A LOT depends on the frequencies you are talking about.
At higher frequencies, the reactance of the caps connecting the 5V plane to
ground will be higher than the direct connection of the 2.6V plane directly
to the driver/receiver of the signal. Since this reactance is higher, the
signal return will tend to be on the path of least resistance (reactance),
and will tend to be on the 2.6V plane. If the distance between that plane
and the signal is not what is needed for the impedance requirement, you are
sunk. Especially if the signal impedances are calculated at 1MHz (as many
are).
--
George Patrick
Tektronix, Inc.
Central Engineering, PCB Design Group
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
Phone: 503-627-5272 Fax: 503-627-5587
http://www.tektronix.com <http://www.tektronix.com>
http://www.pcb-designer.com
It's my opinion, not Tektronix'
-----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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