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Setting Up Lumped-constant Boundary Conditions with S2P File

For the lumped-constant boundary condition, Touchstone file can be used as circuit information.

 

In particular, s2p file (2-port S-parameter file) is supported. When s2p file is used, the S-parameters are obtained from the s2p file

and the impedance is calculated from the S-parameters and the circuit information is used in the analysis.

 

When calculating the impedance from the S-parameters, it is presumed that the circuit has a simple composition (lumped constant) of resistance, inductance, and capacitance.

The simple composition is a circuit expressed by single admittance Y as shown below. A relationship between voltage and current in this circuit is as below.

 

Y * (V2 - V1) = i

 

This relationship can be written by a matrix as in equation (1). It can be interpreted that the circuit has two ports. The matrix on the right side of the equation (1) is

a Y-parameter and calculated from the S-parameter. In a simple circuit like this, by calculating Y-parameter from the S-parameter

and making Y(1, 1) component an admittance (reciprocal of impedance), the circuit can be incorporated into the analysis.

 

It must be noted however, that the calculation is performed correctly only for a simple circuit that can be written with a single admittance.

If the distributed voltage and current exist in the circuit, the calculation will not be performed correctly. Especially, if each component of the calculated Y-parameter has a different value,

it is not known which impedance of the two ports will be applied to the lumped constant boundary, and the analysis may be different from your expectation.