RE: Ribbon Cable

From: Bil Herd <>
Date: Tue, 17 Dec 2013 16:28:32 -0500
Message-ID: <>
>On 2013-12-17, at 19:46, Bil Herd <> wrote:
>> Cool.  I missed the exact problem, I kind of responded more to the
question of the behavior of ribbon cables.

>It was a very relevant answer! Thank you.

>Out of those you wrote about - is there a software you use and would
recommend for PCB impedance adjustments?

First one I found is at
culator  but it doesn't model a path, just a crossection but will help
visualize the baseline.  I used to use HSPICE when I didn't want to model
a coax or a transmission line as a lump sum but as a length of
Capacitor,Inductor,Resistor. HSPICE made a change to their Coax model for
me in 92 when I showed it deviated from real life at low frequencies,
which points outs that tools are manmade things and not real life.

To think of adjusting impedance along the path, basically to counteract
things that raise or lower impedance too much, keep visualizing the
cross-section.  A good appnote from Altera is here and  where they talk more about
TDR with some good examples.

LVDS example:

I really liked the book
when it came out as I was able to point to some of the math in support of
my more intuitive understanding of noise and propagation.

I have an app note somewhere that has a little demonstration of changing
the ground plane around the trace as a signal goes through a series
termination resistor, basically as it hits the pad, climbs up onto the
resistor, through and back down, you have a ground trace one layer under
it that gets wider and skinnier with the result being a smoother
transition with little abrupt change in impedance.  If I find the app I
can send a link, it's a great way to test visualization of a real life


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Received on 2013-12-17 22:03:04

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