Virtuoso Meets Maxwell: Custom Passive Devices in RF Circuits - Devices or Interconnects?

'Virtuoso Meets Maxwell' is a blog series aimed at exploring the capabilities and potential of Virtuoso RF and Virtuoso MultiTech. So, how does Virtuoso meet Maxwell? Now, Virtuoso supports RF designs, and the RF designers measure the physical and radiation effects by using 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. We are posting on Mondays.

The Virtuoso Electromagnetic Solver integration allows layered parasitic extraction and electromagnetic simulation with the Cadence Quantus Extraction Solution and the Cadence EMX Planar 3D Solver. In one of our previous blogs, we described how users benefit from combining highly accurate EM simulation for RF-critical parts and high-capacity parasitic extraction for less critical parts of the design (Virtuoso Meets Maxwell: Layered Electromagnetic Modeling). The seamless integration automatically ensures that parasitic effects are neither missed nor double-counted during post-layout circuit simulation.

But what do you do with custom passive devices? The foundry PDK that you are using may have a few inductors or even a transformer. But if you want your RF circuit to match the challenging specs that you received from your customer, you may need to use optimized custom passive devices. With the VEM-EMX integration, you can easily extract an S-parameter model and stitch it into the schematic. But how do you include the parasitics of the remaining circuit? The standard approach is to stitch the S-parameter model into Smart View. But the Quantus Extraction Solution requires a layout versus schematic (LVS) check. And the LVS run does not recognize the custom passive device, as there are no extraction commands for it in the foundry LVS rules file.

Fortunately, there is an article on Cadence Online Support that describes how to use the Pegasus-Quantus blackbox and graybox flows  (Blackboxing Flow on Virtuoso RF using Pegasus, Quantus and EMX). These flows let you define the custom passive device either as a blackbox or as a graybox. The devices inside these boxes are not extracted during LVS. Only the connections to the blackbox are checked during netlist comparison with a schematic. In a Smart View, the layout view or the symbol view of the blackbox is placed. And for post-layout simulation, you can either stitch an electromagnetic model into the Smart View or use the Hierarchy Editor to define the S-parameter view as the simulation view.

Now, what is the difference between a blackbox and graybox? Let’s have a look at an example. This picture shows an inductor. Let’s assume that it is a custom passive device. For Pegasus LVS, we define it as a blackbox  or a graybox. In blackbox mode, the data inside the box (which has a magenta outline in the picture) is not transferred to the Quantus Extraction Solution. (The box is cell-based and nets that are crossing the boundary at higher hierarchy, such as the netA and netB shown, are not part of the box.) Because the shapes inside the blackbox don’t have any impact on parasitic extraction in Quantus Extraction Solution, the coupling capacitors to adjacent nets netA and netB are not taken into consideration. This implies that the total capacitance values for netA and netB are underestimated.

To consider parasitic coupling between custom passive device and adjacent nets, you must switch to graybox mode. This transfers the shapes inside the box as graybox data to the ground connection, and coupling capacitance between netA/netB and graybox data is extracted. The parasitic capacitors are connected to the ground. There is no cross-coupling from netA/netB to the device itself but the capacitive load for netA/netB is taken into account.

To summarize, the Pegasus LVS - Quantus Parasitic Extraction flow supports both blackbox and graybox modes. Both flows differ in the way parasitic coupling between elements inside the box and adjacent nets is treated. In blackbox mode, metal shapes inside the box are not considered during capacitance extraction, and capacitive coupling between the custom passive device and adjacent nets is ignored. In graybox mode, the metal shapes inside the box are considered for capacitance extraction. The extracted capacitors are lumped to the ground, and the capacitive effect is added to the adjacent net but not to the passive device.

In RF circuits, capacitive coupling may have a significant effect on circuit characteristics. Underestimating its effect during post-layout circuit simulation might result in products that are not matching the initial specification or entire product failure. The Virtuoso Electromagnetic Solver integration of the EMX Planar 3D Solver allows the extraction of the custom passive device as interconnect.

Remember, a custom device is not extracted during LVS. The Electromagnetic Solver Assistant allows components and nets to be included in the electromagnetic model. The capability of defining a net as an electromagnetic model provides the possibility to consistently define the custom passive device as an interconnect by introducing metal resistors as net splitters.

In this approach, Pegasus LVS is run without any blackbox or graybox statement. The Quantus Extraction Solution’s parasitic extraction treats the inductor metal as interconnect, including the extraction of all cross-coupling to adjacent nets. And, electromagnetic stitching into the Smart View replaces the Quantus Solution-extracted RC model for inductor_net with the highly accurate S-parameter model extracted by EMX Planar 3D Solver.

It is worthwhile to take a closer look at how you treat custom passive devices in your current RFIC design and verification flow. Are you sure you are capturing all parasitic coupling between the device and adjacent nets? Layered extraction with the Quantus Extraction Solution and EMX Planar 3D Solver and the tight integration into the Virtuoso RF Solution gives you all the flexibility to correctly capture cross-coupling.

Stay tuned to the Virtuoso Meets Maxwell blog series to hear more exciting news and valuable hints about RF design methodologies in the Virtuoso RF Solution.

Related Resources

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

Claudia Roesch

Contact Us

For any questions, general feedback, or even if you want to suggest a future blog topic, write to custom_ic_blogs@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! To receive notifications about the new blogs in this series, click Subscribe and submit your email ID in the Subscriptions box.