ΣΔ ADC with fractional sample rate conversion for software defined radio receiver

Abstract

An analog-to-digital converter (ADC) in a software defined radio (SDR) receiver must be able to support the specifications of various wireless standards. It should also work with the low power supply voltage of CMOS circuits. A ΣΔ ADC is one of the solutions for the SDR receiver. However, the power consumption of the circuits, such as a lowpass filter (LPF) and a decimator after the ΣΔ modulator is large due to the high sampling speed. On the other hand, since a different wireless communication standard has a different master clock rate, it is necessary to provide the clock rate suitable for each standard. As a solution for the problem, direct insertion/deletion based sample rate conversion (SRC) has been proposed. Nevertheless, in order to apply this method to the ΣΔ ADC, the problem of the high clock speed in the circuits of the LPF after ΣΔ modulator remains. In this paper, a novel ΣΔ ADC with fractional SRC is proposed. In the proposed scheme, SRC and filtering are combined. By filtering and decimating the output of the ΣΔ modulator in parallel with Q transversal filters the clock speed of the circuits of the SRC and the LPF can be reduced P times for Q/P SRC.