Can a complete 4-port S-parameter (including differential and common mode) simulation of a RF broadband system be done using the RF Toolbox 2.5 (R2009a)?

I have the following requirements:

a) Sources are broad band – DC to ~30GHz frequency content (they are not narrow band RF signals). 4 port S pars are typically touchstone files.

b) Zs1, Zs2, ZL1, ZL2 are frequency dependent impedances.

c) V2 is not necessarily equal to –V1, i.e., it will contain a common mode signal component

d) The aim is to find the differential signal and the common mode signal at the loads.

Can a complete 4 port simulation be done which takes mode conversions into account?

Answer 1

[MathWorks Support Team]

This can be done using the RF Toolbox 2.5 (R2009a). The following functions can be used to realize this application.

S2MM - Convert 4N-port single-ended S-parameters to 4 2N-port mixed-mode S-parameters.

SMM2s - Convert 4 2N-port mixed-mode S-parameters to single-ended S-parameters.

S2DD - Convert 4-port S-parameters to differential 2-port S-parameters Sdd.

S2DC - Convert 4-port S-parameters to cross mode 2-port S-parameters Sdc.

S2SCD - Convert 4-port S-parameters to cross mode 2-port S-parameters Scd.

S2SCC - Convert 4-port S-parameters to common mode 2-port S-parameters Scc.

S2TF - Calculate transfer function from 2-port S-parameters.

In order to model mixed-mode simulation, there might be multiple approaches to go about it.

The RF Toolbox documentation includes an example "Modeling a High-Speed Backplane (Part 2: 4-Port S-Parameters to a Rational Function Model)" showing a differential-model application.

If you replace S2SDD with S2SCC, you will get the common-mode. Please note that when using S2TF, the impedances are different for differential-mod and common-mode:

 Zs(common-mode) = (Zs1 * Zs2)/(Zs1 + Zs2)
 Zl(common-mode) = (Zl1 * Zl2)/(Zl1 + Zl2)
 Z0(common-mode) = Z0/2

vs.

 Zs(differential-mode) = Zs1 + Zs2
 Zl(differential -mode) = Zl1 + Zl2
 Z0(differential -mode) = 2*Z0