Notch filter

A notch filter ( English notch filter ) is an electronic filter , with the frequencies within a narrow frequency range may be filtered out. A notch is clearly inserted into the frequency diagram.

Notch filters represent a particularly narrow-band type of band-stop filters which have only one zero in the transfer function and thus not a wide frequency band , but ideally precisely one frequency attenuated as strongly as possible.

application

Magnitude frequency response of a notch filter for 50 Hz in order to suppress disruptive components of the mains frequency

Notch filters are used as “cleaning filters” in high and intermediate frequency technology , for example in radio receivers. In low-frequency technology they are also used when a particularly narrow-band effect is desired, e.g. B. in audio technology . These notch filters have a large quality factor (Q factor), corresponding to a small bandwidth.

In music electronics there are also notch filters as tunable bandstop filters with adjustable frequency. In this way, unwanted feedback and interference frequencies can be weakened by lowering the signal level of the corresponding frequency. In order to be able to find a disturbing point in a piece of music that needs to be removed, it is advisable to set the filter to a narrow presence increase and then to hear at which point the frequency is particularly intrusive. Then you switch to “Notch” and remove this narrow error area.

Special uses

in modern powerline networks, in order to keep sensitive shortwave applications free from the powerline signal
when controlling piezo actuators , in order to suppress their resonance frequency and to increase the bandwidth of the control loop .
in the measurement technique , in order for laser-induced Raman scattering (LRS) the interfering Rayleigh signal hide
in high-frequency reception stages (radio radio receivers) as a blocking filter for unwanted frequencies (such as interference )
in VHF receivers to filter the pilot tone ; also in cassette recorders to filter this pilot tone ( MPX filter )
in medical technology , for example in EEG recording
in older hearing aid models to reduce the tendency to feedback
in measuring devices in order to suppress disruptive components of the mains frequency
in GNSS receivers to suppress interference signals

literature

Thomas Görne: Sound engineering . 1st edition. Carl Hanser Verlag, Leipzig 2006, ISBN 3-446-40198-9 .
Roland Enders: The home recording manual . 3. Edition. Carstensen Verlag, Munich 2003, ISBN 3-910098-25-8 .