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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