Hi There is a standard NTP driver that "talks Thunderbolt". The full blown NTP package is pretty easy to set up. It will hold ms accuracy slaved to a GPS.
Bob On Jul 29, 2010, at 7:29 PM, Hal Murray <[email protected]> wrote: > >> The telescope can point to within arc seconds of a star and that implies >> that the computer clock needs to be within 50 ms. > >> If he does a Windows NTP sync first thing in the evening after a few hours >> there's too big an error. > > That looks like the classic time vs frequency problem. > > What is the primary goal? Pointing or tracking? Pointing requires time. > Tracking requires frequency. > > If you are otherwise happy with "Windows NTP sync", you may be able to solve > your problem by doing a sync before pointing at another object if it's been > more than N hours since the last sync. > > > Long song and dance.... [Remember, I don't run Windows so I may screwup > anything that's Windows specific.] > > > The typical PC has 2 crystals. One runs at 32 KHz. The other is usually > 14.xxx (from early PC days) that gets PLLed up to make clocks for the CPU and > PCI and USB and ... > > The 32KHz crystal runs the battery backed RTC/TOY/CMOS clock. It's a watch > crystal so it should be pretty good. But it's not very convenient for > keeping time at the microsecond level. > > The 14 MHz crystal is stable, but typically not very accurate. (Remember low > cost.) That's accurate at the PPM level, it will be fine if you just put a > scope on it. It may be off by 50 PPM. Even if the hardware is good, the > software can screw things up. (Linux is good at this. Current kernels don't > get a consistent answer on the same hardware. Jumps by 200 PPM from boot to > boot are not uncommon.) > > [Network and audio and ??? cards typically have a separate crystal. They are > usually not convenient for timekeeping but if you do serious audio work you > can measure it's actual frequency.] > > Let's see if I can do the math right... > > 3 hours is 10,000 seconds. 50 PPM times 10,000 seconds is 500,000 > microseconds. So if the clock is off by 50 PPM, it will drift 1/2 second in > 3 hours. Even 5 PPM will drift 50 ms in 3 hours. > > > The main reason for running real ntpd rather than just setting the time > occasionally is that ntpd will figure out how far off the frequency is and > correct for it. ntpd calls that fudge factor "drift" and prints it out in > PPM with 3 digits to the right of the decimal point. > > If all you need is 50 ms, you should be able to get that most of the time by > just running ntpd over the net. It's sure worth a try. It may not be good > enough if you have a crappy net connection or change from no-load to > uploading tons of data from observations earlier in the evening. (Contact me > off list if you want help in monitoring a ntp server and/or setting up and > interpreting its log files.) > > > > Odds and ends to keep in mind... > > Modern PCs use spread spectrum clocking. That fudges things by 1 or 1/2 % or > so which is huge in terms of PPM. The point is that you have to measure it. > Just doing the math from the nominal CPU frequency isn't good enough. > > The actual frequency is temperature dependent, so things will change if you > open the roof and let the cold air in or the CPU changes from idle (or off) > to working hard. The ballpark is 1 PPM per 10 F. > > One of the classic ways to screwup timekeeping is to miss interrupts, > typically because some other interrupt routine is running too long. This was > easy to tickle on (very) old Linux systems that used PIO rather than DMA for > disk transfers. I only mention it because you might have some strange > hardware with buggy interrupt routines. > > Normal Windows clocks tick every 10 ms. Windows has a multimedia mode that > does much better. There is a switch in the Registry or something. It may > help to use that mode. I think you want to leave it on. (The Meinberg > ntpd-installer package turns it on.) > > ntpd is both a client and server. A system will act as a client to get time > from lower stratum servers and act as a server to provide time to higher > stratum servers. > > > > -- > These are my opinions, not necessarily my employer's. I hate spam. > > > > > _______________________________________________ > 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.
