Beginning with version 188.8.131.52 of the SPARQ and Signal Integrity Studio software application, users can import Touchstone files and convert the S-parameters to mixed mode, or to reassign port numbers. This application note describes how to use these features and provides examples for 4 and 12-port Touchstone files.
How to Import S-parameters
The first step is to go to the SParam Import dialog. To get to this screen, click the Setup button at the top of the application, and subsequently select the SParam Import tab. (Alternatively, select File → Recall Touchstone.) The SParam Import tab includes user interface elements for loading and converting Sparameter files, and for selecting individual S-parameters to display as well as time domain responses.
Once in the SParam Import dialog, click the folder icon on the left side to open a Windows “Open File” dialog. Navigate to the folder with your Touchstone 1 formatted S-parameter file, select it and click “Open” to load. When the file is successfully loaded, the dialog will display some basic information about the file, including number of points, ports, and end frequency. If the S-parameter measurement was taken using a LeCroy SPARQ, this information is displayed.
Converting to Mixed-Mode
To perform a mode conversion, click the Convert… button in the SParam Import dialog. This will open the SPARQ Import Config window. The key elements of this window include selectors for configuring the mode conversion, which includes the configuration of input and output port assignments, and the Apply button to invoke the conversion.
Descriptions of Import Config Dialog Elements
The selectors can be understood as follows:
Blue Selectors: These refer to the port numbers in the Touchstone file being imported (i.e., the “input” file)
Green Selectors: These refer to the user-defined port numbers to be used after applying the conversion.
Yellow Selectors: These refer to the ordering of the ports in the S-parameter matrix and the subsequent assignments when saving the converted S-parameters to a new Touchstone file. When converting to mixedmode, these selectors default to values that put differential-to-differential S-parameters (SDDij) in the upper left quadrant of the S-parameter matrix, common-to-common (SCCij) in the lower right quadrant, and mode conversion parameters (SDCij and SCDij) in the upper-right and lower-left quadrants.
Example1: Converting a Typical 4-port Touchstone File to Mixed Mode
S-parameters are should almost always be saved in single-ended format, where each port in the Touchstone file corresponds to a single-ended port on the device containing a signal pin and return path. When converting a 4-port Touchstone file to mixed mode, the user will need to know which ports are to be associated as plus (+) / minus (-) pairs so that the conversion to mixed mode can be done correctly. Figure 3 and Figure 5
show the two typical ways in which ports in a network with one differential lane could be assigned, and how the Import Config window should be configured to convert to the Sparameters to mixed mode
In Figure 3 the blue selectors have been ordered (1, 3, 2, 4), which corresponds to the “input” differential pair having single-ended assignments of 1 & 3, and the “output” pair having assignments 2 & 4. Figure 4 shows that with these assignments, 1 goes through to 2 and 3 goes through to 4.
In Figure 5 the blue selectors have been ordered (1, 2, 3, 4), which corresponds to the “input” differential pair having single-ended assignments of 1 & 2, and the “output” pair having assignments 3 & 4. Figure 6 shows that with these assignments, 1 goes through to 3 and 2 goes through to 4.
In both examples, the “User-defined Ports” are configured such that the first pair of Touchstone ports is to be considered differential port 1, and the second pair as differential port 2. In both examples, the orange “Output” selectors are configured to organize the output mixed-mode S-parameters in the industry standard manner: differential-to-differential S-parameters (SDDij) in the upper left quadrant of the S-parameter matrix, common-to-common (SCCij) in the lower right quadrant,
and mode conversion parameters (SDCij and SCDij) in the upper-right and lower-left quadrants, respectively.
After configuring the dialog, click Apply.The S-parameters are converted to mixed-mode, and the selectors in the main window for selecting S-parameters to view show SDxSy notation, as shown in Figure 7.
Note: Users can save the converted S-parameters to a new Touchstone file, but this is not recommended. The Touchstone 1 format does not encapsulate any information about mixed-mode results, which leads to confusion for port assignments and reference impedence. However, when saving in mixed-mode format with the SPARQ and Signal Integrity Studio Software, header comments are written to the Touchstone file indicating that the S-parameters are saved in mixed-mode representation. When subsequently importing such a file, the application will correctly interpret the file as being mixed mode and assign ports correctly.
Interlude: What’s the Best Way to Assign Ports?
We recommend assigning ports using the following convention: odd ports for inputs, even ports for outputs. Why? This convention easily extends to higher port counts and leaves port assignments intact. In a 2-port network, S21 is always the insertion loss, so it makes sense to retain this for higher port counts as well. See Figure 4 and Figure 8. In both networks, S21 and 43 are insertion loss measurements. Now compare Figure 6 and Figure 9. The singleended port assignments have changed when expanding the network from 4 to 12 ports, such that S21 in the 12-port measurement is no longer insertion loss,
but is now near-end crosstalk! S71 is now the insertion loss that corresponds to S21. Obviously, this can easily lead to confusion. Our preferred method of “odds left evens right” retains the port assignments, preventing confusion and any subsequent misinterpretation. This consistency in port assignments becomes important when “scaling up” models from 4 to 8, 12 or even much higher (e.g. 144) port counts. When using “odds left evens right”, S21, S43, S65 and S87 don’t change – they will be the same meaning for both 8 ports measurements as well as, for example, 88 port measurements.
Example 2: Converting a 12-port Touchstone File to Mixed Mode
Figure 8 and Figure 9 show the two typical ways in which ports in a 3 differential lane network could be assigned, and how the SPARQ Import Config window should be configured to convert to mixed mode.
In Figure 8, the blue selectors have been ordered with our preferred convention: the input differential pairs have single-ended assignments (1, 3), (5, 7) and (9, 11), and the output pairs have assignments (2, 4), (6, 8), (10, 12). The figure shows the equivalent diagram. The output touchstone file ports have been assigned to place the mixed-mode S-parameters in the matrix in the preferred locations : differential-to-differential Sparameters (SDDij) in the upper left quadrant of the S-parameter matrix, common-to –common (SCCij) in the lower right quadrant, and mode conversion parameters (SDCij and SCDij) in the upper-right and lower-left quadrants, respectively.
After configuring the dialog, click Apply. The S-parameters are converted to mixed-mode, and the S-parameter selector in the main window show SDxSy notation, as shown in Figure 7.
In Figure 9 the blue and green selectors have been ordered with an alternative assignment convention, with sequentially ordered ports. While this leads to the “cleanest” configuration in the dialog, this convention has limitations (See previous section, Interlude: What’s the Best Way to Assign Ports?)
Example 3: Using the Convert Dialog to Reassign Port Numbers
The dialog can be used to reassign port numbers in Touchstone files. This can be done to correct errors made in the original assignments, or to switch from one numbering convention to another. Figure 10 shows how to convert from a sequential ordering such as in Figure 9 to the recommended convention in Figure 8. In this example, the Import Config dialog is configured for singleended to single-ended conversions via the Config Mode selectors. The green User Port selectors are configured to reassign ports 1 through 6, which were assigned to the DUT’s input ports in the input file, to be odd-numbered ports in the output file. The yellow ports are set to the same values as the green selectors. After configuring the selectors, click Apply to invoke the conversion. Then click the floppy disk icon to save the converted S-parameters.