Method and apparatus for providing coarse and fine tuning control for synthesizing high-frequency signals for wireless communications

• US 6,388,536 B1
• Filed: 06/27/2000
• Issued: 05/14/2002
• Est. Priority Date: 05/29/1998
• Status: Expired due to Term

• Current Assignee: Silicon Laboratories Incorporated
• Original Assignee: Silicon Laboratories Incorporated
• Inventors: Welland, David R.
• Primary Examiner(s): KINKEAD, ARNOLD M
• Application Number:
  US09/604,228
• Time in Patent Office: 686 Days
• Field of Search: 331/117.R, 331/36.C, 331/177.V, 331/167, 331/177.R, 327/107, 327/159, 455/260, 455/262
• US Class Current: 331/177.00R
• CPC Class Codes: H03B 5/1212 the amplifier comprising a pair of transistors, wherein an output terminal of each being connected to an input terminal of the other, e.g. a cross coupled pair
• CPC Class Codes: H03B 5/1228 the amplifier comprising one or more field effect transistors
• CPC Class Codes: H03B 5/1265 switched capacitors
• CPC Class Codes: H03D 7/165 at least two frequency changers being located in different paths, e.g. in two paths with carriers in quadrature combined with amplitude demodulation H03D1/2245, combined with angle demodulation H03D3/007; N-path filters H03H19/002
• CPC Class Codes: H03L 7/087 using at least two phase detectors or a frequency and phase detector in the loop
• 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 7/113 using frequency discriminator
• CPC Class Codes: H03L 7/187 using means for coarse tuning the voltage controlled oscillator of the loop H03L7/191 - H03L7/195 take precedence
• CPC Class Codes: H03L 7/199 with reset of the frequency divider or the counter, e.g. for assuring initial synchronisation

Abstract

A method and apparatus for synthesizing high-frequency signals is disclosed that overcomes integration problem associated with prior implementations while meeting demanding phase noise and other impurity requirements. In one embodiment, a phase-locked loop (PLL) frequency synthesizer is disclosed having a voltage controlled oscillator (VCO) with a variable capacitance that includes a discretely variable capacitance in conjunction with a continuously variable capacitance. The discretely variable capacitance may provide coarse tuning adjustment of the variable capacitance, and the continuously variable capacitance may provide a fine tuning adjustment of the variable capacitance. In a more general terms, a frequency synthesizer is disclosed having a first variable and a second capacitance circuits and frequency control circuitry to coarsely tune the output frequency by adjusting the first control signal and to finely tune the output frequency by adjusting the second control signal.

Claims

1. A frequency synthesizer having at least one variable frequency output, the output frequency varying as a capacitance within the synthesizer varies, comprising:

a discretely variable capacitance circuit integrated within the frequency synthesizer and having a first control signal, the first control signal determining which of a plurality of discrete capacitance amounts are included within an overall capacitance for the discretely variable capacitance circuit;

a continuously variable capacitance circuit integrated within the frequency synthesizer and having a second control signal, the capacitance amount for the continuously variable capacitance circuit comprising a plurality of continuous capacitance amounts from a plurality of capacitance circuits, and the second control signal comprising a plurality of variable control signals that determine how much capacitance from each of the plurality of capacitance circuits are included within an overall capacitance for the continuously variable capacitance circuit, such that a relatively linear circuit behavior for the continuously variable capacitance circuit is achieved over a selected frequency range; and

frequency control circuitry to coarsely tune the output frequency by adjusting the first control signal and to finely tune the output frequency by adjusting the second control signal.

2. The frequency synthesizer of claim 1, wherein the second control signal is configured to be set to an initial value while the first control signal is being adjusted to coarsely tune the output frequency.

3. The frequency synthesizer of claim 2, wherein the initial value is at about the middle of a capacitance range for the continuously variable capacitance circuit.

4. The frequency synthesizer of claim 1, wherein the first control signal comprises a multiple-bit digital control signal, and the second control signal comprises a plurality of continuously variable signals.

5. The frequency synthesizer of claim 1, wherein the discretely and continuous variable capacitance circuits provide a variable capacitance for an LC tank oscillator.

6. The frequency synthesizer of claim 5, wherein the frequency synthesizer comprises a phase-locked loop circuit.

7. The frequency synthesizer of claim 1, wherein the first control signal is adjusted by the frequency control circuitry until the output frequency of the frequency synthesizer is within a first selected amount of a desired output frequency.

8. The frequency synthesizer of claim 7, wherein the first selected amount is within about 0.1 percent of the desired output frequency.

9. The frequency synthesizer of claim 7, wherein the second control signal is adjusted by the frequency control circuitry to calibrate the output frequency to about the desired output frequency.

10. The frequency synthesizer of claim 9, wherein the second control signal is further adjusted by the frequency control circuitry to compensate for post-calibration drift of the output frequency.

11. The frequency synthesizer of claim 10, wherein the second control signal is adjustable to compensate for a post-calibration drift within a range of about 1 percent of the desired output frequency.

12. A controlled oscillator, the output frequency varying as a capacitance within the controlled oscillator varies, comprising:

a discretely variable capacitance circuit integrated within the frequency synthesizer and having a first control signal, the first control signal determining which of a plurality of discrete capacitance amounts are included within an overall capacitance for the discretely variable capacitance circuit;

a continuously variable capacitance circuit integrated within the frequency synthesizer and having a second control signal, the capacitance amount for the continuously variable capacitance circuit comprising a plurality of continuous capacitance amounts from a plurality of capacitance circuits, and the second control signal comprising a plurality of variable control signals that determine how much capacitance from each of the plurality of capacitance circuits are included within an overall capacitance for the continuously variable capacitance circuit, such that a relatively linear circuit behavior for the second capacitance circuit is achieved over a selected frequency range; and

wherein the capacitance of the controlled oscillator is dependent upon a combination of the variable capacitance circuits.

13. The controlled oscillator of claim 12, wherein the capacitance amount for the discretely variable capacitance circuit is greater than the capacitance amount for the continuously variable capacitance circuit.

14. The controlled oscillator of claim 12, wherein the discretely variable capacitance circuit comprises a plurality of capacitor circuits with each capacitor circuit including a capacitance amount that may be selectively included or excluded from a total capacitance amount for the discretely variable capacitance circuit.

15. The controlled oscillator of claim 12, wherein the first control signal comprises a multiple-bit digital control signal, and wherein each capacitor circuit includes a control node coupled to receive one bit from the multiple-bit digital control signal to selectively include or exclude the capacitance amount.

16. The controlled oscillator of claim 12, wherein the second control signal is configured to be set to an initial value while the first control signal is being adjusted.

17. A The controlled oscillator of claim 12, wherein each of said plurality of capacitance circuits includes a control node coupled to receive one of the plurality of variable control signals.

18. A method of operating a frequency synthesizer, comprising:

providing a discretely variable capacitance circuit integrated within the frequency synthesizer and having a first control signal, the first control signal determining which of a plurality of discrete capacitance amounts are included within an overall capacitance for the discretely variable capacitance circuit;

providing a continuously variable capacitance circuit integrated within the frequency synthesizer and having a second control signal, the capacitance amount for the continuously variable capacitance circuit comprising a plurality of continuous capacitance amounts from a plurality of capacitance circuits, and the second control signal comprising a plurality of variable control signals that determine how much capacitance from each of the plurality of capacitance circuits are included within an overall capacitance for the continuously variable capacitance circuit, such that a relatively linear circuit behavior for the continuously variable capacitance circuit is achieved over a selected frequency range;

coarsely tuning the output frequency by adjusting the first control signal; and

finely tuning the output frequency by adjusting the second control signal.

19. The method of claim 18, wherein the discretely variable capacitance circuit and the continuously variable capacitance circuit provide variable capacitance for an LC tank oscillator.

20. The method of claim 18, wherein the coarsely tuning step comprises adjusting the first control signal until the output frequency of the frequency synthesizer is within a first selected amount a of a desired output frequency.

21. The method of claim 20, wherein the first selected amount is within about 0.1 percent of the desired output frequency.

22. The method of claim 20, wherein the finely tuning step comprises adjusting the second control signal to calibrate the output frequency to about a desired output frequency.

23. The method of claim 22, further comprising compensating for drift of the output frequency by further adjusting the second control signal.

24. The method of claim 23, wherein the second control signal is adjustable to compensate for drift within a range of about 1 percent of a desired output frequency.

25. The method of claim 18, wherein the second control signal is configured to be set to an initial value while the first control signals being adjusted to coarsely tune the output frequency.

26. The method of claim 25, wherein the initial value is at about the middle of the capacitance range for the continuously variable capacitance circuit.

27. A method for calibrating an output frequency of a controlled oscillator, the output frequency varying as a capacitance within the controlled oscillator varies, comprising:

providing a discretely variable capacitance circuit integrated within the frequency synthesizer and having a first control signal, the first control signal determining which of a plurality of discrete capacitance amounts are included within an overall capacitance for the discretely variable capacitance circuit;

adjusting the first control signal to vary a capacitance of the discretely variable capacitance circuit and thereby vary the output frequency by a first amount;

providing a continuously variable capacitance circuit integrated within the frequency synthesizer and having a second control signal, the capacitance amount for the continuously variable capacitance circuit comprising a plurality of continuous capacitance amounts from a plurality of capacitance circuits, and the second control signal comprising a plurality of variable control signals that determine how much capacitance from each of the plurality of capacitance circuits are included within an overall capacitance for the continuously variable capacitance circuit, such that a relatively linear circuit behavior for the continuously variable capacitance circuit is achieved over a selected frequency range; and

adjusting the second control signal to vary a capacitance of the continuously variable capacitance circuit and thereby vary the output frequency by a second amount.

28. The method of claim 27, wherein the first amount is greater than the second amount.

29. The method of claim 27, wherein the first control signal comprises a multiple-bit digital control signal.

30. The method of claim 27, further comprising compensating for drift in the output frequency by adjusting the second control signal.

31. The method of claim 27, wherein the second control signal is configured to be set to an initial value while the first control signal is being adjusted to coarsely tune the output frequency.

32. The method of claim 31, wherein the initial value is at about the middle of the capacitance range for the continuously variable capacitance circuit.