Phase Noise of Unsaturated Two-Stage Voltage-Controlled Ring Oscillator Without Tail Current

This paper investigates the phase noise (PN) of two-stage voltage-controlled ring oscillators. The delay cell is source-coupled pair with no tail current source, suitable for reduced voltage swing as supply reduces under technology scaling. With two stages, the output can become unsaturated, introducing extra cycle-to-cycle correlation and adding PN. The new cycle-to-cycle PN model is developed and applied to two different cell topologies. The design was fabricated in 1.2-V 0.13-μm CMOS, operating at 0.89-GHz oscillator frequency with 200-μW power consumption. PN measurement and simulation agree with theory over circuit parameter variations and corresponding design insights are drawn. Specifically as tuning voltage varies from 0.5 to 0.6 V, PN increases by around 5 dB.