Dave,
     It sounds like you are confusing dual stripline with broadside coupled
     differential striplines.  Dual stripline and offset stripline are
     essentially the same animal, from a modeling standpoint.  Dual
     stripline implies two signal layers at 90 degrees to one another
     centered between two planes.  The fact of the matter is that each of
     the two signal layers is an offset stripline.  Offset simply means
     that the trace is not centered between the planes.  There is, in
     reality some small degree of interaction between the signal layers of
     a dual stripline since a potential capacitor is created at every
     crossover.  My guess is that it is pretty insignificant, since this is
     a very common approach to getting more controlled signals but keeping
     the overall layer count down.  I'm not a EE but have built my share of
     impedance boards.

     Hope this helps...Andy Slade


______________________________ Reply Separator _________________________________
Subject: Re: [TN] Z-control
Author:  David Curtis <[log in to unmask]> at SMTPLink-Hadco
Date:    01/08/99 11:43 AM


Thanks for the reply Dave and Steve.

Please correct me if I am wrong, but from my little bit of digging here I found
that the difference between
dual stripline and offset stripline (with mirrored dielectric thicknesses) is
that in the former the signal
layers interact and in the latter they do not (usually because the traces are
aligned 90 degrees from one
layer to the other).

Question: Under what other circumstances is it appropriate to use the offset
stripline, that is when can I be
sure the interaction is minimal and when is it not (despite what the designer
says).


Obviously I'm new to this, so a good handbook/tutorial/reference specifically
for PCBs would be nice. Know of
anything good.?

Thanks again,

Dave

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