A low-phase-noise 25-GHz quadrature voltage-controlled oscillator (VCO) using 180-nm TSMC CMOS is presented. To equalize the oscillation frequency to the LC-resonant frequency, a phase shifter at a transistor gate adjusts a phase delay due to an RC delay in a transistor. This phase-adjusting architecture extends operation bias range and suppresses phase noise by 6 to 9 dB compared with a conventional differential VCO.

Quadrature voltage-controlled oscillators (VCOs) with current-weight-average and voltage-weight-average phase-adjusting architectures are studied. The phase adjusting equalizes the oscillation frequency to the LC-resonant frequency. The merits of the equalization are explained by using Leeson's phase noise equation and the impulse sensitivity function (ISF). Quadrature VCOs with the phase-adjusting architectures are fabricated using 180-nm TSMC CMOS and show low-phase-noise performances compared to a conventional differential VCO. The ISF analysis and small-signal analysis also show that the drawbacks of the current-weightaverage phase-adjusting and voltage-weight-average phase-adjusting architectures are current-source noise effect and large additional capacitance, respectively. A voltage-average-adjusting circuit with a source follower at its input alleviates the capacitance increase.