[USRP-users] Accuracy and time

g_iftah at yahoo.com g_iftah at yahoo.com
Wed Sep 10 01:28:46 EDT 2014


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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