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

Todd vr at vrillusions.com
Thu Dec 13 10:27:26 EST 2007

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.

My PGP Key ID: 0xBC90230C
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