Circuit Theory/Filters

thumb|Low pass filter thumb|High pass filter thumb|Band Pass filter thumb|Band Stop filter thumb|Comb filter 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.