[time-nuts] Chooses for a desktop/server NTP external 1PPS reference

[Alexander Sack]

On Tue, Dec 8, 2009 at 3:43 PM, Bruce Griffiths
<bruce.griffiths at xtra.co.nz> wrote:
> Alexander Sack wrote:
>>
>> On Tue, Dec 8, 2009 at 3:01 AM, Robert Atkinson<robert8rpi at yahoo.co.uk>
>>  wrote:
>>
>>>
>>> Hi Alexander,
>>> Welcome to the list.
>>> Another professional option is the Trimble Accutime Gold.
>>> http://www.trimble.com/timing/  The older Accutime 2000 and Palisade turn up
>>> on ebay. A desk top box is the CNS Clock II
>>> http://www.cnssys.com/cnsclock/CNSClockII.php cost about $3000 complete.
>>> Some idea on if you want new / professional or used / amateur and if you
>>> are a keen constructor would allow more targeted advice.
>>>
>>>
>>>
>>
>> Thanks Robert!
>>
>> Yeah I didn't really specify price or performance metrics in my
>> request.  I think I should document this stuff and write up a
>> beginner's FAQ (I didn't see anything on LeapSecond.com).
>>
>> Alright, the reason WHY I didn't actually specify much is because I
>> really don't know WHAT to look for.  I found out today that the
>> rinky-dinky Endrun Technologies Cf/Ct receivers are 1k each (GPS and
>> CDMA variants)!  I was kinda shocked because there ain't much to these
>> units AFAIK.  So I want to step back for a second (pun intended?) and
>> ask everyone, WHAT SHOULD I BE LOOKING FOR?  Say I want nanosecond
>> accuracy with respect to the PPS rising edge to absolute UTC....I know
>> that for sure.  My target system is FreeBSD though I would like to use
>> something with Snow Leopard as well (even if that's just ntpd synced
>> locally to the FreeBSD box).  I am interested in running NanoBSD on a
>> stand alone system.
>>
>> As for as assembly, I'm open, though I haven't soldered anything since
>> undergraduate engineering school in a lab.  I am a software guy by
>> trade.
>>
>> Another issue is what governs the price of these units?  I'd like to
>> think its not going to be like the audio industry.
>>
>> Hope that helps,
>>
>> -aps
>>
>>
>
> Alexander
>
> If you want the best performance possible from nanobsd then using an M12M or
> M12+T is advisable.
> In this case hardware timestamping (with a resolution of about 100ns) of the
> PPS signal is used together with a customised version of the driver created
> by Poul-Henning Kamp.
>
> The cheapest option would be to obtain used  M12+T timing receiver boards.
> however using these would entail constructing a board with a TTL/CMOS to
> RS232 transceiver and adding a 50 ohm driver for the PPS signal (required
> for highest time stamping precision as it avoids the delay of an RS232
> driver and receiver). The only complication being that the receiver board
> uses a mating connector with 1.27mm pitch rather than the more common 2.54mm
> pitch connectors used by older receivers.

Ahh thanks for this info.  I was gravitating toward trying to setup
something similar using phk's ntpd driver?  I need to read more.

> Of course network congestion and queueing delays add to the timing noise
> seen by client machines on the network.

Right.

> If one used PTP (IEEE 1588) instead of ntp then the performance can be
> substantially improved if one can avoid standard network switches with their
> queueing delays.
> However PTP is relatively new and as yet IEEE 1588gear hasn't yet hit the
> surplus market.
> Usually PTP uses hardware timestamping of the relevant network packets,
> however there have been some efforts at software timestamping.

Yes but in fact I would like to play with IEEE 1588 as well for work reasons!

-aps