An optimally coupled 5 GHz quadrature LC oscillator

A 5 GHz quadrature LC oscillator is realized which is based on a new architecture for multi-phase LC oscillators. Each section in the oscillator is coupled with an explicit phase shift of 180 degrees divided by the number of sections. Analysis on behavioral level shows that this maximizes the quality factor, and as a result, the carrier-to-noise ratio and robustness. An effective quality factor is derived which quantizes the degradation in phase noise performance if the sections of a multiphase LC oscillator are non-optimally coupled. The realized 5 GHz quadrature LC oscillator demonstrates that even at high frequencies the additional complexity of the proposed architecture yields a CNR improvement. The oscillator is realized in a BiCMOS process with a cut-off frequency of 30 GHz using an LC resonator with a quality factor of 4. A tuning range from 4.91 to 5.23 GHz is obtained with a CNR better than 113 dBc/Hz at 2 MHz offset. The VCO core power dissipation is only 21.2 mW at 2.7 V supply voltage.