[time-nuts] Thunderbolts, time references, NTP etc.

Ralph Smith ralph at ralphsmith.org
Mon Jun 15 23:13:10 EDT 2009


On Jun 15, 2009, at 9:07 AM, Dave Baxter wrote:

> Is this good value, and a trusted seller?

I know nothing about the seller, but the price (US $159) looks  
reasonable, considering the inclusion of the antenna and power supply.

> My intended main application, is to drive a local NTP server (Network
> Time Protocol) as I'm getting more and more P'd off with my ISP's
> ineptitude in maintaining a network where NTP (or anything else not
> related to web crawling or email) works with any reliability.  (Huge  
> and
> variable latency, ping to ping, at different times of day, confirmed  
> by
> other users of the same ISP.)
>
> I run (just because I can!) a HF beacon monitor station, running the
> Faros software from Alex VE3NEA.  That uses NTP and only NTP to
> synchronise it's software clock.  (http://www.dxatlas.com/Faros/)  The
> results of that can be seen at http://g8kbv.homeip.net:8008/  The  
> white
> delay spots illustrate where the NTP source is messed up.
>
> The 10MHz output would perhaps be useful, once I have modified a radio
> or two to use that as a reference for their synthesizers, but that is
> not necessary as yet.   I had a GPS with a 1pps output, but due to a  
> PSU
> malfunction, it doesnt work any more sadly.  :-(
>
> Mind you, I'm, still having difficulty configuring a FreeBSD box for  
> NTP
> server use.   The instructions to do so I have are good it seems
> (http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm) just that the system
> does not seem to co-operate, and I'm not that familiar with
> Linux/FreeBSD etc at the basic user level, let alone re-compiling the
> kernel.  As evidenced by a non booting system, the two times I
> eventually got it to compile (that takes hours to complete) and that  
> was
> not even with modified sources, just what came on the CD  
> unmolested.  As
> each reload takes an age, I sort of lost the plot with that method!

You should be able to get an operating NTP configuration off of the  
stock FreeBSD install.  Please note that the web page you cited is  
somewhat outdated, the current production FreeBSD release is 7.2.  The  
first thing I would suggest is to get a default FreeBSD install  
running with NTP using servers from the internet (if you're in the UK  
I would start with the UK NTP pool servers 0.uk.pool.ntp.org,  
1.uk.pool.ntp.org, etc.).  You should be able to do this without any  
recompiling.

Once you have the FreeBSD system operating with NTP I would then work  
on the GPS synchronization.  Note that the Thunderbolt PPS width is 10  
microseconds, and for PPS detection to work on a FreeBSD serial port  
the pulse should be on the order of a couple of milliseconds wide.   
You can use a pulse stretcher, such as the TAPR FatPPS (http://www.tapr.org/kits_fatpps.html 
) to generate a long enough pulse.  Using PC hardware, with FreeBSD  
you should be able to get the NTP server synchronized within a few  
tens of microseconds of UTC, using PPS into the serial port.

You may also consider a device such as the Garmin GPS18 if all you  
want is an economical stratum 1 time server.

> The local Linux User Group is not that much help either, no one has  
> any
> experience with this sort of "engineering" thing.
>
> Ultimately, I'd like to run something like this on a non-PC hardware
> platform.

Now you get into the fun stuff.  A device such as the Soekris Net4501  
(124 Euro, US $148 new) <http://www.soekris.com/> makes a wonderful  
NTP server.  Unmodified it works quite well.  One simple hardware  
modification allows much more precise timestamping of the PPS,  
allowing synchronization within a couple microseconds.  Going further,  
replacing the stock crystal with a precision frequency source, which  
you can derive from the Thunderbolt's 10 MHz, will allow the NTP  
server to synchronize itself within 150 ns.  John Ackerman describes  
this at <http://www.febo.com/pages/soekris/>.  I have built one based  
on John's description, with a few alterations, and it works extremely  
well.

First things first, though.  For a beginner I would: 1) NTP  
synchronized to internet servers using stock FreeBSD install, 2) then  
get NTP working with PPS, 3) Then look at embedded-type device such as  
Net4501.

I, and I'm sure there are others here, would be happy to help with any  
questions, going off-list if need be to avoid cluttering the list.

Ralph




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