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

Urban Hakansson uhakansson at tecore.com
Fri Dec 5 16:05:11 EST 2014


I just wanted to follow up with a question. Could you tell me how many dB loss the Octo-Clock G clock reference signals can experience and the USRP B210 still be able to use the signal? 

Regards, 

Urban Hakansson 

----- Original Message -----

From: "Marcus D. Leech" <mleech at ripnet.com> 
To: "Urban Hakansson" <uhakansson at tecore.com>, usrp-users at lists.ettus.com 
Sent: Monday, December 1, 2014 6:25:24 PM 
Subject: Re: [USRP-users] RF sensor time stamp synchronization across long distances 

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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