Concatenation of S-Parameter and Networks | Getting Started with S-Parameters, Part 2

Welcome to this second installment in the ongoing MathWorks technical series on the use of RF Toolbox. In this installment, we are going to build from the S-parameter basics covered previously and combine or concatenate S-parameters of separate networks together to create a new network. This will be done using two separate methods, and the motivation for using the different approaches will be established by the end of this video.

Let's begin by outlining the principal functions that are going to be used today, namely, circuit, add, setports, cascadesparams, rfinterp1, and snp2smp. Again, 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 help browser.

There are a couple of RF Toolbox examples that I find particularly useful for these functions, which include, first, MOS interconnect and crosstalk. This example shows the user how to construct a circuit network and set reference ports by using the circuit and setports functions. Time domain analysis is also included in the example, which permits the user to do a transient analysis of the constructed circuit.

Second, the Help page for cascadesparams. There are five hand-worked examples that show how arbitrarily sized S-parameter networks can be combined to form a single S-parameter network representation.

Let's go over an example where a four-port S-parameter network is combined with analog components to create a new filtered representation of the original four-port S-parameter representation. The first thing that we will need to do is import the four-port network default.s4p using the sparameters function. Then we will need to characterize the analog components, resistors and capacitors, which smooth the sharp edges of the input signal.

With the building blocks characterized, we can start to construct the new network. With the use of the add function, networks and analog components can be combined to create the new network. In this case, filtering RC networks are added to each port of the S-parameter network. Next, ports for the new networks need to be defined using the setports function. Finally, the S-parameters of the new network can be calculated. This is done by using the sparameters function.

Now let's go over an example where two S-parameter networks are combined to create a new network. Let's combine two four-port networks together and create a new four-port network. We will use the four-port S-parameter file default. s4p once again to represent each of the networks being concatenated together.

After importing the data from the file, the first thing that needs to be done is to reorder the ports of each of the respective networks that will be concatenated together. This is accomplished by using the snp2smp function. For the default.s4p file, the input terminals are labeled ports 1 and 3, while the output terminals are ports 2 and 4.

The function cascadesparams automatically connects port 1 of a given network to port n of a first given network. To properly realize the combined network, the ports need to be reordered. For the first network, port 2 needs to be relabeled port 3, and port 2 will be relabeled port 3. For the second network, port 1 needs to be reallocated to port 2, port 3 needs to be reassigned to port 1, and port 2 needs to be reassigned to port 3.

Before the concatenation occurs, we will also need to set the number of connections that will be made between the two networks. That value is 2. With the ports properly reordered, the networks can be concatenated together, with the S-parameters of this new network being calculated using the cascadesparams function.