Marcus is right, "_external_" is a B200- and N2x0-only feature. I also think 8 ms should be fine, but just as an FYI, "our" PPS signals have a 25% duty cycle (so, 250 ms). Since we only register the pos. edge, anything in the ms-range should be working. When you say "occasionally", do you mean within a single session? I.e., you init the device, set the time, and then don't touch the device any more? As an experiment, can you try a time that's not on the PPS edge (like. .5 seconds off)?
Really, the PPS shouldn't be the issue here, since it's only used once (to set the time). --M On Fri, Sep 10, 2021 at 8:49 PM Marcus D. Leech <[email protected]> wrote: > On 2021-09-10 2:35 p.m., Mike wrote: > > 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? > > That should be plenty. > > 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? > > My recollection is that this is a leading-vs-trailing option in PPS > processing--I'm not even certain that the B2xx family supports that > flexibility. > > It may be that what you're seeing is a sampling anomaly with respect to > the PPS. If the PPS is *sampled* by the FPGA at 50MHz, you would expect > there > to be occasional anomalies of 1 sample time. But I am not intimately > familiar with the FPGA code, so I'm not sure. > > > To the extent it matters, we are operating under Ubuntu 20.04 with UHD > 3.15. > > > Thanks! > > >
