Low-phase-noise LC quadrature VCO using coupled tank resonators in a ring structure

The concept of coupled resonators is employed in a ring VCO structure to reduce the phase noise. This architecture allows the design of low-phase-noise voltage controlled oscillators (VCOs) using integrated low-Q inductors. Quadrature differential outputs are also realized in this design. Two monolithic LC tanks are coupled together to implement a transimpedance resonator with an effective Q close to twice that of a single tank. In addition, the coupled tank's transimpedance resonator provides 90/spl deg/ phase shift. Four such stages are cascaded in a ring structure to provide I-Q differential outputs, and to further reduce the phase noise. A prototype of the VCO is built in a 0.35-/spl mu/m CMOS technology. The measured phase noise is -122 dBc/Hz at 600-kHz offset from 1.93 GHz. The VCO draws 9.2 mA from a 3-V supply, and occupies a chip area of 1.1/spl times/1.1 mm/sup 2/.