Re: [Discuss-gnuradio] Delay of synchronization with PPS and UHD on USRP2

[Josh Blum]

> These days I am trying to synchronize two USRP2 with PPS signal  and UHD. The two USRP2 
> is connect with same reference clock and PPS signal. I use function 
> "set_time_next_pps(uhd::time_spec_t(0))" to set time to be zero, and make two 
> USRP2 start sampling after 3 seconds. The result shows that there is still phase 
> difference, which means a delay( about 30ns) between USRP2s.
>
> I used 10MHz reference clock as input and set DDC frequence to 10MHz. Then I 
> get two complex DC signal. The phase difference is about 0.3*2pi.   It seems 
> that the PPS impedence difference between USRP2s is the reason of delay. Any 
> clues or solution???

You need to configure the usrp2 to use the external reference. Did you 
run set_clock_config(...) prior to setting the time?

(making an example of this is on my whiteboard TODO list)

> By the way, the stream mode "
> STREAM_MODE_NUM_SAMPS_AND_DONE" does not work with "set_time_next_pps" function. It works with 
> "set_time_now". But stream mode " STREAM_MODE_START_CONTINUOUS " works well with   
> "set_time_next_pps" function.

When performing set_time_next_pps(...) you will want a sleep(1) prior to 
starting streaming. Because it could take as much as 1 second for the 
registers to latch the new time value.

-Josh