The Mother of All Free Filter Software Websites: Iowa Hills Software

Update: Unfortunately, this website is no longer being published.

Über RF/microwave engineer and Ham radio aficionado Cornell D. sent me an e-mail about a website called Iowa Hills Software. Per the site's tag line, "The analog and digital filter design programs on this site are FREE. This web site, and the programs on it, are a retired RF engineer's winter time hobby." If your primary interest is in calculating theoretical values for filters and not being able to simulate "real world" effects of component geometry and materials, packaging, and circuit boards, then it would be hard to find a more comprehensive program for investigating the potential for filter designs.

However, the software goes beyond presenting only the canonical form (basic) of the circuits because, particularly for bandpass and bandstop filters, it is often difficult or impossible to realize with actual components. To wit: "The canonical form, or simplest form, is described in most textbooks, but is almost useless in practice. These filters cannot be built unless the bandwidth is greater than about 50 percent. Even then, the component values required to build the filter become impractical. The series inductors are prohibitively large in the sense that their self resonant frequency becomes a major problem. It is also impractical, and usually impossible, to obtain the nominal values needed, not to mention the problem with tolerances. In general, the only place for a canonical band pass filter is in applications requiring very wide filters. If you are new to RF filter design, take some free advice and don't waste any time with these, except for the time required to understand their limitations. Engineers understood this many years ago, and devised methods that add parts to the design which in turn gives the engineer some latitude in the selection of component values." Nearly 70 topologies are available to choose from based on type of filter being designed.

Both FIR (finite impulse response) and IIR (infinite impulse response) are handled for digital filters and opamp and RF are handled for analog filters. Lowpass, highpass, bandpass, and bandstop (notch) responses, including all common transfer functions like Butterworth, Chebyshev, Gaussian, and Papoulis (aka Optimum "L" filter) are included. A choice of ideal components or standard component values is available, as well as being able to perform a Monte Carlo analysis based on user-defined tolerances. Gain (attenuation), group delay, phase, and return loss plots are available.

A Smith Chart program is also included for designing impedance matching circuits. Buried within the Smith chart designer is a handy resistive attenuator designer.

In addition to the free software, there is also a huge amount of tutorial information available from an obviously very experienced filter designer. An explanation of the differences between IIR and FIR filters, a discussion group delay "without equations," a page on discrete inverse Laplace transforms, and much more is presented. Even if you are familiar with filter design, you can probably find something new and useful to you. There is a lot to explore.

Screen shots below show what I got with a few minutes of playing around with the various features. The circuits shown are not meant to represent any particular kind of filter, Smith Chart, or attenuator.

There is no contact information on the website except for a submission form and an e-mail in the Help screen, so it seems the author prefers anonymity. I, however, as the result of some Internet sleuthing, was able to locate a fairly certain identity, but I will keep it confidential.