Microwave filters are among the most essential of high-frequency components, necessary for keeping signals separate in applications ranging from communications to radar systems. High-frequency filters come in many shapes, sizes, and response types, including bandpass, bandstop, lowpass, and highpass filters. They may be simple in concept, but the performance of a filter design is affected by many parameters, from the widths of transmission lines to the consistency of a printed-circuit-board (PCB) laminate's dielectric constant and dissipation factor.

Fortunately, computer-aided-engineering (CAE) software support may be as plentiful for RF/microwave filters as it is for any high-frequency component area, with diverse CAE tools available, including many free of charge.

All of the major CAE design "suites" of software tools, such as the Advanced Design System (ADS) from Agilent Technologies, Microwave Office from AWR, and the CST Studio Suite 2011 from CST, provide the combination of linear circuit simulation, electromagnetic (EM) simulation, and circuit optimization tools needed to design microwave filters in a variety of transmission-line technologies, including microstrip, stripline, coplanar waveguide, and waveguide.

Many of these suites includes tools dedicated to filter design. for example, ifilter is a filter synthesis module integrated within awr's Microwave office. it features an intuitive user interface and supports a wide range of filter designs, including lumped-element and distributed filter types.

EM simulation software programs are also powerful tools for designing microwave filters, with such programs as the Sonnet Suites from Sonnet Software, Momentum from agilent technologies, and AXIEM from AWR supporting the design of high-frequency filters with a variety of responses. The finite-element EM software from ANYS, now on version 13.0, has been used for everything from simulation to analog, audio, digital, and microwave filters to complete biomedical systems. Cadence Design Systems offers modeling tools not only for filter design and PCB layout, but for analyzing manufacturing yields on each design.

The IE3D EM design and verification program from Mentor Graphics can be applied to filter design as well as higher-level circuits, including monolithic microwave integrated circuits (MMICs). Many of these EM simulation tools, including aXieM and sonnet lite from sonnet software, are available as free or trial versions which can be upgraded to full versions.

In addition to software, many filter CAE suppliers offer guidance on the use of their tools for specific filters. for example, CST provides a free application article with details on the design of a waveguide filter on their website. The filter (see figure) has a passband from 7.9 to 8.4 GHz and is intended for military X-band satellite-communications (satcom).

Filpro is a modular filter design software that handles virtually all types of lumped-element and distributed filters, multiplexers, and coupling structures. it incorporates nearly all of the synthesis and circuit transformation techniques reported in the technical literature as programmable modules. All necessary techniques for a design can be selected from different toolboxes and linked as needed. Owing to this approach, the software can be expanded to cover new problems simply by adding more modules.

The software includes a synthesis menu for starting a design, but also allows a user to design a filter by modifying a built-in LC prototype, based on various lowpass, highpass, bandpass, and bandstop Chebyshev and Butterworth responses. Equivalent filters can be produced by replacing ideal inverters with practical inverters and replacing resonators by desired lumped-distributed practical equivalents. Filpro performs circuit transformations based on synthesis toolboxes (menus). The software also includes a large number of programmed, stored transformations.

ALK Engineering offers a pair of powerful filter design programs in PCFILT and S/FILSYN. In addition, LCLAYOUT is a low-cost drawing program for LC filters designed by the PCFILT or S/FILSYN design programs. It provides drawings for circuits built using air wound inductors and single-layer capacitors formed on alumina or PTFE.

Along with the advanced software suites and powerful EM simulators, this industry features a wide range of low-cost and free software tools for designing filters, many with example designs. Circuit Sage provides extensive listings of online design tools for analog and digital filters as well as online application notes and technical articles on designing analog and digital filters.

FilterCAD is a free design program from Linear Technology that helps create filters based on the firm's integrated circuits (ICs). It can be used to design lowpass, highpass, bandpass and notch filters with a variety of responses, including Butterworth, Bessel, Chebychev, and custom responses.

Raltron offers a simple, online bandpass filter design tool at http://www.raltron.com/cust/tools/band_pass_filters.asp that computes the ideal component values (C1, C2, C3, L1, L2, and L3) for a bandpass filter when given the image impedance level, frequency, and desired loaded quality factor (Q).

The Student Edition of the Elsie filter design software from Tonne Software is free and included in the ARRL Handbook. It can be upgraded to the Professional Edition with the purchase of a key for $195 USD. Elsie handles the design of lumped-element filters and networks, at frequencies from audio through the microwave range.

One of the more popular free tools is AppCAD, originally developed by Hewlett- Packard Co. (now Agilent Technologies). Version 3.0.2 of RF and Microwave AppCad can be downloaded from www.hp.woodshot.com. It provides useful Sparameter analysis routines, including functions for analyzing filter responses.

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