Re: [SI-LIST] : Re: Differential Impedance

Mike Jenkins (jenkins@lsil.com)
Thu, 05 Feb 1998 17:42:44 -0800

Fred,

I agree with your desire for tight coupling, but the problem that's
avoided by separating the traces is mainly one of tolerance. In
board layout, the off-diagonal terms are quite sensitive to etch
tolerance. Another practical issue is measurement -- a lot more
people have (and understand) single-ended TDRs than have dif'l
TDRs.

What I didn't say in the first note was to keep the traces apart
BUT NOT TOO FAR -- about 3 to 5 trace widths is a guideline
that I've seen. Add to this the rule of keeping other traces
away and I think you're reasonably safe from noise, and you can
still get the field cancellation you mention when it's needed,
as when the traces run over a split ground plane or through a
connector.

You are right to bring up this issue that I neglected to mention.

Mike

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