[USRP-users] USRP N200 + octoclock MIMO channel sample time alignment

Matt Ettus matt at ettus.com
Fri Sep 6 12:09:03 EDT 2013


Juha,

You should expect the timing on all the devices to be aligned to much
better than 1ns.  If it is not, then there is another issue.  Can you tell
me how you are computing the time difference?  Is it by RF phase or by
correlation?  Also, what daughterboards are you using?  Are all the N200s
the same rev and running the same rev of the FPGA image?

Matt


On Fri, Sep 6, 2013 at 7:29 AM, Juha Vierinen <x at mit.edu> wrote:

> Hi,
>
> I've been testing MIMO alignment of a usrp N200 setup. I've got four usrps
> hooked up to a octoclock-g, which is showing the gps lock light. I then
> periodically start 8 channel recordings with external 10 MHz and PPS
> locking. All channels are fed the same signal, originating from a 8 way
> splitter.
>
> The relative phase differences between the channels is relatively stable,
> although over tens of hours, I can detect some drift in the relative phase
> differences between the channels. Also, from some reason, a few of the
> USRPs seem to be consistently either 2 microseconds behind, or 2
> microseconds ahead of each other, ie., the samples aren't aligned for some
> reason. The timing cables aren't exactly equal length, but the  differences
> in the cable lengths is the order of 1 m or less, not 300 m.
>
> What kind of timing alignment should I expect for a four usrp N200 system?
> Is it expected that in a MIMO setup there will be this much difference in
> sample alignment? The rx_time tags suggest perfect channel alignment.
>
> juha
>
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