CAE: Do Designers Trust Their Tools?

Nov. 8, 2011
To keep customers satisfied, EDA vendors offer a combination of simulation techniques to ensure accuracy, robust modeling libraries, and compatibility with emerging standards.

Electronic-design-automation (EDA) tools are used for a variety of simulation and modeling tasksfrom developing matching circuits for amplifiers through complete system simulation. Some RF/microwave engineers may work with EDA software every day, but how much do they trust these tools? Among their biggest concerns is model accuracy. The good news is that the EDA houses are involved in a constant feedback loop with their customers. They are striving to provide the most accurate models possible, as well as the necessary support and tools to verify them. In addition, EDA vendors have an eye on the entire system design, allowing engineers to create components and then combine their functions for a full system-level simulation. The myriad of complementary tools include statistical analysis to help translate simulated results into reproducible products.

Martin Timm, CST's Marketing Director, sums up the challenge facing EDA vendors and their customers: "The accurate modeling of input geometry, materials, and physicsas well as the accuracy and efficiency of the simulation processrepresent the software vendors' challenge. There is, of course, the basic worry of most design engineers: Have they chosen the right tool for the job, or would another tool deliver another, probably more accurate answer?"

This demand for the best possible accuracy is what forces EDA vendors to remain in such close contact with their customers. "We're really in a unique situation in that, for many years, we have had hundreds of design engineers use our EDA tools to design Agilent's RF/microwave instruments," notes Charles Plott, Agilent EEsof EDA Product Planning & Marketing Manager - Core Products. "Along with our customers, these internal designers provide regular feedback on what their challenges are and how we can improve their productivity (Fig. 1)."

Plott also cites model accuracy as the number-one design worry. "This is true for both passive circuit design and nonlinear circuits. For example, for passive circuit design (filters, matching circuits, connectors, antennas, etc.), accuracy has been the main driver for the existence of electromagnetic (EM) simulation. Of equal or greater importance is the accuracy of the device model for nonlinear designs with active components. That accuracy is mission critical. This is something in which foundries, design engineers, and EDA companies all invest."

Timm points out that simulation accuracy depends on various factors, including whether a simulation model truly represents its real-life counterpart and whether structural and material effects have been considered. These concerns are generally outside of the software vendor's control. Yet he believes that it is important to deal with these complex structures in order to develop accurate, realistic models.

Shawn Carpenter, Vice President, Sales and Marketing, Sonnet Software, agrees that RF/microwave designers are most concerned with accuracy. He adds, "A second concern, but closely related, is whether they can come to terms with whatever complexity the tool presents in order to get the necessary level of accuracy out of that tool." Designers are clamoring for tools that are more intuitive and easier for them to master in order to feel confident that they are extracting the most accurate designs and simulations.

Sherry Hess, Vice President of Marketing, AWR Corp., echoes the call for ease of use: "First and foremost, designers want to make sure their circuits are functionally sound (that they work) rather than spending undue time on understanding the nuances of the software used to first design them."

Of course, all of these concerns must be weighed against how long it takes to achieve a solution. Tradeoffs must be made in terms of simulation time, points out Markus Kopp, Corporate Product Manager, Electronics, ANSYS, Inc. One way that ANSYS addresses this tradeoff is by combining its core technologyadaptive meshing with a new technology called Solver on Demand. "Combining these technologies ensures that the simulation is extremely easy to create," explains Kopp. "But it will always have a fine, accurate mesh where it is most important to provide accuracy and a coarse mesh elsewhere for efficiency."

Achieving a sound design can be challenging. In today's increasingly complex designs, how do designers know that their signal chains will not be influenced by nearby components? "Today, it's rare that an RF design is standalone," observes Per Viklund, Product Marketing Director, Mentor Graphics. "RF always shares limited board space with non-RF circuits. Implementing an RF circuit in sync with other circuitrywhile making sure there are no harmful interactionsis a serious challenge."

CST's Timm concurs: "What we have learned from interacting with our customers is that there is an increasing need to deal with the high levels of complexity found in real-world examples in a simple and straightforward way. Here, real world' means not just a single componentfor example, an antennabut also the influence of the placement and feed and potentially the effect of thermal stress or general EMC considerations."

To boost their customers' confidence in the accuracy and usability of their designs, analysis, and simulations, EDA companies are making strides both to allay designers' fears and build design confidence. CST, for example, addresses the accuracy question on different levels. Within the different CST STUDIO SUITE simulators, methods such as the Perfect Boundary Approximation (PBA), True Geometry Approximation, and arbitrary order curved finite elements are designed to increase simulation accuracy and performance. The firm also offers a System Assembly and Modeling (SAM) feature, which allows designers to set up an EM system in schematic diagram form. The user can assign simulation tasks to a single component, sections of the schematic diagram, or the entire design as a three-dimensional (3D) structure. As a result, he or she can study or optimize it in part or as a whole.

Designers want to run complex simulations for accuracy, but this has to occur within a reasonable amount of time to be useful. By adding Solver on Demand technology to its HFSS software, ANSYS enables engineers to access the HFSS solver from an enhanced-computer-aided-design (ECAD) interface (Fig. 2). This aspectcombined with the adaptive meshing technique in HFSSallows engineers with very little experience in 3D modeling to create, set up, and solve extremely complex EM simulations.

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ANSYS further addresses simulation time by deploying fast solver algorithms and a variety of high-performance-computing (HPC) options, including multiple compute cores. Among the HPC techniques offered is domain decomposition. Here, a mesh can be subdivided into multiple mesh domains that can then be solved in parallel on networked compute nodes. Alternatively, multiple frequency points for a broadband model can be solved on networked compute nodes. ANSYS refers to this as the spectral decomposition method.

Keeping one eye on accuracy and one on design time, AWR has refined its simulation tools over more than 11 years. “The unified data model that spans all of these tools ensures that everything in the schematic is addressed in the layout and that everything in the layout is included in the simulation,” notes Hess. “The software is intuitive to learn and use so that the designer can spend more time designing.”

To address circuit complexity, Mentor Graphics has developed its Enterprise approach, allowing multiple design teams to work together (Fig. 3). To help to eliminate potential interactions as the design progresses, each team designs its technology in the context of the complete mixed-technology environment. The Expedition Enterprise system synchronizes the parametric-shapes library with the RF simulation tool to ensure that the simulation matches what is built.

For its part, Sonnet places a strong emphasis on mesh control. Careful edge meshing is key in RF designs, Carpenter explains, because electrons congregate on the edges of transmission-line structures at high frequencies. Sonnet’s EM simulation technique uses a shielded-domain method of moments (MoM) that exhibits monotonic error convergence as designers refine the mesh. To preserve geometries when porting designs between different simulators, Sonnet incorporates interfaces to other simulators, such as Agilent ADS, AWR Microwave Office, and Cadence Virtuoso. The firm also provides port calibration for both internal and external ports.

To best serve its customers, Agilent strives to fully engage in EDA as well as test and measurement. In addition to its circuit-simulation products like ADS and Genesys, for example, the company offers a dedicated companion platform for device model extraction (IC-CAP). Plott reports that many of Agilent’s customers and foundry partners use IC-CAP to extract device models for use with circuit simulators. Designers can use Agilent hardware (network analyzers, sources, etc.) to make the extraction measurements as well. “In RF/ microwave EDA, offering a credible modeling solution for our customers is a pretty critical requirement,” notes Plott. “In other words, simulation alone is a partial solution. You have to have a core expertise in modeling.” Agilent’s commitment to modeling incorporates its EDA team as well as its internal foundry.

Perhaps one of the greatest challenges for EDA is adequately dealing with multiple standards and the resulting need for increased RF and microwave functionality. Plott notes, “Our customers, and our own internal designers, often have to ship products when the ink is barely dry on a new standard. So it’s absolutely critical to make sure that the EDA software that they use is ‘standard-aware’ as early as possible—sometimes concurrent with early versions of the standard as it is being built. Accuracy is dependent on having good models, but also a real-world signal and spec to use as a source and sink while you’re designing.”

The Agilent SystemVue platform is built on a mathematical-core dataflow engine, allowing designers to access (or modify) popular standards. Alternately, they can prototype and explore their own formats. The company’s Wireless Library team works on prototyping the standards even when they are still under development.

Generally, Hess suggests that designers have a high level of confidence in the accuracy of models and simulators in EDA tools. The challenge lies in the circuit complexity and the stringent standards to which they must comply. This issue requires a system-level view as well as concern for other influences. Additional tools may be required, which are dedicated to specific tasks. Examples include thermal management, filter synthesis, and manufacturability.

“Engineers have long been stereotyped as conservative and risk-averse. However, all designs pose risks, so engineers are really taking risks every day—and their job is to minimize them,” observes Hess.

Fortunately, today’s EDA vendors are enabling engineers to simulate designs more quickly by using the latest standards and model libraries across platforms and networks. They are providing the application support to verify these designs against known or physical test results. And perhaps above all else, they are listening to and responding to their customers.

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