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Design Software for Virtual Prototyping of RF and Microwave Devices

October 14, 2004
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The use of simulation software is becoming an essential part of the design cycle of any new device, and has allowed the concept of “virtual prototyping” to become commonplace. Not only does this provide engineers with the ability to test new ideas before building prototypes, it also allows for the “what if” scenario. Radical designs can be created and tested at a fraction of the cost of traditional methods. In some applications, this saving in time and effort can be dramatic; for example, one software user managed a reduction in time spent from one person-year per prototype using traditional methods, to one designer analyzing 60 designs in a single month.


The CONCERTO software suite is specifically for RF and microwave design, and complements the OPERA software (for static and power frequency electromagnetic analysis) also produced by Vector Fields. The latest version of CONCERTO — version 4.0 is being released in conjunction with the 20th anniversary of the company.

There are major enhancements to the new version, many of which are in the user interface. The 3D Modeler has been extended to allow models to be imported from other CAD systems, or to be created directly using the extensive set of tools available. An Expert Mesher has been introduced that automatically defines the initial mesh, while still giving users complete control to make changes as required. The Simulator has also been enhanced to allow greater levels of interaction with the solution as it progresses, using new visualization tools.

The CONCERTO Software Suite

CONCERTO is the environment for microwave design incorporating the robust finite difference time-domain analysis (QUICKWAVE), an easy to use 3D geometric modeler, and powerful visualization facilities. The environment uses the most reliable modules, giving the user all the tools required for fast and accurate designs. All the tools are intuitive to use, and together make CONCERTO an advanced solution for RF and microwave design.

There are numerous reasons why CONCERTO is an ideal tool for virtual prototyping, including:

• CONCERTO is easy to use. Models are created using a fully 3D geometric modeler, allowing simple or complex shapes to be created quickly and efficiently. CAD import is included for all major CAD data formats. Less time is spent with the mechanics of inputting a design into the software, and more time can be spent on design.

• CONCERTO is fast, accurate and reliable. Using the unique Conforming Meshing technology, complex shapes and curved boundaries can be modeled accurately, without adding to the overall size of the model or requiring a reduction in time step. A rapid turn around of designs is therefore assured, with the confidence that an accurate solution is being produced.

• CONCERTO includes advanced visualization tools. It is possible to monitor the solution as it progresses in time (sometimes giving extra insight into the way the device actually works), or display the final result. Using the dynamic multi-window visualization tool, one can see what you want, where you want, when you want.

•CONCERTO models can be easily parameterized. There is a sophisticated macro language that can be used to parameterize models. A command script is automatically created, and can be modified using the integrated application sensitive editor. Changes to a design can be achieved by simply setting a few parameters.

• CONCERTO is fully supported. The company has a large number of application engineers who can give expert support. Professional support tools mean questions will be answered efficiently and reliably, giving the designer more time for the job in-hand.

The CONCERTO Modeler

Models are created using a fully 3D geometric modeler, shown in Figure 1, allowing simple or complex shapes to be created quickly and efficiently. The Modeler includes basic building blocks, and uses Boolean operations (Union, Subtract, Intersect, Trim, Cutaway) to build more complex structures. Bodies can be transformed and faces swept (to form waveguides, for example). Other functions are included for blending, chamfering, twisting, lofting and “morphing.” In addition, thin sheets and wires can be embedded within the finite difference grid. All the data required for the analysis is defined in the Modeler, which then launches the QUICKWAVE analysis.

Fig. 1 Screenshot of the Geometric Modeler showing the toolbars and pulldown menus used to define complex 3D designs.

The data required for the QUICKWAVE Simulator is generated directly from the Modeler. The data includes global settings (e.g., units), material properties and boundary conditions (including ports). Efficient absorbing boundary conditions ensure the boundaries do not generate spurious reflections (both PML and high order ABC are used). Symmetry condition can be used to reduce the size of an analysis by only requiring part of the complete structure to be analyzed.

The Expert Mesher used in the Modeler uses the frequency and material properties to correctly set initial cell sizes. The relevant geometric features are captured using the Mesher, enabling special facilities within the analysis to be effective (for example, a feature to accurately represent field singularities near metallic corners). Users can easily override the automatic settings if desired.

An alternative method for building models is using a library of predefined, parameterized objects. Complex structures can be built very quickly and efficiently, with the facility for users to create their own library items. The Optimizer can then be used to optimize a design using one or more parameters in the final design.

The QUICKWAVE Simulator

The QUICKWAVE Simulator (developed by QWED Sp. z o.o) is based on the FDTD technique, and uses the unique Conforming Meshing technology to ensure complex geometries are easily and efficiently modeled. Conforming elements allow material boundaries to intersect with finite difference cells, so that cells can contain more than one material. This technique used in QUICKWAVE is unique in that it requires no extra terms to be added (no increase in memory) and the time step does not need to be reduced (no increase in solution time).

Fig. 2 A Vivaldi antenna (a), with enlargement of the used mesh (b).

Figure 2, showing a Vivaldi antenna, displays how the Conforming Meshing technology allows cells to have more than one material property. The results of the simulation can be viewed dynamically in the multi-window Simulator in a number of ways, including:

• S-Parameters: Advanced facilities for producing the S-parameters are available. Enhanced techniques unique to QUICKWAVE allow accurate S-parameter extraction, including below cut-off, and features to reduce errors. Any number of S-parameters can be displayed on the same graph, including previously saved data (allowing easy comparison of design changes).

• Fields Displayed on Geometry Surface: The fields can be displayed on planes throughout the model. In addition, the fields and surface currents can be shown on the surface of the model itself.

Fig. 3 QUICKWAVE analysis showing the dynamic display.

Figure 3 shows the dynamic display of the QUICKWAVE analysis. The user has complete control over what is viewed in the multi-window visualization tool.

Other ways of viewing and saving the results of the simulation include:

• multi-modal, multi-port S-matrices

• radiation and scattering patterns, including point sources and plane waves

• pattern of field, dissipated power, SAR

• time-domain reflectometry

• E-field integration along user-specified contours

• direct extraction of Q-factors (space and media-selective)

• eigen value extraction.

Advanced Features in CONCERTO

The CONCERTO environment includes extra unique features. The 2D QUICKWAVE analysis module allows very fast initial designs and optimization to be carried out for axi-symmetric geometries. Only a 2D cross section is modeled, although the full vector equations are solved. Higher order axi-periodic modes can also be defined and modeled, as shown in Figure 4.

Fig. 4 Axi-symmetric corrugated horn antenna (a) solved using the QUICKWAVE-2D Simulator with results compared with measurements (b).

If temperature effects are important, a Thermal module is available. This option models the temperature rise in lossy materials due to heating. Material properties can be defined as a function of temperature, giving a true prediction of temperature rise in applications involving microwave heating.

For high Q devices, the use of the QProny module can reduce the solution time by a factor of 10 or more. By filtering out the oscillations in the frequency response, an accurate prediction of the S-parameters can be obtained without waiting for all the energy to dissipate in the system. Figure 5 shows an example of QProny using a cavity model (the red line is the smoothed result computed after 2,000 time steps, reproduced for comparison). An advantage of modeling in the time domain is that as well as obtaining the complete frequency response in a single analysis, true time-domain analyses can also be performed, such as Time-domain Reflectometry.

Fig. 5 Cavity model (a) and graphs of S21 after 2000 time steps (b) (showing oscillations in frequency response with the smoothed result using QProny shown in red), 20,000 time steps (c) and 40,000 time steps (d).

Applications

The applications of the CONCERTO suite are varied and include industrial applications (telecommunications, electronics, microwave ovens, food processing, automobile and transport, defense, industrial microwave chemistry), applications in science (space and atmosphere research, microwave heating, electromagnetic impact on biological tissues, electronics) and as a teaching aid (software such as CONCERTO makes an ideal tool in teaching microwave engineering).

Some typical applications listed below indicate how special features within the CONCERTO software are used to advantage in order to obtain fast and accurate simulations.

Antennas: Thin wires can be used for antenna modeling, and with near-to-far field transformation, 2D and 3D radiation plots can be produced. For more complex structures (such as horn antennas), the conforming mesh is essential to accurately model the complex geometry. Patch antennas are efficiently modeled by using infinitely thin metal layers (finite thickness sheets would require very small grids and would lead to excessive solution times).

Cavities: CONCERTO can be used to quickly obtain the resonant frequencies of cavity structures (such as RF cavities and microwave ovens), including lossy walls and lossy loads. For each resonant mode, the Q-factors can be computed, and the modal shape interrogated.

Waveguide Components: Enhanced S-paramater calculations ensure the highest level of accuracy, including below cut-off. Multi-mode analyses can easily be performed.

Filters: This is a very demanding application, and the advanced techniques within CONCERTO are required to give accurate solutions. The fields near corners must be modeled accurately (including the field singularity), thin sheet models can be used for planar filters, and local grid refinement can be used where extra precision is required. With the unique QProny module, the analysis time can be reduced significantly without loss of accuracy.

The Future of CONCERTO

The new version of the CONCERTO suite is showing itself to be one of the most advanced software tools for RF and microwave design, allowing fast and accurate virtual prototyping. The future of CONCERTO is also ensured, with additions to the suite to include all the main solution techniques.

To complement the FDTD analysis already in place, the 3D Moment Method module CLASP (developed by Culham Electromagnetics and Lightning Ltd., part of the Chelton Group) has recently been included in the suite. CLASP is primarily used for the simulation of radar signatures, antenna coupling, antenna clusters and EMC. In addition, the 3D Finite Element module SOPRANO (which has been in use for many years as part of the OPERA suite for eigen value analysis of RF cavities and other resonant structures) is being made available within CONCERTO.

All three analyses are launched from a common user interface, making CONCERTO unique in offering the three main solution techniques in a single environment, offering the most appropriate method tailored for the application without prejudice.

Vector Fields Inc.,
Aurora, IL
(630) 851-1734,
www.vectorfields.com.

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