The Passive Filter

The original passive filter, still much used today. All the elements are passive LCR, as lumped or transmission line when distributed, thus in contrast to an active filter there will always be a loss factor. These filters come in many formats, response, in many cases used as prototypes for active (digital) filters. Here are a number of links to different filter designers

Butterworth Filters

This is a class of maximally flat filters. A compromise between flat group delay, rolloff and flat magnitude.

Butterworth Filters

Bessel Filters

This is a class of maximally flat group delay filters. Poor roll-off, hence a continuous magnitude droop. Emphasis is a flat group delay

Bessel Filters

LinearPhase Filters

This is a class of equi-ripple linear phase filters. Phase Ripple <= 1 degree.

LinearPhase Filters

Guassian Filters

This is a class of pure state Guassian filters.

Guassian Filter

Legendre Filters

This is a class of Legendre or Papoulis Filters.

Legendre Filter

Chebychev Filters

This is a class of Chebychev Type 1 Filters.

Chebychev 1 Filter

Chebychev Filters

This is a class of Chebychev Type 2 Filters.

Chebychev 11 Filter

Elliptic Filters

A class of equi-ripple in-band and equiripple out-band filters. ie: a superposition of Chebychev type 1 and Chebychev type 11.

Elliptic Filters