RF-CMOS oscillators with switched tuning

TLDR

Fully integrated CMOS oscillators are highly sought for single‑chip wireless transceivers, yet conventional LC designs suffer from limited tuning ranges due to the lack of suitable varactors and large process‑induced capacitance variations, while metal spiral inductors provide more stable self‑inductance. This study aims to design RF CMOS oscillators that achieve a wide tuning range sufficient to cover process variations while maintaining low current consumption and phase noise. The authors built prototypes in a 0.6 µm CMOS process that oscillate up to 1.8 GHz on a sub‑3 V supply, using a tuning scheme that leverages digital logic and MOS analog switches.

Abstract

Fully integrated CMOS oscillators are of great interest for use in single-chip wireless transceivers. In most oscillator circuits reported to date that operate in the 0.9 to 2 GHz frequency range, an integrated spiral inductor sets the frequency. It is generally believed that an LC oscillator, even when it uses a low-Q inductor, displays a lower phase noise than a ring oscillator. However, due to the absence of a good varactor compatible with CMOS technology, the integrated LC oscillator suffers from a very limited tuning range. Although this tuning range may encompass the limited frequency agility required in an RF oscillator, for instance to span the modulation bandwidth in a transmitter, it will seldom cover the much larger lot-to-lot process variations manifest as spreads of up to 20% in capacitance. Fortunately, the self-inductance of a metal spiral does not suffer spreads, because it depends on a precise number of turns and on the geometry of metal traces which is little affected by fluctuations in lithography. This work addresses the practical problem of how to design RF CMOS oscillators with a wide enough tuning range to reliably cover process variations, without compromising current drain or phase noise. Prototypes were developed in the 0.6 /spl mu/m MOSIS CMOS process to oscillate at up to 1.8 GHz with a sub-3V supply. The tuning method exploits digital capabilities and MOS analog switches.