On 9/15/2014 3:04 PM, Tom Miller wrote:
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.
It's not nearly that simple. 8 nF distributed along 100 M is not the same as an 8 nF cap at the source.
The example I gave was measuring 1 M from the source, then 100 M of cable beyond that. If the signal has a rise time of, say, 20 ns (Thunderbolt max spec), then anything added cable of more than 3 M (~ 10 ns one way) simply doesn't matter - there's no time for anything which happens to the signal beyond that distance to return in less than the rise time. The signal during the rise time can't "know" whether there's 3 M or 1000 M of additional cable, or whether the far end is terminated or not.
I don't pretend to fully understand transmission lines, but I do know the basics.
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