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Thu, 20 Aug 1998 14:31:53 -0400 |
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Cameron -
Things you need to know going in to a controlled impedance circuit board
include:
1.) The effective frequency of the signals to be impedance matched.
Don't try control impedance on a global basis the head and heart aches
aren't worth it. The effective frequency can be calculated as F in Hz
=0.35/Signal rise time in nsec.
2.) The target impedance for the lines in question. For FR4 the
practical range is around 30-100 ohms anything outside that range will
require a material change.
3.) The relative Permittivity(sp?) of the substrate being used. For FR4
it will be between 3.8 and 4.5 depending on Frequency.
4.) Layer stack-up. This is critical in controlled impedance.
5.) Critical Length for the controlled impedance lines.
Calculate as Critical Length in inches = [(Trise * Speed of
light(inches/sec))/sqrt (Er)]/2.
Don't be surprised, however if the engineer gets a puzzled look when you ask
for the rise time, and is a long time getting back to you with it.
This is where the real fun of PCB design starts >:->
Jeffrey A. McGlaughlin
Printed Circuit Board Designer
Battelle Memorial Institute
Columbus Ohio
-----Original Message-----
From: [log in to unmask] [SMTP:[log in to unmask]]
Sent: Thursday, August 20, 1998 10:35 AM
To: [log in to unmask]
Subject: [TN] More Controlled Impedance
Thanks to those who responded to my previous question about trying
to find some information about how to design a board with controlled
impedance. Unfortuneatly, I still haven't been able to find any information
on the topic.
I now have an engineer who wants me to design his board with some
controlled impedance. Can anyone tell me what questions I should ask him,
and give me some design considerations that I should pay attention to.
Thanks,
Cameron
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