[USRP-users] Phase Alignment of Multiple X310s

ROBIN TORTORA tilla at comcast.net
Tue Nov 7 11:28:19 EST 2017


On the UBX results, Y0 -> Y1, I assume that is in the same motherboard?


Y2 and Y3 on a different motherboard (delta seems to be much larger, but very very similar across Y2 and Y3 relative to Y0)?


So if I wanted to improve on the "error" on channels 3 and 4 or add more channels, are there any options?


Lets assume I have done the easy things like match cable lengths and use splitters with low phase balance etc. :)

> On November 6, 2017 at 6:33 PM Rob Kossler via USRP-users <usrp-users at lists.ettus.com> wrote:
> 
>     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
> 
> 
>     [Inline image 1]
> 
> 
>     On Mon, Nov 6, 2017 at 5:11 PM, Marcus D. Leech via USRP-users <usrp-users at lists.ettus.com mailto: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 https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices
> > 
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