Circuit Theory/Filters

Some terms used to describe and classify linear filters:

Frequency Response

 * Low-pass filter – low frequencies are passed, high frequencies are attenuated.
 * High-pass filter – high frequencies are passed, low frequencies are attenuated.
 * Band-pass filter – only frequencies in a frequency band are passed.
 * Band-stop filter or band-reject filter – only frequencies in a frequency band are attenuated.
 * Notch filter – rejects just one specific frequency - an extreme band-stop filter.
 * Comb filter – has multiple regularly spaced narrow passbands giving the bandform the appearance of a comb.
 * All-pass filter – all frequencies are passed, but the phase of the output is modified.


 * Cutoff frequency is the frequency beyond which the filter will not pass signals. It is usually measured at a specific attenuation such as 3dB.
 * Roll-off is the rate at which attenuation increases beyond the cut-off frequency.
 * Transition band, the (usually narrow) band of frequencies between a passband and stopband.
 * Ripple is the variation of the filter's insertion loss in the passband.
 * The order of a filter is the degree of the approximating polynomial and in passive filters corresponds to the number of elements required to build it. Increasing order increases roll-off and brings the filter closer to the ideal response.