On CIC decimator variants - from shifting zeros to the sparse FIR-CIC structure

The cascaded integrator-comb (CIC) filter is an efficient multiplier- free structure for decimation and interpolation. However, due to its narrow stopband notches, it is useful only when the signal of interest has a small bandwidth compared to the input and output sample rates. Consequently, it can only be used in the first (last) few stages of multistage decimators (interpolators). Various modifications have been proposed in order to alleviate this shortcoming. We investigate the modified CIC structure proposed by Saramaki and Ritoniemi and show that its response can be further improved without increasing the implementation complexity. This remodification results in a FIR filter nested inside a CIC decimator, a structure that is already known. We then propose the use of a sparse FIR filter in this nested structure. This results in remarkable attenuation improvements for given complexity, as shown by design examples.