RE: [SI-LIST] : via effect

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From: Ron Miller (rmiller@brocade.com)
Date: Fri Jun 01 2001 - 15:20:27 PDT


Stub effect can also cause excess capacitance.

1/4 wavelength open stub at any frequency is equivalent to a short.

less than 1/4 wavelength open stub looks capacitive.

When the signal only traverses a part of the via and the via continues
to the surface of the board it creates a stub which appears capacitive.

At gigahertz rates with higher frequency components it becomes a real
problem

Ron Miller

-----Original Message-----
From: Mike Jenkins [mailto:jenkins@lsil.com]
Sent: Friday, June 01, 2001 11:58 AM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : via effect

V R,

I wanted to try to be the first with the answer....YES!! :)

v r wrote:
>
> Hi SIgurus,
> i have a question .is the presense of a via along a trace inductive or
capacitive?
> regards
> vr
....

Seriously, the via itself is probably capacitive, due to increased
coupling to the ground planes thru which the via extends.

However, depending on the route the return current is forced to
take (i.e., separating it from the signal current thru the via),
the net effect of a via may be inductive.

Regards,
Mike

-- 
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