A low phase noise quadrature LC VCO using capacitive common-source coupling

A low phase noise quadrature LC VCO with inherent tail-current shaping is presented. Two identical differential LC VCOs are locked in quadrature with a capacitor connected between their common-source nodes. This capacitor further drives the oscillators into a tail-current shaping mode, which increases their oscillation amplitude and reduces their phase noise. A multi-band 1.9 GHz differential nMOS quadrature LC VCO prototype has been fabricated in a 0.25 mu m BiCMOS process. The measured phase noise is -124.4 dBc/Hz at a 1 MHz offset from a 2.07 GHz carrier while the circuit dissipates only 3 mW from a 1.5 V supply. The quadrature VCO is tunable from 1.75 GHz to 2.1 GHz and the VCO core only occupies 0.625mm(2).