Microwave Journal

Two Synthesis Tools Speed RF/Microwave Filter Design

February 9, 2022

Engineers with filter design skills are in demand. 5G development is driving the need for filters covering new frequency bands, using new topologies for an ever-increasing number of products and systems. High performance microwave and mmWave filter design has traditionally required highly experienced and specialized engineers. Yet the number of designers with filter design experience can’t meet the demand. Due to the complexity, high Q of resonators and sensitivity of the filter elements, the design and tuning process is counterintuitive and often tedious. Taking shortcuts that compromise the design process to meet schedule demands can lead to performance shortfalls in the prototypes, requiring iterative design and testing cycles.

Figure 1 The quick synthesis screen of Nuhertz FilterSolutions.

Figure 2 Lowpass filter generated by Nuhertz FilterSolutions in Ansys HFSS.

Figure 3 SynMatrix filter synthesis showing the response and coupling matrix.

Figure 4 SynMatrix automated tuning shows optimization progress with error levels.

To simplify the complexity of filter design, Ansys is introducing two tools for RF/microwave filter design and analysis. Both can save weeks of design and test time for filter development, enabling engineers lacking deep filter design experience to successfully design complex filters. Used with Ansys multiphysics analysis, first-pass success is virtually assured.

Nuhertz FilterSolutions

The first new tool is Ansys Nuhertz FilterSolutions. It dramatically speeds the design of lumped element surface-mount and planar filters (see Figure 1) by providing a catalog of standard designs with a library of supplier components for lumped element designs, which enable instant and accurate prediction of circuit performance. For distributed planar filters, Nuhertz FilterSolutions has a list of geometric layout topologies relevant to the filter type and can instantly predict the filter response of each. In addition to simulating the filter response, the tool automatically generates the planar geometry and predicts the size of the filter.

The frequency response predictions are built on models derived from finite element analysis of the filter components. With a single click, the filter geometry can be exported to Ansys HFSS for high accuracy electromagnetic (EM) analysis and optimization of the filter’s performance (see Figure 2). Nuhertz FilterSolutions includes synthesis tools for active, switched-capacitor and digital filters. The netlist of active filters can be exported in SPICE format, and the digital filter synthesis module exports C code for the generated filter.

SynMatrix Optimization

The second filter design tool is from Ansys’ partner company SynMatrix. The SynMatrix solution provides an arbitrary topology filter synthesis capability, with computer-aided tuning and optimization (see Figure 3). It is well-suited for cavity, waveguide and ceramic filters and supports the synthesis of diplexers and triplexers, as well as multiband filters. The SynMatrix tool performs sensitivity, power handling, dispersion and coupling matrix calculations from a Touchstone file (.s2p) generated by a network analyzer or by Ansys HFSS. SynMatrix also features a function that can generate 3D models automatically in Ansys HFSS.

Optimization really sets SynMatrix apart from other design tools. It interfaces with HFSS to take a roughly tuned filter model and adjusts the physical parameters until the filter is properly tuned. With a physical filter, it interfaces with a network analyzer to guide tuning on the bench. The utility of a completely tuned filter model, one that accurately predicts tuning range, power handling and sensitivity cannot be over-emphasized. Having an accurate and complete 3D model saves the time and cost of prototyping and redesigning—perhaps several times—during development.

Complementary Tools

A common question is which solution is better? This is the wrong question because the Nuhertz and SynMatrix tools are complementary. Nuhertz FilterSolutions is recommended for planar and lumped element surface-mount filter designs. The SynMatrix tool is better suited for cavity filter designs and where flexible filter topologies are required. The two will work together, as the SynMatrix tool can use AI to optimize planar distributed filters designed using Nuhertz.

Together with the other unique 3D EM simulation capabilities of HFSS, such as analytical derivatives and automatic adaptive meshing, these two filter design tools make RF and microwave filter design a more predictable process that is accessible to more engineers.

Ansys, Inc.
Canonsburg, Pa.