[SI-LIST] Re: FEM/BEM/MOM
- From: Geoff Stokes <gstokes@xxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 28 Aug 2003 10:44:09 +0100
Hi Ben
FEM Finite Element Method - I think maybe as used in Ansoft HFSS
www.ansoft.com
BEM Boundary Element Method
http://www.wessex.ac.uk/conferences/2003/bem25/
MoM Method of Moments
e.g. NEC, FEKO, AWR Microwave Office
3D solvers include Ansoft HFSS and CST Microwave Studio (Finite Integral
method).
www.cst.de
I can see that the problem usually comes down to formulation of a
lumped-element model of high frequency effects. The field solvers give
linear network parameters, e.g S or Y to which the model must be fitted, or
the parameters might be used directly in a simulator - that is possible for
small systems in Cadence.
Planar solvers are useful for dealing with PCB traces in the x-y plane,
usually called 2 1/2 D because they give useful but not full description in
the z-direction. Some 3D solvers are full-wave descriptions in terms of
Maxwell's equations and that's clearly going to be the most accurate
approach. It wasn't really practical until recently, when high speed desk
top computing power became available at a reasonable price. The differences
are mathematical - don't ask me too much about that! The main thing to
understand is that a nodal circuit schematic analysis of any kind is a gross
approximation, and the error increases with increasing frequency. This is
because varying currents produce varying fields which cause couplings and
radiation not represented in the schematic. A further complication is that
the fields themselves inherently involve a time delay, and that makes
extraction of lumped-element models extremely difficult.
Just a little diversion to show the error in a schematic:
An old example often quoted is the problem of connecting two one farad
capacitors together by a switch; one is at 1V potential, the other zero.
The initial energy is 1/2 CV**2 =3D 0.5 joule. After closing the switch th=
ere
is charge distribution, and energy =3D 1/8 +1/8 =3D 0.25 joule. Where did =
the
missing 0.25 joule go? If you did an electrical degree, you'd see the
paradox. If you studied high frequencies, you'd know the answer. (I did a
Physics degree, then built RF circuits, so for me a capacitor is not a
capacitor.)
The answer is
1) it's not physically possible to put two capacitors together at one point
2) therefore they are separated by a distance
3) therefore on closing the switch, the discharge current travels a distance
4) the conductors have finite conductivity
5) therefore there is a varying electromagnetic field and energy is
dissipated and radiated
6) please don't talk about too much about inductance because it's only an
approximation.
Best wishes
______________________________________________=20
Geoff Stokes
Systems Engineer, Signal Management Group
Zetex plc=20
Lansdowne Road, Chadderton, Oldham, OL9 9TY,=A0 UK=20
Tel direct:=A0 +44-161-622-4857=A0=A0 Switchboard: +44-161-622-4444
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e-mail:=A0 gstokes@xxxxxxxxx=20
> -----Original Message-----
> From: Ben Rothchild [mailto:benrothchild@xxxxxxxxx]
> Sent: 28 August 2003 09:31
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] FEM/BEM/MOM
>=20
>=20
> Hello Everybody,
>=20
> When looking at Signal Integrity tools one
> comes across terms like FEM, BEM, MOM
> as well as some others like 2D, 2.5D,3D
> field solvers.=20
>=20
> Can somebody explain in extremely layman
> terms what they mean ( for a=20
> person with a background of undergraduate
> degree in Electrical engineering).
>=20
> Thank you very much
>=20
> Ben
>=20
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