Virtuoso Meets Maxwell: Defining Ports in EMX Planar 3D Solver

'Virtuoso Meets Maxwell' is a blog series aimed at exploring the capabilities and potential of Virtuoso RF Solution and Virtuoso MultiTech. So, how does Virtuoso meets Maxwell? Now, the Virtuoso platform supports RF designs, and the RF designers measure the physical and radiation effects by using the Maxwell's equations. In addition to providing insights into the useful software enhancements, this series broadcasts the voices of different bloggers and experts about their knowledge and experience of various tools in the Virtuoso IC-Packaging world along with the nuances of RF, microwave, and high frequency designs. Watch out for our posts on Mondays.

Meaningful electromagnetic (EM) simulation results depend greatly on the quality of port definitions. And since the need to perform EM simulations is expanding beyond strictly RF applications and into high frequency analog, more of you are grappling with this issue. For everyone who is new to EM simulation, let me start with a definition of what ports mean in an EM analysis. Simply put, ports can be thought of as the network analyzer probes that you use in the lab to measure S-parameters.  In the EMX® Planar 3D Solver as well as other EM solvers, these are used to excite the circuit that is created by meshing your physical shapes. Think of this as the injection of EM energy into the structure. Currents on the metal are measured, and the simulator uses this information to calculate S-parameters. If your port definitions are not correct for your system, you may get results, but most likely your results cannot be trusted.  

Fortunately, the Virtuoso RF Solution’s integration of the EMX Solver can take the guesswork out of correctly defining ports. With this powerful combination, you have the flexibility of specifying edge or internal ports, as well as whether the port is unreferenced or referenced to a specific net and location in the design. (Referenced ports are *critical* to have at high frequencies.) Determining port locations could not be simpler – EMX Solver has always used pins to decide where to put ports, and in the Virtuoso RF Solution the same is true. Let’s look at some of the automation that can help guide you in port creation.

In the figure below I show an example PA design that you’ll recognize if you’ve read any of our Virtuoso Meets Maxwell blogs. I’ve selected two structures to include in my EM simulation, L0 and L12. Selecting multiple components means that I will get the coupling effects between the two inductors, and not just the results from simulating them individually. Once added to the model, it’s just two button clicks to create ports at the location where I am likely to need them: at the pin locations of the inductor PCells:

These ports are created without references, which is the default in the EMX Solver. More on that later in the blog. You can easily check your port locations by selecting the ports in the table. In fact, if you look closely at the image above, you’ll see that the port on the right side of the topmost inductor is highlighted, corresponding to the L0/MINUS pin. If I select all the ports listed in the table and zoom in, I’ll get an even better idea of what was automatically created for me:

You might recognize these as internal ports based on pins with no specific access direction, which means that we will model current flow as going into the ‘face’ of the ports highlighted above. For some applications that may be sufficient, but it could be that an edge port is a better representation of current flow. Using the controls on the Electromagnetic Solver Assistant, it’s a trivial task to change the port type.

First, select the Add edge connection icon at the bottom of the Assistant:

Once you invoke this command, your cursor will change to prompt you to select an edge to define the port. In the figure below you can see this depicted by the yellow dotted line:

Just click, and the new port will be added to the table. Delete the existing internal port and your edge port is ready for use.

Things get more interesting if I am designing at high frequencies. My default unreferenced ports probably will not give me the accuracy that I need – for that I will need to define a specific reference for each port. Once again, this is a very simple task in the Electromagnetic Solver Assistant.

The first step is to define the physical location of the reference and create a port that will act as a placeholder for the reference node. The easiest way to do this is to create a temporary pin, and then use the same flow as described above to create the port. Once the port is created, you can delete the pin, retaining the integrity of your golden layout. Then it’s just a matter of setting the reference port Type to Disabled, and then entering the reference port net name in the Reference column for the port needing the reference:

The result will be an edge port defined for L0/PLUS that is referenced to the gnd port. In the command that is sent to the EMX Solver, you’ll see this shown with the <portname>:<referencename> syntax that is used for this purpose.

I hope this blog has shown you how quick and easy it is to define ports in the Virtuoso RF Solution flow and motivates you to take a closer look at capability. This streamlined approach to defining ports, along with our powerful shape simplification and the unparalleled performance and accuracy of the EMX Solver will significantly improve your design flow. For more information, take a look at the Related Resources link below. There's an opportunity to test drive the software yourself by accessing the Free Trials link. 

Thanks for reading!

Kristin Fullerton

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Virtuoso Meets Maxwell series includes posts about the next-generation die, package, and board design flow with a focus on reinventing and optimizing the design process to ensure that the designer remains a designer! Keep watching!

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