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Jump-Start EM/Circuit Co-Simulation with Keysight RFPro and Modelithics Models

Parent Category: 2021 HFE

By Dan Schwarz and Chris DeMartino

No question, electromagnetic (EM) analysis is critical for many of today’s designs. Specifically, for designs that include surface-mount components, it is often necessary to perform an EM/circuit co-simulation that combines circuit analysis of the component models with EM analysis of the surrounding interconnects. This article presents an EM/circuit co-simulation workflow made possible by combining Keysight Technologies’ RFPro EM environment with Modelithics® passive-component models.

Performing EM Analysis as Easily as Circuit Simulation

RFPro is an EM environment dedicated to RF/microwave circuit design that is included within several available Keysight Pathwave Advanced Design System (ADS) packages. RFPro is available for integration within ADS, Cadence® Virtuoso®, and Synopsys Custom Compiler. Its goal is to make EM analysis as easy as performing circuit simulations, thereby simplifying the EM/circuit co-simulation process.

RFPro includes presets for correct EM analysis of user-identified physical circuit layout structures. It also features automatic EM/circuit co-simulation partitioning for integration of EM results into circuit designs for interactive tuning and optimization. The end results of using RFPro include the following:

  • Two hours to two weeks can be saved per simulation versus using external EM simulators.
  • Users can instantly set up correct EM/circuit co-simulations without layout modifications.
  • EM ports and materials are automatically assigned.
  • There is no need to wait for someone else to set up and run batched external EM analysis.

Finally, RFPro also includes EM capabilities with respect to solver, meshing, visualization, and predictive analysis settings. As a result, complex IC, PCB, or multi-technology modules can be quickly set up and analyzed with few or no manual operations.

Measurement Precise Modeling from Modelithics

Here, we will provide a specific example to demonstrate how easy it is to use RFPro alongside Modelithics Microwave Global Models™ for RLC components. Modelithics provides accurate measurement-based simulation models for all types of passive and active RF/microwave components from a wide range of today’s popular vendors. Modelithics models are currently available for Keysight Pathwave ADS for everything from surface-mount RLC passive components to transistors, amplifiers, attenuators, filters, and more. One of the benefits associated with the models is suitability for EM/circuit co-simulations, which now extends to the RFPro EM platform. As mentioned earlier, EM/circuit co-simulation involves simulating the component models within a circuit simulator, while the surrounding microstrip elements are analyzed by an EM simulator.

We can demonstrate how Modelithics models can be used in conjunction with RFPro by presenting a design example of a lowpass filter designed using a 4-mil-thick Rogers RO4350B substrate.1 Figure 1 shows a schematic of the filter, which is designed for a passband to 1.5 GHz. For the inductors and capacitors, we are using Modelithics models for the Coilcraft 0402CS and AVX UQCL part families, respectively. Note that for this example, ADS 2021 Update 1 is being used along with the Modelithics COMPLETE Library v21.0.

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Figure 1 • Lowpass filter schematic in Keysight ADS.

Configuring the Models and Building the Layout

For this example, we want to simulate the component solder pads within the EM simulator rather than within the models themselves. Modelithics Microwave Global Models for passive components include a parameter called Sim_mode, which allows users to specify whether solder-pad effects should be included within the model. Setting the Sim_mode parameter to 2 removes pad effects from the electrical model. With this setting, the solder pads are also removed from the layout by default. The Sim_mode 2 setting is intended to be used when it is desired to simulate the pads within an EM simulator, as is the case here. Hence, the Sim_mode parameter of all models is set to 2 in this example. With this setting, the component pins for EM/circuit co-simulation will be located at the edges of the component bodies rather than the edges of the solder pads as will be shown later.

The Pad_mode parameter provides further independent control of the pad layout treatment.2 In this example, we will set the Pad_mode parameter to 1 for all models. This setting overrides the default configuration for Sim_mode 2, as it forces the solder pads to be included in the layout and allows them to be analyzed within the EM simulator. Failing to set the Pad_mode setting to 1 (when using Sim_mode 2) would result in the solder pads being removed from the layout, thus preventing them from being simulated within the EM tool.

Note that in some cases, users may want to implement custom solder pads that do not correspond to the standard pad topology within the model. For example, one may want to create solder pads of a different shape as opposed to the standard implementation. In that scenario, the user would want to set Sim_mode to 2 and the Pad_mode parameter to 0 (default setting) or 2 and then separately add the custom solder-pad layouts within the layout editor. However, in this example, we are simply using the default pad configurations for the models. Thus, the Pad_mode parameter is set to 1 for all models to allow the default solder pads to be included in the layout.

The filter can now be viewed in layout mode by selecting “Window>Layout” in the toolbar. To easily maintain the named nets and synchronization between schematic and layout, “Net-Based” connectivity should be used. This option can be set globally from the main ADS window by selecting “Options>Preferences>Connectivity.” Alternatively, users can select this option for an individual layout by selecting “Tools>Set Connectivity Options.” Figure 2 shows the suggested layout connectivity settings.

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Figure 2 • Shown are the suggested settings for layout connectivity.

 

In layout mode, users can select “Schematic>Design Differences” from the toolbar to build the layout from the schematic. Clicking “Update” in the subsequent user interface makes it possible to place all the schematic elements in the layout (Fig. 3). These elements include the component models, microstrip interconnects and vias, and input/output pins. Figure 4 shows the final layout after placing and properly connecting all the elements from the schematic. We must also configure the substrate in the Substrate Editor (Fig. 5).

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Figure 3 • The “Unplaced Components” can be placed in the layout.

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Figure 4 • Filter layout after properly placing all the elements.

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Figure 5 • Substrate Editor for configuring the substrate parameters.

 

Launching the RFPro platform

After configuring the substrate, we can launch RFPro by selecting “Tools>RFPro>New” in the toolbar. Figure 6 shows the RFPro user interface, which includes the filter layout along with the “Project” and “Setup” interfaces in the upper and lower left areas, respectively.

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Figure 6 • RFPro user interface. Note the “Project” (top left) and “Setup” (bottom left) interfaces.

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Figure 7 • The pins for the input and output must be dragged from the “Project” interface to the “Setup” interface.

 

Before performing a simulation, some amount of configuration is required. For one, the input/output pins in the “Pins” location in the “Project” interface must be selected and then dragged to the “Ports” location in the “Setup” interface (Fig. 7). Also, to perform an EM/circuit co-simulation, the models must be specified as circuit elements. To do so, right-click the model in the “Project” interface and then select “Change Component Role>To Circuit” (Fig. 8).

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Figure 8 • The component models must be specified as circuit elements. Also, for this example, we must select “Component Includes Area Pins in Net.”

 

 

Right-clicking a model in the “Project” interface also lets users select an option called “Component Includes Area Pins in Net” (Fig. 8, again). Selecting this option enables the component solder pads to be included in the EM analysis, which is what we want to do here. Therefore, this option must be selected for both models. In addition, Figure 9 shows the pin locations for each component in the design. Notice how the pins are correctly located at points that correspond to the edges of the component bodies rather than the edges of the solder pads. Finally, we need to select both models and drag them to the “Component Models” location in the “Setup” interface.

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Figure 9 • The location of the component pins corresponds to the edges of the component bodies.

 

At this point, we can double-click “Options” in the “Setup” interface to configure the EM analysis. For this analysis, the start and stop frequencies are set to 50 MHz and 8 GHz, respectively. We can also select the simulator we wish to employ, which in this case is Momentum Microwave. We can then simulate this filter by selecting “Run” directly underneath “Options.” Once the simulation is complete, we can expand the “Results” selection located directly underneath “Run” and then select “Generate Sub Circuit” (Fig. 10). Users can then choose to “Create View in Existing Cell” or “Create new Cell/View.” Here, we will choose “Create View in Existing Cell.”

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Figure 10. We can generate a subcircuit after the simulation is complete.

 

Final Subcircuit Analysis

Selecting “Generate Sub Circuit” followed by “Create View in Existing Cell” adds a new schematic to our cell called “schematic_Full_EM_Analysis,” which represents a subcircuit of the filter design (Fig. 11). This subcircuit includes a block that contains the EM analysis data along with all the component models. Keep in mind that we must also add a substrate element to this schematic. For this example, the substrate element for 4-mil-thick Rogers RO4350B is chosen from the Modelithics SUBSTRATE Library, which is a collection of many of today’s popular substrates.

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Figure 11 • This schematic represents the subcircuit of our filter design.

Generating a subcircuit also adds a new symbol to our cell called “symbol_Full_EM_Analysis.” For the final step, we can drag this symbol onto a new schematic (Fig. 12). We can then select the element followed by “Choose View for Simulation.” Selecting “schematic_Full_EM_Analysis” enables us to perform a full EM/circuit co-simulation that incorporates the EM analysis data. Note that we can also perform a circuit analysis of the filter that does not include EM effects by selecting “schematic.” Hence, designers can easily switch between circuit and EM analysis, allowing for fast comparisons between circuit- and EM-based results.

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Figure 12 • Final schematic that includes the filter subcircuit element.

 

Finally, the filter was built and measured. For comparison purposes, Figure 1 shows the results of the circuit simulation along with the measured data. For S21, the circuit simulation adequately predicted the in-band filter performance. However, the out-of-band performance was not predicted very well.

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Figure 13 • Results of the circuit simulation (solid traces) along with measured data of three filters (dashed traces). S11 is on the left; S21 is on the right.

 

Figure 14 shows the EM/circuit co-simulation results. We can see that the results agree well with the measured data. Notice how the out-of-band prediction is much more accurate in comparison to the results of the circuit simulation. Thus, we have validated the overall EM/circuit co-simulation model results.

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Figure 14 •EM/circuit co-simulation results (solid traces) along with measured data of three filters (dashed traces). S11 is on the left; S21 is on the right.

 

In summary, a workflow that combines the RFPro EM platform with Modelithics models has been demonstrated. The described method makes it easy to perform EM/circuit co-simulations, thus providing designers with an efficient means for accurate analysis. Interested designers may want to consider this approach for their next project.

About the Authors

Dan Schwarz is with Keysight Technologies and Chris DeMartino is with Modelithics.

References

1. C. DeMartino, “Application Note 071: Filter Design Flow in Keysight ADS with Substrate Scalable Models.” Modelithics literature: https://www.modelithics.com/Literature/AppNote.

2. E. O’Dell, L. Levesque, and I. Bedford, “Application Note 057: Advanced Pad Model Features in the Modelithics Library.” Modelithics literature: https://www.modelithics.com/Literature/AppNote.

July 2021

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