CST2013: Results / Single Ended S-Parameter

Home: MacrosMacrosResultsSingle-Ended S-ParameterMultiline Common+differential mode -> single-ended

The macro performs the transformation of common/differential S-Parameters into single-ended S-Parameters, in view of their usage within a circuit simulator that only allows single-ended elements. It works for several pairs of lines that are considered to be independent (far enough) from each other.

Please note that this macro implements a special case of the also available single-ended postprocessing feature, that can be activated for multipin port definitions.  A necessary condition for this special case is that the impedance of the odd mode is four times the impedance of the even mode. This condition can be forced to be true by renormalizing the S-Parameters before starting the macro.

Definitions

Consider two conductors (transmission lines) above a common ground plane.  

Differential (odd) excitation: simultaneously excite two identical signals of opposite polarity

Common-mode (even) excitation: simultaneously excite two identical signals of identical polarity

Single-ended excitation: excite each line independently (with independent signals)

How to proceed

1) First define all ports as multipin ports as described below under ”Port numbering for multiple coupled lines”. Define two modes for each port (the odd and the even mode), taking care to choose the same order for all ports. For example, at all ports define the first mode as odd, and the second mode as even mode:

2) Perform the simulation for all ports, to obtain the S-parameters corresponding to the defined modes.

3) Then use the macro for converting these S-parameters, as described below.

Port numbering for multiple coupled lines

If the structure contains several pairs of coupled lines, then please take care to define the ports such that each line  n  has ports  (2n-1) and 2n. The example below illustrates the numbering, for a structure with 4 pairs of coupled lines.

Usage

First, you must enter the numbering convention for the initial (common/differential) and final (single ended) configurations.

Single-ended line numbering:

Choose one of the two possible numberings for the single-ended ports:

- near-end single-ended ports with consecutive numbers and far-end ports with consecutive  numbers

- for each line, number the two ends consecutively

Common/diff. mode numbering:

Enter the situation that applies, according to the way your ports are defined.

Display single-ended S-Parameters:

Choose whether to just save them, or to also display the resulting single-ended parameters. Please note that for many ports, the display of the S-Parameters may take some time.

After pressing OK, the single-ended S-Parameters are calculated, and for each line the information regarding the impedances are displayed, as in the example below.

S-Parameter normalization

When applying the methodology above, the resulting S-Parameters are automatically normalized to an impedance that is twice the impedance of the even mode. In general they will not be normalized to 50 Ohm!

This procedure corresponds to adapted multipin (odd/even) ports, and will yield S-Parameters that best characterize the lines themselves, without any influence from external connectors, etc.

To obtain single-ended S-Parameters normalized to 50 Ohm:

1) Define the ports as described above

2) Simulate

3) Renormalize the S-Parameters: those corresponding to odd modes to 100 Ohm, those corresponding to even modes to 25 Ohm. In the example below, the first mode is the odd mode at each port

.

4) Apply the macro

The resulting single-ended S-Parameters will be automatically normalized to 50 Ohm. This procedure will yield single-ended S-Parameters which correspond to those obtained by measurement with 50 Ohm connectors.

Touchstone Export

Home: MacrosMacrosResultsSingle-Ended S-ParameterExport single ended to Touchstone

The macro performs the export of the previously by the above macro  generated single-ended S-Parameters to Touchstone.

How to proceed

1) Generate the single ended S-Parameters, by using the macro ”Multiline common/differential to single ended”.

2) Start the ”Export single ended to Touchstone” macro

3) If desired, enter a name for the generated output file.

Please note that there will be NO further renormalization of the single-ended S parameters. Therefor the export macro will only work if all single-ended S-Parameters were normalized to the same impedance.

Limitations

• All the ports of the structure need to be multipin ports with common/differential modes.

• The number of ports must be even.

• The resulting single-ended S-Parameters cannot be renormalized from within CST MWS to an impedance different from Z11.

OK

After pressing OK, the single-ended S-Parameters are calculated and for each line the information regarding the impedances are displayed.

Cancel

Closes this dialog box without performing any further action.

Help

Shows this help text.