[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.

[Hal Murray]

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 
pictures.

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.

 


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