Voltage controlled oscillator having improved phase noise

• US 7,492,228 B2
• Filed: 02/27/2006
• Issued: 02/17/2009
• Est. Priority Date: 10/07/2003
• Status: Expired due to Term

• Current Assignee: MediaTek, Inc.
• Original Assignee: MediaTek, Inc.
• Inventors: Strange, Jonathan Richard
• Primary Examiner(s): KINKEAD, ARNOLD M
• Application Number:
  US11/362,935
• Publication Number:
  US 20060145771A1
• Time in Patent Office: 1,086 Days
• Field of Search: 331/185, 331/16, 331/34, 331/179, 331/17, 331/175, 327/156
• US Class Current: 331/16
• CPC Class Codes: H03B 2200/0038 including a current mirror
• CPC Class Codes: H03B 2200/0048 including measures to switch the frequency band, e.g. by harmonic selection
• CPC Class Codes: H03B 2200/0062 Bias and operating point
• CPC Class Codes: H03B 2200/0066 Amplitude or AM detection
• CPC Class Codes: H03B 2200/0068 Frequency or FM detection
• CPC Class Codes: H03B 2200/0072 Frequency hopping and enabling of rapid frequency changes
• CPC Class Codes: H03B 5/04 Modifications of generator to compensate for variations in physical values, e.g. power supply, load, temperature
• 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 5/1253 the transistors being field-effect transistors
• CPC Class Codes: H03B 5/1271 the frequency being controlled by a control current, i.e. current controlled oscillators
• CPC Class Codes: H03B 5/1293 having means for achieving a desired tuning characteristic, e.g. linearising the frequency characteristic across the tuning voltage range
• CPC Class Codes: H03J 2200/10 Tuning of a resonator by means of digitally controlled capacitor bank
• CPC Class Codes: H03L 5/00 Automatic control of voltage, current, or power
• CPC Class Codes: H03L 7/099 concerning mainly the controlled oscillator of the loop

Abstract

An apparatus for compensating for variations in loop gain of a phase locked loop as a function of frequency, comprising a correction calculator for introducing a loop gain correction as a function of target frequency of a oscillator controlled by the phase locked loop.

Claims

1. An apparatus, for performing frequency compensation of a “

constant of proportionality”

of a voltage controlled oscillator, comprising a constant of proportionality modifier for modifying the constant of proportionality used in a frequency control loop as a function of a target frequency of the voltage controlled oscillator, in which the constant of proportionality modifier produces a correction signal which varies as the square of the target oscillator frequency.

2. An apparatus as claimed in claim 1, in which a loop gain correction is solely a function of the desired target frequency.

3. An apparatus as claimed in claim 1 in which, for a given combination of phase detector, charge pump and voltage controlled oscillator, a loop gain correction is solely a function of a target frequency.

4. An apparatus as claimed in claim 1, in which a loop gain correction is performed by modifying a bias current of a charge pump used to provide a control voltage to a voltage controlled oscillator.

5. An apparatus as claimed in claim 1, further including a filter for low pass filtering an output of the charge pump.

6. An apparatus as claimed in claim 1, in which the constant of proportionality modifier includes a look-up table in order to derive the correction signal.

7. An apparatus as claimed in claim 1, wherein the constant of proportionality is a loop gain of a phase locked loop.

8. An apparatus as claimed in claim 1, in which the output of the voltage oscillator is divided in frequency by a divider to form a divided signal, and the frequency of the divided signal is compared with a reference frequency and the result of the comparison is used to control a charge pump for generating a control signal of the voltage controlled oscillator.

9. An apparatus as claimed in claim 8, in which a bias current for the charge pump is modified by the constant of proportionality modifier.

10. A method of performing frequency compensation of the constant of proportionality of a voltage controlled oscillator within a phase locked loop, the method comprising the steps of:

1) receiving a target frequency for the voltage controlled oscillator;

2) deriving a correction value based on the target frequency;

3) performing amplitude correction so as to set the amplitude to a desired value; and

4) supplying the correction value for use, either directly or indirectly, by the voltage controlled oscillator, wherein the correction value varies as (target frequency)<sup>−

2</sup>.

11. A method as claimed in claim 10, in which the correction value is used to control a bias current of a charge pump used within the phase locked loop to generate a control voltage for the voltage controlled oscillator.

12. A method of performing frequency compensation of the constant of proportionality of a voltage controlled oscillator, the method comprising forming a frequency error signal, using the frequency error signal to drive the oscillator, and using a target frequency to derive a constant proportionality for converting the error signal into a frequency change of the voltage controlled oscillator, where the constant of proportionality is corrected by a correction factor which varies as (frequency)⁻

2.

13. An apparatus for compensation for variations in loop gain of a phase locked loop as a function of frequency, comprising a correction calculator for introducing a loop gain correction which varies as the inverse square of a target frequency of an oscillator controlled by the phase locked loop.

14. An apparatus as claimed in claim 13, in which loop gain control is performed by modifying a bias current of a charge pump used to provide a control voltage to a voltage controlled oscillator.

15. An apparatus for compensating for variations in loop gain of a phase locked loop as a function of frequency, comprising a correction calculator for introducing a loop gain correction as a function of target frequency squared of an oscillator controlled by the phase locked loop.

16. An apparatus for compensating for variations in loop gain of a phase locked loop as a function of frequency, comprising a correction calculator for introducing a loop gain correction as a function of a target frequency of an oscillator controlled by the phase locked loop, wherein the phase locked loop further includes a frequency divider having a divide ratio of N, and the gain correction varies as a function of N².

17. An apparatus as claimed in claim 16, in which a correction factor varies as 1/N².

18. An apparatus for performing frequency compensation of a “

constant of proportionality”

of a voltage controlled oscillator, comprising a constant of proportionality modifier for modifying the constant of proportionality used in a control loop as a function of a target frequency of the voltage controlled oscillator, wherein the constant of proportionality modifier produces a correction signal which varies as (target frequency)<sup>−

2</sup>.

19. A method for performing frequency compensation of a “

constant of proportionality”

of a voltage controlled oscillator, comprising providing a constant of proportionality modifier for modifying the constant of proportionality used in a frequency control loop as a function of a target frequency of the voltage controlled oscillator, in which the constant of proportionality modifier produces a correction signal which varies as the square of the target oscillator frequency and the correction signal is applied to the voltage controlled oscillator.

20. The method of claim 19 wherein said correction signal is solely a function of the desired target frequency.

21. The method of claim 19 wherein said correction signal modifies a bias current of a charge pump used to provide a control voltage to said oscillator, which is a voltage controlled oscillator.

22. The method of claim 19 wherein the constant of proportionality modifier includes a look-up table for deriving the correction to the control signal.

23. The method of claim 19 wherein the constant of proportionality is the loop gain of a phase locked loop.

24. The method of claim 19 further including dividing the output of the voltage oscillator by a frequency divider to form a divided signal and comparing the frequency of the divided signal with a reference frequency and using the result of the comparison to control a charge pump for generating a control signal for the voltage controlled oscillator.

25. The method of claim 24 in which a bias current for the charge pump is modified by the constant of proportionality modifier.

26. A method of controlling a voltage controlled oscillator within a phase locked loop, the method comprising the steps of:

1) receiving a target frequency for the voltage controlled oscillator;

2) receiving a target amplitude for the voltage controlled oscillator;

3) adjusting the amplitude of the voltage controlled oscillator so as to set it within an acceptable range of the target amplitude,

4) forming a frequency error signal representing a difference between the target frequency of the voltage controlled oscillator and an actual frequency of the voltage controlled oscillator;

5) using the frequency error signal to form a control signal for the oscillator;

using the target frequency to derive a correction to vary a loop gain of the phase locked loop as a function of the target frequency, in which the correction of the loop gain varies as (target frequency)<sup>−

2</sup>.

27. A method as claimed in claim 26 where the loop gain correction of the phase locked loop is an open correction.

28. A method as claimed in claim 26 in which the loop gain correction is solely a function of the desired target frequency.

29. An apparatus for controlling a voltage controlled oscillator, the apparatus comprising:

a phase frequency detector;

a phase locked loop comprising a programmable frequency divider and a charge pump;

an amplitude controller for the voltage controlled oscillator; and

a correction calculator,

wherein upon receipt of a target frequency for the voltage controlled oscillator and a target amplitude;

a) the amplitude controller adjusts the voltage controlled oscillator amplitude to set it within an acceptable range of values;

b) the programmable divider is set to a divide ratio N such that the target frequency F_out=F_ref×

N, where F_ref is a reference frequency supplied to the phase frequency detector;

c) the phase frequency detector compares the output of the programmable divider with the reference frequency and generates an error signal;

d) the error signal is provided to the charge pump an output of which is connected to a control terminal of the voltage controlled oscillator; and

e) the target frequency is further provided to the correction calculator, which derives a correction to vary the loop gain of the phase locked loop according to the target frequency, in which the loop gain varies as (target frequency)<sup>−

2</sup>.

30. An apparatus as claimed in claim 29 in which correction from the correction calculator is used to modify a bias current of the charge pump.

31. An apparatus as claimed in claim 29 in which the loop gain correction of the phase locked loop is an open loop correction.

32. An apparatus as claimed in claim 29 in which the loop gain correction is solely a function of the target frequency.

33. An apparatus for controlling a voltage controlled oscillator having an inductor—

capacitor resonant circuit therein, the apparatus comprising;

a phase frequency detector;

a phase locked loop comprising a programmable frequency divider and a charge pump;

an amplitude controller for the voltage controlled oscillator; and

a correction calculator,

wherein upon receipt of a target frequency for the voltage controlled oscillator and a target amplitude;

a) the amplitude controller adjusts the voltage controlled oscillator amplitude to set it within an acceptable range of values;

b) the programmable divider is set to a divide ratio N such that the target frequency F_out=F_ref×

N, where F_ref is a reference frequency supplied to the phase frequency detector;

c) the phase frequency detector compares the output of the programmable divider with the reference frequency and generates an error signal;

d) the error signal is provided to the charge pump an output of which is connected to a control terminal of the voltage controlled oscillator; and

e) the target frequency is further provided to the correction calculator, which derives a correction to vary the loop gain of the phase locked loop according to the target frequency, in which the loop gain varies as(target frequency)<sup>−

2</sup>.