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Notch filter

A notch filter (also called a narrow band-pass filter or T-notch filter) is a electronic filter typically used when the high frequency and the low frequency are less than 1 to 2 decades apart (that is, the high frequency is less than 10 to 20 times the low frequency).

Example 1:

High Freq: 2000 Hz
Low Freq: 120 Hz

Example 2:
High Freq: 500 Hz
Low Freq: 30 Hz

See also: FM radio.