A 6-GHz capacitively-coupled Colpitts CMOS quadrature voltage controlled oscillator

In this work a new low-noise low-power Colpitts quadrature voltage controlled oscillator (QVCO) made by coupling two identical current-switching differential Colpitts voltage controlled oscillators (VCO) is proposed; coupling of the VCOs is done using some capacitors in an "in-phase anti-phase" scheme. In this coupling configuration first harmonics (as well as higher harmonics) from each VCO are injected to the other VCO, as opposed to coupling schemes in which only even harmonics are injected. An analysis of the linearized circuit which confirms 90A degrees phase difference between output signals of the proposed circuit is presented. Since no extra noise sources or power consumption are introduced to the core VCOs, the proposed QVCO achieves low phase noise performance and low power consumption. The proposed circuit is designed and simulated in a commercial 0.18 mu m CMOS technology. The simulated phase noise of the proposed QVCO at 3 MHz offset frequency is -138.3 dBc/Hz, at 6 GHz. The circuit dissipates 8.16 mW from a 1.8 V supply and its frequency can be tuned from 5.6 to 6.3 GHz.