Transmission Line S-Parameter Generation | Getting Started with S-Parameters, Part 5

Welcome back to the ongoing MathWorks technical series on the use of RF Toolbox. In this installment, we are going to use RF Toolbox to instantiate a behavioral transmission line and generate S-parameters from the behavioral model. We will go over the key functions used for generating the behavioral model, as well as documenting the workflow that will be followed. Then, a demonstration will be given that shows the process being used to generate S-parameters for a behavioral micro-strip transmission line.

Let's begin by outlining the principal functions that are going to be used today, txlineMicrostrip, txlineCPW, txlineCoaxial, txlineTwoWire, txlineParallelPlate, S-parameters, RF plot getZ0. Help for each of these RF Toolbox functions can be found by typing in Help and the function name at the MATLAB command prompt, or typing in the function name in the MATLAB help browser.

One RF Toolbox product example that I find particularly useful for demonstrating this workflow is analysis of coplanar waveguide transmission line in x-band application. This example shows the user how to create a coplanar waveguide transmission line and then calculate S-parameters when different backing dielectric materials are used. The process to create a behavioral transmission line and generate its S-parameters using MATLAB is straightforward.

First, you need to select which one of the nine different transmission line models that you are going to use, and then set the appropriate transmission line properties for the transmission line that you select. The associated documentation for each of the respective transmission line elements will aid you in the creation of a behavioral transmission line. Take, for instance, the case where you want to create a micro-strip transmission line.

You will be able to set transmission line properties, such as its height and width, the length of the transmission line, the thickness and conductivity of the conductor, along with the dielectric properties of the board material that the conductor is mounted upon. You will then be able to use RF Toolbox functions to calculate the transmission line's RF behavior. Typical parameters that you can readily calculate include S-parameter analysis, group delay analysis, along with calculation of the transmission line's characteristic impedance.

Let's go through an example of a high performance micro-strip transmission line. Specifically, we are going to analyze the performance of a micro-strip transmission line that has a 10 mil width mounted on an aluminum board that has a thickness of 10 mil. We are going to use MATLAB to create this behavioral transmission line model, and then we are going to analyze the behavior of this model.

After creating the behavioral model for the transmission line, we will generate the S-parameters and then plot the same S-parameters using MATLAB's RF plot function. Finally, we can obtain the characteristic impedance of the transmission line at either a single frequency or over a range of frequencies, using a for-loop.