RE: [SI-LIST] : What is effect of adjacent signal layer on PCB on

Ingraham, Andrew ([email protected])
Thu, 17 Jun 1999 09:15:38 -0400

One way to look at this is that each crossover on the adjacent signal layer,
is a small capacitance, whose value depends on the trace widths, separation,
etc. For first order you can consider it a capacitance to AC ground, if the
two signals aren't correlated with one another, do not usually switch at the
same time in the same/opposite direction.

Small capacitors along a transmission line both reduce Zo and increase
delay. There are simple formulas for this; it depends on the concentration
of capacitors (crossovers) per unit length. (Such as: section 4.4.3 in
"High Speed Digital Design" by Johnson & Graham, or ? in the "MECL Design
Data Book" - my copy is temporarily unavailable).

The closer you put each signal layer to its nearest plane layer and further
from the other signal layer, the better.


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