Nirmal,
All very true. But my question is really about capacative
mechanisms in laminate providing a path for return currents.
If we take the example layup, with two signal layers in a
stripline configuration, conventional layout technique is
to contain a net within this stripline cavity. If this is
done, it appears to me that the capacitor formed by the
three sheets of laminate between the reference planes may
provide a part or all of the path for return currents of
signals changing layers within that cavity.
------------- reference plane
--- wiring, vertical
--- wiring, horizontal
------------- reference plane
The reason a typical layout will take these inner two layers
as a pair is that the outer layers are usually too congested
with components to make a viable pairing with a clear layer.
(if your horizontal layer is 50% covered by component lands
then a 100% clear vertical layer doesn't do you much good)
So my question remains, True (no other d-caps needed)?
Poor (rely on this path and I'll step on your signal)?
False (too high a capacitance to accomodate return currents -
discrete caps needed)?
Best regards,
--
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
jseeger "at" appliedcad "dot" com 978 649 9800