It also appears that crosstalk is much more of an issue than the changes in
> Good question Chris.
> A paper was written on this exact subject. Author was Thomas J. Buck.
> To summarize: you will see an impedance drop if the signal in question
> enough crossover of the orthogonal signal layer. However
> layer separation plays a part in this. In Buck's paper, he saw *approx*
> 8 ohms
> drop in impedance on a 60 ohm trace crossing about 30 traces/in in the
> orth. layer.
> You might want to build some sample boards of your planned design and
> test them.
> The experience can be very enlightening.
> Hope this *helps*.
> Bill Dempsey
> DNA Enterprises, Inc.
> p.s. I can send you a copy of Buck's paper (snail mail) if you send me
> an address.
> Chris Bobek wrote:
> > When you have a PCB stackup that has a signal plane sandwiched between
> > ground planes, you have stripline. Zo at this point is fairly easy to
> > characterize.
> > Now, if you add a second signal layer in between the two ground planes,
> > in the X direction and the other in the Y, you still have stripline
> > (asymetric), but now you have a bunch of traces crossing the path above
> > below) your signal layer. I understand this introduces crosstalk, but
> > does it do to the characteristic impedance of your traces? Is the
> > still controlled? Does it vary widely, or not have much effect at all?
> > I see a lot of stackups with this arrangement, and I wonder how Zo is
> > maintained.
> > Thanks,
> > Chris
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