How to choose filter design software

Microwave filters are one of the most important high-frequency components used to isolate signals in a wide range of applications, from communications to radar systems. High-frequency filters come in many shapes, sizes, and response types, including bandpass, bandstop, lowpass, and highpass filters. These filters may be conceptually very simple, but the performance of a filter design is affected by many parameters, ranging from the width of the transmission line to the consistency of the dielectric constant and dissipation factor of the printed circuit board (PCB) laminate.

Fortunately, in the field of high-frequency components, computer-aided engineering (CAE) software support is abundant, just like RF/microwave filters. There are a variety of CAE tools on the market, many of which are free.

All of the major CAE software design tool “suites,” such as Agilent Technologies’ Advanced Design System (ADS), AWR’s Microwave Office, and CST’s CST Studio Suite 2011, offer a combination of linear circuit simulation, electromagnetic (EM) simulation, and circuit optimization tools to meet the needs of designing microwave filters using a variety of transmission-line technologies, including microstrip, stripline, coplanar waveguide, and waveguide.

Many of the above suites include tools dedicated to filter design. For example, ifilter is a filter synthesis module integrated into AWR's Microwave Office suite. ifilter has an intuitive user interface and supports a wide range of filter designs, including lumped element and distributed filter type designs.

Electromagnetic simulation software is also a powerful tool for designing microwave filters. Such software programs include Sonnet Suites from Sonnet Software, Momentum from Agilent Technologies, and AXIEM from AWR, all of which support the design of high-frequency filters with various responses. ANYS's finite element electromagnetic software (current version 13.0) has been widely used in everything from simulating analog, audio, digital and microwave filters to entire biomedical systems. The modeling tools provided by Cadence Design Systems can be used not only for filter design and PCB layout design, but also for analyzing the manufacturing yield of each design.

Mentor Graphics' IE3D electromagnetic design and verification program can be used for filter design and higher-level circuit design, including monolithic microwave integrated circuits (MMICs). Many electromagnetic simulation tools, including aXieM and Sonnet Lite from Sonnet Software, are available free of charge or in trial versions that can be upgraded to full-featured versions in the future.

In addition to software, many filter CAE suppliers also provide design guidelines for using their tools for specific filters. For example, CST's website has a free application article that details the design of a waveguide filter. This filter (see figure) has a passband from 7.9 GHz to 8.4 GHz and is mainly used for military X-band satellite communications (satcom).

Figure: This waveguide filter design is suitable for X-band military satellite communication systems.

Filpro is a modular filter design software that can handle virtually all types of lumped element and distributed filters, multiplexers, and coupled structure devices. Filpro integrates almost all synthesis and circuit conversion technologies reported in technical literature and uses them as programmable modules. All the technologies required for a design can be selected from different tool boxes and linked as needed. Thanks to this new technology, Filpro software can solve new problems simply by adding more modules.

Filpro software includes a comprehensive menu for starting the design, and also allows users to design filters by modifying built-in LC prototypes based on various low-pass, high-pass, bandpass and band-stop Chebyshev and Butterworth responses. Equivalent filters can be generated by replacing ideal inverters with actual inverters and resonators with required lumped-distributed actual equivalent circuits. Filpro can complete circuit conversion based on the comprehensive tool box (menu). The software also includes a large number of programmed and stored conversion designs.

PCFILT and S/FILSYN from ALK Engineering are a pair of powerful filter design programs. LCLAYOUT is a low-cost drawing program for LC filters designed by PCFILT or S/FILSYN design programs. LCLAYOUT can provide schematics for circuits built using air-core wirewound inductors and single-layer capacitors grown on alumina or PTFE.

In addition to advanced software suites and powerful electromagnetic simulation tools, the industry has also introduced a large number of low-cost and free software tools for filter design, and many tools provide design examples. Circuit Sage not only provides an extensive list of online design tools for analog and digital filters, but also provides many online application notes and technical papers for analog and digital filter design.

FilterCAD is a free design program from Linear Technology that helps create filters based on the company's integrated circuits (ICs). FilterCAD can be used to design low-pass, high-pass, band-pass, and notch filters with a variety of responses, including Butterworth, Bessel, Chebyshev, and custom responses.

Raltron provides a simple online bandpass filter design tool on its website that calculates the ideal component values ​​(C1, C2, C3, L1, L2, and L3) for a bandpass filter given the target impedance level, frequency, and required loading quality factor (Q).

Tonne Software's student version of Elsie filter design software is free and included in the ARRL Handbook. An upgrade to the professional version is available by purchasing a $195 key. Elsie can be used to design lumped-element filters and networks and supports frequencies from audio to microwave.

One of the more popular free tools is AppCAD, originally developed by Hewlett-Packard, now Agilent Technologies. Version 3.0.2 of RF & Microwave AppCAD can be downloaded free of charge from the website. This tool provides useful S-parameter analysis routines, including functions for analyzing filter response.

Finally, Nuhertz Technologies also offers several design tools for filters, including QuickFilter and Filter Solutions. The company also offers a free version of Filter Solutions, called Filter Free. These tools can be used to design active and passive filters, lumped element RF filters, and digital filters.