Design of IIR multiple notch filters based on all-pass filters

A new method for the design of multiple notch fitters is presented using an all-pass filter of order 2N, N being the number of notch frequencies. The all-pass filter is realized as a cascade of N second-order all-pass sections. Besides meeting the notch frequency specifications exactly, the method ensures that the realized 3-dB bandwidths are lower than those specified. Illustrative examples are given.