Hi > On Feb 18, 2017, at 4:53 AM, David J Taylor <david-tay...@blueyonder.co.uk> > wrote: > > Hi, > > I was wondering whether there is some data/information available on the > claimed +/- 100 ns jitter?
I guess the previous was not complete enough. I routinely measure PPS jitter on GPS modules down well below 10 ns on a 53131. That’s after sawtooth correction is applied. Without sawtooth correction, the +/- 24 or +/- 12 or +/- 1.25 ns of the sawtooth adds into that. The reference used is an HP 5071 with a high performance tube option. Bob > > Regarding the PPS -> USB (using the CTS line of a FTDI FT232R), I > plotted, using some lines of Python, the time offset as attached. Just > to get an overview how it is 'worst case', i.e., user program, python, > etc. The 1PPS signal comes from a GPS rx. > Looks like a standard deviation of around 150 us. > y-axis: measured pps offset from full second (computer time) in us, > x-axis pps pulse number. > > On the long term it looks interesting (while measuring I played with the > NTP server on this computer) > Until ca. second 10000: ntpd synchronization via internet > Until ca. second 17000: made an additional LAN NTP server (GPS) available > Until the end: replaced ntpd with chrony (still using internet and local > servers) > > Interesting points: > -It looks surprisingly bad with using the normal ntpd (especially, there > is not really an improvement having an local GPS based server > available, did I do something wrong? Only the offset changes by ca. 3 > ms.) > -It looks surprisingly good with chrony. But there are continuously > outliers of up to 4500 us, is this a result of the chrony control loop? > The time is wandering around with ntpd, but has less jitter. > > Conclusion: > Despite the 150 us stddev, the using PPS over USB gives some interesting > inside of what the local ntp server is actually doing. It looks to me > like it would be an improvement to use it when using ntpd, but not when > using chrony. > > Best regards, > Thomas > DK6KD > SA6CID > > PS: > Raw data is here, if you want to zoom in: (1.7 MiB, one row per PPS > offset in us) > http://petig.eu/pps-usb.txt > ================================================= > > Thomas, > > I've done some tests with PPS over USB with Windows some time back, which > showed that PPS?USB could be better than LAN-sync alone, but that also > included a reduction of the poll interval from possibly 64 seconds for LAN > sync to 16 seconds for PPS sync, which may have influenced the results. > > <pet-peeve>It would be helpful to have some units on the axes - 10000 what? > I'm guessing microseconds....</pet-peeve> > > For comparison, here is a Raspberry Pi running NTP, with the reported offset > plotted against time. > > http://www.satsignal.eu/mrtg/raspi14_ntp_3.html > > This Raspberry Pi (running a seismic detector) is using an Ethernet > connection via Power-line Ethernet (yes, I know, QRM etc. etc.), and a couple > of switches to a very good stratum-1 server. I would estimate from your > graph that the jitter in offset is about 1 millisecond peak-to-peak, but it > seems that I get less than that - perhaps 100 microseconds peak-to-peak with > occasional excursions outside that. This is with the latest reference > version of NTP. > > remote refid st t when poll reach delay offset jitter > ============================================================================== > *192.168.0.20 .GPS. 1 u 17 32 377 12.351 0.000 0.428 > +192.168.0.11 .uPPS. 1 u 2 32 377 12.432 -0.106 0.824 > -192.168.0.3 .kPPS. 1 u 13 32 377 21.366 -4.524 0.804 > +192.168.0.83 .kPPS. 1 u 27 32 377 21.614 -4.511 1.206 > uk.pool.ntp.org .POOL. 16 p - 64 0 0.000 0.000 0.001 > -193.150.34.2 133.150.251.233 3 u 38 64 137 32.343 2.738 1.477 > -80.87.128.17 94.125.129.7 3 u 30 64 375 53.337 -1.225 1.516 > -192.146.137.13 82.148.230.254 2 u 56 64 377 46.089 2.220 2.535 > -129.250.35.250 249.224.99.213 2 u 169 64 214 42.499 -3.015 12.507 > -213.130.44.252 145.238.203.14 2 u 487 64 200 37.210 -0.725 13.232 > > 73, > David GM8ARV > -- > SatSignal Software - Quality software written to your requirements > Web: http://www.satsignal.eu > Email: david-tay...@blueyonder.co.uk > Twitter: @gm8arv > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.