Voltage controlled oscillator having improved phase noise

• US 20060145771A1
• Filed: 02/27/2006
• Published: 07/06/2006
• Est. Priority Date: 10/07/2003
• Status: Active Grant

• Current Assignee: MediaTek, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Strange, Jonathan Richard
• Granted Patent: US 7,492,228 B2
• Time in Patent Office: Days
• 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 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.

2. An apparatus as claimed in claim 1, in which the loop gain correction is an open loop correction.

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

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

5. An apparatus as claimed in claim 1, in which the 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.

6. An apparatus as claimed in claim 1, in which the gain correction is a function of target frequency².

7. An apparatus as claimed in claim 6 in which the gain correction varies as frequency

$\frac{1}{{\mathrm{frequency}}^2}.$

8. An apparatus as claimed in claim 1, further including a frequency divider in the phase locked loop having a divide ratio of N, and in which the gain correction varies as a function of N².

9. An apparatus as claimed in claim 8, in which the correction factor varies as

$\frac{1}{N^2}.$

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

11. 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.

12. An apparatus as claimed in claim 11, in which the constant of proportionality modifier produces a correction signal which varies as the square of the target oscillator frequency.

13. An apparatus as claimed in claim 11, in which the constant of proportionality modifier produces a modification which varies as

$\frac{1}{{\mathrm{frequency}}^2}$

14. An apparatus as claimed in claim 11, in which the control signal modifier includes a look-up table in order to derive the correction to the control signal.

15. An apparatus as claimed in claim 11, wherein the constant of proportionality is the loop gain of a phase locked loop.

16. An apparatus as claimed in claim 11, 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.

17. An apparatus as claimed in claim 16, in which a bias current for the charge pump is modified by the constant of proportionality modifier.

18. 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 receiving a target frequency for the voltage controlled oscillator. deriving a correction value based on the target frequency. supplying the correction value for use, either directly or indirectly, by the voltage controlled oscillator.

19. A method as claimed in claim 18, 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.

20. A method of performing frequency compensation of the constant of proportionality of a voltage controlled oscillator, the method comprising the forming of 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.

21. A method of compensating for variations in a loop gain of a phase locked loop as a function of a target frequency, comprising means for introducing a loop gain correction as a function of the target frequency.