[USRP-users] Phase drift issue of N310

Piotr Krysik perper at o2.pl
Thu Mar 14 03:56:40 EDT 2019

Hi Ali,

The daughterboards have their own clock generators, but they are not
exactly 'independent'. At least they don't have to be, as they share the
same reference clock. Look at the block diagram:


and "Ref Clock" block. I don't have N310 and I know that reality can be
a bit far from expectations (i.e. look at my "What makes sense and what
doesn't in the way carrier frequency is set for TwinRX currently?" post).

But maybe the daughterboards can be configured to use that reference clock.

Best Regards,
Piotr Krysik

W dniu 13.03.2019 o 19:59, Ali Dormiani via USRP-users pisze:
> Hello,
> You sort of said the answer yourself:
> "two dboards are using independent reference clocks"
> RX0 and RX1 use the same LO while RX2 and RX3 use a different LO.
> I like to think of each N310 as two radios in the same box (when it
> comes to phase issues). Additionally, I think UHD/GNUradio start up RF
> components at run-time so all calibration needs to be preformed each
> time you run a flow-graph or C file?
> There are signal processing ways to mitigate this. Consider looking
> into ESPRIT or array shape calibration research.
> The hardware fix to this problem is feeding an external LO into each
> N310 dboard.
> Attached are some old results from our lab with an 8 TX 24 RX system
> set up indoors (all N310s). You can see that our DOA estimate in
> ESPRIT drifts badly (10 degrees or so) over about a second of data.
> The point I'm trying to make is, depending on your application, you
> can avoid buying external LO's, references, ect with some math effort.
> However, if you are looking for hardware I suggest Ettus Octoclocks
> for splitting and the following frequency synthesizer:
> https://www.valonrf.com/frequency-synthesizer-4400mhz.html
> Good luck with your endeavors,
> Ali
> On Wed, Mar 13, 2019 at 5:52 AM guowang qiu via USRP-users
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>     Hi all,
>     There are two daughter boards in N310, the phase difference
>     between channels of different dboards will drift rapidly.
>     In my test, a B200 transmits single tone continuous wave to a
>     1-to-2 splitter, and the outputs of splitter are connected to
>     input ports of RF0 and RF2 of N310.
>     During the single program running, the signals received by the two
>     channels should keep the phase difference approximately stable.
>     But I observed that the phase drift of the two channels was very
>     fast. Within a time span of 20 seconds, the phase difference
>     varied by tens of degrees. Sense that two dboards are using
>     independent reference clocks, not the same reference clock from
>     mboard. By contrast, the phase difference between the two channels
>     of x310 with two ubx is stable.
>     Does anyone know how to explain this issue?
>     Best regards,
>     Damon
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