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) ---------------------------------------------------------------------------- ----- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To set a vacation stop for delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To set a vacation stop for delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------