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Reply To: | TechNet E-Mail Forum. |
Date: | Thu, 6 Apr 2000 12:52:53 -0500 |
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I'm not sure if I fully understand your situation, you might want to go to
http://www.ultracad.com/calc.htm and use their transmission line calculator
to play with the numbers and see what you get.
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From: Doug [SMTP:[log in to unmask]]
Sent: Thursday, April 06, 2000 1:38 PM
To: [log in to unmask]
Subject: [TN] 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
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