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

Unruh unruh-spam at physics.ubc.ca
Fri Jun 12 08:20:36 UTC 2009


"David J Taylor" <david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid> writes:

>David Lord wrote:
>[]
>> Late last night I got round to connecting the Garmin GPS18x-LVC and
>> just after midnight it stepped from around 60s to 0.5ms and 8 hours
>> later is between -74us and +62us. The module is in south facing
>> upstairs window in same location I failed to get anything useful
>> from the Globalsat BR304. Now I need to make a 30m extender cable to
>> get the signals downstairs to back of house to connect to servers.
>>
>> David

>Good news, David.  Sensitive little pill-box, isn't it?

>One suggestion might be wireless - could you get a very atom-based small 
>PC or even a wireless router to accept the GPS signal and serve NTP over 
>the WLAN?  Cable would be nicer and my preference.  With Dave Hart's new 
>PPS support it /should/ be possible to send just the PPS signal around 
>(two wires), and use that to refine an Internet-derived time.  I might be 
>inclined to go for a screened cable if you need a 30m run, but thin 3-core 
>mains cable /might/ suffice.  The RS-232 I used to use was very tolerant 
>of cabling, but it probably had higher signal levels and robustness than 
>the GPS 18x LVC.

I use cat 5e cable since it is cheap, available and is supposed to be
good for better than about 100MMHz
(10nsec) signals. I would not use usb cable or any old wire, because of
the problems of spread of the pulse from the gps. Also I would terminate
it properly as well so that signal  reflections could be problematic. on
long runs.
If the run is less than 10m it should be ok even with improper
termination.


>Cheers,
>David 




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