Reconfigurable digital filter based on fractional delay structure and interpolation technique for wider cutoff frequency range

In fractional delay based filters, each unit delay is replaced by a fractional delay structure (FDS) whose delay value determines the cutoff frequency (f_c) of the filter. In this paper we present the design of interpolated FDS (IFDS) based filter to overcome the lower and upper limits on f_c range of the existing FDS based filter. In the proposed IFDS based filter, the continuously variable fractional delay (CVFD) element is used to replace the unit delay element of the prototype filter. The CVFD element provides wider fractional delay range compared to the FDS used in the existing FDS based filter, and hence, the IFDS based filter overcomes the lower limit on f_c of the FDS based filter. Due to inherent limitations of the FDS, the FDS based filter has an upper limit on f_c of the FDS based filter. In the IFDS based filter, we use the CVFD element to vary f_c of the filter below this limit and then use interpolation technique to obtain variable multiband response. This multiband response of the interpolated filter is used along with its complementary response to obtain lowpass responses with f_c above the upper limit on f_c of FDS based filter. Simulation results show that the magnitude and phase responses of the proposed IFDS based filter are not impaired by the use of FDS. No degradation is observed in the passband or the stopband of the IFDS based filter compared to that of the prototype filter. The proposed reconfigurable filter design technique can be easily extended to design reconfigurable filter banks with variable number of subbands and fine control over the subband bandwidth.