[ntp:questions] NMEA ref.clock better than my ISP's timeserver?
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.
>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
If the run is less than 10m it should be ok even with improper
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