[ntp:questions] Re: Best way to harness multiple stratum 1 servers

Terje Mathisen terje.mathisen at hda.hydro.com
Thu Apr 6 06:59:55 UTC 2006


John Ackermann N8UR wrote:
> Hal Murray said the following on 04/05/2006 03:08 PM:
> 
>> As a practcal matter, the best is probably the one that's that most
>> reliable.  Or the one that's most convenient.  Or the one with the
>> fastest CPU or best network connection.
>>
>> The last few nano/micro seconds won't make much difference
>> after a network hop.
> 
> You're probably right, but my problem (if you can call having too many
> time sources a problem!) is that it's not intuitively clear which one of
> the servers is the best, other than that the WWVB server is
> significantly worse than the GPS and Cs ones.  But the GPS and Cs ones
> have very similar performance.  To the outside world it doesn't matter
> because the internet and even my firewall performance will mask
> differences at this level, but as an academic exercise, and on the local
> wire, I'd like to see if I can wring the best possible performance out
> of the system.
> 
> I'm probably asking NTP to perform at a level it's not really designed
> for, and that perhaps the OS/hardware can't even support (in terms of
> system clock stability and time resolution -- though a longer term goal
> is to replace the mobo clock with a source derived from the Cs to solve
> at least one problem).

The latter is the PHK way: Use a frequency synthesizer to generate the 
required input frequency for your chosen motherboard, using the 10 MHz 
(or whatever) Cs output as the source.

The next step is to build a 10 MHz (or faster!) counter which will reset 
on each PPS signal, then count until latched by a read signal from the 
NTP server. This allows you to get rid of the variable interrupt latency 
problem.

Terje

-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"




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