It doesn't have to be ground. That is why they are always called reference planes.
I personally use + and - voltages with no problems. I also have used split planes
above and below, but I also recommend not to use split plane reference planes in
the areas where controlled impedance traces are or on very high frequencies.
This is just my personal findings.
I like the polar calculator and actually compared my calculator on their findings.
Go to www.pcbdr.com and get the spread sheet calculator. or directly to
http://home.hiwaay.net/~robdne/files/Beta%20ResourceV4.xls to use the
calculator online. (click on the "stack" tab at the bottom)
The updated version is much better. These calculate the entire board and
adds the thickness on the fly.

Chris Robertson
Author "PCB Designer's Reference"
[log in to unmask]
ISBN: 0130674818
Search for it at www.Amazon.com

  ----- Original Message -----
  From: John Parsons
  To: [log in to unmask]
  Sent: Wednesday, November 19, 2003 1:14 PM
  Subject: [TN] Co-planar Impedance Calculations


  Greetings all,

  I have a customer who is doing an impedance design for Intel and he has no
  previous impedance experience.  This is the information I have.

  - double sided board
  - target impedance 50ohm
  - signal line 5mil wide, spaced 5mil from "ground" trace and this pair of
  traces should be spaced a minimum of 20mil from adjacent traces.

  This is what he was given from Intel.  I am using an older Polar model
  (CITS25 calculator) which does have models for co-planar designs but from
  what I can tell they assume that the signal trace is sandwiched between
  ground (return lines) on both sides.  Is this a correct interpretation?  Is
  it possible to model the design as described above?

  While we have some experience with controlled impedance I have no experience
  with the aforementioned design and could use some assistance.

  Best Regards
  John Parsons

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