A study of phase noise in colpitts and LC-tank CMOS oscillators

TLDR

The study investigates phase‑noise characteristics of CMOS Colpitts and LC‑tank oscillators. Closed‑form symbolic expressions for the 1/f² phase‑noise region were derived for both oscillator types, and prototypes were implemented in a 0.35‑µm CMOS process to validate the models. The LC‑tank oscillator achieves a 2‑dB lower phase‑noise figure of merit than the differential Colpitts, with a best‑measured noise of –142 dBc/Hz at 3 MHz offset from a 2.9 GHz carrier (16 mW), corresponding to an FoM of ~189 dBc/Hz, whereas the differential Colpitts shows an FoM about 5 dB higher.

Abstract

This paper presents a study of phase noise in CMOS Colpitts and LC-tank oscillators. Closed-form symbolic formulas for the 1/f/sup 2/ 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-/spl mu/m 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 /spl sim/189 dBc/Hz. For the same oscillation frequency, the FoM displayed by the differential Colpitts oscillators is /spl sim/5 dB lower.