Frequency translation loops for RF filtering : theory and design

Abstract

Modern wireless transceivers are required to operate over a wide range of frequencies in order to support the multitude of currently available wireless standards. Wideband operation also enables future systems that aim for better utilization of the available spectrum through dynamic allocation. As such, co-existence problems like harmonic mixing and phase noise become a main concern. In particular, dealing with interference scenarios is crucial since they directly translate to higher linearity requirements in a receiver. With CMOS driving the consumer electronics market due to low cost and high level of integration demands, the continued increase in speed, mainly intended for digital applications, offers new possibilities for RF design to improve the linearity of front-end receivers. Furthermore, the readily available switches in CMOS have proven to be a viable alternative to traditional active mixers for frequency translation due to their high linearity, low flicker noise, and, most recently recognized, their impedance transformation properties. In this thesis, frequency translation feedback loops employing passive mixers are explored as a means to relax the linearity requirements in a front-end receiver by providing channel selectivity as early as possible in the receiver chain. The proposed receiver architecture employing such loop addresses some of the most common problems of integrated RF filters, while maintaining their inherent tunability. Through a simplified and intuitive analysis, the operation of the receiver is first examined and the design parameters affecting the filter characteristics, such as bandwidth and stop-band rejection, are determined. A systematic procedure for analyzing the linearity of the receiver reveals the possibility of LNA distortion canceling, which decouples the trade-off between noise, linearity and harmonic radiation. Next, a detailed analysis of frequency translation loops using passive mixers is developed. Only highly simplified analysis of such loops is commonly available in literature. The analysis is based on an iterative procedure to address the complexity introduced by the presence of LO harmonics in the loop and the lack of reverse isolation in the mixers, and results in highly accurate expressions for the harmonic and noise transfer functions of the system. Compared to the alternative of applying general LPTV theory, the procedure developed offers more intuition into the operation of the system and only requires the knowledge of basic Fourier analysis. The solution is shown to be capable of predicting trade-offs arising due to harmonic mixing and loop stability requirements, and is therefore useful for both system design and optimization. Finally, as a proof of concept, a chip prototype is designed in a standard 65nm CMOS process. The design occupies < 0:06mm2 of active area and utilizes an RF channel-select filter with a 1-to-2:5GHz tunable center frequency to achieve 48dB of stop-band rejection and a wideband IIP3 > +12dBm. As such, the work presented in this thesis aims to provide a highly-integrated means for programmable RF channel selection in wideband receivers. The topic offers several possibilities for further research, either in terms of extending the viability of the system, for example by providing higher order filtering, or by improving performance, such as noise.