Digital compensation for nonlinearities in a polar transmitter

1. A radio frequency (RF) polar transmitter, comprising:

a digital processor coupled, during a measurement mode, to receive an input complex modulated digital signal including an amplitude-modulated digital signal and a phase-modulated digital signal and a feedback signal including a amplitude-modulated feedback signal and a phase-modulated feedback signal and operable to compare the input complex modulated digital signal to the feedback signal to determine an error signal indicative of a difference between the input complex modulated digital signal and the feedback signal and to store the error signal;

a Digital-to-Analog Converter (DAC) for converting the amplitude-modulated digital signal from digital to analog to produce an amplitude-modulated analog signal;

a translational loop for producing a phase-modulated RF signal from the phase-modulated digital signal;

a power amplifier for producing a modulated RF signal from the phase-modulated RF signal and the amplitude-modulated analog signal; and

a feedback loop coupled to receive the modulated RF signal from the power amplifier and operable to produce the feedback signal from the modulated RF signal.

2. The polar transmitter of claim 1 , wherein the Digital-to-Analog converter includes first and second Digital-to-Analog converters for converting the amplitude-modulated digital signal and phase-modulated digital signal, respectively, from digital to analog to produce a phase-modulated analog signal and the amplitude-modulated analog signal, respectively, and wherein the translational loop is coupled to receive the phase-modulated analog signal and operable to up-convert the phase-modulated analog signal from an intermediate frequency to a radio frequency to produce the phase-modulated RF signal.

3. The polar transmitter of claim 1 , wherein the translational loop is a phase-locked loop coupled to receive the phase-modulated digital signal and operable to produce the phase-modulated RF signal that tracks the phase of the phase component of the input complex modulated digital signal.

4. The polar transmitter of claim 1 , wherein, during an operating mode:

the digital processor produces a correction signal from the error signal and adds the correction signal to the input complex modulated digital signal to produce a corrected complex modulated digital signal;

the power amplifier provides the modulated RF signal to an antenna for transmission thereof; and

the feedback loop is closed.

5. The polar transmitter of claim 4 , wherein, during the measurement mode:

the digital processor is operable to compare the phase-modulated feedback signal to the phase-modulated digital signal to produce a phase error signal indicative of a difference between the phase-modulated digital signal and the phase-modulated feedback signal and to compare the amplitude-modulated feedback signal to the amplitude-modulated digital signal to produce an amplitude error signal indicative of a difference between the amplitude-modulated digital signal and the amplitude-modulated feedback signal; and

the digital processor is operable to store the amplitude error signal and the phase error signal for use during the operating mode.

6. The polar transmitter of claim 5 , wherein, during the operating mode:

the digital processor is operable to produce a phase correction signal from the phase error signal and an amplitude correction signal from the amplitude error signal; and

the digital processor is operable to add the phase correction signal to the phase-modulated digital signal to produce a corrected phase-modulated digital signal and to add the amplitude correction signal to the amplitude-modulated digital signal to produce a corrected amplitude-modulated digital signal.

7. The polar transmitter of claim 6 , wherein the modulated RF signal includes an amplitude modulated distortion signal produced by the power amplifier; and wherein the digital processor is operable to measure the amplitude modulated distortion signal as the amplitude error signal and to compensate for the amplitude modulated distortion signal using a quadratic polynomial curve-fit to produce the amplitude correction signal.

8. The polar transmitter of claim 6 , wherein the modulated RF signal includes a phase modulated distortion signal produced by the power amplifier; and wherein the digital processor is operable to measure the phase modulated distortion signal as the phase error signal and to compensate for the phase modulated distortion signal using a linear polynomial curve-fit to produce the phase correction signal.

9. The polar transmitter of claim 6 , wherein the modulated RF signal includes a local oscillator feed through signal produced by the power amplifier, and wherein the digital processor is operable to measure the local oscillator feed through signal and to compensate for the local oscillator feed through signal by biasing the corrected amplitude signal with a DC value determined from the local oscillator feed through signal.

10. The polar transmitter of claim 4 , wherein the input complex modulated digital signal includes a sequence of test signals in the measurement mode, and further comprising:

a source generator for generating the sequence of test signals in the measurement mode and for generating a control signal that drives the digital processor during the measurement mode and the operating mode.

11. The polar transmitter of claim 1 , further comprising:

a baseband processor coupled to the digital processor and for generating the input complex modulated digital signal.

12. A radio frequency (RF) transceiver, comprising:

a polar transmitter including:

a digital processor coupled, during a measurement mode, to receive an input complex modulated digital signal including an amplitude-modulated digital signal and a phase-modulated digital signal and a feedback signal including a amplitude-modulated feedback signal and a phase-modulated feedback signal and operable to compare the input complex modulated digital signal to the feedback signal to determine an error signal indicative of a difference between the input complex modulated digital signal and the feedback signal and to store the error signal;

a Digital-to-Analog Converter (DAC) for converting the amplitude-modulated digital signal from digital to analog to produce an amplitude-modulated analog signal;

a translational loop operable to produce a phase-modulated RF signal from the phase-modulated digital signal; and

a power amplifier operable to produce a modulated RF signal from the phase-modulated RF signal and the amplitude-modulated analog signal; and

a receiver coupled in a feedback loop with the transmitter to receive the modulated RF signal from the power amplifier and operable to produce the feedback signal from the modulated RF signal.

13. The RF transceiver of claim 12 , wherein the Digital-to-Analog converter includes first and second Digital-to-Analog converters for converting the amplitude-modulated digital signal and the phase-modulated digital signal, respectively, from digital to analog to produce a phase-modulated analog signal and the amplitude-modulated analog signal, respectively, and wherein the translational loop is coupled to receive the phase-modulated analog signal and operable to up-convert the phase-modulated analog signal from an intermediate frequency to a radio frequency to produce the phase-modulated RF signal.

14. The RF transceiver of claim 12 , wherein the translational loop is a phase-locked loop coupled to receive the phase-modulated digital signal and operable to produce the phase-modulated RF signal that tracks the phase of the phase-modulated digital signal.

15. The RF transceiver of claim 12 , wherein, during an operating mode:

the digital processor produces a correction signal from the error signal and adds the correction signal to the input complex modulated digital signal to produce a corrected complex modulated digital signal;

the power amplifier provides the modulated RF signal to an antenna for transmission thereof; and

the feedback loop is closed.

16. The RF transceiver of claim 15 , wherein, during the measurement mode:

the digital processor is operable to compare the phase-modulated feedback signal to the phase-modulated digital signal to produce a phase error signal indicative of a difference between the phase-modulated digital signal and the phase-modulated feedback signal and to compare the amplitude-modulated feedback signal to the amplitude-modulated digital signal to produce an amplitude error signal indicative of a difference between the amplitude-modulated digital signal and the amplitude-modulated feedback signal; and

the digital processor is operable to store the phase error signal and the amplitude error signal for subsequent use during the operating mode.

17. The RF transceiver of claim 16 , wherein, during the operating mode:

the digital processor is operable to produce a phase correction signal from the phase error signal and an amplitude correction signal from the amplitude error signal; and

the digital processor is operable to add the phase correction signal to the phase-modulated digital signal to produce a corrected phase-modulated digital signal and to add the amplitude correction signal to the amplitude-modulated digital signal to produce a corrected amplitude-modulated digital signal.

18. The RF transceiver of claim 17 , wherein:

the modulated RF signal includes an amplitude modulated distortion signal produced by the power amplifier;

the digital processor is operable to measure the amplitude modulated distortion signal as the amplitude error signal and to compensate for the amplitude modulated distortion signal using a quadratic polynomial curve-fit to produce the amplitude correction signal;

the modulated RF signal includes a phase modulated distortion signal produced by the power amplifier; and

the digital processor is operable to measure the phase modulated distortion signal as the phase error signal and to compensate for the phase modulated distortion signal using a linear polynomial curve-fit to produce the phase correction signal.

19. The RF transceiver of claim 17 , wherein the modulated RF signal includes a local oscillator feed through signal produced by the power amplifier, and wherein the digital processor is operable to measure the local oscillator feed through signal and to compensate for the local oscillator feed through signal by biasing the corrected amplitude signal with a DC value determined from the local oscillator feed through signal.

20. The RF transceiver of claim 15 , wherein the complex modulated digital signal includes a sequence of test signals in a measurement mode, and further comprising:

a source generator for generating the sequence of test signals in the measurement mode and for generating a control signal that drives the digital processor during the measurement mode and the operating mode.

21. The RF transceiver of claim 12 , wherein the receiver further includes:

radio circuitry coupled to receive the modulated RF signal from the power amplifier and operable to down-convert the modulated RF signal to an IF analog signal;

an analog-to-digital converter for converting the IF analog signal into an IF digital signal;

a filter operable to filter the IF digital signal to produce a filtered IF digital signal; and

a vector de-rotator coupled to receive the filtered IF digital signal and operable to vector de-rotate the filtered IF digital signal to produce the feedback signal.