On 2020-10-07, Andreas Mattheiss <please.post@publicly.invalid> wrote: > Hello, > > I wonder what's a realistic ballpark for the jitter I can expect when > feeding a GPS PPS into ntpd?
around a microsecond or less. > > My setup is: a cheap u-blox NEO6M has its PPS output connected to a MAX232 > level shifter, connected to a true serial port on my desktop PC. I use it Not really needed since almost all serial cards will handle TTL level with not problem. > at the same time for daily work, so no designated ntp server. The GPS You need to set the seconds somehow. Are you doing that via nmea from the GPS? > antenna is sitting inside, close to the window, with a building opposite - > suboptimal view of the sky. I do feed RTMS corrections to the GPS though > (ionospheric corrections etc.). The GPS is set to "stationary" mode, so > it's probably set up as good as it gets. > > The jitter values I get do, sorry, jitter. I guess it's a lot to do with > the poor view to the sky, mainly. PPS from GPS is claimed to be accurate > to say 10's of ns; my jiitter values are around 10 micro s approximately, > on avarege, for a well settled system set up as described. They do zoom > up, occasionally, to 60 micro s though. I would assume that it's > unrealistic to expect ns accuracy from ntpd on a non-designated system. It sounds to me like you are running from the wrong edge of the signal. Is your level shifter also and inverter? > > What do you think, is my 10 micro jitter within experience? While I am No. I get using chronyd (report from chronyc) sources MS Name/IP address Stratum Poll Reach LastRx Last sample #* PPS0 0 4 377 24 +2035ns[+2458ns] +/- 776ns sourcestats Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev PPS0 24 11 367 +0.000 0.016 +1ns 1972ns Of course chrony has long been know to give much better control of the clock than does ntpd > writing this I just realised a post from David Taylor > (<qgmgcu$2ad$1...@dont-email.me>) showing > > remote refid st t when poll reach delay offset > jitter >============================================================================== > o127.127.22.1 .uPPS. 0 l - 16 377 0.000 0.026 > 0.013 > > Pretty much what I get. > > Thanks, regards > Andreas > > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
