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

David Lord snews at lordynet.org
Fri Jun 12 10:34:46 UTC 2009


David J Taylor wrote:
> Unruh wrote:
> []
>> 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.
> 
> Cat 5e cable sounds like a good compromise.  On very long runs, you 
> would not only need to terminate the cable correctly, you would need to 
> drive it correctly as well, requiring powered line drivers and receivers 
> at either end.
> 
> In the OP's case, the run is only 30m (i.e. about 100-150ns) so the 
> effect of reflections on a system (RS-232) which is bandwidth-limited to 
> a microsecond or two should be very little, and I would likely use the 
> cable in an unbalanced configuration.

First I don't think there's any problem getting the nmea down
twisted pair at 4800 baud.

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




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