Über
RF/microwave engineer and Ham radio aficionado Cornell D. sent
me an email 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 userdefined
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.
FIR Filter Designer
Opamp Filter Designer showing Monte Carlo Analysis
Resistive Attenuator Designer

RF analog Filter Designer showing Monte Carlo Analysis
Smith Chart 
There is no contact information on the website except for a
submission form and an email 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.
Posted January 7, 2014