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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 13 21:47:19 UTC 2007


Todd wrote:
> 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.
>
>   
What about making use of cell phone network timing?
Can you actually receive/use a cell phone within the data center?

Bruce




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