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This paper presents a study of phase noise in CMOS Colpitts and LC-tank oscillators. Closed-form symbolic formulas for the 1/f2 phase-noise region are derived for both the Colpitts oscillator (either single-ended or differential) and the LC-tank oscillator, yielding highly accurate results under very general assumptions. A comparison between the differential Colpitts and the LC-tank oscillator is also carried out, which shows that the latter is capable of a 2-dB lower phase-noise figure-of-merit (FoM) when simplified oscillator designs and ideal MOS models are adopted. Several prototypes of both Colpitts and LC-tank oscillators have been implemented in a 0.35-μm CMOS process. The best performance of the LC-tank oscillators shows a phase noise of -142dBc/Hz at 3-MHz offset frequency from a 2.9-GHz carrier with a 16-mW power consumption, resulting in an excellent FoM of ∼189 dBc/Hz. For the same oscillation frequency, the FoM displayed by the differential Colpitts oscillators is ∼5 dB lower.