Differential LC voltage-controlled oscillator

A differential voltage-controlled oscillator (VCO) employs a pair of accumulation-mode varactors driven with a differential control voltage to generate a differential oscillating waveform. The differential control voltage is formed from a pair of level-shifted input differential control voltage components. Level shifting of the input control voltages and driving the varactors with a differential control voltage allows for biasing of the varactors over a substantial range of capacitance variation. Such differential VCO may be employed within a phase-locked loop (PLL) circuit, with the pair of input control voltages being provided by the loop filter of the PLL circuit. The differential VCO comprises a differential control voltage to voltage converter (CV2VC) coupled to an LC-tank VCO. To improve common-mode noise rejection of the LC-VCO, the inductors of the LC-tank may be AC-coupled to the supply voltage, and the output differential oscillating waveform may be AC-coupled to the LC-tank through capacitors.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:

FIG. 1 shows an integrated differential LC voltage-controlled oscillator (VCO) of the prior art employing accumulation-mode varactors and operating with an output frequency above 1 GHz;

FIG. 2 shows a block diagram of an exemplary VCO circuit in accordance with an embodiment of the present invention;

FIG. 3 shows a schematic diagram of an exemplary circuit implementation for the differential control voltage to voltage converter (CV2VC) circuit of FIG. 2;

FIG. 4 shows exemplary characteristic curves of capacitance versus differential gate voltage for different operating conditions of an accumulation-mode varactor that may be employed in the differential VCO circuit of FIG. 2;

FIG. 5 shows a schematic diagram of an exemplary circuit implementation for the LC-VCO circuit of FIG. 2;

FIGS.

6(A,B) shows a schematic diagram of an implementation of the differential CV2VC shown in FIG. 3;

FIGS.

7(A,B) shows a schematic diagram of an implementation of the LC-VCO shown in FIG. 5;

FIG. 8 is a graph of signal level versus time illustrating the overall AC gain for the differential control voltage passing through the differential CV2VC of FIG. 6;

FIG. 9 is a graph of signal level versus time illustrating the overall AC gain for the differential control voltage appearing at different circuit element nodes of differential CV2VC of FIG. 6;

FIG. 10 shows AC gain versus frequency showing signals of differential CV2VC of FIG. 6;

FIG. 11 shows the single-ended waveforms for VCPI and VCNI input to differential CV2VC of FIG. 6; and

FIG. 12 shows the single-ended waveforms for VCPO and VCNO output from differential CV2VC of FIG.

6.

1. A voltage-controlled oscillator comprising:

2. A voltage-controlled oscillator (VCO), comprising:

3. The VCO as recited in claim 2, wherein each varactor includes 1) a common terminal where the pair of back-to-back varactors are coupled back-to-back, the common terminal coupled to one component of the differential control voltage, and 2) a separate terminal coupled to a complement component of the differential control voltage.

4. The VCO as recited in claim 3, wherein the inductance of the LC-tank circuit includes a pair of inductors, each inductor coupled between a source voltage driving the differential amplifier and the separate terminal of a corresponding varactor.

5. The VCO as recited in claim 4, wherein each inductor is coupled between the source voltage and the separate terminal of a corresponding varactor with a capacitor.

6. The VCO as recited in claim 3, wherein the oscillating waveform includes a pair of differential oscillation components, each differential oscillation component being AC-coupled to a corresponding output terminal through a capacitor.

7. The VCO as recited in claim 2, wherein the differential amplifier comprises a pair of cross-coupled transistors.

8. The VCO as recited in claim 2, wherein the gain and the DC level is set to adjust a DC bias point across each varactor so as to increase an available range of capacitance for the varactor.

9. The VCO as recited in claim 2, wherein the voltage-to-voltage converter comprises a voltage-follower amplifier, a first differential amplifier stage, and a second differential amplifier stage, wherein:

10. The VCO as recited in claim 9, wherein each of the first and second differential amplifier stages is coupled to a corresponding voltage driver pair of transistors to isolate the differential amplifier stage and drive its differential output signal.

11. The VCO as recited in claim 2, wherein each varactor is either i) an accumulation mode NMOS varactor, ii) a depletion mode NMOS varactor, iii) a bipolar Base-Emitter Junction varactor, iv) a bipolar Base-Collector Junction varactor, v) a P+ to Ntub Junction varactor, or vi) a N+ to Ptub Junction varactor.

12. The VCO as recited in claim 2, wherein the circuit is embodied in an integrated circuit.

13. A method of generating an oscillation waveform with a voltage controlled oscillator (VCO) comprising the steps of:

14. The method as recited in claim 13, wherein step (a) comprises the step of:

wherein step (b) comprises the step of: