[ntp:questions] Simple but good NTP server

John Ackermann N8UR jra at febo.com
Mon Jan 25 18:21:01 UTC 2010


Jan Ceuleers wrote:
> Terje Mathisen wrote:
>> The canonical DIY ntp server would be to base them on phk's choice, the
>> Soekris single-board computer:
>>
>> http://phk.freebsd.dk/soekris/pps/
>>
>> Since this board has a hw counter capable of accurately timing the PPS
>> signals,Poul-Henning got it to run at sub-us accuracy, using a cheap
>> timing GPS.
> 
> A few more points:
> 
> - It does not explicitly say so at the page above, but the Soekris model that Poul-Henning used was the 4501. I've only got 4801s and they're not as good for timing.
> 
> - The results shown on the above page are of a 4501 that has been significantly hacked by adding a Rubidium oscillator to the mix. Not for the faint hearted and not cheap either.
> 
> - The net4501 costs €136 for the board and case. Add €15 for the power supply. Then add around €100 for a GPS riming receiver and another $1700 for the Rubidium standard. Admittedly the latter is optional if your needs are modest.

You're right that the hardware hacks (documented on my web site at 
http://www.febo.com/pages/soekris) aren't for the faint-hearted.  I'd 
like to stress, though, that you don't need an expensive Rubidium or 
Cesium standard to make a noticeable improvement in timekeeping, 
particularly short-term response to things like ambient temperature 
changes.  An inexpensive (<$20) temperature compensated crystal 
oscillator (TCXO) will be significantly better than the stock crystal on 
the Soekris (or any other PC).

The quality of the oscillator will impact stability over time periods 
shorter than NTP's loop time constant, but will have no effect over 
longer time intervals where NTP steers the clock.  The basic idea is 
that *anything* will be more stable than the crystal on the motherboard. 
  BTW -- if you get a TXCO operating directly at 33.333 MHz, you don't 
need the "ClockBlock" board either, which saves $70 over the 
configuration shown in my article.

And if you can live with a surplus solution, there are tons of LPRO-101 
Rubidium oscillators available on eBay for less than $100.  They require 
soldering to interface, but if you're already hacking a Soekris board, 
the wiring is trivial.

John




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