Latest HFSS Release Improves Speed and Accuracy
Hewlett-Packard Co., HP EEsof Division Westlake Village, CA
Three-dimensional (3D) electromagnetic (EM) simulation software has been a valuable tool of high frequency circuit designers for some time. The benefits of simulating a circuit’s high frequency behavior on a computer prior to building costly multiple prototypes have saved much time and money in the product development stage. However, these tools have had their share of drawbacks that frustrated circuit designers and limited their use. As a result of time-consuming simulation preparations and enormous demands on computer memory, many complex circuits have been forced into a cut-and-try build cycle well before any simulation benefits could be achieved.
The HP high frequency structure simulator (HP HFSS) release 5.0 has addressed many of these limitations for arbitrarily shaped, passive, 3D structures. Improvements in speed, memory usage and the drawing environment have addressed many of the recent user concerns and increased the design tool’s overall effectiveness.
Originally, HP HFSS was created to solve for the EM energy distribution and S-parameters in complex, real-world structures such as high frequency circuits, antennas, connectors and adapters, transitions and launches, cavities and packages. The simulation software was the industry’s first high frequency 3D field solver based on EM theory. HP HFSS utilizes the finite-element method of EM simulation, which subdivides 3D space into simple spatial elements. Electric fields are described for each element and the EM interactions between these elements are computed using Maxwell’s equations in differential form. The resulting matrix yields a representation of the EM fields throughout the structure, and ultimately provides the S-parameters for all excitations.
HP HFSS 5.0 offers powerful new two-dimensional (2D) and 3D simulator engines that provide an average of 10-times improvement in speed and an accuracy improvement, eliminating lengthy simulations and reducing the computer memory demands by approximately 50 percent. The key reason for this improvement lies in the simulator’s method of subdividing and refining the structure in question. Improvements to the meshing algorithm have resulted in better speed, accuracy and memory usage, and tighter integration of the various software modules within HP HFSS increases the program’s efficiency significantly.
New Modeling Capability
Rather than recreate a more efficient and universal drawing package, release 5.0 contains a customized version of the industry-standard AutoCAD™ software package from AutoDesk. This popular drawing tool provides a more comprehensive drawing capability that has been accepted universally in industry for years. Drawings that used to take hours to complete in previous HFSS versions now can be completed in seconds. In addition, the new release allows better file compatibility and translation with other 3D drawing tools by incorporating the industry-standard ACIS® kernel, the foundation of many state-of-the-art modeling packages.
To simplify design entry and editing, a new solid model parts library of parameterized 2D and 3D objects including microstrips, spirals, boxes, cylinders and helixes allows complex structures to be drawn quickly. 
Figure 1 shows a typical helix from the parts library. Mistakes may be corrected using an unlimited undo function, which can step back to the start if necessary.
Component Design
Circuit simulators use mathematical models to describe passive components such as spiral inductors, microstrip junctions and coplanar waveguides. In the past, parasitic coupling was ignored, leading to inaccurate results. HP HFSS takes parasitic coupling into account for increased simulation accuracy. In addition, release 5.0 simulates structures for which no analytical equations exist, such as transitions between different types of transmission lines and coax-to-microstrip launchers. HP HFSS models component packaging easily, as well as via holes, connector interfaces, databus terminations and EM interference shields.
Memory Usage
Several improvements were implemented in the adaptive refinement process, resulting in the need for fewer mesh elements to achieve the specified accuracy. Also due to these improvements, less than half the computer memory used by previous releases is required. This reduction in memory usage along with the speed improvement allow more complicated designs to be simulated that would have been impossible to accomplish in the past. Instead of next-day results from an overnight simulation, multiple iterations can be completed in a single workday. In addition, release 5.0 operates on both UNIX® workstations and PCs, making the program available to many more designers. For the first time, HP HFSS is offered for use on Windows® ’95 and NT PC environments.
Powerful New Displays
New display capabilities provide enhanced visualization and animation capabilities to evaluate simulation results. EM field animation and dynamic structure rotation can be performed simultaneously for optimizing viewing angles. EM fields can be displayed as shaded plots, contour lines or vectors. Smith charts, antenna far-field plots, and magnitude and phase plots accept data from multiple simulations for comparison.
For antenna designers, the entire far-field plot can be displayed even though symmetry planes were used in the simulation. 
Symmetry planes greatly reduce the computational time required for antenna design. In addition, solid 3D polar patterns display antenna beam patterns in both azimuth and elevation simultaneously, as shown in Figure 2 . Antenna radiation patterns, gain, beamwidth and sidelobe levels can be determined from a variety of antenna far-field plots based on a user-defined plane.
Conclusion
With HP HFSS 5.0’s increased speed, reduced memory and additional enhancements, 3D EM simulation can now be used for more complex designs. This new simulator release integrates readily into the design process, supporting the industry-standard SAT format for transferring files from other 3D mechanical CAD tools. HP HFSS 5.0 is fully compatible with earlier HFSS 2.06, 3.0 and 4.0 releases. Release 5.0 will be available in the spring. Price: $43,000.
Hewlett-Packard Co., HP EEsof Division, Westlake Village, CA (800) 452-4844, ext. 1301.