[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

David Lord snews at lordynet.org
Fri Jun 12 11:46:10 UTC 2009


David J Taylor wrote:
> David Lord wrote:
> []
>> First I don't think there's any problem getting the nmea down
>> twisted pair at 4800 baud.
> 
> .. agreed, but .....
> 
>> To play safe with the ttl level pps I've setup to use a simple
>> emitter follower that can drive into 75r coax.
>>
>> I didn't want to use line drivers as I don't have any handy that
>> work off single 5V supply.
>>
>> I need a converter back to ttl before going to pc and don't have
>> any spare gates in the box with compulsory flashing led and
>> ticker, so will use another couple of transistors wired as
>> Schmitt trigger.
>>
>>
>> David
> 
> I think you're needlessly overcomplicating the PPS signal.  I have the 
> GPS 18LVC feeding /two/ parallel RS-232 interfaces, /and/ a low-current 
> LED, so for a 30m length I think you would get away with a simple wire 
> connection.  However, have fun with the soldering iron!

Are you getting better than 10us offsets though? :-)

I've no idea how much signal degradation there would be using
utp for pps and will both put it on spice and keep the spare
pair in cable free to give it a try.

However I'm making up various pcbs so an extra couple of tiny
jobs won't add much to the fun and is playing safe in case I
end up mounting aerials on roof or in garden, although that
looks very much less likely to be needed now.


> If you're seeing 4us offset now, that may be as good as it gets.  You 
> may need a more constant temperature to see better.

That's another project, putting the servers in a cupboard
with temperature control. Alternative of locating the crystals
and fitting heater is also a possibility. Cupboard idea has
advantage of cutting down noise and will help keep the pcs
from overheating.

David




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