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'")
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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


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