Re: [SI-LIST] : Trace impedance

Jim Lyke (
Thu, 20 Aug 1998 13:18:17 -0600

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.


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.
>Fabrizio Zanella
>EMC Corporation

Jim Lyke
Air Force Research Lab
3550 Aberdeen Ave SE
Bldg 887, Room 3
Kirtland AFB NM 87117-5776

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