Wide tuning range voltage controlled oscillator (VCO) with minimized phase noise variation in nanoscale CMOS technology

A cross-coupled differential VCO with low phase noise variation is presented. The VCO core adopts a metal-oxide-metal (MOM) digital switching capacitor array (DSCA), which is connected in series and in parallel to the nMOS varactor in order to reduce the KVCO variation. For further gain linearity, wider tuning range and minor phase noise variations, this varactor bank is connected in parallel to three nMOS varactor pairs. Each pair is biased at a different voltage. The proposed VCO has been designed and implemented in UMC 130-nm, 6-metal CMOS technology and operates from 3.45 GHz to 6.23 GHz with 55.6% tuning range, phase noise variations ranging between -134 dBc/Hz and -130.8 dBc/Hz and power consumption below 6 mW at supply voltage of 3.2 V. The simulated phase noise at 1 MHz offset is 132.4 dBc/Hz at 5.0 GHz and a good FOM performance of approximately 204.5 dBc/Hz is achieved.

I. Introduction

The continuous demand for better coverage and higher capacity of wireless local area networks (WLAN) has resulted in increasing requirements for Worldwide Interoperability for Microwave Access (WiMAX) technology^[1]. The IEEE unlicensed national information infrastructure WiMAX band (as specified by IEEE 802.16) provides both fixed and portable wireless broadband connectivity, independent of a base station. The majority of countries around the world have embraced the 5 GHz spectrum for license-exempt communications. Phase noise may vary dramatically over the tuning range of a VCO, but the published literatures overshadow this issue through measuring or simulating phase noise at the center frequency^[2]. It is difficult to simultaneously obtain a large tuning range and small phase noise variation, by accommodating MOS varactors because the probability of converting AM noise to FM noise incrcascs^[3]. The noise grows in line with the difference between the maximum and the minimum capacitances and may become indistinguishable from phase noise. Thus switching capacitors are compulsory to maintain minimum phase noise variations for wide tuning range and low VCO sensitivity, . Many articles^{[4] [5] [6] [7]} which consider switching capacitors inside the LC tank have been published in order to extend the tuning range and minimize VCO phase noise. However, these studies result in occupying a larger die area as well as exhibiting large phase noise variation and higher power dissipation.