A Study On Phase Noise And Current Efficiency For CMOS LC Oscillators

The oscillator is one of the most important blocks in wireless communication sys-tems, which is responsible for frequency generation and clock reference. In an integrat-ed radio-frequency (RF) transceiver, the phase noise of oscillator will directly impact both the noise figure (NF) in the receiver and the error vector magnitude (EVM) in the transmitter. Therefore, in the last few decades, oscillator phase noise has always been a research hotspot in RF IC.In recent years, demands for low-power circuit design keep increasing with the popularizaton of handheld mobile devices. As oscillator is a power-hungry block in the RF transceiver, research begin to focus on how to design a low-power low-noise oscillator, which ignites the study on energy efficiency of the oscillator. Amongst all, the current efficiency of an oscillator, due to its close relation to output amplitude as well as phase noise, has become an eye-catching issue.CMOS LC oscillators, with its achievable low phase noise at a moderate power consumption, are widely adopted in communication systems. This dissertation focuses on the phase noise and current efficiency in LC oscillators. Specifically, for the tra-ditional class-B oscillator, which is most widely used in both academia and industry, the dissertation presents an accurate current efficiency model and its range. The study shows that the current efficiency in the traditional class-B oscillator is between0.60and0.85at the FoM optimal point, which revises a common assumption that current efficiency in all class-B oscillators is equal to2/π. Moreover, the theory is extended to AC-coupled class-B and class-C oscillators. Analysis shows that the current efficiency of the two kinds of class-B oscillators is identical and the current efficiency of class-C oscillators is between0.85and1.The dissertation also elaborates the phase noise behavior in LC oscillators and links the two linear phase noise models with the oscillation equation, which reveals the mechanism behind. Utilizing the impulse-sensitivity-function (ISF) in LTV mod-el, phase noise models for the class-B oscillator suitable for the large amplitude case and low amplitude case are given, respectively. With the analytical results on current efficiency, a revised closed-form FoM expression is presented. Furthermore, based on the analysis above, some key techniques and design concepts are discussed, includ-ing the impact of tail capacitance, the boundary between voltage-limited regime and current-limited regime, and comparisons in energy efficiency of the class-B and class-C oscillator. Analysis shows that the main difference between the performances of these oscillators lies in voltage efficiency rather than current efficiency. Fundamentals of the LC digitally-controlled-oscillator (DCO) are also introduced in this dissertation, with a summary of the state-of-the-art. Finally, a high resolution tuning scheme for the LC digitally-controlled-oscillator (DCO) based on desensitized tail capacitance tuning is presented. The sensitivity of the oscillation frequency to the tail capacitance is quantitatively analyzed. The impact of the tail capacitance on the phase noise is qualitatively discussed. Analysis and simulation results show that both the phase noise and the resolution can be improved simultaneously if the tail capacitance value is properly designed.