Modern broadband communication systems require highly stable frequency references adjustable over a wide range of frequencies. One such system is a cable tuner, which requires a voltage-controlled oscillator (VCO) capable of tuning between 1.2 and 2.1 GHz and a phase noise below –85 dBc/Hz at a 10 kHz offset from the carrier . Solutions addressing such specifications have traditionally relied on expensive high-end technologies and external components . Today’s CMOS technologies are cost-effective and provide sufficient bandwidth for many RF communication applications. This research project focuses on the design of wideband low-phase-noise frequency VCOs in CMOS.
Although various types of VCOs can achieve a wide tuning range, LC VCOs are most suitable since they generally exhibit lower phase noise. To achieve a wide tuning range using a single resonator, we propose a VCO tuned using a mixed-signal scheme. Its frequency is digitally adjusted in coarse steps and subsequently fine-tuned to the desired value using a varactor device. An amplitude control scheme is also implemented.
The goal of this research project is to establish a framework for the analysis, design, and optimization of wideband low-phase-noise VCOs. Tradeoffs between different candidate LC VCO topologies and between available devices will be investigated. The effects of varactor nonlinearities on phase noise and tuning range will be analyzed. A set of analytical methods will be provided to aid in predicting these effects and any other quantities relevant to the overall VCO performance. Our proposed LC VCO will be integrated within a frequency synthesizer to demonstrate its practical feasibility.