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

Urban Hakansson uhakansson at tecore.com
Mon Dec 8 11:13:22 EST 2014


Thanks, 

That was a relief. I just read http://files.ettus.com/manual/page_usrp_b200.html and it state the max +15 dBm just as you say but no min value. 

The document also lists that the acceptable 1 PPS input range is given in Volts. 1.8 - 5V. If this translates to attenuation, it would allow for ~9 dB attenuation. 

Since the Octoclock G 1PPS output = 5 Volts I should be OK for the 1 PPS requirement as well. 

Is this correct? 

Urban Hakansson 

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

From: mleech at ripnet.com 
To: "Urban Hakansson" <uhakansson at tecore.com> 
Sent: Monday, December 8, 2014 10:58:36 AM 
Subject: Re: [USRP-users] RF sensor time stamp synchronization across long distances 


Actually, the acceptable reference range, according to the ADF4001 (used to process the REFIN on B200) datasheet extends down to -5dBm. 
So you should be good. My initial +10dBm was based on the MAX input (+15dBm), but I didn't realize the ADF4001 had such a broad range 
of acceptable input power levels on REF IN. 



On 2014-12-08 10:45, Urban Hakansson wrote: 


Thanks, 

That is useful information, but I am still in need of knowing the output power of the OctoClock-G to know how much power-loss I can impose between the B210 and the OctoClock-G. 

Based on the Octoxlock & Octoclock G specification sheet I tried calculating the RMS Power of the 10 MHz square wave. 

I assume here that the Vrms = Vpk = Vpp/2 for the 10MHz reference square wave, and the impedance is 50 Ohm. 

This gives the rms power to be 10*log10((((Vpp/2)^2)/Impedance)/1e-3) = 9.9 dBm. 

If the B210 requires input power to be at least +10 dBm, then I can have basically not have any power loss at all, and the length of any coaxial cable must be very short. 

Is this correct? 

To reiterate my objective, I am contemplating to use the OctoClock in a distrubuted RF sensor network inside a building where locking to an external GPS constellation is not a reliable option. I need a very reliable reference clock in unlocked state, that can ensure both frequency and time-stamp synchronization to multiple remote radios, across long distances, anywhere from 100 to to 1000 feet. 

I am looking for example at an 3/8" LMR-400 cable which has a loss of ~0.4 dB/100feet at 10MHz. The total power loss would be between 0.4 dB and 4 dB at 10MHz. 

Based on the information you gave me I am not sure if the Octoclock-G <---100-1000-ft coaxial cable> multiple B210s is a viable solution. 

I am not sure how to calculate the output rms power and also the ensuing power loss of the 1PPS signal since I think this just a very short 5 Volt pulse repeated every 1 second, not a periodic signal like a sinusoid or a square wave. 

Could you please clarify so I know if I can insert a 0.4- 4dB loss between the Octoclock-G and multiple B210s. 

Regards, 
Urban Hakansson 



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

From: "Marcus D. Leech" <mleech at ripnet.com> 
To: "Urban Hakansson" <uhakansson at tecore.com> 
Cc: usrp-users at lists.ettus.com 
Sent: Friday, December 5, 2014 4:23:26 PM 
Subject: Re: [USRP-users] RF sensor time stamp synchronization across long distances 

On 12/05/2014 04:05 PM, Urban Hakansson wrote: 
<blockquote>

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 



Refclock input to B210 should be in the range +10 to +15dBm 
Similar for 1PPS. 



<blockquote>

----- 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: 
<blockquote>

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, 


</blockquote>
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. 



<blockquote>

This e-mail may contain privileged, confidential, copyrighted or other legally protected information, and is intended exclusively for the intended recipient. If you are not the intended recipient (even if the e-mail address above is yours), you may not review, store, use, copy, disclose or retransmit it in any form. If you are not the intended recipient or otherwise have received this by mistake, please immediately notify the sender by return e-mail (or sysadmin at tecore.com ), then delete the message in its entirety. Thank you. 
_______________________________________________ USRP-users mailing list USRP-users at lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com 
</blockquote>


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

This e-mail may contain privileged, confidential, copyrighted or other legally protected information, and is intended exclusively for the intended recipient. If you are not the intended recipient (even if the e-mail address above is yours), you may not review, store, use, copy, disclose or retransmit it in any form. If you are not the intended recipient or otherwise have received this by mistake, please immediately notify the sender by return e-mail (or sysadmin at tecore.com ), then delete the message in its entirety. Thank you. 
</blockquote>


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

This e-mail may contain privileged, confidential, copyrighted or other legally protected information, and is intended exclusively for the intended recipient. If you are not the intended recipient (even if the e-mail address above is yours), you may not review, store, use, copy, disclose or retransmit it in any form. If you are not the intended recipient or otherwise have received this by mistake, please immediately notify the sender by return e-mail (or sysadmin at tecore.com), then delete the message in its entirety. Thank you. 



</blockquote>




                                                                                                                                                                                                                                       This e-mail may contain privileged, confidential, copyrighted or other legally protected information, and is intended exclusively for the intended recipient.  If you are not the intended recipient (even if the e-mail address above is yours), you may not review, store, use, copy, disclose or retransmit it in any form.  If you are not the intended recipient or otherwise have received this by mistake, please immediately notify the sender by return e-mail (or sysadmin at tecore.com), then delete the message in its entirety. Thank you.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/attachments/20141208/896322cb/attachment-0002.html>


More information about the USRP-users mailing list