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Question of the Month Answers, March 2007

March 7, 2007
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Published March 7, 2007

You be the expert: The responses to the February Question of the Month are below

Madhusudhan from VT University, Belgaum, India has submitted this month's question:

Dear Harlan,
I would like to introduce myself as a PhD student in microwave and MEMS doing research in India. I have just started my research in modeling of MEMS devices and I have encountered a problem. I am very much confused by different things like field solvers and SPICE models. Can you provide any clarity between the two and also suggest field solvers or a low cost field solver that I can work with for my degree?

From: Alberto Armoni, Selex Communications SpA Subhead:

Dear Madhusudhan,
Field solvers are tools that simulate the behavior of the electromagnetic field inside a structure. They use a mathematical model to find a solution very similar to the solution of the physical differential equations (deriving from Maxwell's equations) that describe the electromagnetic field in every point of the domain you consider. A field solver uses mathematical methods such as FEM (Finite Element Method) or MOM (Method of Moments). Field solvers work at a physical layer. SPICE models (Simulation Program with Integrated Circuit Emphasis) are general-purpose analog circuit simulators that use nodal analysis to construct the circuit equations: SPICE models take a text netlist describing the circuit elements (transistors, resistors, capacitors, etc.) and their connections, and translate this description into equations to be solved. It links together the various component equations and the Kirchhoff's circuit laws at an electrical level. One of the most popular field solvers is Ansoft HFSS, but I don't think it can be described as "low cost."

From: Zeshan Ahmad, UET Lahore Pakistan

Dear Madhusudhan,
What I think is that you cannot compare them because both are totally different from each other in the sense that the SPICE simulator works on the basis of predefined model parameters, mostly extracted by practical means, while the field solvers are working on numerical solutions to solve Maxwell equations. The smaller the size of increment for each iteration of a part of the solution, the better the response. That is what is represented in the field solvers as the grid size. SPICE, on the other hand, is totally unrelated to the Maxwell equations. It is a solver, too, but of another nature. I hope this is valid information for you.

From: Luis Abril, Alcatel Alenia Space

Dear Madhusudhan,
A field solver tries to solve Maxwell's equations of the entire, normally passive circuit, while the SPICE models are models perhaps validated by measurements, although they do not couple with the other elements of the circuit. Therefore, the field solvers are more accurate but more time-consuming. I guess it is not cheap, but I use ADS by Agilent with Momentum (planar structures simulator in the same package) for field solver and co-simulation of the results obtained with Momentum (PCB effects) with the active models.

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