[time-nuts] Using a Vectron OCXO 5mhz oscilator with ntpd

Hal Murray hmurray at megapathdsl.net
Wed Dec 12 06:04:12 EST 2007

> 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.

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.)
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.

The main advantage you will get from a good OCXO will be that ntpd doesn't 
have to spend any time tracking the thermal drift of your local crystal.  You 
still need to find the seconds ticks.

These are my opinions, not necessarily my employer's.  I hate spam.

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