Re: [SI-LIST] : PCB impedance variation v.s. layout pattern

Doug Brooks (doug@eskimo.com)
Fri, 27 Aug 1999 12:40:26 -0700

Are you looking at a single illustration? We have found that Z varies from
board2board, from trace2trace on the same board, and from place2place on
the same trace! We have never seen a case where ordinary turns changed Z.
Before you jump to conclusions, be sure you can replicate this on more than
one board.

Doug Brooks

At 10:17 AM 8/27/99 -0700, you wrote:
>Dear all:
>
>We are designing a Rambus related product which requires the PCB to
>maintain
>28 Ohm +/- 10% trace impedance.
>However, what we have found that a straight line trace will get a higher
>impedance (through TDR, 1 Ohm or more) than
> a trace with many turns on it. We know a turn on the trace will change
>the
>impedance. However, is it the root cause
>that make our trace impedance not consistent?
>
> Two traces are on the same layer, with the same trace width.
>
>
>+---------------------------------------------------------------------------
>------------------------------------+
> Straight line
>
> +-----------+ +------+ +---- \\\\ -----+
>+------------------------+
> | | | | | |
> | | | | | |
> +-----+ +----+ +---+
> line with turns
>
>
>Allen
>
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>
>
>
.
****************************************************
Doug Brooks, President doug@eskimo.com
UltraCAD Design, Inc. http://www.ultracad.com

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