Filters are indispensible in RF/microwave systems, whether for stopping some signals or separating others. High-frequency filters come in many shapes and sizes, with a variety of different responses, and designing such a filter was once best left to an engineer with not only the expertise, but the experience to know when a given filter construction or technology might be appropriate. In the age of the computer, however, some of that expertise and experience has been captured in the form of program code, and design engineers in the RF/microwave industry have a wide range of software design tools for filtersboth for commercial sale and as free-of-charge "freeware." Although the old adage "you get what you pay for" is usually true, some of the free software tools can be quite useful for different applications.

Commercial RF/microwave design suites, such as the Advanced Design System (ADS) tools from Agilent Technologies, the CST Design Studio from CST, and Microwave Office from AWR Corp. include the circuit design tools needed to create RF and microwave filters in a variety of transmission-line technologies. These include microstrip, stripline, coplanar waveguide, and waveguide. In addition, the Genesys RF and microwave design software from Agilent is a low-cost integrated suite of programs that includes filter design capabilities.

Many of these full-featured computer-aided-engineering (CAE) design suites include dedicated software design tools. For example, the iFilter filter design software module is designed to plug into the Microwave Office design environment. This module was developed specifically for the synthesis of lumped-element and distributed filters, and was written to operate seamlessly within the AWR Design Environment. The iFilter module provides an intuitive interface to simplify filter design, and all tradeoffs in performance to be made as they are connected to other circuits in a design. The software automatically generates layouts for distributed filters and can realize lumped-element filters using a library of commercial components, such as inductors and capacitors.

The iFilter software also provides feedback on higher-order modes and line widths for common characteristic impedances, and warns the user if the desired characteristics of the filter are beyond the capabilities of a specific topology. In addition, it can perform accurate synthesis through electromagnetic (EM) verification with AWR's AXIEM three-dimensional (3D) planar EM simulator.

WaveCon has been providing filter design software for over 20 yearsmost notably, in the form of its Parfil design tool. The company's software targets parallel-coupled interdigital and combline filters, in particular for RF/microwave microstrip and stripline applications. The firm has consistently upgraded Parfil over the years, adding the capability to design flat-group-delay filters. In addition to Parfil, the firm also supplies the Elliptic, LineComp, and ProCAP software tools. Elliptic, as the name suggests, is designed for creating elliptic filters. The software includes design information for both cross-coupled and elliptic filters. Adding rods or lines which couple to the first and last filter rods provides two additional zeros to achieve the desired elliptic filter response.Folding a filter so that rods or lines couple between internal rods achieves cross coupling, which is also possible at lower frequencies through the use of capacitors and inductors.

LineComp handles transmission line and coupled transmission line analysis and synthesis, while ProCAP is a nodal analysis program developed for use with the Parfil and Elliptic programs. It has over 100 circuit elements which may be used in a circuit design.

The Filpro filter design software was developed by a number of engineers and educators, and is available at www.metu.edu.tr. Designed as a teaching tool, it supports fast filter synthesis, circuit transformations, and analysis of lumped-element and distributed filters. The software contains filter synthesis and circuit transformation techniques from technical literature sources as independent modules which can be accessed by means of its user menus. By linking different choices from different menus or toolboxes, a filter design can be created.

Filpro offers a variety of operating modes. Using its "Design by Inserting Elements" option, for example, allows a user to configure a cascaded circuit by inserting elements into a design. The software functions as a sort of virtual network analyzer supported by a large number of library circuit transformations. The software allows quick insertion of elements into any position in a circuit, simply by moving a cursor and clicking on the location of the circuit shown in the screen. In addition to designing lumped-element and distributed filters, the software can also be used to synthesize direct-coupled or evanescent-mode or propagating mode rectangular waveguide filters.

For those starting in filter design, the RF/microwave industry provides many low-cost and free design tools that can not only assist a design project, but serve as excellent educational tools. For example, in addition to the firm's renown for its upgrade work on Klipsch loudspeakers (from famed Arkansas speaker designer Paul Klipsch). ALK Engineering offers a pair of powerful filter design programs in PCFILT (see figure) 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 polytetrafluoroethylene (PTFE) circuit-board substrates. The firm also offers free DOS and Windows-based demonstration versions of its filter design software, along with free copies of the user's guides in PDF form for those interested in reviewing the software's capabilities before buying it.

Elsie from Tonne Software is a 32-b filter design program for a variety of filters from audio frequencies through the microwave range, including Butterworth, Chebyshev, Cauer, Bessel, Gaussian, Constant-K, and M-derived types. It is written for handling lumped-element filters and provides high-quality screen graphics to make the task of designing a filter more enjoyable, along with Monte Carlo analysis functionality to make the final filter more accurate and realistic. The software allows data to be saved in a file format for analysis by another program. A student version of Elsie is included in the latest version of the ARRL Handbook from the American Radio Relay League.

Some of the integrated-circuit (IC) manufacturers offer useful tools for active filter design, including the excellent nine-page application note (No. 1795) from Maxim Integrated Products, "Analog Filter Design Demystified." It shows how to use the company's ICs to assembly some basic filter architectures, including lowpass and highpass filters. Linear Technology provides a free filter design program called FilterCAD (now version 3.0) that helps create active filters based on Linear's IC products. It can be used to design lowpass, highpass, bandpass, and notch filters with a variety of responses, including Butterworth, Bessel, Chebychev, and custom responses. Another free filter design program is from Raltron which can be used to compute the component values for an RF/microwave bandpass filter when given the image impedance level, frequency, and desired loaded quality factor (Q).

In addition to its QuickFilter and FilterSolutions filter design software tools, Nuhertz Technologies 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. QuickFilter, a low-cost (only $2295 USD) filter synthesis program, can be used to create lowpass, highpass, bandpass, and band-reject filters based on lumped-element, distributed-element, active element, switched-capacitor, and digital technologies. QuickFilter provides graphic and text interfaces to guide operators in selecting from among various classic filter response curves, including Bessel, Butterworth, Chebyshev, and Elliptic filter responses. The filter synthesis software is designed for use with the firm's FilterSolutions design software for added flexibility, along with stopbands and adjustments to optimize phase linearity.

Another free design tool is the popular AppCAD set of software tools, originally from Hewlett-Packard Co. (now Agilent Technologies) and now available as Version 3.0.2 from www.hp.woodshot.com. It provides useful S-parameter analysis routines, including functions for analyzing filter responses. The tool can analyze the S-parameters of an active or passive filter design for gain, stability, noise figure, impedance matching, phase linearity, group delay, and isolation, and can deliver calculated results on polar plots, Smith charts, rectangular plots versus frequency, and in tabular form.

In addition to these programs, EM simulators are also useful for designing high-frequency filters, and most of the larger software suites include EM simulation tools that can be used with the collection's circuit simulators. As well, suppliers of EM simulators, like Sonnet Software, offer a powerful tool in the form of their free EM simulator, Sonnet Lite. Although this is a feature-limited version of the firm's popular and powerful EM simulators, it is capable of performing analysis on 3D planar circuits and can be a useful assistant in the design and analysis of RF/microwave filters. In addition to studying stripline and microstrip filters, the software is quite useful for analyzing package resonances, transmission lines, and high-frequency interconnections.