Re: [SI-LIST] : What is effect of adjacent signal layer on PCB on Zo?

Chris Bobek (cbobek@cadence.com)
Tue, 15 Jun 1999 10:52:48 -0700

Thanks for your response. This pretty much answered my question, which is:
"Is there a severe effect in Zo with orthogonal signal layers?" The answer
appears to be "no, but there will be *some* effect".

It also appears that crosstalk is much more of an issue than the changes in
Zo.

Thanks,

Chris

> 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
> experiences
> enough crossover of the orthogonal signal layer. However
> signal-to-signal
> 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.
> bdempsey@dnaent.com
>
> 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
two
> > 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,
one
> > 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
(or
> > below) your signal layer. I understand this introduces crosstalk, but
what
> > does it do to the characteristic impedance of your traces? Is the
impedance
> > 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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