Reactive Power Imbalances in LC VCOs and their Influence on Phase Noise Mechanisms

Phase-noise mechanisms in cross-coupled LC voltage-controlled oscillators (VCOs) are reviewed based on a physical understanding of reactive power imbalances in the tank and in the active part. These phenomena are proven to be the predominant phase-noise degradation mechanism in relatively low- and high-current operations. Based on this analysis, a technique to suppress these detrimental effects is developed and implemented in an LC VCO design. The measured results confirm the dependencies predicted by the analysis, and the usefulness of the proposed technique to simultaneously optimize the phase noise at high and low offset frequencies. The measured VCO tuning range is 600 MHz, ranging from 2.4 to 3 GHz. The VCO-prescaler circuit exhibits a phase noise from - 88 to -92 dBc/Hz at 15 kHz and from -155 to -160 dBc/Hz at 10 MHz, when the power consumption is 6 and 10 mA for the VCO and 2 mA for the prescaler, and the power supply is 2 V.