An LC Quadrature VCO Using Capacitive Source Degeneration Coupling to Eliminate Bi-Modal Oscillation

A phase shift technique using capacitive source degeneration (CSD) is presented for LC quadrature voltage-controlled oscillators (QVCO), where capacitively degenerated differential pairs are used to couple the LC tanks to implement a phase-shifted transconductance and negative input resistance to compensate resonator losses, and to minimize the flicker noise contributions. The CSD technique not only introduces excess phase shift to eliminate undesired bi-modal oscillation, but inherently provides a large coupling ratio to improve quadrature phase accuracy. Compared to existing phase-shift LC-QVCOs, the proposed CSD-QVCO presents excellent phase accuracy and power efficiency. A prototype of the QVCO was fabricated in TSMC 0.18-μm CMOS technology. The measured phase noise is -120 dBc/Hz at 3-MHz offset from 5-GHz carrier, with a power consumption of 6.4 mW from a 1.2-V supply.