Method and apparatus for generating a variable capacitance for synthesizing high-frequency signals for wireless communications

• US 20020034932A1
• Filed: 10/09/2001
• Published: 03/21/2002
• Est. Priority Date: 05/29/1998
• Status: Active Grant

• Current Assignee: Silicon Laboratories Incorporated
• Original Assignee: Silicon Laboratories Incorporated
• Inventors: Welland, David R.
• Granted Patent: US 6,760,575 B2
• Time in Patent Office: Days
• US Class Current: 455/260
• CPC Class Codes: H03B 21/02 by plural beating, i.e. for frequency synthesis ; Beating in combination with multiplication or division of frequency digital frequency synthesis using a ROM G06F1/02; digital frequency synthesis in general H03K; indirect frequency synthesis using a PLL H03L7/16
• CPC Class Codes: H03J 2200/10 Tuning of a resonator by means of digitally controlled capacitor bank
• CPC Class Codes: H03L 7/099 concerning mainly the controlled oscillator of the loop
• CPC Class Codes: H03L 7/113 using frequency discriminator

Abstract

A method and apparatus for synthesizing high-frequency signals, such as wireless communication signals, includes a phase-locked loop (PLL) frequency synthesizer with a variable capacitance voltage controlled oscillator (VCO) that has a discretely variable capacitance in conjunction with a continuously variable capacitance. The discretely variable capacitance may provide coarse tuning adjustment of the variable capacitance to compensate for capacitor and inductor tolerances and to adjust the output frequency to be near the desired frequency output. The continuously variable capacitance may provide a fine tuning adjustment of the variable capacitance to focus the output frequency to match precisely the desired frequency output. The continuously variable capacitance may be formed by using a plurality of separate capacitance circuits. The individual capacitance circuits may include two capacitors coupled to a variable resistance element. The variable resistance element may be a transistor controlled by an analog control voltage. The total capacitance of the continuously variable capacitance may be substantially linear with respect to the phase of the VCO output while the individual capacitance circuits exhibit nonlinear behavior.

Claims

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

a variable capacitance circuit integrated within the frequency synthesizer, the variable capacitance circuit comprising,

a first capacitor and a second capacitor coupled in series between a first node and a second node,

a control node coupled between the first and second capacitors, and

a variable resistance element coupled between the control node and the second node; and

a variable control signal coupled to the variable resistance element.

2. A method of operating a wireless communication frequency synthesizer, comprising:

providing a capacitance circuit integrated within the frequency synthesizer, the capacitance circuit comprising a first capacitor and a second capacitor coupled in series between a first node and a second node, a third node being between the first and second capacitors;

providing a variable resistance element coupled to the third node;

generating a variable control signal; and

controlling the resistance of the variable resistance element with the variable control signal to vary the capacitance of the capacitance circuit.

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

a variable capacitance circuit integrated within the frequency synthesizer, the variable capacitance circuit comprising,

a first capacitor coupled between a first node and a second node,

a control node coupled between the first and second nodes, and

a variable resistance element coupled between the control node and the second node; and

an analog variable control signal coupled to the variable resistance element.

4. A method of operating a wireless communication frequency synthesizer, comprising:

providing a capacitance circuit integrated within the frequency synthesizer, the capacitance circuit comprising a first capacitor coupled between a first node and a second node, a third node being between the first and second nodes;

providing a variable resistance element coupled to the third node;

generating a variable analog control signal; and

controlling the resistance of the variable resistance element with the variable analog control signal to vary the capacitance of the capacitance circuit.

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

a frequency output; and

a substantially linear variable capacitance circuit integrated within the frequency synthesizer, the variable capacitor comprising,

a plurality of nonlinear capacitance circuits, at least some of the capacitance circuits controlled by at least one variable control signal;

wherein the substantially linear variable capacitance circuit has a substantially linear relationship between the phase of the frequency output and the capacitance of the linear variable capacitance circuit.

6. A method of operating a wireless communication frequency synthesizer, comprising:

providing an integrated variable capacitance circuitry, the variable capacitance circuitry formed from a plurality of capacitance circuits;

adjusting a capacitance of each of the plurality of capacitance circuits in a nonlinear manner; and

adjusting the output frequency of the synthesizer by varying a total capacitance of the variable capacitance circuitry in a substantially linear manner.