Stacked Ring Oscillators with Fractional Injection Lock

Ring oscillators are widely used in modern applications due to their simplicity, compact size, and low power consumption. Their oscillation frequency is determined by the number of stages (or inverters) and the average delay per inverter. In advanced submicron technologies, logic gates’ minimal propagation delay allows high-frequency operation with great efficiency. However, ring oscillators become increasingly power-inefficient at lower frequencies (MHz range). This paper presents a novel oscillator topology that combines two injection locked stacked ring oscillators with different stage counts, generating two output frequencies with a fractional ratio. The stacking technique reuses the supply current while splitting the available supply voltage across the two oscillators. This approach effectively reduces the oscillation frequency while maintaining energy efficiency and improving phase noise performance by approximately 3 dBc/Hz across all frequencies. Simulation results in TSMC 65nm confirm that the locking mechanism of two stacked ring oscillators is achieved, even when they contain different numbers of inverters, enabling fractional injection locking.