Inductive-capacitive (LC) voltage controlled oscillator (VCO) having tuning range controlled by a digital-to-analog converter (DAC) with programmable tail current
View Patent ↗A device includes an inductive-capacitive voltage controlled oscillator (LC-VCO) having a tank circuit and programmable tail current, and a control circuit configured to adjust the tail current based on an amount of capacitance provided to the tank circuit.
1. A device, comprising:
a programmable digital-to-analog converter configured to generate a tail current;
an inductive-capacitive voltage controlled oscillator (LC-VCO) having a tank circuit, the tank circuit comprising:
a center-tapped inductor configured to receive the tail current from the programmable digital-to-analog converter; and
a pair of voltage-controlled capacitors coupled in parallel with the center-tapped inductor, the pair of voltage-controlled capacitors having a control voltage coupled into a node between the pair of voltage-controlled capacitors for fine tuning the tank circuit; and
a control circuit configured to provide to the programmable digital-to-analog converter a digital control signal for adjusting the tail current, wherein the digital control signal is based on an output of the LC-VCO.
2. The device of claim 1 , further comprising:
a switchable capacitive network coupled to an output of the tank circuit.
3. The device of claim 2 , wherein the switchable capacitive network is used to coarse-tune the oscillation frequency of the tank circuit.
4. The device of claim 2 , wherein the switchable capacitive network is used to increase a tuning range of the LC-VCO by lowering a low frequency operating point of the LC-VCO.
5. The device of claim 4 , wherein the switchable capacitive network does not alter a high frequency operating point of the LC-VCO when increasing the tuning range of the LC-VCO.
6. The device of claim 5 , wherein the switchable capacitive network comprises switchable metal-insulator-metal (MIM) capacitors.
7. A method, comprising:
using a programmable digital-to-analog converter to generate a tail current;
generating an oscillation frequency in an inductive-capacitive voltage controlled oscillator (LC-VCO) by providing the tail current to a center-tapped inductor of a tank circuit;
fine tuning the tank circuit by varying a control voltage that is provided at a node between a pair of voltage-controlled capacitors connected in parallel with the center-tapped inductor; and
adjusting the tail current based on an amount of capacitance that is coupled to an output of the LC-VCO.
8. The method of claim 7 , further comprising increasing the tail current as the amount of capacitance coupled to the output of the LC-VCO is increased.
9. The method of claim 7 , further comprising using a switchable capacitive network to alter the amount of capacitance that is coupled to the output of the LC-VCO.
10. The method of claim 9 , further comprising:
using the switchable capacitive network to increase a tuning range of the LC-VCO by lowering a low frequency operating point of the LC-VCO.
11. The method of claim 10 , wherein the switchable capacitive network does not alter a high frequency operating point of the LC-VCO when increasing the tuning range of the LC-VCO.
12. The method of claim 11 , further comprising implementing the switchable capacitive network using switchable metal-insulator-metal (MIM) capacitors.
13. A device, comprising:
an inductive-capacitive voltage controlled oscillator (LC-VCO) having a tank circuit, the tank circuit comprising:
a center-tapped inductor that is provided a tail current; and
a pair of voltage-controlled capacitors coupled in parallel with the center-tapped inductor, the pair of voltage-controlled capacitors having a control voltage coupled into a node between the pair of voltage-controlled capacitors for fine tuning the tank circuit;
a switchable capacitive network coupled to an output of the tank circuit for coarse tuning the tank circuit; and
a programmable digital-to-analog converter configured to generate the tail current in accordance with the coarse tuning of the tank circuit.
14. The device of claim 13 , wherein the tail current is increased as an amount of capacitance provided by the switchable capacitive network to the tank circuit is increased.
15. The device of claim 14 , wherein increasing the tail current increases a tuning range of the LC-VCO by lowering a low frequency operating point of the LC-VCO without altering a high frequency operating point of the LC-VCO.
16. The device of claim 1 , wherein the tail current is provided to the center-tapped inductor through a center tap of the center-tapped inductor.
17. The device of claim 16 , further comprising a differential amplifier that includes a first transistor cross-coupled to a second transistor, a drain terminal of the first transistor connected to a first end of the center-tapped inductor and a drain terminal of the second transistor connected to an opposing end of the center-tapped inductor.
18. The method of claim 7 , further comprising:
coarse tuning the tank circuit by varying the amount of capacitance that is coupled to the output of the LC-VCO.