On Design of CIC Decimators

Abstract

The process of changing sampling rate of a signal is called sampling rate conversion (SRC). Systems that employ multiple sampling rates in the processing of digital signals are called multirate digital signal processing systems. Multirate systems have different applications, such as efficient filtering, subband coding, audio and video signals, analog/digital conversion, software defined radio and communications, among others (Jovanovic Dolecek, 2002). The reduction of a sampling rate is called decimation and consists of two stages: filtering and downsampling. If signal is not properly bandlimited the overlapping of the repeated replicas of the original spectrum occurs. This effect is called aliasing and may destroy the useful information of the decimated signal. That is why we need filtering to avoid this unwanted effect. The most simple decimation filter is comb filter which does not require multipliers. One efficient implementation of this filter is called CIC (Cascaded-Integrator-Comb) filter proposed by Hogenauer (Hogenauer, 1981). Because of the popularity of this structure many authors also call the comb filter as CIC filter. In this chapter we will use term CIC filter. Due to its simplicity, the CIC filter is usually used in the first stage of decimation. However, the filter exhibits a high passband droop and a low attenuation in so called folding bands (bands around the zeros of CIC filter), which can be not acceptable in different applications. During last several years the improvement of the CIC filter characteristics attracted many researchers. Different methods have been proposed to improve the characteristics of the CIC filters, keeping its simplicity. In this chapter we present different proposed methods to improve CIC magnitude characteristics illustrated with examples and MATLAB programs. The rest of the chapter is organized in the following way. Next Section describes the CIC filter. Section 3 introduces the methods for the CIC passband improvement followed by the Section 4 which presents the methods for the CIC stopband improvement. The methods for both, the CIC passband and stopband improvements are described in Section 5.