Design of wide-band CMOS VCO for multiband wireless LAN applications

In this paper, a general design methodology of low-voltage wide-band voltage-controlled oscillator (VCO) suitable for wireless LAN (WLAN) application is described. The applications of high-quality passives for the resonator are introduced: 1) a single-loop horseshoe inductor with Q > 20 between 2 and 5 GHz [1] for good phase noise performance; and 2) accumulation MOS (AMOS) varactors with Cmax/Cmin ratio of 6 [2] to provide wide-band tuning capability at low-voltage supply. The adverse effect of AMOS varactors due to high sensitivity is examined. Amendment using bandswitching topology is suggested, and a phase noise improvement of 7 dB is measured to prove the concept. The measured VCO operates on a 1-V supply with a wide tuning range of 58.7% between 3.0 and 5.6 GHz when tuned between ±0.7 V. The phase noise is -120 dBc/Hz at 3.0 GHz, and -114.5 dBc/Hz at 5.6 GHz, with the nominal power dissipation between 2 and 3 mW across the whole tuning range. The best phase noise at 1-MHz offset is -124 dBc/Hz at the frequency of 3 GHz, a supply voltage of 1.4 V, and power dissipation of 8.4 mW. When the supply is reduced to 0.83 V, the VCO dissipates less than 1 mW at 5.6 GHz. Using this design methodology, the feasibility of generating two local oscillator frequencies (2.4-GHz ISM and 5-GHz U-NII) for WLAN tranceiver using a single VCO with only one monolithic inductor is demonstrated. The VCO is fabricated in a 0.13-μm partially depleted silicon-on-insulator CMOS process.