A filter is one or more components placed in a circuit for the purpose of changing or controlling the frequency content of signals passing through the circuit.

Filters have stop bands and pass bands.  Ideally speaking, frequencies that are in the stop band are attenuated, while frequencies in the pass band are not.

Filters constructed using capacitors and/or inductors have resonant frequencies at which signals passing through them will be either significantly attenuated or significantly amplified.  In some cases additional components must be added to a filter design to damp resonant effects to acceptable levels.

Filters may be configured using a variety of topologies.  The basic filter types are as follows:

Low pass -- Allows low frequency signals to pass through without significant attenuation, while higher frequency signals are attenuated.  Most common type of filter for EMI control.

High Pass -- Allows high frequency signals to pass through without significant attenuation, while lower frequency signals are attenuated.

Band Pass -- Allows signal frequencies within the pass band to pass through without significant attenuation, while attenuating signal frequencies below and above the pass band.  Also known as a notch filter.

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