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

Mike S mikes at flatsurface.com
Mon Sep 15 18:10:03 EDT 2014

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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