Hal Murray wrote: >> I have started to see this 5mhz frequency quartz oscilators that are >> really inexpensive (see http://www.alltronics.com/cgi-bin/ >> category.cgi?item=04P010 ). My question, how would you get this to >> work with ntpd? Seems like it work great for my situation where I can >> just put inside the rack behind the server, use some ntp servers to >> get the time to within 1 msec usually and then use this to get into >> the nanosecond accuracy. > > There are two issues with keeping time. > > The first is to get your clock to tick at the right frequency, or to know how > fast your clock is actually ticking. > > ntpd is pretty good at that. If you haven't done it already, turn on > loopstats and graph the offset and drift columns. The drift is basically the > difference between the nominal frequency and the measured frequency, aka the > inaccuracy of the crystal. Most crystals make reasonable thermometers. > > The other issue is lining up the seconds ticks. You can get reasonable > results by asking nearby ntp servers that know the answer. How good you get > depends mostly on your network connection. If you want better than a few > milliseconds, you need a local PPS source. >
So I pretty much had things backwards? using the PPS for the "tick" and then network ntp servers to get it pretty close to the real time. > If you want to get started, I recommend the Garmin GPS 18 LVC. Under $100. > Some soldering required. Not much. (It needs 5V. You can get it from USB.) > http://support.ntp.org/bin/view/Support/ConfiguringGarminRefclocks > I got mine at ProVantage.com There are some on eBay, but the picture is the > USB version. > > You want the OEM version - without maps or software. Yeah I've looked into that. My problem is the server I want to do the really precise measurement of is located in a secure datacenter that they're not going to let me drill a hole in the roof just so I can get really really accurate time when really ntp works just fine. I may end up looking into it at home, but even if I run it at home and then use it as a source for the server at the datacenter you then introduce the network latency into it and I pretty much lose any benefit of using a local time source. That's why I saw this little crystal oscilator and thought that could be used for a PPS signal. For the curious: http://rikku.vrillusions.com/ntp/ (i have a bunch of "noselect" ones I'm just evaluating network latency and such) So basically just from ntp servers it stays under 1msec usually although it has been a little more bumpy recently. -- Todd http://www.vrillusions.com/ My PGP Key ID: 0xBC90230C
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