High-frequency designs once were created with the help of many different computer-aided-design (CAD) toolsmany lacking interfaces between them. Over the past decade, however, it has become possible for a microwave designer to step through design, layout, fabrication, and more as part of a continuous design flow. If that flow lacks any specialty, such as three-dimensional (3D) electromagnetic (EM) simulation, it generally will offer an easy interface to products that provide that capability. As microwave designs evolve toward multi-function modules and complete subsystems, they will confront tighter space constraints, undesired coupling, and increased EM interaction while operating at higher frequencies. Such trends will make the design process increasingly difficult, forcing engineers to rely even more on software tools and necessarily intuitive interfaces between most-used tools.
An example of a complete design flow is Agilent EEsof's Advanced Design System (ADS). ADS 2009 Update 1 promises to allow monolithic-microwave-integrated-circuit (MMIC) and RF-module designers to stay within one design platform, thereby eliminating the stops and starts associated with point tools. To achieve the MMIC/module desktop flow, Joe Civello, Advanced Design System Product Manager, points out that ADS 2009 Update 1 offers several major benefits. With integrated planar and threedimensional 3D EM simulation and analysis, for example, des igner s can account for the EM effects from packages and interconnects without any import or export of data or models. In addition, an easyto- use Optimization Cockpit permits the engineer to interactively modify settings in real time during optimization. For MMIC designers, a MMIC-layout toolbar allows one-click access to commonly used layout functions. In addition, ADS layout functions can be accessed from foundry-endorsed process-design-kit (PDK) components for faster, error-free MMIC/systemin- package (SiP) layout and design synchronization with the schematic. Thanks to the integration of Mentor Calibre Layout-Vs-Schematic (LVS) in the MMIC/SiP design flow, error-free layout connectivity is ensured before hardware fabrication.
The most unusual feature of ADS 2009 Update 1 is the X-parameter generator, which promises to deliver fast, drop-in usable, and accurate nonlinearbehavioral models. X-parameters are mathematically correct scattering coefficients for active components. The firm asserts that they provide a powerful, yet simple and automated process for capturing nonlinear component behavior over arbitrary complex impedances, input powers, DC biases, and more across various frequencies. According to Civello, the models in ADS 2009 Update 1 eliminate the need for datasheets of specification-based characterization parameters and the laborious measurements typically required for accurate designs. Earlier this year, Agilent introduced the option of arbitrary load impedance X-parameters for its PNA-X Nonlinear Vector Network Analyzer (VNA). Using the PNA-X nonlinear VNA, X-parameters are measured and used to create X-parameter models that can be imported into ADS to simulate actual linear and nonlinear component behavior. The full complex Gamma dependence of a device under large-signal operating conditions can be captured and instantly modeled in ADS using the NVNA option, an external Maury Microwave load-pull tuner, and Maury's load-pull software.
ADS is a standard in research and development laboratories across the world along with rival HFSS, which hails from Ansys subsidiary Ansoft (www.ansoft. com). The most notable feature of HFSS 12.0 is domain decomposition (Fig. 1). Dr. Lawrence Williams, Director of Product Management, states, "This technique allows HFSS to exploit high-performance-computing (HPC) capabilities to solve electromagneticfield problems of unprecedented size and scope. With domain decomposition, a single HFSS job can be divided into smaller pieces and then distributed across a network of computers. HFSS can use all of the memory across that network, thus allowing truly giant simulations." Other new features include key updates in mesh generation, solver technologies, and enhancements to the user interface and the modeler.
When HFSS is coupled with Ansoft Designer and SIwave, engineers can tackle complete RF systems. Ansoft Designer allows engineers to dynamically link system and circuit simulation to 3D electromagnetics. Release 5.0 boasts an advanced RF System Engine and Filter Synthesis tool as well as speed improvements for tuning, optimization, sensitivity, and statistical analyses. For its part, SIwave is a finite-element-based simulator that is able to analyze an entire multilayer PCB to extract S-parameters and board resonances as well as electromagnetic interference (EMI). The latest version of SIwave has several desktop improvements, solver enhancements, and automation features.
A product-family approach also is taken by CST or Computer Simulation Technology. According to Dr. Martin Timm, the firm's Director of Marketing, "Within CST STUDIO SUITE, eight tools are embedded in one design environment. The most unusual individual tool is CST PARTICLE STUDIO for the analysis of devices such as microwave tubes and effects such as multi-pacting. This is integrated with our core tool CST MICROWAVE STUDIO, which enables the simulation of structures ranging from electrically small components (under 1 wavelength) to installed antenna performance and radar cross section (hundreds or thousands of wavelengths)."
The CST software is continuously used in seminal research in space, defense, medical, etc. In medical applications, for example, the CST STUDIO SUITE graphical user interface (GUI) has been used to perform 3D EM simulation of cancer treatment by RF thermoablation (Fig. 2). Timm notes, "A catheter is used to apply a 40-W signal at 375 MHz to a tumor in the liver. The bioheat equation solver is used for the realistic simulation of the resulting temperature distribution."
The medical arena also has set the stage for some unusual applications for AWR's software tools. According to Sherry Hess, Vice President of Marketing, "Microwave Office software and TriQuint Semiconductor's foundry process allowed Meridian Medical Systems to develop a radiometer for its unique catheter used to treat cardiac arrhythmia. The catheter can simultaneously deliver microwave radiation for tissue heating while the radiometer fabricated as a MMIC senses the temperature of the heart wall to give doctors much more feedback about the procedure. Microwave Office software was used along with a process design kit (PDK) developed jointly by AWR and TriQuint that accurately represents TriQuint's foundry process. The seamless integration of the two made it possible to design the circuit, select the features from the PDK library, perform design rule checking, and send the result to TriQuintall in a very short time."
Among the standout features of Version 2009 of the Microwave Office and Analog Office design suites is multi-rate harmonic-balance (MRHB) technology, which vows to dramatically increase speed while reducing the computer memory required to perform the steady-state analysis of complex nonlinear systems with multiple signal sources. Hess notes, "MRHB defines harmonic-balance analysis on a block-by-block basis, reducing filtered sources and their harmonics by defining new hybrid tones based on linear combinations of the source tones." Thus, MRHB satisfies the need to perform steady-state analysis of distributive or dispersive systems with more than two or three signal sources. At the same time, it promises to provide a 5X speed increase when simulating large, complex multitone designs.
The firm also has updated the Visual System Simulator (VSS) and AXIEM software. The VSS communications systems optimization software now supports AWR Connected for Rohde & Schwarz, which adds real-world test signals from the R&S WinIQSIM2 instrument software to the simulation to produce much greater accuracy than generic waveforms. For its part, the AXIEM 3D planar EM analysis software features proprietary solver and meshing algorithms that make it possible to migrate EM analysis from a back-end, post-verification tool to an upfront design diagnostic solution. Beyond these individual features, Hess asserts, "The AWR Design Environment is unique in its support of third-party technologies and tools that can be seamlessly employed with AWR's software to deliver an exceptional design flow and value proposition for RF and microwave designers. Examples include HSPICE, the ICED IC design and verification software tool, and Sonnet Software EM simulators."
Integration with major layout flows is provided by Zeland Software as well. The "layout-to- EM-model" flow of the firm's IE3D-SI is fully automatic. It features one-click native integration from Cadence Allegro PCB/Package Designer, AWR Microwave Office, Autocad DXF, and GDSII layout databases. IE3D-SI targets circuit designers and signal-integrity engineering teams developing advanced packages, PCBs, ICs, and MMICs. It offers automatic 3D geometry model creation, which features full support for TRUE dimension for bond wires, solder balls and bumps, and interconnect and dielectric thicknesses. In addition, built-in mixed-domain SPICE simulation performs robust, accurate, and efficient time-domain-simulationbased up-frequency-domain S-parameter models.
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Similarly, Sonnet Suites Release 12 for 3D Planar High Frequency Electromagnetic Analysis from Sonnet Software can be used as a standalone product or as a co-simulation EM analysis engine to major EDA frameworks from Cadence, Agilent, and AWR. It also promises accuracy for extreme technologies and model extraction for nearly any 3D planar geometry including conductor thickness. For microwave designers, the software's most valuable advantage is its ability to accurately and reliably extract models for arbitrary planar conductors and vias in stratified (flat) dielectrics.
The product's most unusual capabilities are its Co-Calibrated Ports and Uniplanar Anisotropic Dielectric Modeling. Shawn Carpenter, VP, Sales and Marketing, states, "Co-Calibrated Ports are perfectly de-embedded, global ground-referenced ports that may be placed anywhere within the circuit. All the tiny stray inductances and capacitances associated with (and between) all the internal ports in a group are now exactly removed. Take, for example, a power FET amplifierperhaps in the form of an RF IC. EM analysis cannot analyze the FETOther lumped components may be left out of the EM simulation as well, such as MIM capacitors or spiral inductors, and their models connected later to the ports in a favorite framework. The circuit design could be quickly optimized in the framework tool by adjusting these key component values, taking into account circuit-layout interactions from EM analysis of 90+ percent of the circuit layout. A final layout can be synthesized in the framework, and a new EM analysis conducted on a final layout containing optimized components."
The software also introduces the ability to model uniplanar anisotropy in dielectric layers. As a result, Sonnet can accurately model materials that have different vertical and horizontal dielectric and magnetic properties. According to Carpenter, "Consider a coupled microstrip resonator consisting of two parallelcoupled transmission-line sections. The even-mode coupling between these two transmission lines depends largely upon the capacitance to the ground plane for the two lines; this capacitance is influenced by the vertical dielectric properties. Odd-mode coupling between the pair depends upon the capacitance between the two lines. This is influenced strongly by the horizontal dielectric properties.
If the horizontal and vertical dielectric properties differ significantly (and this will depend on your application and error requirements), modeling your resonator with an isotropic assumption could lead to design failure. Sonnet's uniplanar anisotropy modeling for dielectrics overcomes this limitation in EM modeling for planar circuits, enabling designers to model real materials with higher fidelity than before." Dielectric Laboratories has observed good first-pass success in using this feature for microstrip filter designs (Fig. 3).
Clearly, microwave designs are on a path of evolution as they grow in complexity, rise to higher frequencies, and encounter new design complications. The microwave software industry is responding with teamwork as much as competition to make sure that engineers have design flows that actually do what they need and interface with the right outside tools. Such tools reflect a growing shift from component to system design. As noted by Agilent's Civello, they point toward increasing integration as planar and full 3D EM are integrated into an RF EDA platform that designs, simulates, and can generate manufacturing artwork for MMIC/RF IC, package and module/laminate, PCB, etc. Whether the inspiration is MMIC design, emerging medical applications, or something else, microwave software will be critical to the cutting-edge designs of tomorrow.