Voltage-controlled oscillator with improved tuning curve linearization

1. A voltage-controlled oscillator circuit comprising:

a gain element;

an LC resonator coupled with the gain element, the LC resonator including an inductor section and a capacitor section, the capacitor section having at least two branches connected in parallel and an analog voltage control input coupled to the at least two branches for tuning the LC resonator to adjust an output frequency of the VCO, wherein at least one branch of the at least two branches is DC-coupled, at least one other branch of the at least two branches is AC-coupled and any of the remaining branches of the at least two branches is selected from the group of DC-coupled and AC-coupled,

wherein an analog voltage control signal applied to the analog voltage control input is received by each branch.

2. The voltage-controlled oscillator circuit of claim 1 , wherein the at least two branches comprise one DC-coupled branch and one AC-coupled branch.

3. The voltage-controlled oscillator circuit of claim 1 , wherein the remaining branches of the at least two branches comprise at least two AC-coupled branches, each AC-coupled branch comprising a first coupling capacitor connected in series to a first end of a first varactor, a second end of the first varactor connected in series to a first end of a second varactor, a second end of the second varactor connected in series to a second coupling capacitor, a first resistor connected between a bias voltage and a first node between the first coupling capacitor and the first end of the first varactor, a second resistor connected between the bias voltage and a second node between the second coupling capacitor and the second end of the second varactor, the voltage control input connected to a node between the first varactor and the second varactor.

4. The voltage-controlled oscillator circuit of claim 3 , wherein the bias voltage is different for at least one of the at least two AC-coupled branches.

5. The voltage-controlled oscillator circuit of claim 3 , wherein the bias voltage for each of the at least two AC-coupled branches is selected to provide a tuning curve of the voltage-controlled oscillator circuit that is approximately linear.

6. The voltage-controlled oscillator circuit of claim 3 , wherein a characteristic of the first varactor and second varactor is different for at least one of the at least two AC-coupled branches.

7. The voltage-controlled oscillator circuit of claim 3 , wherein a characteristic of the first varactor and second varactor for each of the at least two AC-coupled branches is selected to provide a tuning curve of the voltage-controlled oscillator circuit that is approximately linear.

8. The voltage-controlled oscillator circuit of claim 1 , wherein the at least two branches comprise at least two DC-coupled branches, each DC-coupled branch comprising a pair of varactors connected in series, the voltage control input connected to a node between the pair of varactors, wherein a characteristic of the pair of varactors is different for at least one of the at least two DC-coupled branches.

9. The voltage-controlled oscillator circuit of claim 8 , wherein the characteristic of the pair of varactors for each of the at least two DC-coupled branches is selected to provide a tuning curve of the voltage-controlled oscillator circuit that is approximately linear.

10. The voltage-controlled oscillator circuit of claim 1 , further comprising a current source coupled to the LC resonator.

11. The voltage-controlled oscillator circuit of claim 1 , wherein the gain element comprises a cross-coupled transistor pair.