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

Lum Wee Mei (lweemei@dso.org.sg)
Sat, 28 Aug 1999 11:05:29 +0800

I have heard for meander spacing of 1x of the trace width is more than sufficient. From all the discussions, it seems that Rambus tracks requires more than that. What is this Rambus and why it require a little bit of special
treatment?

Thanks and regards.
- If one do not ask, he will never know -

Scott McMorrow wrote:

> Allen,
>
> Trace meanders will couple from edge to edge across the
> meander. This will cause an effective reduction of the
> characteristic impedance of the trace, unless the meanders
> are spaced quite wide. If you are not currently using them
> you should be using meander spacing that is 2X or 3X of the
> trace width for Rambus tracks. This should be equivalent to
> about a 20 to 30 mil edge to edge seperation of the meander
> tracks.
>
> regards,
>
> scott
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
> 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.
> >
> >
> > +---------------------------------------------------------------------------
> > ------------------------------------+
> > Straight line
> >
> > +-----------+ +------+ +---- \\\\ -----+
> > +------------------------+
> > | | | | | |
> > | | | | | |
> > +-----+ +----+ +---+
> > line with turns
> >
> > Allen
> >
> > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com

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