Supports active, passive and digital filter design. Freeware version available.
http://www.nuhertz.com/filter/
with wire parameter, RMS value calculator & SMPS note
http://www.geocities.com/CapeCanaveral/Lab/9643/index.htm
By Tony Fisher. Generates C code for several types of FIR and IIR filters, including low-pass, bandpass, high-pass, band-stop, raised cosine, Hilbert transformer, and more. Includes frequency, phase, impulse, and step response plots.
http://www-users.cs.york.ac.uk/~fisher/mkfilter/
FIR filter design.
http://www.iowegian.com/
Both active and passive filter design.
http://www.numberone.com/products.htm
http://www-smirc.stanford.edu/spiralcalc.html
Provides digital filter design software which can produce specialised filters e.g. Windowed FFT, Optimal FIR filters, and IIR filters.
http://www.tyder.com/
By Tony Fisher. Designs several types of LC filters, including low-pass, bandpass, high-pass, and band-stop, in Butterworth and Chebychev forms. Includes frequency and phase plots.
http://www-users.cs.york.ac.uk/~fisher/lcfilter/