Mail*Link(r) SMTP FWD>RE>Controlled Impedance - Reference Planes Adding more stuff to Bob's posting-- There are other types of transmission lines other that multiple layered types. A single wire in free space is a transmission line, we frequently call it an antenna. All free space is the capacitively coupled "ground plane", the free space impedance of a wire is about 377 ohms. There are transmission lines (and wave guides) that are located in one plane, these are frequently called "slot" or "slotted" and "coplanar" transmission lines, or more frequently wave guides; others require some form of a "conductive" enclosure. The following are some of the more common: Slot or Slotted Coplanar Waveguide gnd. cond sig gnd. cond cccccccccccs____sccccccccccc ccccccccs__scccccs__scccccccc dddddddddddddddddddddddddddd ddddddddddddddddddddddddddddd dddddddddddddddddddddddddddd ddddddddddddddddddddddddddddd Fin Line Suspended Stripline "Trapped" Inverted Microstrip cccccccccccccccccccc ccccccccccccccccccc c ddc c c c ddddddddddddddddddddd c ddc c c cccc c ddddddddddddddddddddd c dd| c cdddddddddddddddddc ccccc ccccc ccccc c ddc c cdddddddddddddddddc ccccc ccccc c ddc c c c ccccccccccccccccccccc cccccccccccccccccccc ccccccccccccccccccc ccccccccccccccccccccc "c" is a conductor, either a metal pattern or a metal box "s" is where the primary signal current flows __ and | is a gap in the conductive patterns (i.e. a non-conductive pattern) "d" is the dielectric as a "solid" material " " the spaces with the illustrations are a fluid dielectric, generally air. (;-) The above is more stuff that you probably did not want to know. (;-) Ralph Hersey e-mail: [log in to unmask] -------------------------------------- Date: 5/30/96 5:40 AM From: R_R_HOLMES The question concerns the proper relationship between reference planes and signal lines in a controlled impedance design. The reference plane(s) for a controlled impedance signal line is the plane or planes that determine the capacitance to ground for that line. In the case of stripline (planes on both sides of the conductor), these are the nearest power/ground planes above and below the conductor. In the case of microstrip (plane on one side only), it is the first plane below the conductor. The reference plane(s) do not have to be adjacent to the signal. Intermediate signal layeres will have little effect on the capacitance coupling. The only issue is the separation between the signal and the reference plane(s). As that increases, the capacitance coupling decreases and the signal impedance increases. The critical issue for controlled impedance is that the ground planes be the correct distance from the trace, not that they be adjacent to the trace. Bob Holmes PhD Lucent Technologies (Formerly AT&T) [log in to unmask] #012#------------- Begin Original Message ------------- From: ig4.att.att.com!ipc.org!TechNet-request Date: Wed May 29 15:15:31 -0700 1996 Subject: Controlled Impedance - Reference Planes To: !"'IPC To: [log in to unmask] (!TechNet'") Content-Type: Text Content-Length: 412 This controlled impedance stuff is new to us... All the reading I have done seems to imply that reference planes are adjacent to the signal plane. Is this typically the case, or always the case? I have a customer who keeps submitting designs which call for a controlled impedance line where the reference plane is not adjacent. Any input in helping us understand this would be appreciated. Regards ------------------ RFC822 Header Follows ------------------ Received: by quickmail.llnl.gov with SMTP;30 May 1996 05:40:22 -0700 Received: from ipc.org by simon.ipc.org via SMTP (940816.SGI.8.6.9/940406.SGI) id HAA25529; Thu, 30 May 1996 07:36:13 -0700 Resent-Date: Thu, 30 May 1996 07:36:13 -0700 Received: by ipc.org (Smail3.1.28.1 #2) id m0uP6du-0000DGC; Thu, 30 May 96 07:15 CDT Resent-Sender: [log in to unmask] Old-Return-Path: <[log in to unmask]> Message-Id: <[log in to unmask]> Date: 30 May 96 08:02:34 -0400 From: [log in to unmask] (R_R_HOLMES) Original-Date: Thu May 30 08:02:34 EDT 1996 Phone: 804-226-5114 Subject: Re: Controlled Impedance - Reference Planes In-Reply-To: your message of Wed May 29 15:15:31 -0700 1996 Content-Type: Text To: ipc.org!att!TechNet Resent-Message-ID: <"EvI-B.0.kGK.g5Phn"@ipc> Resent-From: [log in to unmask] X-Mailing-List: <[log in to unmask]> archive/latest/4425 X-Loop: [log in to unmask] Precedence: list Resent-Sender: [log in to unmask]