
1. Is there a phase difference between the two USRP-B210 RX channels?
2. If there is a difference - what causes it?
Thanks in Advance.

[1] https://www.ettus.com/product/details/UB210-KIT
[2] http://www.analog.com/media/en/technical-documentation/data-sheets/AD9361.pdf
[3] http://ettus.80997.x6.nabble.com/USRP-users-Phase-jumps-in-USRP-B210-with-GPSDO-td7260.html
[4] http://ettus.80997.x6.nabble.com/Re-USRP-users-Random-Phase-Offset-Between-Two-Rx-Channels-of-B210-td10094.html
[5] http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2017-January/023422.html

  • 0
    •  Analog Employees 
    on Jul 24, 2018 12:31 PM

    Your links discuss two different issues.  The first is a phase difference between channels on the same chip and the second is phase differences between boards (multiple chips). 

    On the same chip the phase of the receive pair are aligned at the balls of the chip.  However, due to the layout internally of the transceiver you must polarity invert the second channel AD9361 Rx1 and Rx2 ports phase inversion(ad9361_pp_port_setup)  .  On the B210 the trace length may be different to the SMA connectors which will cause phase differences.  I would ask Ettus Research about the design.  I do not know if they implement this polarity inversion as well in their software.

    In regards to multiple boards,  the transceiver has no internal mechanism to perform phase alignment.  It must be done externally, as demonstrated on the FMCOMMS5 boards.

    -Travis

  • Thank you Travis.
    Regarding the phase between the two RX channels on B210:
    1. Is the phase constant for a given frequency? Meaning, can a table be used for phase alignment after power on/off?
    2. Is the phase stable over time(if no parameters are updated and no power on/off is performed)?
    Thanks Again.

  • 0
    •  Analog Employees 
    on Jul 25, 2018 10:13 AM

    On the same chip both RX channels are driven by the same LO so they will always be aligned.

    -Travis

  • Thank you Travis.

    I assume that the answers to the questions 1 and 2 above are:

    1. The phase is constant for a given frequency. A table can be used for phase alignment after power on/off.

    2. The phase is stable over time for a given temperature range.

    Wanted however to be sure.

    Can you approve if this is correct?

    Thanks.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 1:22 AM

    Yes.

    Since both channels are on the same chip they should drift together if temperature changes.

    -Travis