A study of phase noise in CMOS oscillators

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 . 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- 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. OLTAGE-CONTROLLED oscillators (VCO's) are an integral part of phase-locked loops, clock recovery cir- cuits, and frequency synthesizers. Random fluctuations in the output frequency of VCO's, expressed in terms of jitter and phase noise, have a direct impact on the timing accuracy where phase alignment is required and on the signal-to-noise ratio where frequency translation is performed. In particular, RF oscillators employed in wireless tranceivers must meet stringent phase noise requirements, typically mandating the use of passive LC tanks with a high quality factor . However, the trend toward large-scale integration and low cost makes it desirable to implement oscillators monolithically. The paucity of literature on noise in such oscillators together with a lack of experimental verification of underlying theories has motivated this work. This paper provides a study of phase noise in two induc- torless CMOS VCO's. Following a first-order analysis of a linear oscillatory system and introducing a new definition of , we employ a linearized model of ring oscillators to obtain an estimate of their noise behavior. We also describe the limitations of the model, identify three mechanisms leading to phase noise, and use the same concepts to analyze a CMOS relaxation oscillator. In contrast to previous studies where time-domain jitter has been investigated (1), (2), our analysis is performed in the frequency domain to directly determine the phase noise. Experimental results obtained from a 2-GHz ring oscillator and a 900-MHz relaxation oscillator indicate that, despite many simplifying approximations, lack of accurate MOS models for RF operation, and the use of simple noise