Sensitivity of notch filters realized by parallel connection of allpass subfilters

The different structures of digital notch filters realized as parallel connection of two allpass subfilters are investigated in this paper. The research was done for notch filters with only one notch frequency. The influence of filter coefficients quantization on filter characteristics is analysed for three configurations and the results are displayed. Unlike Butterworth and elliptic filters it is shown that notch filters realised as parallel connection of two allpass filters remain selective even in case the coefficients are represented with only three fractional bits. All given results are obtained using Matlab® package.