Influence of novel MOS varactors on the performance of a fully integrated UMTS VCO in standard 0.25-/spl mu/m CMOS technology

A novel MOS varactor design is compared to standard MOS varactors and its influence on the tuning range, phase noise, and pushing of a CMOS voltage-controlled oscillator (VCO) for UMTS is presented. Three fully integrated CMOS VCOs have been fabricated in standard 0.25-/spl mu/m technology, two with different versions of a novel device, and one with a conventional nMOSFET as the tuning element. All of the fully integrated VCOs fulfill UNITS tuning and phase noise specifications with a power consumption of only 7.5 mW at a 2.5-V power supply. The new varactors outperform the nMOSFET by increasing the frequency tuning from /spl plusmn/7% to /spl plusmn/11% or /spl plusmn/13%, while the measured phase noise of all three VCOs is -117 dBc/Hz at a 1 MHz offset from a 4-GHz carrier.

I. Introduction

Fully integrated voltage controlled oscillators (VCOs) are one of the most challenging parts of radio transceivers to integrate in standard CMOS technologies. This is due to the most important and demanding parameters of a VCO: large frequency tuning range, low power consumption, and low phase noise. In LC-tank VCOs, frequency tuning range and phase noise are mainly determined by the passives of the tank. Many designs have been reported using different varactor (variable capacitor) and inductor implementations [1] [2] [3].