IMPLEMENTATION OF DIGITAL DOWN CONVERTER FOR SOFTWARE RADIO

Abstract

The idea of software radio requires an expansion of Digital Signal Processing (DSP) towards the antenna. Hence, for converting the received signal to baseband, the need of efficient high speed digital down converters (DDC) arises. DDC performs the frequency translation necessary to convert the high sample rates down to lower sample rates for further and easier processing. It is identified as one of the `critical functionalities' because it has to run at a relative high sample rate, and has to provide high resolution. For this purpose, a class of digital linear phase finite impulse response (FIR) filters for decimation (sampling rate decrease) is proposed. They require no multipliers and use limited storage making them an economical alternative to conventional implementations for certain applications. A digital filter in this class consists of cascaded integrator-comb (CIC) stages operating at a high sampling rate and an equal number of comb stages operating at a low sampling rate. Since their inception, CIC filters have become an important building block for DSP systems. They have found a particular niche in digital transmitters and receivers. They are currently used in highly integrated chips from Intersil, Graychip, Analog Devices, as well as other manufacturers and custom designs