Questions on B200 PPS

Hello,

We are using an external GPSDO to provide 10 MHz and PPS into a B200.

Our application collects a fixed number of samples at 50 MSPS using a stream command (UHD_STREAM_MODE_NUM_SAMPS_AND_DONE and the timespec for the next PPS).  In this way we rely on the USRP to provide samples that begin at the instant of PPS arrival.

This works almost all the time.  Occasionally, it appears we miss one sample (20 ns) and thus our timing is off.  We suspect this is due some behavior in the B200 related to the PPS.

We are using a Stanford Research FS740 for our GPSDO and have it set to provide a 5 volt PPS pulse with a duration of 8 microseconds.

Questions:

1.  Is 8 milliseconds long enough for a PPS signal?  If not, what value would be appropriate?

2.  Documentation shows an alternate for the

usrp->set_time_source("external");

command that uses underscores on either side of the word, as

usrp->set_time_source("_external_");

What exactly does this command do relative to the leading edge of the PPS signal?

To the extent it matters, we are operating under Ubuntu 20.04 with UHD 3.15.

Thanks!