Understanding Phase Noise in LC VCOs: A Key Problem in RF Integrated Circuits

The design of low-phase-noise inductor-capacitor voltage-controlled oscillators (LC VCOs) has been one of the most important topics in the field of radio frequency (RF) integrated circuits in last 20 years. A huge number of IEEE papers have been devoted to this subject, which is quite a unique circumstance for a circuit that typically embeds from three (sometimes two) to five transistors. The reasons for this widespread interest are well known, i.e., the phase noise of VCOs is one major obstacle encountered during the design of RF transceivers, and the power dissipated to meet the noise specs is often a nonnegligible portion of the total budget. A second reason is the intrinsic time-variant nature (often also nonlinear) of these circuits, which makes their analysis very difficult.