- The constant-k filter is so named because the product of its series and parallel impedances remains a constant designated k at all frequencies. These impedances can be inductive or capacitive reactances. A constant-k filter can be configured as any of the basic filter types.
- The m-derived filter is a modified form of a constant-k filter based on a constant called m, the ratio of the cutoff frequency to the infinite attenuation frequency. An m-derived filter exhibits a sharper attenuation or roll-off curve than a constant-k filter because it has more poles. It can also be configured as any of the basic filter types.
- The Butterworth filter exhibits an essentially flat ripple response in the passband and a sharp attenuation or roll-off curve at its cutoff frequency. It has a wide operating frequency range that extends from DC into RF. These filters can be configured as low-pass, high-pass, and bandpass. Their transient responses are much better than those of Chebyshev filters.
Filters can be identified by one or more of the following classifications:
- The Chebyshev filter has characteristics that are similar to those of the Butterworth filter, but it trades off higher amplitude ripple response to obtain an even sharper frequency roll-off curve at its cutoff frequency. Because these are constant-k filters, they can be configured as low-pass, high-pass, and band-reject.
- The Bessel filter is named for the mathematical functions used to design it. Its frequency cutoff characteristics are not as sharp as those of the Butterworth filter.
- The elliptical filter is similar to a Chebyshev filter, but its passband contains even higher amplitude ripple response.
- A filter can be further characterized by its number of poles, as determined by the number of reactive components (inductors or capacitors) within the filter. (Resistors do not count as poles because they are not reactive.) The steepness of the attenuation curve or roll-off is determined by the number of poles. For example, a six-pole filter has a steeper attenuation curve than a two-pole filter.
