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

Juha Vierinen x at mit.edu
Mon Sep 9 10:13:18 EDT 2013


I now switched from the FM band as the test signal to white noise and now
I'm getting nicely aligned relative sample delays and relative phases
across all the channels. The problem was in my test setup: the ambiguity
function of a random 1 MHz sliver FM band really isn't good enough for this
type of a measurement. I should have known better.


On Fri, Sep 6, 2013 at 12:09 PM, Matt Ettus <matt at ettus.com> wrote:

> 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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