A Low-Phase-Noise 5-GHz CMOS Quadrature VCO Using Superharmonic Coupling

A new concept for quadrature coupling of oscillators is introduced and demonstrated on a 5-GHz CMOS voltagecontrolled oscillator (VCO). It uses the second harmonic of the outputs to couple the oscillators. The technique provides quadrature over a wide tuning range without introducing any increase in phase noise or power consumption. The VCO is tunable between 4.57 and 5.21 GHz and has a phase noise lower than 124 dBc/Hz at 1-MHz offset over the entire tuning range. The worst-case measured image rejection is 33 dB. The circuit draws 8.75 mA from a 2.5-V supply.