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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