[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
hmurray at megapathdsl.net
Mon Sep 15 17:51:39 EDT 2014
tmiller11147 at verizon.net said:
> So does adding ~80 pF per meter or 8 nF for 100 meters (RG58) to your output
> have any effect on the risetime? Because that is what it will see with an
> open cable.
That way of thinking only works if the risetime is long relative to the cable
length. In this context, long enough is ballpark of 4 to 10 times the prop
time of the cable.
For long cables, the cable initially looks like the characteristic impedance
of the cable. You can see that on a scope with the classic reflection
In vacuum, the speed of light is very close to 1 ft/ns. Coax varies from
60-90% of that.
So if you have 10 ft of cable, it's unlikely that your driver is slow enough
for the lumped capacitor approximation to be valid. With shorter cables and
old gear using HC chips, you might get there.
These are my opinions. I hate spam.
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