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

Scott McMorrow (scott@vasthorizons.com)
Fri, 27 Aug 1999 22:17:04 -0700

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Lum,

What you have heard is wrong. At 1X trace spacing there can be
around 10% coupling of the signal across the meander. This is
seen as an apparent lowering of the characteristic impedance
in the vicinity of the meander. Additionally it causes waveform
distortion, overshoot and undershoot (negative overshoot for the purist.)

These effects are especially critical with Rambus due to
the extremely low skew and noise margins required by the bus.
Total signal swing is 800mV and total timing skew allowance
is on the order of 150pS. Operating frequency is from
0 to 400MHz.

They are additionally critical to any high resolution, low noise
signal, such as skew controlled clocks. As a result, I always
recommend trace meander spacing of 2X the trace width.

With a good simulator the negative effects of trace meanders
can be predicted accurately.

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
Lum Wee Mei wrote:

> 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 -

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