A voltage controlled oscillator (VCO) has a plurality of series-connected inverters. Within each inverter a first transistor has a first current electrode coupled to a first power supply voltage terminal, a second current electrode, a first control electrode coupled to an output terminal of another inverter of the plurality of series-connected inverters, and a second control electrode for receiving a first bias signal. A second transistor has a first current electrode coupled to the second current electrode of the first transistor, a second current electrode coupled to a second power supply voltage terminal, and a first control electrode coupled to the first control electrode of the first transistor. The second control electrode of the first transistor of each inverter receives a same or separate analog control signal to adjust the threshold voltage of the first transistors thereof to affect frequency and phase of the VCO's signal.