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       Adding a Dynamic Link N-Port from HFSS >
           Complex Port Reference Impedances               


Complex Port Reference Impedances

Design engineers accustomed to circuit and electromagnetic simulation tools often have different expectations when matching ports with complex port impedances. This section discusses these differences and highlights the potential pitfalls when linking HFSS designs with complex port impedances in Designer.

The underlying issue is one of consistency between how designers interpret results generated by Ansoft’s field solvers and how these same results are subsequently applied in the circuit analyses of Designer. The technical basis of the issue is a difference in the commonly accepted definition of S-parameters used in electromagnetic (EM) theory and in circuit theory when complex port impedances are used. Specifically, from the EM perspective S11=0 when the source impedance is equal to the load impedance where the circuit perspective defines S11=0 when the source impedance is the conjugate of the load impedance. The definitions are identical when the port impedances are real.

The generality of Designer implies there are a number of capabilities that may be affected differently by this issue:

• Basic linear circuit analysis with complex valued reference impedances

• Touchstone, NMF and other data file components (NMF is a common format for data transfer among microwave simulators. Designer supports the linear table-based data subset of the NMF specification. The full MDE Neutral Model Format Specification must be obtained from the MAFET consortium.)

• Dynamic Link HFSS component

• Dynamic Link port impedance references

• Touchstone port impedance references

Following is a summary of how S and Y parameters with complex reference impedances are treated in Designer. HFSS results are superscripted, that is, these are the results the design engineer would see in the HFSS post processor. Designer results are subscripted with the first set of letters representing the type of data component and the second set of letters representing the type of port reference impedance specification:

• DL is the dynamic link

• NMF is an unnormalized Neutral Model File

• NMF50 is Neutral Model File that has been normalize to 50 ohms

• Z1P is a port reference based on a Touchstone file

For example:

1. YHFSS = YDL,DL = YDL,Z1P but SHFSS = SDL,DL SDL,Z1P

The Designer/HFSS dynamic link component transfers the Y-parameters from HFSS to Designer and yields identical Y parameter results in both tools. This meets the expectations of an EM-focused designer. However, when the designer references the port impedances to the Z1P file the conjugate definition of S-parameters is used and the EM-focused designer may see unexpected results.

2. YHFSS YNMF,DL = YNMF,Z1P and SHFSS = SNMF,DL SNMF,Z1P

YHFSS = YNMF50,DL = YNMF50,Z1P and SHFSS = SNMF50,DL SNMF50,Z1P

Un-renormalized HFSS-generated NMF data may not be transferred as expected by the EM designer from HFSS to Designer but renormalized data (to 50 ohms in this case) is transferred properly. When working with NMF files it is highly recommended to renormalize to a reference impedance with a real value before use in Designer.



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