Oscillator with differential tunable tank circuit

• US 6,650,195 B1
• Filed: 07/30/2001
• Issued: 11/18/2003
• Est. Priority Date: 07/30/2001
• Status: Expired due to Term

• Current Assignee: Xilinx, Inc. (Advanced Micro Devices, Inc.)
• Original Assignee: Xilinx, Inc. (Advanced Micro Devices, Inc.)
• Inventors: Brunn, Brian Taylor, Harjani, Ramesh
• Primary Examiner(s): Tokar, Michael
• Assistant Examiner(s): Mai, Lam T.
• Application Number:
  US09/918,752
• Time in Patent Office: 841 Days
• Field of Search: 331/117.R, 331/116.FE, 331/115, 331/177.V, 331/167, 331/172
• US Class Current: 331/177.00V
• CPC Class Codes: H03B 5/1215 the current source or degeneration circuit being in common to both transistors of the pair, e.g. a cross-coupled long-tailed pair
• CPC Class Codes: H03B 5/1228 the amplifier comprising one or more field effect transistors
• CPC Class Codes: H03B 5/1243 the means comprising voltage variable capacitance diodes
• CPC Class Codes: H03L 2207/06 Phase locked loops with a controlled oscillator having at least two frequency control terminals
• CPC Class Codes: H03L 7/0891 the up-down pulses controlling source and sink current generators, e.g. a charge pump
• CPC Class Codes: H03L 7/099 concerning mainly the controlled oscillator of the loop
• CPC Class Codes: H03L 7/18 using a frequency divider or counter in the loop H03L7/20, H03L7/22 take precedence

Abstract

Many electronic devices, such as computers and printers, communicate data to each other over wireline or wireless communications links. One component vital to such communications is a voltage-controlled oscillator (VCO)—a circuit that outputs an oscillating signal having an oscillation frequency based on a control voltage. Conventional VCOs adjust frequency based on a single control voltage input, which makes them vulnerable to unintended changes in the control voltage (and power-supply voltages relative to the control voltage.) These voltage changes cause frequency deviations that can make communications between devices less reliable. Accordingly, the present inventors devised a VCO that includes differential frequency control—frequency control based on the difference of two control voltages. The VCO, which uses a differentially tunable impedance, rejects common-mode noise (undesirable voltage variations that affect both control voltages) and thus bolsters reliability of communication circuits, such as phase-lock loops, receivers, transmitters, transceivers, and other devices that use it

Claims

1. An oscillator comprising:

first and second supply nodes;

a differentially tunable tank circuit;

a first negative-resistance circuit coupled between the tank circuit and the first supply node;

a second negative-resistance circuit coupled between the tank circuit and the second supply node;

a current source coupled between the first negative-resistance circuit and the first supply node; and

a current sink coupled between the second negative-resistance circuit and the second supply node.

2. The oscillator of claim 1, wherein the differentially tunable tank circuit includes:

an inductance; and

a differentially tunable capacitance coupled to the inductance.

3. The oscillator of claim 2, wherein the oscillator includes first and second outputs and the differentially tunable capacitance comprises:

first and second control inputs;

first and second varactors coupled respectively between the first output and the first control input and between the second output and the first control input; and

third and fourth varactors coupled respectively between the first output and the second control input and between the second output and the second control input.

4. The oscillator of claim 3, wherein each of the varactors have positive and negative nodes, with the positive nodes of the first and second varactors coupled to the first control input and the negative nodes of the third and fourth varactors coupled to the second control node.

5. The oscillator of claim 3, wherein the inductance is coupled in parallel to the differentially tunable capacitance.

6. The oscillator of claim 3, wherein the inductance comprises a center-tapped inductance having first and second end nodes and a center tap, with the first and second end nodes coupled respectively to the first and second outputs, and the center tap coupled to receive a common-mode control voltage.

7. The oscillator of claim 1, wherein the first negative-impedance circuit comprises a first differential cross-coupled pair of transistors and the second negative-impedance circuit comprises a second differential cross-coupled pair of transistors.

8. The oscillator of claim 7, wherein the first differential cross-coupled pair of transistors consists of transistors of a first size and the second differential cross-coupled pair of transistors consists of transistors of a second size different from the first.

9. The oscillator of claim 1, further comprising a bias-current-control input coupled to the current source and the current sink.

10. A phase-locked loop comprising:

a phase-detector having a first and second inputs and first and second outputs;

a charge pump coupled to first and second outputs of the phase detector;

a common-mode voltage circuit coupled to the charge pump;

the oscillator of claim 1 coupled to the common-mode voltage circuit;

a frequency divider coupled between first and second outputs of the tunable oscillator and the first and second inputs of the phase detector.

11. A transceiver comprising the phase-locked loop of claim 10.

12. A programmable integrated circuit comprising the transceiver of claim 11.

13. An oscillator comprising:

a tank circuit including an inductance and a differentially tunable capacitance coupled to the inductance;

a negative-impedance circuit coupled to the tank circuit; and

first and second outputs;

wherein the differentially tunable capacitance comprises;

first and second control inputs;

first and second varactors coupled respectively between the first output and the first control input and between the second output and the first control input; and

third and fourth varactors coupled respectively between the first output and the second control input and between the second output and the second control input.

14. The oscillator of claim 13, wherein the negative-impedance circuit comprises a differential cross-coupled pair of transistors.

15. A phase-locked loop comprising:

a phase-detector having a first and second inputs and first and second outputs;

a charge pump coupled to first and second outputs of the phase detector;

a common-mode voltage circuit coupled to the charge pump;

the oscillator of claim 13, coupled to the common-mode voltage circuit;

a frequency divider coupled between first and second outputs of the tunable oscillator and the first and second inputs of the phase detector.

16. A transceiver comprising the phase-locked loop of claim 15.

17. A programmable integrated circuit comprising the transceiver of claim 16.

19. The oscillator of claim 14, further comprising:

a first negative impedance circuit coupled between the first supply node and the differentially tunable tank circuit; and

a second negative impedance circuit coupled between the second supply node and the differentially tunable tank circuit.

18. An oscillator comprising:

first and second supply nodes;

first and second differential tuning inputs for receiving first and second tuning voltages;

a differentially tunable tank circuit having first and second output nodes coupled between the first and second supply nodes and to the first and second differential turning inputs, with the tank circuit including;

a differentially tunable impedance means for providing an impedance based on a difference between the first and second tuning voltages.

20. The oscillator of claim 18, further comprising:

a current source coupled between the first supply node and the differentially tunable tank circuit; and

a current sink coupled between the second supply node and the differentially tunable tank circuit.

21. The oscillator of claim 18, further comprising a common-mode control input, coupled to the differentially tunable impedance means, for receiving a common-mode control voltage.

22. An oscillator comprising:

first and second supply nodes;

a tank circuit;

a current source coupled between the tank circuit and the first supply node; and

a current sink coupled between the tank circuit and the second supply node.

23. The oscillator of claim 22, wherein the tank circuit comprises a differentially tunable impedance means for providing an impedance based on a difference between first and second tuning voltages.

24. A transceiver comprising the oscillator of claim 22.

25. A tunable capacitance circuit comprising:

first and second nodes;

first and second control inputs for receiving first and second complementary control voltages;

first and second varactors coupled respectively between the first node and the first control input and between the second node and the first control input; and

third and fourth varactors coupled respectively between the first node and the second control input and between the second node and the second control input.

26. The tunable capacitance circuit of claim 25, wherein each of the varactors have positive and negative nodes, with the positive nodes of the first and second varactors coupled to the first control input and the negative nodes of the third and fourth varactors coupled to the second control node.

27. A method of tuning an oscillation frequency of an oscillator having a total capacitance, comprising:

providing first and second control signals;

changing a first portion of the total capacitance based on the first control signal; and

changing a second portion of the total capacitance based on the second control signal; and

wherein changing the first portion based on the first control signal comprises changing the bias voltage on a varactor.

28. The method of claim 27, wherein changing the second portion based on the second control signal comprises changing the bias voltage on a varactor.

29. The method of claim 27, wherein the first and second control signals are complementary.

30. A method of operating an oscillator having a total capacitance, the method comprising:

changing a first portion of the total capacitance in response to a noise signal; and

offsetting at least a portion of the change in the first portion of the total capacitance; and

wherein offsetting at least a portion of the change in the first portion of the total

capacitance comprises increasing or decreasing a second portion of the total capacitance in response to the noise signal.

31. The method of claim 30, wherein offsetting at least a portion of the change in the first portion comprises offsetting substantially all of the change in the first portion.