High-Performance LC-VCOs and their Analysis using Time-Varying Root-Locus

This thesis focuses on the analysis and development of High-Performance LC Voltage-Controlled Oscillators (VCO) tailored for wireless communications. The frequency instabilities of the VCO signal, or poor phase noise performance, may degrade the quality of signal transmission and reception in communication systems. Furthermore, advanced transceivers used in modern terminals should cover a wide frequency range. Unfortunately, there is a fundamental tradeoff between wide tuning range and low phase noise in oscillators. Therefore, solutions that break the tradeoff between phase noise performance and wide tuning range are needed. The thesis evaluates two approaches to mitigate the abovementioned fundamental tradeoff: circuit approach and theoretical approach. The goal of the circuit approach is to develop oscillator architectures that can generate wide tuning range, or multiple frequency bands, without significant effect on phase noise performance. The goal of the theoretical approach is to gain deeper understanding of the phase noise mechanisms in oscillators. Once the phase noise mechanisms are well understood, the acquired knowledge can be used to build high-performance oscillators, which would indirectly mitigate the tradeoff between phase noise and tuning range. The main outcome of circuit approach presented in the thesis is the proposed dual-band VCO based on fourth-order resonator. The principle is validated with analytical derivations and prototype measurement. Furthermore, a tapped-inductor topology has been proposed to reduce silicon area. The introduced in the thesis programmable VCO gain concept for LC-VCOs is another circuit approach to mitigate the tradeoff. The main outcome of the theoretical approach is the developed Time-Varying Root-Locus (TVRL) method for the analysis of large-signal oscillators. The method allows oscillator architecture comparison and extraction of effective quality factor. The root trajectories are further linked with physical phenomena in oscillators, whereas the effective quality factor estimation links the TVRL method with oscillator phase noise. The verified by simulations and measurement method could become an auxiliary tool for oscillator characterization.