True EM-Circuit Co-Simulation with Genesys-Momentum Integration
By Rick Carter, Field Engineer, Agilent Technologies

With the increasing frequencies and complexities of today’s RF circuits, electromagnetic (EM) simulation has become a necessity. Due to the complexities with incorporating discrete parts into EM simulations with existing tools, many designers abandon EM when simulating circuits such as amplifiers and LC filters. Linear and non-linear simulation alone does not have the accuracy of combined EM-circuit co-simulation. Stray couplings, parasitics and box effects, not captured by circuit simulation, can result in additional design turns and increased time to market.

When to Use an EM Simulator
Most EM software is of two types: 2.5D (sometimes called planar 3D) which simulate electromagnetic interactions in the X and Y direction, but allow only vertical currents, such as a via hole; and 3D, which can simulate in X, Y, and Z directions. This article focuses on 2.5D or Planar 3D EM simulators, which are primarily used for planar geometries such as printed circuit boards.

Planar 3D EM modeling is especially valuable in the following design situations:

When parasitic coupling is present:
Even when circuit models are physically far apart, unexpected coupling can take place. Examples include stubs that seem sufficiently separated, but are actually inductively coupled to each other because of a resonance condition, and footprint pads for discrete parts which can add additional capacitance to the circuit.

When a circuit model does not exist:
For example, if a designer wants to analyze a microstrip Y-junction for which there is no model.

When there are slots in ground planes:
Designers remove portions of ground planes for a variety of reasons, such as reducing the capacitance to ground of a spiral inductor, or to allow a via to pass through a ground plane.

When the model range is exceeded:
All circuit simulator models are developed with a number of range limited control parameters (such as width, length, height, or dielectric constant). Some models break down gradually, while others generate significant errors as soon as the range limits are exceeded.

When the circuit is within a box, especially with a cover:
Box effects, especially cover effects, can drastically affect circuit performance by coupling to the walls and cavity resonances. EM is usually the only way to predict these box effects accurately for the complete circuit.

Most of the 2.5D EM tools on the market simulate transmission lines and distributed circuits directly. Problems arise when discrete parts such as transistors, inductors and capacitors are included in the design. Many EM tools have limited or no capability for including these parts, or require purchase of a separate circuit simulator to do so. Software that claims integrated co-simulation usually requires an intricate setup to include the discrete parts. The setup varies, but usually requires the following steps (also shown in Figure 1).

1. Generate schematic
2. Generate layout
3. Place internal ports on the layout for each part connection, usually two or three per part
    (very time-consuming and error prone)
4. Run EM
5. Generate EM S-parameter file
6. Create a separate schematic to combine EM file and parts (again, very time consuming,
    error prone)
7. Run circuit simulation (sometimes limited to linear simulation)
8. Finally, view combined response

A Better Solution
Agilent has solved most of these problems with Momentum GX. Momentum GX is the integration of the industry-proven Agilent Momentum EM engine from Agilent’s Advanced Design System into the easy-to-use and affordable Eagleware Genesys platform. Within Genesys, True EM-circuit co-simulation is an easy, four-step process (see Figure 2):

1. Generate schematic
2. Generate layout
3. Run EM and linear or non-linear circuit co-simulation simultaneously
4. View co-simulation -- circuit and EM are combined automatically

The Agilent Genesys platform, with its new Momentum GX functionality, combines integrated linear, layout, harmonic balance, SPICE, synthesis modules and RF system architecture with powerful 3D-planar EM simulation into a single, affordable environment. As a result, the solution is most suitable for helping designers achieve cost-effective, first-pass design accuracy without ever leaving the Genesys design flow.

It accurately characterizes and improves circuit performance with the ability to analyze arbitrary design geometries, including multi-layer structures, and can accurately simulate complex EM effects as described earlier.

Momentum GX Features
Momentum GX features both full-wave and quasi-static EM solvers. The full-wave EM solver (MW mode) provides accurate simulation of open-boundary problems and is capable of full dispersion and radiation simulation. The quasi-static solver (RF mode) enables significantly faster simulation of much larger, complex circuits without sacrificing accuracy.
Other advanced features include:

1. Automatic conformal mesh – many EM solvers use a rectangular grid-based system where the grid must be set manually (Figure 3). The grid based system also suffers from inaccuracies due to stair step approximations of curved portions. Momentum GX’s automatic conformal mesh uses both triangular and rectangular mesh which is set without user intervention. This gives not only better accuracy, but also faster simulation times and less memory usage.

2. Adaptive frequency sweeps – Adaptive Frequency Sampling is an intelligent data interpolation algorithm that selects frequency samples automatically. Important circuit details are modeled by sampling the response more often when the S-parameters are changing rapidly, while minimizing the overall total number of samples. The result is a high-resolution S-parameter response analysis that minimizes overall simulation time.

3. No box requirements – many EM solvers require the circuit to be inside a closed “box.” If your circuit is not inside an enclosure or it is in the middle of a larger circuit away from the enclosure walls, inaccuracies can result unless the box walls are moved far away from the circuit to be simulated. This also results in additional simulation time and memory usage. Momentum GX can simulate with or without a box.

4. Tuning, Optimization, Monte Carlo, Yield Analysis – tuning, circuit optimization, Monte Carlo and yield analyses are key to ensuring manufacturability and are usually limited to circuit simulators. Momentum GX has the capability to tune, optimize, and perform Monte Carlo and yield analyses on not only the layout (transmission lines) but also on any schematic element and even the substrate parameters. Results can then be plotted on a graph or a histogram (see Figure 4).

Momentum GX: True Co-Simulation Example
The Genesys suite of tools includes 12 synthesis modules for synthesizing a variety of components using design data as input (see Figure 5).

For this example, we use the microwave filter synthesis module to design a Bandpass, Chebychev, Combline, 7th-order filter with a passband from 2.3GHz to 2.8GHz. These entries are made in the “Topology” and “Settings” tabs in panels A and B of Figure 6. The Combline topology was chosen because it uses discrete capacitors and therefore, demonstrates the ease of co-simulation. The “Options” tab is shown in panel C of Figure 6. The “Select Manufacturing Process” button allows selection of 13 available processes (Panel D). The FR4 substrate was selected from a library of substrates. “Create a Layout” and “Prepare Layout for Simulation – Momentum” are selected to automatically generate a layout and insert ports for a Momentum GX simulation.

The synthesis module creates the schematic, layout, linear simulation, and graphs, runs the simulation, and sets up an optimization automatically. In addition, it prepares the layout for a Momentum GX simulation by adding the input and output ports. The seven capacitors in the schematic are automatically linked to the layout through the footprints. There is no need to manually add ports – all ports are inserted and the layout is set up for the Momentum GX simulation. The schematic, layout and graphs are shown in Figure 7.

All that is necessary is to add a Momentum GX analysis from the available analyses in the workspace tree in Genesys (see Figure 8).

The results of the Momentum GX analysis and the layout mesh are shown in Figure 9. This simulation took less than 4 minutes in RF mode. The response shows a passband shift of approximately 140MHz. What could cause this? Genesys automatically created the layout. In so doing, it connected the nodes of each part. The nodes of the lines are on the ends, whereas the nodes of the capacitor pads are in the center of the pad. Connecting these nodes has effectively lengthened our resonant lines by half a pad. We need to move the pads down to remove this extra length.

To adjust the pads, all are selected by drawing a box around them and moved with the arrow keys on the keyboard, as shown in Figure 10.

All that is necessary is to re-run the Momentum GX co-simulation. The response is shown in Figure 11. There is now only about a 30-MHz shift in the passband. If necessary, the capacitors could be tuned using the sliders added to the graph and watch the Momentum GX response update in real time. An optimization could be run to automatically adjust the values to specified design limits. DC or SPICE simulations could also be run to investigate power handling or pulse response.

Ideal capacitors were used in this example. They could also be replaced with manufacturers’ models, S-parameter models, or many other types of more accurate models.

Gerber Import Simulation Example
Many engineers want to simulate PCB layouts that exist in board layout tools that do not have EM simulation capability. The Genesys import/export capability simplifies the process. The user simply imports the design in DXF, Gerber or GDSII format, places the discrete parts in a schematic and places the resulting footprints in their proper place on the layout. Finally, run the Momentum GX simulation as previously demonstrated. Figure 12 shows a complete board imported, a matching circuit section isolated, schematic generated and resultant component pads placed in the layout. In this case, substrate dependant LC models from Modelithics (http://www.modelithics.com) were used. Then an optimization on all discrete components was run. The graph shows significant improvement in the match after optimization.

Summary
Planar 3D electromagnetic simulation is an extremely important tool for obtaining first-pass design success. However, most EM simulators are not practical to use for discrete circuits. Agilent has combined the power and accuracy of the industry proven ADS Momentum EM solver with the ease-of-use and affordability of the Genesys platform. Starting at under $15K (US list), the Agilent Genesys/Momentum GX bundle provides high-performance system and circuit design tools that reduce time to market by reducing board design turns.

The example files used in this article can be downloaded from our Momentum GX webpage: www.agilent.com/find/eesof-momentum-gx.

For more information about Agilent’s Genesys product line, visit www.agilent.com/find/eesof-genesys.

About the Author
Rick Carter received his Bachelor of Electrical Engineering degree from the University of South Florida in Tampa, FL. He worked in a variety of engineering and sales positions before joining Eagleware in 1999, where he held positions in technical support, application engineering, marketing, and sales. Rick joined Agilent with the purchase of Eagleware by Agilent in 2005, and has held positions in both applications engineering and sales.

The author wishes to thank Lance Lascari of www.rfdude.com for providing content for this paper.

Agilent Technologies
www.agilent.com
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