Band-stop filter

A generic ideal band-stop filter, showing both positive and negative angular frequencies

In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a range to very low levels. It is the opposite of a band-pass filter. A notch filter is a band-stop filter with a narrow stopband (high Q factor).

Other names include 'band limit filter', 'T-notch filter', 'band-elimination filter', and 'band-rejection filter'.

Typically, the width of the stopband is less than 1 to 2 decades (that is, the highest frequency attenuated is less than 10 to 100 times the lowest frequency attenuated). In the audio band, a notch filter uses high and low frequencies that may be only semitones apart.

Audio example 1: Anti-hum filter

• Low Freq: 59 Hz
• High Freq: 61 Hz

This means that the filter passes all frequencies, except for the range of 59–61 Hz. This would be used to filter out any noise from a 60 Hz power line.

Homemade bandstop filter tuned for 153MHz pager band

Audio example 2: Anti-presence filter

• Low Freq: 1 kHz
• High Freq: 4 kHz

RF example 1: Non-linearities of power amplifiersFor instance, when measuring non-linearities of power amplifiers a very narrow notch filter could be very useful to avoid the carrier so maximum input power will not be exceeded.

See also

• High-pass filter
• Low-pass filter
• Mains hum

References

• Federal Standard 1037C
• MIL-STD-188

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