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

D. C. Sessions (dc.sessions@vlsi.com)
Fri, 27 Aug 1999 12:47:53 -0700

Allen Wang 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.

My first suspicion is that your trace width is *not* the same in both cases
because the turns increase the mean trace width. Are you radiusing your turns?

> +---------------------------------------------------------------------------
> ------------------------------------+
> Straight line
>
> +-----------+ +------+ +---- \\\\ -----+
> +------------------------+
> | | | | | |
> | | | | | |
> +-----+ +----+ +---+
> line with turns
>
> Allen
>
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-- 
D. C. Sessions
dc.sessions@vlsi.com

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