Gm-enhanced differential colpitts VCO
Date of Issue2012
School of Electrical and Electronic Engineering
This thesis proposes a gm-enhanced differential Colpitts voltage-control oscillator (VCO) that initiates oscillation with a lower biasing current and exhibits a better phase noise performance. The proposed gm-enhanced topology can improve performance of VCO, whereby the oscillator constructed based on the suggested topology demonstrated operation from 1.53 GHz to 2.1 GHz. The tank inductors engaged in the oscillator are each designed with a quality factor of 9. To evaluate the design, a conventional cross-coupled differential LC VCO, a conventional differential Colpitts VCO and the proposed VCO are simulated using 1.5 V supply voltage, under the 0.13 μm standard CMOS process. Biased with a current of 3 mA, the simulation results show that the proposed VCO's phase noise performance is of -128.9 dBc/Hz at 1 MHz offset, which is 3.6 dBc/Hz better than that of the cross-coupled differential LC VCO and 2 dBc/Hz better than that of the conventional differential Colpitts VCO. In the post-layout simulations of the proposed VCO and conventional cross-coupled differential LC VCO, the former has a 2.1 dBc/Hz phase noise better than that of the conventional one. Based on the developed designed gm-enhanced topology, a gm-enhanced differential Colpiits Quadrature VCO (QVCO) is designed. Operating at 1.8 GHz, the proposed QVCO shows a phase noise of -132.4 dBc/Hz at 1-MHz offset, when biased with 4-mA current and 1.5 V supply voltage. Compared with the conventional Colpitts QVCO, the proposed gm-enhanced differential Colpiits QVCO demonstrates better phase noise of 4 dB under the same operating condition.