Hi > On Oct 31, 2017, at 11:33 PM, Leo Bodnar <l...@leobodnar.com> wrote: > >> From: Bob kb8tq <kb...@n1k.org> >> Working all this back into a holdover spec in an unknown temperature >> environment is not at all easy. >> Bob > > > This is true, it is too easy to multiply figures from the datasheet and then > start believing in them. > > We did extensive testing of real units in real life before committing to any > specification figures. They are based on statistical measurements followed > by an expanded safety margin. > Here is a typical holdover offset curve over 24 hours in non-DC environment > (i.e. 5-10 degrees ambient temperature change during day/night period.) > > http://leobodnar.com/balloons/NTP/24hr-holdover.png > <http://leobodnar.com/balloons/NTP/24hr-holdover.png>
Looking at the data, the DUT did some sort of discrete frequency shift around 4,000 seconds. The rest of the time it plodded along do nothing much ( = it was very stable). None of that is terribly unusual in terms of a holdover plot. The nasty question that always gets asked is “what if shift happened earlier?”. Depending on the test profile, that may be unlikely or …. . Doing a lot of testing is about the only way to sort things out. > > Time drift over 24h on this particular unit was below 0.7ms. This is pretty > good for the device that consumes 1W of power (via PoE or USB) and fits in > the pocket. > > I have used typical Raspberry Pi with a GPS add-on run-of-the-mill timeserver > as suggested by Attila to monitor relative offsets - this is why reported > timing is jittery and local (to RPi) 1PPS has an offset. > > It is really puzzling why holdover has suddenly come into focus. This is TimeNuts …. > Due to NTP redundancy feature it is trivial to put several inexpensive time > servers around the local or campus network and let clients do the standard > NTP sanity checking and server selection. And those building an NTP system > able to cope with 24h+ global GPS outage know what they are doing anyway. Based on some other posts, it appears that some of the applications are in *very* unusual environments. They are far more outage prone than one would normally expect. Bob > > Leo > _______________________________________________ > 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.
