[USRP-users] N310: Channel pairs are drifting in phase whith external LO setup

Marcus D. Leech patchvonbraun at gmail.com
Fri Mar 22 09:17:29 EDT 2019

On 03/22/2019 05:31 AM, Janos Buttgereit via USRP-users wrote:
> Hi,
> after having struggled a lot with network config issues I finally got out N310 up and running. We chosed it for an application that needs perfectly constant phases between all four channels to build up DOA estimation applications.
> I’ve set up an N210 as a signal source, outputting a sine round about 1.5GHz feeding a 4-way splitter that than feeds all four input channels of the N310. A Rhode&Schwarz signal generator outputting a 3GHz sine wave feeds a two way splitter that is connected to both external rx LO inputs.
>  From this setup I expect all four channels to maybe have a static phase offset that does not change over time. However, it can be noticed that the pairs of channels 0 and 1 and the pairs of channels 2 and 3 show those behavior but both pairs drift against each other over time. Any idea of what could be wrong with this setup?
> I attached the GNURadio flow graph for this setup.
> Thanks in Advance!
Change the "Sync" option to "Unknown PPS".   Why this is required is a 
bit of a mystery, but another user experienced the same issue.

The N310 is conceptually two radios in a single box, and each radio 
within the FPGA has its own "timekeeper".  For some reason, unless those
   "timekeepers" are started synchronized with each other, sample 
timestamps drift relative to one another.  This should not be the case, but
   it appears to be.

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