Virtuoso Meets Maxwell: Full CellView EM Extraction

22 Jun 2020 • 7 minute read

'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.

Today, we are going to extract a 3D S-parameter model for an inductor using the Cadence AWR AXIEM 3D planar method-of-moments solver. The S-parameter model will be a new cellview that we can easily use in a schematic for circuit simulation.

Here is the inductor we want to simulate:

In Virtuoso Layout Suite EXL I switch my workspace to “Electromagnetic”. This pulls up the Electromagnetic Solver Assistant on the right of the window. This is where we will run the AWR AXIEM solver for the 3D simulation.

Previously I had created my process setup in a “typ.emproc” file. This is actually a pretty short file, it just contains the path to the ICT or qrcTechFile from my PDK, as well as the Silicon substrate parameters. The Virtuoso environment found my “typ.emproc” file and automatically applied it to my simulation model.

The Electromagnetic Solver Assistant would let us pick the nets and instances we want to simulate. But for this project, we simply want to simulate all the geometry. So all we have to do is check “Extract Full CellView”. That is all the model setup we have to do!

All that is left to do before we can run the simulation is to define ports. Here we simply want an edge port on each inductor pin. One click and our ports are automatically generated. Each port is tagged with the name of its pin. This will be used later when our S-parameter model instance is netlisted.

At this point we can click on “Mesh and Simulate” to run the AWR AXIEM solver. A small inductor like this runs very quickly of course.

After the run we can review the mesh in the Virtuoso 3D Viewer. The layers are shown in the same color as in the 2D canvas. And from the Nets assistant we see there is only one net, as expected for an inductor like this.

The S-parameter result file is now visible in the “Results” tab of the Electromagnetic Solver assistant.

Next to the S-parameter file is a convenient “Plot” button. This loads the s2p file into the results viewer. All we need to do is double-click on the “L” and “Q” signals to get plots for the inductance and quality factor.

Now that we have our S-parameter file, the next question is how can we use this in a schematic? If you have used the Electromagnetic Solver assistant before, you will know that the “Create Extracted View” button normally stitches the S-parameters into our golden schematic. But, in this case, we didn’t just extract a few nets or instances – we extracted the entire cellview! So there is nothing to stitch, our S-parameters model the entire cellview already.

Actually, it would make a lot more sense to create a new “sparam” cellview. That way we could use it as a simulation view in the Hierarchy Editor. And that is exactly what that same button does when we have a “full cellview” model! It brings up the “Create S-parameter cellview” form. I am going to leave the suggested view name “sparam”. And I already have a symbol, so I don’t need to create another one.

If we check now in the Library Manager, we will see the new “sparam” cellview that the Electromagnetic Solver Assistant created for us a moment ago. It seems simple, but it actually has several neat features: First, we can double-click it and the s2p file will open in our default text editor – a great way to check header comments like which solver version was used in the simulation.

Another really nice benefit is that the s2p file is stored in the cellview folder! So it gets automatically checked into our design management system. Gone are the days where Touchstone files were floating around the project in various unmanaged folders.

And the third benefit is that the CDF was automatically updated to include the typical attributes we would expect of an n-port instance, e.g. the interpolation method.

To see this in action, I created a simple filter where I used that sparam cellview we just created. And I can directly control the parameters of the n-port that will be written to the netlist.

Actually, you might be thinking: “wait a minute, we didn’t use the sparam view – we used the symbol view in the schematic!” And that is exactly right, we will of course use Virtuoso Hierarchy Editor to switch in the sparam view for the symbol view. And because the sparam view is tagged as a leaf cell (a fourth benefit if you will), we don’t even need to change anything in the Global Bindings.

It may come as a surprise to you, but these simple steps are all we need to netlist our circuit. Here I just invoked Virtuoso ADE Explorer, set the target simulator to Spectre, and created a netlist. We can see two instances of our inductor S-parameter model (in my screenshot above I showed the attributes of one inductor, but the other one was also an instance of our model).

Just to convince ourselves that everything worked okay, we can look at the connectivity of L0: port 2 is connected to p1. And if we look back all the way to the screenshot where we generated our ports we can see that in fact pin p1 was generated as port 2.

I also want to point out again how the file path is written to the netlist: we never created an n-port instance with some hard-coded path to an s2p file! Instead, the path to the file is automatically computed based on the lib, cell and view name of our sparam view. If we ship this project to our colleague on the other side of globe and they generate a netlist, then the file path will be automatically computed based on their library location!

This concludes our little project for today. We started with a layout for which we wanted to create an S-parameter model using Cadence’s method-of-moment AWR AXIEM solver. It just took a couple of mouse clicks to specify “full CellView” and automatically generate a port for every pin. After the simulation we generate an “sparam” view for our model and that view contained the actual s2p file. Then as a test we instantiated our sparam view into a simple filter and created a netlist. The netlister automatically derived the correct path to the s2p file based on the lib, cell and view names. The Virtuoso environment removes a lot of the mundane and error-prone workaround S-parameter files, letting you focus on your actual design work.

Of course, we can simulate much more complicated structures than just an inductor. For example, we can use “Extract Full CellView” for a complete bandpass filter like this one:

With just a few mouse clicks, we can send the entire cellview to the simulator and view the mesh:

Related Resources

For more information on Cadence circuit design products and services, visit www.cadence.com.

About Virtuoso Meets Maxwell

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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Johannes Grad