On the Suitability of Discrete-Time Receivers for Software-Defined Radio

CMOS radio receiver architectures, based on radio frequency (RF) sampling followed by discrete-time (D-T) signal processing via switched-capacitor circuits, have recently been proposed for dedicated radio standards. This paper explores the suitability of such D-T receivers for highly flexible software-defined radio (SDR) receivers. Via symbolic analysis and simulations the authors analyze the properties of D-T receivers, and show that at least three challenges exist to make a D-T receiver work for SDR: 1) the sampling clock frequency is related to the radio frequency, complicating baseband filter design; 2) a frequency-dependent 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. Compared to a mixer based radio receiver, extra costs are needed to solve these problems.