A Low-Phase-Noise CMOS Ring Voltage-Controlled Oscillator Intended for Time-Based Sensor Interfaces

We describe in this paper an improved ring voltage-controlled oscillator (VCO) showing a reduced phase noise while allowing an extended frequency tuning range. The phase noise improvement is obtained through the minimized contribution of tuning line noise while maintaining a rail-to-rail swing. The proposed VCO features a linear tuning characteristic yielding a constant gain over a wide range of operating frequencies. An analytical model is extracted resulting in closed-form expressions for the VCO phase noise. Employing the analytical expressions, the contributed noise and phase noise limitations are fully addressed, and all the effective factors are investigated. The VCO prototype was fabricated in a 0.35
$\mu \text{m}$
CMOS process. It consumes 0.903 mW from a 3.3 V supply when running at its maximum oscillation frequency of 9.37 MHz. The measured VCO phase noise is −147.57 dBc/Hz at 1 MHz offset from the 9.37 MHz oscillation frequency, and the circuit occupies a silicon area of 0.005 mm2. A state-variable MATLAB® model of a time-based sensor interface has been developed including the impact of phase noise nonideality. The system-level simulations demonstrate that the PLL-based sensor interface exploiting the proposed VCO characteristics can achieve a 88.43 dB signal-to-noise ratio over a 1-kHz bandwidth.