Co-existence problems in a mobile terminal environment pose strict requirements on the linearity of a front-end receiver. In this paper, active feedback is explored as a means to relax such requirements by providing channel selectivity as early as possible in the receiver chain. The proposed receiver architecture 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 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.A prototype designed in a standard 65nm CMOS process occupies < 0.06mm 2 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.

The impedance transformation property of passive mixers enables integrated high-Q channel selection at RF with a programmable center frequency through a clock [1,2]. As such, this technique is suitable for addressing both linearity and flexibility requirements in wideband and cognitive radio applications. However, given the typically low resistance level at the RF side of the receiver chain, the RC product necessary for filtering results in large capacitors, and, consequently, large die area that does not scale with technology. In addition, filter rejection at the RF side is limited by the resistance of the switches of the passive mixer. Thus, large switches are typically needed for moderate rejection values (5Ω switches for 16dB rejection [2]), which translates to higher power consumption in the LO buffers. Furthermore, filtering prior to the LNA [1] or eliminating it altogether [3] improves linearity at the expense of noise and switching harmonics being injected directly at the antenna node. Conversely, an LNA first architecture offers an opposite trade-off. This work demonstrates a highly compact design of a direct conversion receiver with an active feedback frequency translation loop to perform channel selection at the LNA output while simultaneously cancelling its distortion. Figure 9.3.1 (top) shows the proposed architecture. Signals at the antenna are down-converted and amplified. Along the feedback path, the desired signal BW, now centered at DC, is rejected using a HPF, while interferers are up-converted and fed back at the LNA output. With a high loop gain, node 'A' becomes a virtual ground for interferers beyond the corner frequency of the HPF, which effectively creates a channel select filter at node 'A'. Since the feedback loop sinks current, filter rejection at the RF side is not limited by the resistance of the down-conversion mixer switches. As opposed to other feedback-based receivers [4,5], the proposed architecture incorporates the receiver's downconversion path within the loop to provide an IF output instead of having a separate rejection loop after the LNA with an RF output that needs further down-conversion.The channel bandwidth is now determined by the corner frequency of the HPF (ωHPF) divided by 1+TO, where TO is the loop gain ( Fig. 9.3.1-bottom). That is, for a given bandwidth, the capacitance needed for channel selection, and therefore die area, is reduced by the available loop gain. The loop bandwidth (ωDOM) sets an upper limit on the frequency range of interferers that can be suppressed. Such a filtering loop is therefore suitable for implementation in a modern high speed process and its performance is expected to improve with technology scaling.To cancel LNA distortion, the V-to-I conversion circuits of the LNA and feedback amplifier are matched (Fig. 9.3.2). Under this condition, a downconverted inverted replica of input interferers is forced at the output via the feedback action, causing the feedback amplifier to perfectly sink the distortion currents sourced by the LNA. This arr...

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 iii Abstract 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 < 0.06mm 2 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 present...