Scott,
        I can give you an answer, however your engineers may choose to not
accept it. It is from one of the preeminent figures in signal integrity, Lee
Ritchey, it published in the March 1999 PCD magazine.
        Contrary to popular belief you do not have to deal with differential
impedances in your PCB traces. This is the fallacy of this design dilemma.
The simple solution is two equal length (+/- 500 mils) singular controlled
impedance lines routed along roughly the same path. The two lines should
each have an impedance to Gnd of 1/2 Zo or 25 ohms. Each trace should be
terminated to Gnd in it's characteristic impedance. Voila, you have a 50 ohm
balanced impedance differential pair.
        Your calculations are definitely out there, I don't know how you
have been calculating this figure but here is my solution using your basic
characteristics. Dielectric thickness: 0.008 inches, total copper weight: 1
oz (0.0014in), FR4 (Er - 4.7).

Your trace width should be 27.5 mils (0.0275 inches). Don't forget to
terminate each line with a 25 ohm terminator.


        If you need a copy of the article, let me know, I will scan it and
forward it to you.

Sincerely,
Brad Velander.

Lead PCB Designer
Norsat International Inc.
#300 - 4401 Still Creek Drive,
Burnaby, B.C., Canada, V5C 6G9.
Tel   (604) 292-9089 (direct line)
Fax  (604) 292-9010
Website: www.norsat.com


-----Original Message-----
From: Scott Ellis [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, December 05, 2001 4:17 PM
To: Protel EDA Forum
Subject: [PEDA] 50 ohm differential impedance connections and traces


A couple of questions. 

Does anyone have a good way of doing board  to board connection (only about
an inch) for a differential 50 ohm connection?

Can I make two parallel connections with 100 ohm twisted pair?

We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm thick,
and we get traces 140thou, 8thou spacing over a gound plane for a 50 ohm
differential impedance. Can anyone confirm our result? 140 thou is getting
up there!

The reason for the questions is that we need to take a differential PECL
signal from one board to another and maintain the signal integrity.

Scott Ellis
Manager
Novatex Research - Excellence in Electronic Research & Development
[EMAIL PROTECTED]
41 Yule Road, Merewether, Newcastle, NSW 2291, Australia
Ph 0412 988408   Fax 02 49636058

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