Synthesis of variable IIR digital filters with complex coefficients

Sometimes it is required to change the filter characteristics of a digital filter in real time during its filtering operation. To meet this requirement, variable cutoff frequency lowpass filters with real coefficients have been reported using only one parameter. Moreover, variable bandpass filters with complex coefficients have been reported where only one parameter was to be changed and where either the upper or lower band‐edge frequency of the passband was fixed. However, in those filters, the frequency response was degraded with the change in characteristics which could not be resolved. This paper proposes the synthesis of a variable IIR digital filter using one parameter and the frequency response is not degraded when the characteristics are changed. In the variable filter proposed here, the cut‐off frequency and the bandwidth may be changed without any restriction. Moreover, a simple circuit realization technique is proposed so that the filter circuit may actually be implemented. In this technique, any type of real coefficient filter circuit may be used as the prototype filter. Finally, the simulation of the synthesis problem is carried out and the validity of the proposed method is confirmed.