High tuning accuracy design of variable IIR filters as a cascade of identical sub-filters

The limits of application, tuning accuracy and sensitivity of variable IIR digital filters realized as a cascade of several identical sub-filters are investigated in this paper. Sub-filters of first and second-order are realized with bilinear and biquadratic sections with independent tuning, developed by the authors, while sub-filters of higher order are realized as parallel allpass structures using first and second-order allpass sections with minimized sensitivities. It is shown that filters so designed have higher accuracy and wider ranges of tuning compared to other known variable filters and behave much better in a limited word-length environment.