FreeLists / si-list / [SI-LIST] Impedance measurement using TDR

[Bob Patel <whizplayer@xxxxxxxxx>]

Hi! I was conversing with my colleague about the way
impedance measurements are done using a TDR and the
rise times that are employed.
Our discussion is regarding impedance measurement and
looking at discontinuity on a trace using TDR.
My colleague says that I should use a longer coaxial
cable for injecting the signal from TDR into the short
trace and that way I can mitigate the settling effects
of the TDR.
Well, then the real discontinuities on the trace will
get masked out because the signal will settle down and
I could see this effect for the same length of the
trace except the launch point was far away and so the
signal could settle down. Then the question arises how
do you look at discontinuity and if you do with a fast
rising edge then you could but this will not show up
with the slower or actual rise time of your signals.
My thinking is leading me to the fact that in a PCB
there can be a lot of discontinuity but they could be
will not be present with the rise time of your signal.
I conclude that for a trace which is going to carry a
signal of rise time 100ps then the impedance of the
trace and any discontinuity has to be measured with 
a signal whose rise time = Fknee.
I needed to know how should the impedance be measured
and also look at the discontinuity at the same time.
Bob

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