Abstract:Infinite Impulse Response(IIR) digital filter is designed usually in S-domain firstly, then the S-domain transfer function is transferred to Z-domain. To simplify the design process, a direct design method of filter based on Single Degree of Freedom(SDOF) model is presented, including the types of LP filter, HP filter, BP filter, BS filter. Simulation results show that the design process is direct and simple,and the designed filter bears good frequency characteristics. Another advantage of the proposed method is that the filter parameters needn’t change with system sample time. Therefore, it is a new method for IIR digital filter design with good usability.