Re: [SI-LIST] : Trace impedance

Dima Smolyansky (dima@tdasystems.com)
Thu, 20 Aug 1998 18:25:59 -0700

I believe there are some conformal mapping equations that actually take
conductor thickness into account.

Has anybody considered that the discrepancy between the calculation and
measurement can actuall be due to poor measurements? I am not necessarily
talk about impedance measurements (although, that too), but measurements
and variations of board thickness and trace width, thickness etc.?

Thanks,

-Dima Smolyansky

----------
> From: Jim Lyke <lyke@plk.af.mil>
> To: fabrizio=zanella%eng%emchop1@fishbowl02.lss.emc.com
> Cc: casperdd <Dominic.Casperson@cern.ch>;
owner-si-list@silab.Eng.Sun.COM; si-list@silab.eng.sun.com
<si-list@silab.Eng.Sun.COM>
> Subject: Re: [SI-LIST] : Trace impedance
> Date: Thursday, August 20, 1998 12:18 PM
>
> Most of these formulae were based on conformal transforms, often based on
> zero thickness conductors. In a lot of modern work, those assumptions
are
> not very good, particularly when integrated circuits or multichip modules
> are involved. I compared a lot of these expressions (Wheeler,
Hammerstad,
> Jensen, Chang, etc) against real-life measurements on 10u and 25u t-line
> structures of different types in my Master thesis work and got results
all
> over the map.
>
> jim
>
> At 12:25 PM 8/20/98 -0400, fabrizio zanella wrote:
> >Gentlemen, formulae for calculating impedances of stripline/microstrip
> >traces are only somewhat accurate for certain geometries. The formulae
> >were developed many years ago, when line widths were .010"+, dielectric
> >thicknesses differed from today's designs, and most designs used 50 ohm
> >transmission lines. For high impedance tlines, (80+ ohms), the accuracy
of
> >the formulae gets terrible.
> >If you want to match your TDR measured impedances to calculations, you
need
> >to use a field solver tool. There are many good tools available that
use
> >boundary element or finite element analysis. Try Apsim's 'RLGC' tool,
or
> >Ansoft's Maxwell EZ2D.
> >
> >Regards,
> >Fabrizio Zanella
> >EMC Corporation
> >fzanella@emc.com
>
> Jim Lyke
> Air Force Research Lab
> AFRL/VSSE
> 3550 Aberdeen Ave SE
> Bldg 887, Room 3
> Kirtland AFB NM 87117-5776
> 505-846-5812/5815
>
>
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