A 5GHz LC VCO with extended linear-range varactor in purely digital 0.15um CMOS Process

MOSFET varactors with two different threshold voltages are used for extending linear tuning range of a voltage controlled oscillator (VCO). This method is suitable for VCOs implemented in purely digital CMOS processes where MIM-capacitors are not available and only MOS transistors can be used for tuning element. The effectiveness of this technique is demonstrated in a 5 GHz LC VCO with divide-by-2, output buffer, and integrated low-dropout voltage regulator. The VCO consumes 4 mA of current and achieves a tuning range of 4.18-4.94 GHz as well as phase noise of -109 dBc/Hz at 1 MHz from 4.5 GHz carrier. The effective linear range has been extended by 38 percent.