Re: [SI-LIST] : Impedance calculator

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From: Ray Anderson (raymonda@radium.eng.sun.com)
Date: Tue Nov 09 1999 - 16:14:51 PST


As you pointed out, the addition of vias in a transmission line's
path can (will) alter the lines impedance. however, I think you will
find that the alteration is in the form discontinuities at discrete
locations.

Adding a bunch of vias won't change a distributed 50 ohm line to
say a distributed 44 ohm line. The addition of the vias will place
primarily capacitive loading at those locations where the vias are.

(A more accurate representation is actually a small pi network LC
discontinuity). Thus the impedance of the once 50 ohm line will
take on different values at different locations along the trace.

I'd first create a model for the via (based on via length, # of layers
it traverses, diameter, and Er). You may need several different via
models if you need a highly accurate simulation. Then place these
vias in your spice deck at the appropriate locations thereby dividing
your constant impedance t-line up into a number of shorter segments.
Programs like Ansoft Maxwell and Contec RLGC amongst others can
help you create both the via models and the appropriate t-line models.

If you have lots of vias this will force you to create lots of short
t-line segments which can equate to long simulation run times.

Anyway, that's one approach, I'm sure there are others but the parking
lot beckons....

-Ray Anderson
Sun Microsystems

>
> Dear All,
>
> Most of the impedance calculation softwares only calculate the
> impedance of straight line without through hole or via. But in
> a backplane, there are lots of through holes along with the lines.
> Obviously, these through holes will reduce the impedance of the line.
>
> Does any one know any tool or method which can calculate the
> impedance of a line with vias?
>
> Thanks,
> CT Yang
> ADLink Technology, Inc.
>

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