Highly narrow rejection bandwidth finite impulse response notch filters for communication

Design of finite impulse response (FIR) notch filters (NFs) with highly narrow rejection bandwidth (RBW) is suggested. RBW is the distance between 3 dB points on the response. Reduction in the RBW is achieved progressively in three stages. In the first stage, an FIR NF is designed from a second order infinite impulse response (IIR) prototype filter. For a given length L of the NF, the authors choose maximum permissible value of `r´ (the pole length of IIR prototype filter) to achieve very narrow RBW of the FIR filter. In the next stage, by using an amplitude change function (ACF): H(z)(2 - H(z)), the designed filter is sharpened. Consequently, the RBW of the resulting NF is reduced to almost half of the earlier value. This reduction of bandwidth makes the resulting NF of length 2L. In the next stage, RBW can be further reduced by the repeated sharpening of the filter by the same ACF.