A phase noise analysis of CMOS colpitts oscillators

This paper presents a closed-form symbolic formula for phase-noise in the 1/f₂ region of Colpitts oscillator. Calculations based on the formula are verified by simulations, showing accurate results under general assumptions. A new topology of differential Colpitts is proposed, coupling two single-ended Colpitts oscillators by a center tapped inductor insteod of a common mode capacitor. A comparison of phase noise between the differential Colpitts oscillator and the cross-coupled LC-tank oscillator is carried out, showing the LC-tank oscillator presents a better phase noise, working in the same condition. Several prototypes of both Copitts and cross-coupled oscillators have been implemented in a 0.35μm CMOS process. The best performance of the LC-tank oscillators shows a phase noise of-139dB/Hz at 3MHz offset frequency from a 3GHz carrier with a 10mW power consumption, resulting in an excellent phase noise figure-of-merit (FoM) of 189dBC/Hz. Under the same working conditions, the FoM displayed by the differential Colpitts oscillators is between 4dB and 8dB lower than the FoM displayed by the cross-coupled LC-tank oscillators.