Improving linearity in MOS varactor based VCOs by means of the output quiescent bias point

This paper presents a CMOS based LC tank VCO topology improving the tuning range linearity. The VCO tuning range is linearized with PMOS varactors which remain in the inversion region for an extended range of the control voltage. This is achieved with the design of the quiescent operating point in the VCO's output nodes with a value close to the voltage rails, letting the varactors to behave quasi linearly in the achievable VCO tuning range. The experimental results of a VCO in a CMOS 0.35 µm process show a linear tuning range improvement of 75% of the control voltage in the (1.43-1.55) GHz range, with a minimum VCO gain variation compared to similar architectures. The results show a phase noise improvement from −94 dBc/Hz to −124 dBc/Hz @600 kHz offset from the carrier with an overall reduced amplitude noise for the VCO.