[SI-LIST] Re: PCB Trace impedance algorithms

Sam,

Formulas are available for the calculation of trace characteristic
impedance (and velocity of propagation) for simple trace topologies.
Many are reasonably accurate over specified ranges of geometric ratios.

Enterprise PCB design tools tend not to use formulas because the
geometric range limitations.
Instead they use 2D plus finite element fields solvers.
They are called 2D plus because they assume the trace segment or trace
segments being analyzed have a uniform cross-section, so they calculate
the inductance, capacitance, resistance and conductance per unit length
for the representative cross-section, then multiply the values by the
actual length. These types of solvers can usually handle systems of many
coupled traces.

Obviously there is a lot more information.
The reference kindly provided by Dave Instone is certainly good.
You might also try Signal Integrity Simplified by Eric Bogatin for a
wider view.

I hope this helps

Ian Dodd
Mentor Graphics Inc
=20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of David Instone
Sent: Tuesday, January 23, 2007 3:46 AM
To: si.rules@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PCB Trace impedance algorithms

Sam,
  A good place to start would be  'Transmission Line Design Handbook' by
=3D

Brian C Wadell published by Artech House ISBN 0-89006-436-9 which =
has=3D20
various formulae for just about any type of transmission line =
you'll=3D20
ever come across.

Dave Instone
Oxford Semiconductor Ltd
25 Milton Park
Abingdon
Oxon ox14 4sh
UK
www.oxsemi.com
+44 (0)1235 824963



Sam Sam wrote:
> Dear si-list members,
>   =3D20
>   I am learning tool support for pcb designs. I have some questions=20
> reg=3D
arding calculating impedance of a traces in PCB. I use allegro's built
im=3D pedance calculator. I am also aware that there are various other
calculat=3D or tools from UltraCAD, Polar Instruments etc. I am =
wondering
how efficie=3D nt and accurate these calculations are. I guess most of
them use some kin=3D d of assumptions and have simplified closed form
formulas to qucikly exti=3D mate the impedance. But can you people guide
me as what is the exact tech=3D nique or algorithm to calculate the
impedance of a pcb trace say for a mi=3D crostrip structure. Any papers =
or
links to this study would be appreciate=3D d. In specific, since i am
using allegro 's calculator i would like to kn=3D ow how they calculate
the impedance and what are the assumptions they tak=3D e. I have seen =
most
calculator allow single ended and differential trace =3D calculations. =
Is
it possible to extend these techniques to multiple trace=3D s. More
importantly the accuracy of the
>  formulas is of concern to me. When compared to full wave results=20
> these=3D
 formulas from different tools give different result. So i am looking to
=3D learn what is the background behind these? Please advise me on this.
Than=3D ks in advance. Looking forward for your answers....
>   =3D20
>   =3D20
>   Sam
>       =3D09
> ---------------------------------
> All new Yahoo! Mail =3D20
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