"After that all you need to do is write some code to..." Oh - if I had a nickel for every time I've heard that!
Brent On Fri, Jul 22, 2011 at 6:07 PM, Bob Camp <[email protected]> wrote: > Hi > > The easy way is to take a pps off of your external oscillator and feed that > into a port on your NTP server. Let NTP tell you where that pps is. Don't let > NTP lock to the pps, just let it report it's position. > > After that all you need to do is write some code to read the location of the > pulse and implement a *long* time constant loop. Taking the 1 ms number and a > 1x10^-10 goal, the time constant would need to be around 4 months. There are > a few minor details about drift of the local reference and how valid 1 ms is > over long time periods. A reasonable GPSDO will likely be much more stable > and much more accurate. > > To get it into the range of being practical, you have to get the NTP setup > into single digit microseconds. In the us range you still would not beat the > GPSDO, but at least you would have a useful device. 1 us can be done on a LAN > with NTP. It's tough to do better than 1 ms over the net. Since PTP suffers > the same issues over the net that NTP does, it's not a lot of help in this > situation. > > Bob > > On Jul 22, 2011, at 5:30 PM, Javier Herrero wrote: > >> I've found a plot of the ntp-synthesized GPS output compared with the >> UTC-aligned GPS from a Thunderbolt. The generated PPS output was 1us wide, >> and it is represented in infinite persistence to get an idea of the jitter. >> The offset was around 50us, and the jitter around 8us, so not very bad (it >> was at least one order of magnitude better than my requirements, so I did >> not bother to optimize it further). >> >> The ntp source was a M12-based ntp server (a blackfin running uClinux, not a >> Soekris :) ). >> >> Driving the PPS output to a serial port from the ntp is not as trivial as >> you think. This PPS output is from an "oscillator" disciplined to the system >> clock - really a stearable divider from the system clock (it is an embedded >> system using uClinux). If you try to drive a digital output directly from >> the timer interrupt to get the PPS, you would get far more jitter and error. >> >> Best regards, >> >> Javier >> >> P.S. not sure if the attachment will show up... >> >> El 22/07/2011 22:19, Chris Albertson escribió: >>> This is exactly what an NTP server does. It adjusts the rate of a >>> local clock so that the local clock advances at the same rate is the >>> set of Internet servers that have passed a clock selection test. NTP >>> does this very well considering the uncertainly of the lag over the >>> internet. There is a good argument the NTP is optimal but the best >>> you can get using Internet time servers is "about a milli second" or >>> 0.001 second. >>> >>> It would be nearly trivial to write software to produce a PPS output >>> on one of the control line of a serial port. SO you NTP disciplined >>> computer can produce PPS with about .001 second error >>> >>> Send this PPS to the "normal" GPSDXO in place of the GPS' PPS. The >>> computer is "only" about 1000 times worse than a GPS and might work OK >>> if you drastically increase the time constant on the GPSDXO's control >>> loop. >>> >>> I think your "NTPDXO might be as good as GPSDXO is measured over a >>> long enough period, like months. Short term it might be about as good >>> as the TTL can oscillator inside the PC. >>> >>> On Fri, Jul 22, 2011 at 11:27 AM, Jason Rabel >>> <[email protected]> wrote: >>>> I was just thinking (dangerous I know)... Has anyone attempted to build a >>>> stand-alone oscillator that is disciplined via NTP? >>>> >>>> i.e. NTP keeps it on-frequency... And I'm not talking about NTP that is >>>> locked to a local GPS, I'm curious about purely syncing to >>>> other NTP servers over a network. (The presumption is that you have no >>>> access to GPS, WWVB, Cellular, or similar.) >>>> >>>> Is it even possible or am I just day dreaming? >>>> >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> >>> >>> >> <PPSAGPS.pdf>_______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
