High-frequency LC VCO design using capacitive degeneration

In this paper, we evaluate the high-frequency performance limitations of traditional LC voltage-controlled oscillators (VCOs) that use a cross-coupled negative resistance cell and propose a new topology that overcomes these limitations. The proposed cell is based on a capacitively emitter degenerated topology which uses a cross-coupled MOS pair as the degeneration cell. The cross-coupled MOS pair contributes additional conductance and results in a higher maximum attainable oscillation frequency and better negative resistance characteristics as compared to the other topologies at high frequencies. These properties combined with its small effective capacitance enable low-power low-noise high-frequency VCO implementations. The proposed topology is demonstrated through a 20-GHz fully integrated LC VCO implemented in the IBM SiGe 0.25-μm BiCMOS process. A comparison of its figure of merit with previously reported 20-GHz VCOs shows the effectiveness of the proposed topology.