Crystal oscillator

• US 6,337,604 B1
• Filed: 05/12/2000
• Issued: 01/08/2002
• Est. Priority Date: 05/14/1999
• Status: Expired due to Term

• Current Assignee: Infineon Technologies AG
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Clarke, David
• Primary Examiner(s): MIS, DAVID C
• Application Number:
  US09/569,857
• Time in Patent Office: 606 Days
• Field of Search: 331/116.R, 331/116.FE, 331/117.R, 331/117.FE, 331/158, 331/177.V, 331/179
• US Class Current: 331/116.00R
• CPC Class Codes: H03B 5/366 and comprising means for varying the frequency by a variable voltage or current
• CPC Class Codes: H03J 2200/10 Tuning of a resonator by means of digitally controlled capacitor bank

Abstract

A crystal oscillator comprises a crystal 1 and a crystal oscillator circuit 3 for providing an output signal 5. In order to adjust the output frequency, for example during the testing stage of production, a bank of load capacitors C1 to C6 are provided. Each capacitor C1 to C6 may be switched into or out of the circuit by a respective switch 9 to 19. The switches are controlled by control lines D0 to D5, which are set by a 6 bit register 21, known as the trim register. The value in the trim register is set according to the frequency of the output signal. To counteract any swing in the output signal, a bank of current switches T1 to T4 are provided, which enable resistors R1 to R4 to be switched into or out of the circuit. Each resistor R1 to R4 is switched by a respective one of the control lines D2 to D5. Thus, adjusting the load capacitance to obtain a desired frequency causes the oscillator core current to be changed automatically, thereby maintaining a constant swing in the output signal.

Claims

1. A crystal oscillator circuit comprising:

an adjustable load capacitance including a bank of capacitors;

first switches switching the capacitors into or out of the circuit to obtain a particular centre frequency;

a bank of resistors; and

second switches switching the resistors into or out of the circuit in order to vary an oscillator core current which is drawn in accordance with the chosen load capacitance.

2. A crystal oscillator circuit as claimed in claim 1, wherein the adjustable load capacitance comprises a bank of capacitors, and means for switching the capacitors into or out of the circuit.

3. A crystal oscillator circuit as claimed in claim 2, wherein each capacitor in the bank may be individually connected or disconnected from the oscillator circuit.

4. A crystal oscillator circuit as claimed in claim 1, wherein the means for drawing a crystal core current comprises a bank of resistors, and means for switching the resistors into or out of the circuit.

5. A crystal oscillator circuit as claimed in claim 4, wherein each resistor in the bank may be individually connected or disconnected from the oscillator circuit.

6. A crystal oscillator circuit as claimed in claim 1, wherein the means for controlling the load capacitance comprises a plurality of control lines, which determine which capacitors are connected to the oscillating circuit.

7. A crystal oscillator circuit as claimed in claim 6, wherein the control lines also determine which resistors are connected to the oscillating circuit.

8. A crystal oscillator circuit as claimed in claim 5, wherein the status of the control lines are set by a register containing a binary value, each bit of the binary value representing the status of a respective control line.

9. A crystal oscillator circuit as claimed in claim 8, wherein the binary value is set according to the frequency measured at the output of the oscillator.

10. A method of providing a desired output frequency from a crystal oscillator circuit, the method comprising the steps of adjusting the load capacitance of the crystal oscillator circuit to obtain the desired output frequency, the load capacitance being adjusted by switching one or more capacitors from a bank of capacitors into or out of the circuit;

and

switching one or more resistors into or out of the circuit, in order to vary an oscillator core current which is drawn in accordance with the chosen load capacitance.

11. A method as claimed in claim 10, wherein the load capacitance is adjusted by switching capacitors, from a bank of capacitors, into or out of the circuit.

12. A method as claimed in claim 11, wherein each capacitor in the bank may be switched individually into or out of the circuit.

13. A method as claimed in claim 10, wherein the crystal core current is drawn by switching resistors, from a bank of resistors, into or out of the circuit.

14. A method as claimed in claim 13, wherein each resistor may be switched individually into or out of the circuit.

15. A method as claimed in claim 10, wherein the capacitors and resistors are switched into or out of the circuit by a plurality of control lines.

16. A method as claimed in claim 15, wherein the status of each control line is set by a register containing a binary value, each bit of the binary value representing the status of a respective control line.

17. A method as claimed in claim 16, wherein the binary value is set according to the frequency measured at the output of the oscillator.

18. A method as claimed in claim 10, wherein the desired frequency is set during production of the crystal oscillator circuit.