NOTCH AND COMB FILTERS
A notch filter is a filter that contains one or more deep notches or ideally perfect nulls in its frequency response characteristic. Notch filters are useful in many applications where specific frequency components must be eliminated. Example Instrumentation and recording systems required that the power-line frequency 60Hz and its harmonics be eliminated.
To create nulls in the frequency response of a filter at a frequency ω0, simply introduce a pair of complex-conjugate zeros on the unit circle at an angle ω0.
comb filters are similar to notch filters in which the nulls occur periodically across the frequency band similar with periodically spaced teeth. Frequency response characteristic of notch filter |H(ω)| is as shown