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This paper presents a low voltage on-chip oscillator which can compensate process, voltage, and temperature (PVT) variation in an all-digital manner. The relative reference modeling applies a pair of ring oscillators as relative references and estimates period of the internal ring oscillator. The period estimation is parameterized by a second-order polynomial. Accordingly, the oscillator compensates frequency variations in a frequency division fashion. A 1-20 MHz adjustable oscillator is implemented in a 90-nm CMOS technology with 0.04 mm area. The fabricated chips are robust to variations of supply voltage from 0.9 to 1.1 V and temperature range from 0°C to 75°C. The low supply voltage and the small area make it suitable for low-cost and low-power systems.