I think the assumption is generally correct. RF follows the path of least
Impedance. Regardless of the plane's function, if there is a return path of
lower impedance, the RF will find it and use it. You have better control
over the impedance value (50 ohms if that is your target) if the return path
and dielectric are controlled and their stack up is such that there is not a
lower impedance path through the power planes or some other feature on the
board.. shielding will most likely be required between potential low
impedance paths for the matched impedance lines.. Closer or farther, the
return path follows the path of least impedance.

Bill Brooks, CID


-----Original Message-----
From: Jack C. Olson [mailto:[log in to unmask]]
Sent: Monday, July 14, 2003 11:56 AM
To: [log in to unmask]
Subject: [DC] Reference Planes for Impedance Controlled Signals

Reference Planes for Impedance Controlled Signals

I might be showing my ignorance by asking this question.
I thought I knew, but now I'm being challenged on it.

If I have impedance controlled signals that are part of
a circuit that is being run on +2.6V or +3.3V, but there
is a closer plane available, like +5V, the signal returns
will snap to the closer one, right? Does it matter which
TYPE of plane is being used a reference? The text I
received from a colleague implies that if my 2.6V plane
is too far from my 4th signal layer, which is next to the
+5V plane, then they will not be impedance controlled.
Doesn't sound right to me...

Jack (the new guy)

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