[USRP-users] Phase Alignment of Multiple X310s

Rob Kossler rkossler at nd.edu
Mon Nov 6 18:33:28 EST 2017


Sarah,
I just happen to be in the middle of an evaluation presently of the
channel-to-channel phase coherence among various 4 chan receivers, among
them an X310/UBX (pair) and X310/TwinRx.  For this test, I inject a CW tone
(-48 dBm) into a 1:4 power divider which then feeds the four receiver
channels. I then compute the cross correlation (in 1 msec blocks, zero lag)
between each of channels 2-4 and channel 1 (arbitrarily selected as the
reference channel).  The following figure shows the channel-to-channel
phase variation over a 2 sec window.  The left plots are for the four
channel X310/TwinRx.  The right plots are for the pair of  X310/UBX
(appropriately synced).   The 3 plots for each device show channels 2-4 in
comparison to channel 1.  Note that the TwinRx results show much smaller
variation (as expected since there is a shared LO).  Also note that for UBX
results, the variation is much smaller for the two channels that are housed
in the same X310 (top right plot).  I suspect that the much larger
variations shown for the 3rd and 4th channels (relative to channel 1) are
related to a 10 MHz PLL circuit in each X310.  Bottom line is that I see at
least +/- 1 degree "wobble" in the channel-to-channel phase over time.
You'll need to determine if this is acceptable for your application.

Rob


[image: Inline image 1]


On Mon, Nov 6, 2017 at 5:11 PM, Marcus D. Leech via USRP-users <
usrp-users at lists.ettus.com> wrote:

> On 11/06/2017 04:57 PM, Sarah Tran via USRP-users wrote:
>
> Hi everyone!
>
>
>
> I was wondering if anyone knew if it was possible to phase align more than
> 2 X310s (with UBX daughterboard) to form a coherent array? I know that 2x2
> MIMO is possible using a single X310, but I want to be able to sync 4 of
> them. I thought I read somewhere that it was dependent on the
> daughterboard, but I can’t find that thread anywhere.
>
>
>
> Thanks for all the help!
>
> Yes, use an external 10MHz reference and 1PPS source, like the Octoclock,
> or Octoclock-G, or similar.
>
> Also, see this App note:
>
> https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices
>
>
>
>
>
>
>
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