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 AXIEM^TM 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 EMSight^TM 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.