[USRP-users] GPSDO and USRP Clock

Nick Foster nick at ettus.com
Mon Feb 11 20:01:05 EST 2013


On 02/11/2013 04:43 PM, Perelman Nathan (Nathan) wrote:
>
> Sorry, by clock I meant the time registers on the USRP (what is set by 
> calling set_time_next_pps()).
>

The 1PPS signal is only sampled by the FPGA when set_time_next_pps() is 
called. Net drift over long time periods should be around zero, as the 
1PPS pulse is synchronous with the 10MHz, which is in turn synchronized 
by the USRP to its own master clock -- so as GPS time and thus 1PPS 
drifts around, 10MHz and thus the USRP clock should drift around as 
well. Over shorter timescales and while the oscillator is warming up, 
the 1PPS may skip around a little with respect to the 10MHz clock. As 
such, periodically issuing set_time_next_pps() probably isn't a bad idea 
when operating the USRP over long timescales.

--n
>
> *From:*USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On 
> Behalf Of *Nick Foster
> *Sent:* Monday, February 11, 2013 7:06 PM
> *To:* usrp-users at lists.ettus.com
> *Subject:* Re: [USRP-users] GPSDO and USRP Clock
>
> On 02/11/2013 03:51 PM, Perelman Nathan (Nathan) wrote:
>
>     How accurate is the USRP clock once it has been synced to the
>     GPSDO? Is there a need to periodically re-sync it? While the 10
>     MHz clock on the GPSDO is warming up is it causing the clock to
>     drift away from GPS time?
>
>
> When the GPSDO clock is selected as the reference source, the USRP 
> main oscillator is phase-locked to the GPSDO 10MHz reference. There is 
> no drift and no necessity to resynchronize while the USRP is operating.
>
> --n
>
> Thanks.
>
> -Nathan Perelman
>
>
>
>
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