Free version of program designs and analyzes passive and active filters

Filter design simplified with Matlab-focused program.

James Squire, Virginia Military Institute, Lexington, VA; Edited by Brad Thompson and Fran Granville -- EDN, July 7, 2005

At one time or another, most electrical engineers encounter a requirement to design or analyze an analog filter. Despite an abundance of graphical-user-interface-based digital-filter-design tools, such as The MathWorks ( Matlab Signals toolbox, which includes the FDATool filter-analysis package, few general-purpose, intuitive, and free GUI tools exist for synthesis of arbitrary active analog filters. To fill the need for a powerful and intuitive filter-design tool, this Design Idea describes an active-filter-design tool that bioengineering students at the Massachusetts Institute of Technology and at least four other universities use. Although originally implemented to run under Matlab, you can download a free copy of the program's stand-alone version at Select the "Research" menu and scroll to the software section at the bottom of the page. From the program list, select "Active Filter Design for Matlab" to download a copy of Filter Free 4.0.

Filter Free's functions include third-order analog and IIR (infinite-impulse-response) filters and 10-tap FIR (finite-impulse-response) filters. The program synthesizes filter designs and analyzes the frequency, time, and reflection responses of the ideal, unmodified filters. You can also view transfer functions in standard formats and pole-zero patterns. Using Filter Free, you can select any of 11 filter topologies ranging from gaussian to delay in bandstop, bandpass, highpass, and lowpass responses in five passive, transmission-line, active, switched-capacitor, and digital implementations.

As a design tool, Filter Free simulates a filter's frequency and time-domain responses as assembled using idealized component values. For component-approximation purposes, a round-off option reduces the number of significant figures in components' values. Data-display options include time or frequency response, pole-zero plots, transfer function, and reflection coefficient. You can select graphical plots' axis format, scale factors, and units of measurement.

As a teaching tool, Filter Free can load a user-supplied data file containing a stimulus waveform and simulate a filter's time- and frequency-domain responses. You can download 2000-point data files containing sample waveforms from Although the program's user interface is self-explanatory and includes built-in help menus, you can obtain a copy of the program's user's manual in Adobe's pdf format from the download site.