[USRP-users] Accuracy and time

Marcus Müller marcus.mueller at ettus.com
Wed Sep 10 08:41:16 EDT 2014


I'm a bit confused -- I think I answered to this email already yesterday:
http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2014-September/010578.html

Greetings,
Marcus

On 10.09.2014 07:28, g_iftah--- via USRP-users wrote:
> Hi Marcus and all,
>
> Thanks for the quick answer.
>
> In your answer you wrote:
>
>> 1.      What is the accuracy of this:
>> tod = uhd::time_spec_t::get_system_time();
>> usrp->set_time_now(tod);
> The two USRPs should then be sample-synchronous. However, setting the
>
> time without any trigger is not going to be very exact, give or take
>
> 10ms, because the USRP just sets its time when it receives the command.
>
> If you want more accuracy, you need a pulse that will go up at the exact
>
> time you want to set at the PPS input and use set_time_unknown_pps or
>
> one of its sister methods. To be honest, though, when using the system
>
> time, 10ms isn't that much, so "accuracy" is always relative.
>
> And my new question now is:
>
> If I don't have an external source, and I don't use a GPSDO, so were will
> that pps will come from?
>
> Actually what you saying if I got you right is that If I want both usrp to
> have the same time with accuracy butter then 10msec, then I HAVE to either
> use to GPSDO PPS or an external PPS
>
> Or I won't get the accuracy in time between the two usrp, right?
>
> And what if I won't use either? They will still be sample synced right? Only
> the time base will be biased, right?
>
> Plus and that has nothing to do with the time issue the two channel  will
> have a phase difference dew to the different PLL who created the A2D clock,
> right?
>
> Thanks again,
>
> iftah
>
> Sent from Yahoo Mail on Android
>
>
>
>
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