Filter

A filter is the field of signal processing removes unwanted components from a signal. Filters can fall into many categories, with some instances overlapping, such as analog/digital, linear/non-linear, passive/active, etc.

Technical Details
By far the most common type of filters are linear continuous-time filters. These filters are meant to remove certain frequencies while allowing others to pass.

Common Terminology

 * Low Pass Filter – frequencies below a threshold are passed while high frequencies are attenuated
 * High Pass filter – frequencies below a threshold are passed while low frequencies are attenuated
 * Band Pass filter – only frequencies withing a certain band are passed
 * Band Stop filter – only frequencies withing a certain band are attenuated.
 * Notch filter – a very small band Band-stop filter that cuts out one specific frequency
 * Comb filter – has multiple, narrow pass bands, regularly spaced
 * All Pass filter – passes all frequencies but modifies the phase
 * Cutoff frequency - frequency beyond while signals will be passed
 * Roll-off - rate of attenuation above the cutoff frequency
 * Transition band - the band between the start- and stop-band
 * Ripple - the variation insertion loss in the passband of the filter