A class of digital filters with variable cut-off based on EMQF filter sections and sharpening method

A new filter class with variable cut-off frequency is designed based on elliptic filter sharpening technique. A higher order IIR filter with a sharp transition band is constructed by using several identical low-order EMQF (elliptic minimal Q-factors) filters and an additional compensating section. The stopband attenuation is achieved by cascading EMQF subfilters, and the compensation section is designed to minimize the overall passband ripple. For the tuning of the cut-off frequency we propose a simple version of the spectral parameter approximation technique. Low-degree approximating polynomials are developed to represent coefficients of the EMQF subfilters and coefficients of the compensating section in terms of the passband cut-off frequency. In the tuning procedure, the new coefficient values are obtained by the polynomial evaluation consisting of several multiplications and additions. The analytical methods are derived for the filter design and for the tuning procedure. Design and implementation of sharp variable filters is illustrated by means of examples.