A 5 GHz CMOS quadrature VCO with precise quadrature phase

A 5 GHz CMOS quadrature voltage-controlled oscillator (VCO) with precise quadrature phase is developed. The developed quadrature VCO mainly uses two PN complementary cross-coupled oscillators. The coupling transistors operate in parallel to the VCO core transistors. Performance analysis of the developed oscillator shows that the additional coupling transistors not only reduce VCO output voltage swing, but also attenuate VCO quality factor (Q), which substantially degrades phase noise performance. The proposed solution is to minimize the quadrature VCO coupling factor. However, a small coupling factor worsens the quadrature phase and amplitude imbalance. Therefore, a coupling factor of 0.33 is applied to optimize the phase noise performance and quadrature phase. The 5 GHz quadrature VCO is implemented in a TSMC 0.18 μm CMOS process. Measurement results show that the phase noise of the quadrature VCO is lower than -127 dBc/Hz at 3 MHz offset frequency. The quadrature phase error is lower than 0.36 degrees over the wide operating range of 4.99 ~ 5.73 GHz.