[ntp:questions] Isolated Network Drift Problem

Richard B. Gilbert rgilbert88 at comcast.net
Wed Nov 26 19:25:23 UTC 2008

Unruh wrote:
> David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>> Unruh wrote:
>>> With your 100m setup you really want a buffer amp on the line. At 100m, the
>>> one way trip is .3ms, with reflections every .6ms which might make the
>>> system a bit weird. To get rid of the reflections you need a 100ohm
>>> termination if you use cat5e cable to lengthen the 5m wire on the receiver.
>> Round trip time for 100m of velocity factor 2/3 cable is approximately:
>> 3 * 2 * 100 / (2 * 3E8)
>> = 3E2/3E8
>> = 1 microsecond.
> I was taking the speed to be c. And it is only a one way trip-- from the
> GPS to the computer, not round trip. So I will buy your .5us (having no
> idea what the velocity factor is of cat 5e cable) but not your
> round trip ( which is twice that but is irellevant on a properly terminated
> cable)

"c" is not correct.  "c" is the speed of light in vacuum!  Electricity 
in copper is somewhat slower.  1 nanosecond per foot is a useful rule of 
thumb!  It's an approximation but a useful one.  If you need something 
more exact, you might consult the manufacturer.  Cat 5 cable consists of 
four twisted pairs with two turns to the inch.  The physical and 
electrical lengths of the cable are slightly different!

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