An efficient decimation sinc-filter design for software radio applications

During these last years, the software radio has emerged as the only way to design multimode multistandard transceivers that support, everywhere, any communication standard. Software radio, however, requires not only powerful transceiver architectures, but also efficient digital signal processing algorithms. A method to design and implement a very efficient decimation filter for a sigma-delta converter is proposed. The main idea of this article is to free the positions of the zeros of the classical decimation sinc-filter so to attain a high selective frequency behaviour. The filtering stage is realized by a classical sinc-filter cascaded with two other filters obtained from the first one by rotating its zeros by two opposite angles. The whole cascade yields to a simple recursive structure with a frequency behaviour more selective than a sinc-filter around the noise fold frequency bands. Although the design method may be extended to a multistage implementation, the filter design proposed deals with the first filtering stage. Since the designed filter eliminates the most part of the quantization noise, the following filtering stages may be designed with relaxed specifications adopting classical filter design methods.