This innovative simulator speeds and simplifies EM analysis to the extent that it can now be used as an integral early part of the RF/microwave design process.
Jack Browne
Technical Director
Electromagnetic (EM) simulation software has evolved from a laboratory curiosity to a valuable modeling tool during the last decade! But many users still consider the technology as a "back-end" design tool, often applied as a way to check the results on "more trusted" S-parameter-based circuit simulators. To change that way of thinking, Applied Wave Research (AWR, www.appwave.com) has developed the AXIEMTM EM design software, which provides EM analysis as a true front-end design tool. The software enables high-frequency designers to diagnose circuits and structures early in the design cycle in order to avoid flaws and problems, leading to significantly shorter design cycles.
EM simulators come in many shapes and sizes. For studying the electrical properties of a unit length of conductor, for example, a two-dimensional (2D) cross-sectional EM simulation tool based on the method of moments (MoM) or finite-element method (FEM) analysis can provide useful predictions on loss and impedance with fairly quick computational time. But such a simulator only shows current flow in two dimensions. For greater detail, a planar 3D EM simulator can show current flow in three dimensions and can be applied to many planar circuits. For analysis of more complex, 3D structures, such as antennas and waveguide, a full 3D EM simulator based on finite-difference time-domain (FDTD) technology can provide the greatest amount of detail on current flow, albeit with the greatest demands for computer memory and processing time.
In development for more than five years, the new AXIEM (www.axiem3d.com) EM simulator was created to not only provide the best combination of performance per unit of computer processing time and memory, but also to work seamlessly with other computer-aidedengineering (CAE) tools, such as Sparameter- based circuit simulators in analyzing planar circuits. It is well suited for predicting the broadband or narrowband performance of a wide range of high-frequency circuits, such as RF printed- circuit boards (PCBs) and modules, low-temperature-cofired-ceramic (LTCC) circuits, monolithic-microwave integrated circuits (MMICs), and RF integrated circuits (RF ICs).
AXIEM software actually becomes the second EM simulator in AWR's product lineup, joining the EMSightTM software, a design component within the firm's Microwave Office suite of design tools. Where EMSight is designed for closed-boundary problems, assuming that the circuit for analysis is contained within a rectangular, conducting box and fitted to a predetermined dimensional grid, the AXIEM tool is an open-boundary solver that assumes a circuit or structure for analysis is in free space. Furthermore, given AXIEM's ability to succinctly handle arbitrary shapes, it can readily solve problems that are an order (or two) of magnitude over EMSight.
In general, EM simulators are becoming an increasingly important part of the high-frequency circuit design process for their capabilities in predicting current flow and field behavior. The trend mirrors the evolving nature of RF/microwave circuits, growing in density and complexity as designers attempt to fit more functionality into smaller circuits. As circuits are more tightly spaced, an EM simulator can provide insights into unexpected field interactions and coupling effects. By spotting design flaws or problems early through simulation, delays in a design process can be avoided. Traditionally, circuit simulators have provided this earlystage design analysis while EM simulators have been applied at the final stages of the design process as a verification tool. With the speed and versatility of AXIEM software, however, it can work as a stand-alone early-stage simulator or as part of a suite of programs, such as layout and optimization tools, to achieve shorter design cycles with greater accuracy.
The software incorporates a methodof- moments (MoM) solver with solution methodology similar to a fast multipole method, but adapted for full-wave analysis. It has the capabilities of broadband analysis at frequencies from DC through millimeter-wave frequencies. The capability to perform at low frequencies is essential for obtaining accurate bias conditions and setting reliable DC operating points for active devices. It also provides processing speed advantages due to its advanced solver algorithm, scaling much better than existing MoM solvers in both memory usage and solution times, with processing time more closely approximated by Nlog(N) than the exponential N3 function for the number of unknowns, N (Fig. 1).
The solver has been developed to provide accuracy that exceeds traditional 3D planar MoM solutions as well as 2D cross-sectional analysis tools for the most dense and complex circuit structures and matches or exceeds the accuracy of 3D FEM and 3D FDTD solvers for less dense circuits and structures. The solver was developed to overcome the limitations of existing 3D planar EM simulators that rely on the Sommerfeld integral (or similar algorithms), which provides good simulation speed but sacrifices accuracy and dynamic range for speed. The AXIEM product is extremely accurate for broadband simulations, such as an example rectangular spiral inductor (Fig. 2).
AXIEM software incorporates automatically calibrated internal ports that help eliminate some of the complexities in achieving accurate results with an EM simulator. These automatically calibrated ports maintain their own current return paths with relatively low error. They eliminate the need for manual de-embedding or for creating an explicit current path to an edge port in order to provide the necessary conditions for an EM simulation. This capability simplifies the simulation of complex subjects, such as PCBs with a large number of surface-mount pads.
As an example of how the internal ports can be applied, the AXIEM tool was used to analyze a rat-race millimeterwave mixer designed for use at 37 GHz supplied by Farran Technology (www.farran.com). The mixer features internal ports on non-orthogonal edges, normally a structure that would require the use of edge ports to create a current path. The software was also used to simulate the current flow through a spiral inductor at 20 GHz, predicting eddy currents through the spiral inductor.
AXIEM software gains speed through its use of a hybrid meshing technology that uses both triangular and rectangular elements to create a mesh around a circuit or structure. The fully automated process is designed to apply the mixture of meshing elements that provides the highest accuracy with the minimal number of unknowns, thus keeping the computer memory and processing requirements to a minimum for a given simulation. Even for structures with curved forms or extruded planar geometries, AXIEM's hybrid meshing technology creates a true 3D mesh and provides a final solution that includes all X, Y, and Z currents on all surfaces.
The adaptive hybrid meshing technology will mix triangular and rectangular elements if the combination results in a more efficient and accurate mesh and simulation. It seeks the best combination of elements for the least amount of processing time needed to reach the highestaccuracy results.
As an example of the hybrid meshing technology at work, a 1000-mil-diameter microstrip structure fabricated on 32- mil Duroid 5870 substrate material was meshed and simulated within EMSight and AXIEM, respectively. When this structure was meshed with rectangular elements alone, it required a total of 3299 unknowns (rectangular elements) for full meshing and more than 40 s per frequency to simulate within EMSight. Employing AXIEM's hybrid meshing approach on this same structure required a combination of only 630 rectangular and triangular elements to fully mesh it and consequently resulted in considerably faster simulation time of only a few seconds per frequency.
This efficient meshing approach supports the analysis of circuits and structures with relatively thick metal conductors, providing accurate current flow simulations and solutions for thick conductors, even when the thickness of the conductor is greater than the width of the line. This thick-metal capability is essential for analyzing designs fabricated with 90- and 65-nm RF complementary metal oxide semiconductor (CMOS) processes. In these processes, the line thickness is often equal or even greater than the width of the line. The thick metal support is also useful when characterizing such structures as silicon spiral conductors or complete GaAs MMIC devices.
As a performance comparison, AXIEM and the company's closed-boundary EM simulation tool, EMSight, were both used to simulate the performance of a quarter- wavelength, 50O stripline conductor. In reviewing the results of the percent error as a function of analysis time, the new AXIEM simulator offers accuracy that is remarkably close to EMSight (Fig. 3).
AXIEM software was created for tight integration in the AWR design environment consisting of the Microwave Office, Analog Office, Visual System Simulator (VSSTM), and Signal IntegrityTM software tools. It is seamlessly tied to the circuit and system simulation, layout, and verification functions of these various programs through a proprietary AWR unified data model (UDM). The UDM provides features such as extraction directly from circuit simulation without the need to perform explicit layout and EM setup steps. In addition, when used in conjunction with AWR's Automated Circuit Extraction (ACETM) technology, AXIEM provides fast, automatic interconnect model extraction, allowing EM structures to be modified and redesigned quickly.
Automated extraction flow allows seamless inclusion of EM results in any of the circuit simulations that are supported by the AWR design environment. It enables an operator to select the interconnects that are to be EM simulated and associate them with an extraction block. Any selected interconnections are automatically sent to the AXIEM simulator. AXIEM results are automatically merged back into the circuit description for simulation with any supported circuit simulator.
The AXIEM product is presently undergoing beta testing with select existing customers. In addition, AWR will send a free copy of AXIEM to all customers for an extended evaluation period (about 90 days). The software is geared for computers based on dual-core or quadcore microprocessors with at least 4 GB of random-access memory (RAM). It supports the latest MicrosoftTM operating systems, including Windows 2000, Windows XP, and Vista.
AWR
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Suite 430, El Segundo, CA
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e-mail: info@appwave.com
www.appwave.com