Unruh wrote: > David Woolley <[EMAIL PROTECTED]> 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! _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
