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

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Fri Jun 12 12:31:27 UTC 2009

David Lord wrote:
>> 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? :-)

No, but I'm using Windows which is known to be not as good as FreeBSD.

> 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.

I'd be interested to know the results.

> 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.

Yes, my GPS 18x LVC is indoors, under a sloping roof, in Edinburgh (at 56 
degrees north), and has a fairly poor sky view.  I managed to get my GPS 
18 LVC outside just on the roof.  Both appear to have adequate coverage.

>> 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

Cupboard keeps the PCs from overheating?  Doesn't it restrict air flow? 
If you add air conditioning to the cupboard, a little noisy and power 
hungry?  I would try and either go for a low-power PC (perhaps Intel atom 
based, perhaps flash-disk), or cut down the power consumed by the PC and 
try to keep the load constant.  Turn down the clock in the BIOS, remove or 
disconnect the power from unwanted devices (CD/DVD drives), use "green" 
HDs etc. etc.

All way over the top for time-keeping "better than your ISP", of course!


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