Frequency-locked loop calibration of a phase-locked loop gain
View Patent ↗The present invention relates to a calibrated phase-locked loop (PLL), which has a calibration mode for measuring a tuning gain of a variable frequency oscillator (VFO) and a PLL mode for normal operation. Calibration information based on the tuning gain is used during the PLL mode to regulate a PLL loop gain. During the calibration mode, the calibrated PLL operates as a frequency-locked loop (FLL) for low frequency lock times, and during the PLL mode the calibrated PLL operates as a PLL for high frequency accuracy and low noise. By regulating the PLL loop gain, the desired noise spectrum and dynamic behavior of the PLL may be maintained in spite of variations in the operating characteristics or in the characteristics of the PLL components.
1. A circuit comprising:
a variable frequency oscillator (VFO) adapted to receive a control signal and provide an output signal based on the control signal, such that:
during a first calibration mode, the output signal has a first calibration frequency;
and during a phase-locked loop (PLL) mode, the output signal has a locked frequency;
and loop control circuitry adapted to: during the first calibration mode, form a frequency-locked loop (FLL) using the VFO, and regulate the first calibration frequency based on a first calibration frequency setpoint by controlling the control signal, which is associated with a first control value, such that calibration information is based on the first control value and the first calibration frequency setpoint; and
during the PLL mode, form a PLL using the VFO, and regulate the locked frequency based on a locked frequency setpoint by controlling the control signal, such that the PLL has a PLL loop gain, which is based on the calibration information; wherein:
the loop control circuitry is further adapted to receive a first reference signal having a reference frequency and a second reference signal having a reference phase;
the regulation of the first calibration frequency is further based on a frequency difference associated with the first calibration frequency and the reference frequency;
during the PLL mode, the output signal has the locked frequency and a locked phase; and
the regulation of the locked frequency is further based on a phase difference associated with the locked phase and the reference phase;
wherein the loop control circuitry comprises a frequency reduction circuit adapted to:
receive the output signal; and
provide a frequency reduced output signal based on applying a frequency reduction to the output signal, wherein:
during the first calibration mode, the frequency reduced output signal has a first reduced calibration frequency;
the regulation of the first calibration frequency is further based on a frequency difference associated with the first reduced calibration frequency and the reference frequency;
during the PLL mode, the frequency reduced output signal has a reduced locked frequency and a reduced frequency locked phase;
the regulation of the locked frequency is further based on a phase difference associated with the reduced frequency locked phase and the reference phase;
a calibration division ratio is about equal to the first calibration frequency divided by the first reduced calibration frequency;
a PLL division ratio is about equal to the locked frequency divided by the reduced locked frequency; and
a PLL-to-calibration ratio is about equal to the PLL division ratio divided by the calibration division ratio.
2. The circuit of claim 1 wherein the PLL-to-calibration ratio is equal to about a power of two.
3. The circuit of claim 1 wherein:
during the first calibration mode, the FLL has an FLL bandwidth; and
during the PLL mode, the PLL has a PLL bandwidth, such that the FLL bandwidth is greater than the PLL bandwidth.
4. The circuit of claim 1 wherein:
during a coarse tuning mode, the output signal has a coarse tuning frequency; and
the loop control circuitry is further adapted to, during the coarse tuning mode, form the FLL using the VFO, and regulate the coarse tuning frequency based on a coarse tuning frequency setpoint.
5. The circuit of claim 4 wherein the coarse tuning mode is followed by the first calibration mode.
6. The circuit of claim 4 wherein:
the regulation of the coarse tuning frequency is controlled by a coarse tuning signal; and
the VFO is further adapted to receive the coarse tuning signal, such that the output signal is further based on the coarse tuning signal.
7. The circuit of claim 6 wherein the VFO further comprises a plurality of discrete tuning elements and is further adapted to select at least one of the plurality of discrete tuning elements based on the coarse tuning signal, and the output signal is further based on the selection of the at least one of the plurality of discrete tuning elements.
8. The circuit of claim 7 wherein the plurality of discrete tuning elements comprises a plurality of capacitive elements.
9. The circuit of claim 6 wherein the loop control circuitry is further adapted to hold the coarse tuning signal about constant during the first calibration mode and during the PLL mode.
10. The circuit of claim 1 further comprising control circuitry adapted to select one of the first calibration mode and the PLL mode.
11. The circuit of claim 1 wherein the PLL division ratio is based on the locked frequency setpoint and the calibration division ratio is based on the first calibration frequency setpoint.
12. The circuit of claim 1 wherein the frequency reduction circuit is a fractional-N divider.
13. The circuit of claim 1 wherein the frequency reduction circuit comprises a mixer.
14. The circuit of claim 1 wherein:
the VFO is further adapted to receive the control signal and provide the output signal based on the control signal, such that during a second calibration mode, the output signal has a second calibration frequency; and
the loop control circuitry is further adapted to, during the second calibration mode, form the FLL using the VFO, and regulate the second calibration frequency based on a second calibration frequency setpoint by controlling the control signal, which is associated with a second control value, such that the calibration information is further based on the first control value and the second calibration frequency setpoint.
15. The circuit of claim 14 wherein the second calibration frequency is a desired operating frequency.
16. The circuit of claim 14 wherein one of the first calibration frequency and the second calibration frequency is greater than a desired operating frequency and another of the first calibration frequency and the second calibration frequency is less than the desired operating frequency.
17. The circuit of claim 1 wherein the loop control circuitry further comprises:
a PLL phase detector adapted to:
receive the frequency reduced output signal and the second reference signal; and
during the PLL mode, provide a phase error signal based on a phase difference between the frequency reduced output signal and the second reference signal, such that the regulation of the locked frequency is further based on the phase error signal; and
a charge pump adapted to:
receive the phase error signal; and
during the PLL mode, provide a charge pump output signal based on a charge pump current and the phase error signal, such that the regulation of the locked frequency is further based on the charge pump signal, and the PLL loop gain is based on the charge pump current.
18. The circuit of claim 17 wherein:
during the first calibration mode, the VFO has a tuning gain based on a relationship between the first control value and the first calibration frequency setpoint;
the calibration information is based on the tuning gain;
during the PLL mode, the PLL loop gain is based on a product of the tuning gain and the charge pump current; and
during the PLL mode, the loop control circuitry is further adapted to set the charge pump current based on the calibration information.
19. The circuit of claim 18 wherein during the PLL mode, the loop control circuitry is further adapted to set the charge pump current to approximately maintain a constant PLL loop gain over operating conditions of the circuit.
20. The circuit of claim 18 wherein the loop control circuitry further comprises an FLL frequency detector adapted to:
receive the frequency reduced output signal and the first reference signal; and
during the first calibration mode, provide a frequency error signal based on a frequency difference between the frequency reduced output signal and the first reference signal, such that the regulation of the first calibration frequency is further based on the frequency error signal.
21. The circuit of claim 20 wherein the loop control circuitry further comprises a PLL gain calibration circuit adapted to:
receive the frequency error signal;
during the first calibration mode, provide the first control value based on the frequency error signal; and
during the first calibration mode, provide a digital-to-analog converter (DAC) output signal based on the first control value, such that the controlling of the control signal is based on the DAC output signal.
22. The circuit of claim 21 wherein the PLL gain calibration circuit comprises a PLL gain calibration loop filter adapted to:
receive the frequency error signal;
during the first calibration mode, provide the first control value based on filtering the frequency error signal; and
during the first calibration mode, provide the DAC output signal.
23. The circuit of claim 22 wherein:
during a coarse tuning mode, the output signal has a coarse tuning frequency;
the loop control circuitry is further adapted to, during the coarse tuning mode, form the FLL using the VFO, and regulate the coarse tuning frequency based on a coarse tuning frequency setpoint by controlling a coarse tuning signal; and
the VFO is further adapted to receive the coarse tuning signal, such that the output signal is further based on the coarse tuning signal.
24. The circuit of claim 22 wherein the loop control circuitry further comprises an FLL loop filter adapted to:
receive the frequency error signal; and
during the coarse tuning mode, provide the coarse tuning signal based on filtering the frequency error signal.
25. The circuit of claim 1 wherein the circuit is used to form a frequency synthesizer in a wireless communications system.