Voltage-controlled oscillator with tunable tail harmonic filter

Abstract

Disclosed is a voltage-controlled oscillator (VCO) including at least an inductor-capacitor (LC) resonant circuit (including varactors that receive a variable input voltage), cross-coupled transistors connected to the LC resonant circuit, and an LC filter connected to a shared source node of the cross-coupled transistors. The cross-coupled transistors can have back gates connected to receive a variable back gate bias voltage (Vbg), which is dependent on Vin to ensure that an optimal relationship between the oscillating frequency (ω) of the LC resonant circuit and the resonant frequency (ω) of the LC filter is continuously maintained to minimize phase noise. For example, if Vin is increased to increase varactor capacitance and, thereby decrease ω, then Vbg is also increased, thereby increasing the voltage (Vs-s) and the capacitance (Cs-s) on the shared source node connected to the LC filter, decreasing ω, and maintaining an optimal relationship of ω=ω/2.