[SI-LIST] Re: Calculating the Z0 of a trace
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: "Si-List@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 26 May 2005 11:01:33 -0700
Dear Farid,
I agree with the previous postings that a 2-D field solver
is the best way to estimate trace impedance, however, my
reading of your question indicates you are interested in
perhaps two slightly different topics.
1) when a trace is placed between a power plane and a ground
plane I can confirm for you that the impedance is nominally
the same as if it had been placed between two ground planes.
I say nominally, not exactly, because in theory very poor
power-to-ground bypassing could affect the result, however,
if your bypassing is poor enough to cause this phenomenon
then you will have unbelievable crosstalk everywhere
throughout the board. Our usual assumption is that the
bypassing is good enough so the power-to-ground noise is a
small fraction of the signal swingh, and so the voltages on
the power plane affect the trace impedance in only a minor
way.
2) The whole approximation involving "k" in my book is
useful for understanding how crosstalk works, but not
necessarily for computing exact levels. The value of "k" is
usually near one, but can be different. For example, if you
have a LOT of current on one line due to excessive
capacitive loading, then "k" gets bigger. The IBIS-based
crosstalk simulation engines do, in my opinion, a terrific
job of calculating crosstalk for parallel traces over a
solid plane; I encourage you to get one. Anyway, the big
lesson from the crosstalk discussion in my first book is
that crosstalk in microstrips varies quadratically with
distance. That is, once "d" (the distance between trace
centerlines) becomes larger than "h" (the trace height),
then crosstalk goes inversely with the square of "d". This
means two things: first, that moving the traces a little
further away dramatically improves crosstalk (a good thing),
and second, that moving the traces a little too close makes
crosstalk skyrocket. Crosstalk is very sensitive to trace
placement; another good reason to get a simulator to
calculate it for you.
Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505, howie03@xxxxxxxxxx
http:\\sigcon.com -- High-Speed Digital Design seminars,
books, and articles
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of farid syed
Sent: Thursday, May 26, 2005 3:23 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Calculating the Z0 of a trace
Dear all,
I am new to PCB design and i have a few doubts
as listed below
1. I have a board with the stack up S G S S P G S S P S if i
want to find the Z0 of a trace on the 4th layer then how do
i do it?
2. is that type called a stripline, i have read that
stipline is a trace bounded by ground planes on either side.
and microstrip is the one with ground on one side and air on
the other. what is the type of trace on the 4th layer of my
board?
3. I have been through Dr. howard jhonson's handbook where
crosstalk is calculated as = k/1+(d/h)^2 what is K and when
we get a number as a result of the calculation how to use
that number in determining the crosstalk or use it for our
purpose.
can anybody please help/guide me
thanks in advance
warm regards
Fareed
Yahoo! India Matrimony: Find your life partneronline.
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