A low-phase-noise LC QVCO with bottom-series coupling and capacitor tapping

A novel LC Quadrature VCO (QVCO) employing bottom-series coupling and capacitor tapping to suppress phase noise at low voltage operation is presented. Placing the coupling transistors at the bottom of the switching pairs, the bottom-series coupling method yields zero resonator-phase-shift (RPS) which maximizes the quality factor (Q) of the resonator. The capacitor tapping technique allows a large signal swing at the resonator outputs while maintaining the current sources and the turned-on switching transistors in saturation. Therefore, the signal-to-noise ratio (SNR) and the loaded Q of the resonator are both boosted resulting in a low phase noise. The proposed QVCO achieves the figure of merit (FOM) of 189 dBc/Hz with a phase noise of -120.3 dBc/Hz at 1-MHz offset when operating at 6 GHz, and draws 5 mA from a 1-V power supply.