Circuit Theory/Filters

Low pass filter

High pass filter

Band Pass filter

File:Example14Bode.png

Band Stop filter

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.