A study of phase noise in CMOS oscillators

TLDR

The study investigates phase noise characteristics in two inductorless CMOS oscillators. The authors use first‑order linear analysis to derive a noise‑shaping function and a new Q definition, model CMOS ring and relaxation oscillators, and evaluate noise simulation techniques with fabricated prototypes. The analysis identifies additive, high‑frequency multiplicative, and low‑frequency multiplicative noise components, and the calculated phase noise of the 2‑GHz ring and 900‑MHz relaxation oscillators matches measurements within about 4 dB at a 5‑MHz offset.

Abstract

This paper presents a study of phase noise in two inductorless CMOS oscillators. First-order analysis of a linear oscillatory system leads to a noise shaping function and a new definition of Q. A linear model of CMOS ring oscillators is used to calculate their phase noise, and three phase noise phenomena, namely, additive noise, high-frequency multiplicative noise, and low-frequency multiplicative noise, are identified and formulated. Based on the same concepts, a CMOS relaxation oscillator is also analyzed. Issues and techniques related to simulation of noise in the time domain are described, and two prototypes fabricated in a 0.5-/spl mu/m CMOS technology are used to investigate the accuracy of the theoretical predictions. Compared with the measured results, the calculated phase noise values of a 2-GHz ring oscillator and a 900-MHz relaxation oscillator at 5 MHz offset have an error of approximately 4 dB.