Analysis of phase noise in CMOS ring oscillator due to substrate noise

A novel method is presented for analyzing and modeling the phase noise of CMOS ring oscillators. It leads to the conclusion that the phase noise caused by substrate noise has a 1/f₄ frequency dependency, and that the impact of substrate noise on phase noise performance depends largely on the switching time in the digital circuitry dominating the substrate noise. The proposed method is utilized to study the phase noise in a single-ended ring oscillator, which is fabricated in the SMIC 0.13 μm 1P6M standard CMOS process. The substrate noise is coupled from a digital counter circuit that is adjacent to the ring oscillator and operates at a controllable frequency that determines the switching rate for the aggressing signal. The proposed substrate-noise model is verified through measurements. The measurement show that the -40 dB/decade characteristics of the ring-oscillator´s phase noise are more apparent as the switching rate is increased, whereas when the switching rate is reduced, the phase noise exhibits a steeper slope of -60dB/decade.