Discrete-Time Mixing Receiver Architecture for RF-Sampling Software-Defined Radio

A discrete-time (DT) mixing architecture for RF-sampling receivers is presented. This architecture makes RF sampling more suitable for software-defined radio (SDR) as it achieves wideband quadrature demodulation and wideband harmonic rejection. The paper consists of two parts. In the first part, different downconversion techniques are classified and compared, leading to the definition of a DT mixing concept. The suitability of CT-mixing and RF-sampling receivers to SDR is also discussed. In the second part, we elaborate the DT-mixing architecture, which can be realized by de-multiplexing. Simulation shows a wideband 90° phase shift between I and Q outputs without systematic channel bandwidth limitation. Oversampling and harmonic rejection relaxes RF pre-filtering and reduces noise and interference folding. A proof-of-concept DT-mixing downconverter has been built in 65 nm CMOS, for 0.2 to 0.9 GHz RF band employing 8-times oversampling. It can reject 2nd to 6th harmonics by 40 dB typically and without systematic channel bandwidth limitation. Without an LNA, it achieves a gain of -0.5 to 2.5 dB, a DSB noise figure of 18 to 20 dB, an IIP3 = +10 dBm, and an IIP2 = +53 dBm, while consuming less than 19 mW including multiphase clock generation.

I. Introduction

Rf-Sampling receivers have recently drawn both academic [1], [2] and industrial interests [3]–[6]. They sample the signal early in the receiver chain before or simultaneously with downconversion, instead of after downconversion as done in conventional continuous-time (CT) mixing receivers. Direct RF-sampling provides the potential to move A/D converters (ADC) closer to antenna aiming at software radio (SWR) or software defined radio (SDR). As initially proposed by Mitola [7] and BellSouth [8], the ADC in an ideal SWR operates on the RF signal, while in a (more practical) SDR the ADC may be located at IF, after frequency downconversion, still achieving certain flexibility. Reconfigurability by software should allow it to support different wireless standards in one device, bringing cost and size reductions. SDR might also allow for upgradable radios and cognitive radio (CR), to improve the efficiency of utilizing scarce spectrum resources.