Granted Patent B2

US 7,548,128 · App. 11/612,666 · Granted Jun 16, 2009

Abstract

Claims (34)

1. A system comprising:

a crystal oscillator circuit; and

a digital automatic level control circuit operative to: convert an oscillation amplitude of the crystal oscillator circuit from a differential output signal to a proportional DC voltage; convert the DC voltage to a corresponding digital code representation; and adjust bias current and oscillator loop gain such that a desired oscillation amplitude is set, and

wherein the crystal oscillator circuit comprises a differential crystal oscillator that includes

first and second NMOSFET transistors with a programmable tail current source electrically connected as a differential pair;

an RLC tank electrically connected as a load;

a crystal electrically connected in series mode and operative as a filter with high quality factor; and

a capacitance transformer operative to block DC bias from the crystal and to couple the RLC tank to the crystal.

2. The system of claim 1 , wherein the series mode crystal is a fundamental crystal.

3. The system of claim 1 , wherein the series mode crystal is an overtone crystal.

4. The system of claim 1 , wherein:

the system comprises a broadband chip set;

the crystal oscillator circuit and the digital automatic level control circuit form a portion of the broadband chip set; and

the RLC tank is an off-chip RLC tank.

5. A system comprising:

a crystal oscillator circuit comprising:

means for limiting an oscillation amplitude of the circuit to a programmable level; and

means for providing increased gain at startup such that drive level dependency variation in a crystal of the circuit is compensated;

means for digital automatic level control comprising:

means for converting the oscillation amplitude of the crystal oscillator circuit from a differential output signal to a proportional DC voltage.

6. The system of claim 5 , further comprising means for providing differential output signals with reduced reference spurs.

7. The system of claim 5 , wherein the means for digital automatic level control comprises:

means for converting the proportional DC voltage to a corresponding digital code representation; and

means for adjusting a bias current and an oscillator loop gain such that a desired oscillation amplitude is set.

8. The system of claim 7 , wherein the means for digital automatic level control further comprises means for rectifying the oscillation amplitude into the proportional DC voltage.

9. The system of claim 7 , wherein the means for digital automatic level control further comprises means for converting the proportional DC voltage into the corresponding digital code representation.

10. The system of claim 7 , wherein the means for digital automatic level control further comprises a means for selecting the bias current and the oscillator loop gain corresponding to the desired oscillation amplitude.

11. The system of claim 7 , further comprising a means for increasing gain at startup such that drive level dependency variation in a crystal of the crystal oscillator circuit is compensated.

12. The system of claim 7 , further comprising a means for filtering signals of a crystal of a differential crystal oscillator circuit with an RLC tank electrically connected as a load.

13. A method for reducing reference spurs in an output of a crystal oscillator circuit, the method comprising:

converting an oscillation amplitude of the crystal oscillator circuit from a differential output signal to a proportional DC voltage;

converting the proportional DC voltage to a corresponding digital code representation;

adjusting bias current and oscillator loop gain such that a desired oscillation amplitude is set; and

filtering signals of a crystal of a differential crystal oscillator circuit with an RLC tank electrically connected as a load.

Assignments (9)