Bandsperrfilter

Bandsperrfilter, in English commonly called a band-stop filter or notch filter, is a filter that attenuates signals within a specified stopband while allowing frequencies outside the stopband to pass with minimal attenuation. It is characterized by a center frequency f0 around which the attenuation is strongest and a bandwidth Δf that defines the width of the suppressed region. The quality factor Q = f0/Δf describes how sharp the notch is; higher Q means a narrower stopband.

Typical implementations include both passive and active designs. Passive band-stop filters often use LC networks or a twin-T arrangement to create a notch at a chosen frequency. Active versions combine amplifying stages with feedback to achieve deeper attenuation and greater flexibility without relying solely on reactive components. Digital notch filters implement the same principle in software or firmware, using finite-impulse-response (FIR) or infinite-impulse-response (IIR) structures to remove a narrow frequency component from sampled data.

Key considerations in design and use include the level and width of attenuation, insertion loss in the passbands, phase response, impedance matching, and temperature stability of the components for passive designs. Stability and component tolerances affect the exact notch frequency and depth. Digital implementations offer tunability and easy adjustment but require sufficient sampling rate and processing power.

Applications span audio engineering (hum and interference rejection), instrumentation and measurement (isolation of a target signal from a known interference), communications (suppression of adjacent-channel signals), and electromagnetic compatibility (EMI filtering. In German texts, the term bandsperrfilter is used for this concept, while English-language literature often uses band-stop or notch filter.