The SHORT answer is NO, you cannot change the width of the ground to equal the width of the trace and keep your 50 ohms! The slightly longer answer is, If you look and the field lines (lines of flux?) around a trace in free air, they will be circular around the trace, but more dense close to the trace and less strong the further away they get. It is a square of the radius I think. Now, if you move a plane close to the trace you will see the lines snap towards the plane, and they will be dense right between the trace and the plane, but moving out from the edges there will be lines curving gently towards the plane. I can't draw this with ASCII characters, sorry, but if you look at a cross section with a plane on either side it will have a sort of hourglass shape, ok? So now if you cut the planes down to the width of the trace most of them will try to get to the plane, but many will be fringing around the sides, and you won't have your 50 ohms anymore. What we used as a general rule (we have done the same thing you are, kind of making a coax effect on board layers) is to make the ground width FIVE TIMES the trace width. Even so this will not be ideal, but past that the effect is so minimal you probably wouldn't care. onward thru the fog... Jack p.s. unfortunately, to calculate at what point the trace is 10% or 20% of ideal takes a fairly interesting calculation, something like delta2V=0, and you keep calculating all different points of potential in the field until they all add up to zero or something like that, I just went with the 5X rule and it worked great. Good Luck -----Original Message----- From: Doug [mailto:[log in to unmask]] Sent: Thursday, April 06, 2000 10:38 AM Subject: Long Question Needs Short Answer ... Hello All, I might as well jump right into the question. Suppose there's a trace construction as follows. It's 50 ohm controlled impedance with a separation "d" and a width of 5 mils between two ground planes in a symmetrical construction. I'm assuming the controlled impedance is calculated by assuming the ground planes go off to infinity. CASE #1: ground plane --------------------------- 5 mils d ----- d --------------------------- ground plane Now, for some reason, someone wants to start cutting back the ground planes to a width W without changing separation "d" of the trace to the ground planes. Shown as below. CASE #2: ground plane | <---- W ----> | ----------------- 5 mils d ----- d ----------------- | <---- W ----> | ground plane Question: Ignore fab tolerances for the moment. At what point does the controlled impedance of the trace begin to be compromised by 1%, 10%, 20% ... ? Could I in fact reduce the width of the ground planes down to 5 mils, equal to the width of the trace, without compromising impedance? Regards, Doug McKean ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################