[USRP-users] RF sensor time stamp synchronization across long distances

Marcus D. Leech mleech at ripnet.com
Mon Dec 1 18:25:24 EST 2014


On 12/01/2014 05:29 PM, Urban Hakansson via USRP-users wrote:
> Hi everybody,
>
> I am considering an application with multiple USRPs in an RF sensor 
> network, the sensors being located on the order of at least 100 feet 
> apart.
>
> I am thinking of a way of ensuring that at least the UHD packet time 
> stamps sent by each RF sensor over Ethernet to a central server  for 
> processing are synchronized to each other.
>
> Would it be possible to just feed each USRP with a 1 PPS signal, or do 
> I need to feed the USRPs both the 10MHz and the 1 PPS reference 
> signals as a pair. Everything I have read indicates they are both 
> needed, but I just want to make sure.
>
> Could I use the Octoclock G and run +100 feet of coax to multiple URSP 
> (RF sensors)? In other words how much attenuation could the 10MHz and 
> the 1 PPS reference signals suffer due to path-loss through extremely 
> long runs of cable before becoming unusable?
>
> Is there another and better way of synchronizing the UHD packet 
> time-stamps between RF sensors located far apart (one the order of 100 
> feet or more)?
>
> Thanks for you consideration.
>
> Regards,
>
If you really want the clocks to be synchronized, you'll need common 
10MHz and 1PPS reference signals.  Without the common 10MHz, the units
   will start out synchronized (using something like 
set_time_unknown_pps()), but they'll drift apart, because they're each 
using their own internal
   clocks.

Here's a handy coax loss table:

http://www.w4rp.com/ref/coax.html

If you're just looking at time-stamp synchronization, you could install 
separate GPSDOs on each unit as well.  They won't agree on fine-phase of the
  10MHz signal, but they'll all agree "roughly" on 1PPS timing and 
phase, so depending on your requirements, this might be adequate.


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-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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