Discrete-Time Receivers for Software-Defined Radio: Challenges and Solutions

CMOS radio receiver architectures, based on radio frequency (RF) sampling followed by discrete-time (DT) signal processing via switched-capacitor circuits, have recently been proposed for dedicated radio standards. This paper explores the suitability of such DT receivers for highly flexible softwaredefined radio (SDR) receivers. Via symbolic analysis and simulations we analyze the properties of DT receivers, and show that at least three challenges exist to make a DT receiver work for SDR: 1) the sampling clock frequency is related to the radio frequency, complicating baseband filter design; 2) a frequencydependent phase shift is introduced by pseudo-quadrature and pseudo-differential sampling; 3) the conversion gain of a charge sampling front-end is strongly frequency-dependent. Some potential solutions are also suggested for each challenge. Compared to a mixer based radio receiver, extra costs are needed to solve these problems. Index Terms Software-Defined Radio, Discrete-Time Receiver, RF Sampling, Frequency Dependence