A SYNTHESIS OF VARIABLE IIR DIGITAL-FILTERS

It is sometimes required to change the frequency characteristics of a digital filter during its operation. In this paper a new synthesis of variable even-order IIR digital filters is proposed. The cut-off frequ ency of the filter can be changed by a single parameter. The fundament al filter structure is a cascade of second-order sections. The multipl ier coefficients of each section are determined by using the Taylor se ries expansion of the lowpass to lowpass frequency transformation. For this method any second-order section can be used as a prototype, but here in this paper only the direct form and the lattice form are descr ibed. Unlike the conventional method, any transfer functions can be us ed for the proposed method. Finally a designed example shows that the proposed filter has wider tuning range than the conventional filter, a nd the advantage of the proposed filters is confirmed.