In-pixel Voltage-Controlled Ring-Oscillator for Phase Interpolation in ToF Image Sensors

The design and measurements of a CMOS pseudodifferential voltage-controlled ring-oscillator (VCRO) are presented. It is aimed to act as time interpolator for arrayable picosecond time-to-digital convertors (TDC). This design is incorporated into a 64×64 array of TDCs for time-of-flight (ToF) measurement. It has been fabricated in a 0.18μm standard CMOS technology. Small occupation area of 28×29μm2 and low average power consumption of 1.17mW at 850MHz are promising figures for this application field. Embedded phase alignment and instantaneous start-up time are required to minimize the offset of time interval measurements. The measured gain of the VCRO is of 477MHz/V with a frequency tuning range of 53%. Moreover it features a linearity of 99.4% over a wide range of control frequencies, namely from 400MHz to 850MHz. The phase noise is of 102dBc/Hz at 2MHz offset frequency from 850MHz.