Method and apparatus for linear amplification of a radio frequency signal

• US 6,449,465 B1
• Filed: 12/20/1999
• Issued: 09/10/2002
• Est. Priority Date: 12/20/1999
• Status: Expired due to Term

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Gailus, Paul H., Turney, William J.
• Primary Examiner(s): Urban, Edward F.
• Assistant Examiner(s): BEAMER, TEMICA M
• Application Number:
  US09/468,025
• Time in Patent Office: 995 Days
• Field of Search: 330/10, 330/127, 330/135, 330/136, 455/126, 455/127, 455/108, 455/112, 455/260, 455/295
• US Class Current: 455/126
• CPC Class Codes: H03F 1/0227 using supply converters
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/565 using inductive elements
• CPC Class Codes: H03F 2200/102 A non-specified detector of a signal envelope being used in an amplifying circuit
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

A method and apparatus is provided that amplitude modulates a modulated radio frequency (RF) signal (411) by modulating the supply voltage of a power amplifier (410). The method and apparatus further provides an impedance modulator (412) that reduces output signal (415) errors in response to an error signal generated by a feedback circuit (416) that includes a quadrature modulator (506), a limiter (520), a comparator (502), and a quadrature downconverter (510). Intermodulation distortion generated in the feedback circuit (416) by delay mismatches between amplitude and phase feedback paths, and non-linear effects of AM/PM conversion in a limiter (520), are suppressed by placing limiter (520) and quadrature downconverter (510) in a forward path of the overall amplifier loop.

Claims

1. An apparatus for the linear amplification of a radio frequency signal, the apparatus comprising:

a modulating amplifier that is effective to modulate, in response to at least one control signal, and to amplify a radio frequency input signal to produce an output signal, wherein the modulating amplifier comprises;

at least one input port coupled to receive the radio frequency input signal;

an amplifier that amplifies the radio frequency input signal;

an impedance modulator that modulates the radio frequency input signal in response to the at least one control signal;

an output port;

a signal coupler, coupled to the modulating amplifier, that is effective to sample a portion of the output signal to produce an attenuated output signal; and

a feedback circuit, coupled to the signal coupler and the modulating amplifier, that is effective to determine a modulation error between a baseband input signal and the attenuated output signal and is effective to generate the at least one control signal to reduce the modulation error.

2. The apparatus of claim 1, wherein the at least one input port of the amplifier includes a supply port, wherein the linear transmitter further comprises a switching power supply coupled to the supply port of the amplifier, wherein the switching power supply is capable of generating a modulated supply voltage, and wherein the modulated supply voltage corresponds to an amplitude variation of the baseband input signal.

3. The apparatus of claim 1, further comprising a phase modulator coupled to the at least one input port of the amplifier, wherein the phase modulator is capable of generating the radio frequency input signal and phase modulating the radio frequency input signal to produce a phase modulated input signal, and wherein the phase modulated input signal corresponds to a phase variation of the baseband input signal.

4. The apparatus of claim 3, wherein the phase modulator is further coupled to the feedback circuit, wherein the modulation error comprises at least a phase modulation error, and wherein the phase modulator is effective to adjust a phase of the radio frequency input signal based on the at least one control signal to reduce the modulation error.

5. The apparatus of claim 1, wherein the feedback circuit comprises:

a feedback phase modulator that is effective to produce a phase modulated reference signal corresponding to a phase variation of the baseband input signal;

a first mixer, coupled to the signal coupler and the phase modulator, that is effective to produce a downconverted signal;

a comparator, coupled to the first mixer, that is effective to produce a first control signal of the at least one control signal representing an amplitude modulation error based on a difference between the downconverted signal and the baseband input signal;

a phase-shifting circuit, coupled to the feedback phase modulator, that is effective to introduce an approximately ninety degree phase offset to the phase modulated reference signal; and

a second mixer, coupled to the phase-shifting circuit and the signal coupler, that is effective to produce a second control signal of the at least one control signal representing a phase modulation error.

6. The apparatus of claim 5, wherein the first control signal comprises a high frequency component, a low frequency component, or both a high frequency component and a low frequency component, and wherein the feedback circuit further comprises:

a low pass filter coupled to the subtractor and to the switching power supply, wherein the low pass filter is effective to receive at least a portion of the first control signal, filter out the high frequency component, and convey the low frequency component to the switching power supply; and

a high pass filter coupled to the subtractor and to the impedance modulator, wherein the high pass filter is effective to receive at least a portion of the first control signal, filter out the low frequency component, and convey the high frequency component to the impedance modulator.

7. The apparatus of claim 5, further comprising an input phase modulator coupled to the input port of the amplifier and the feedback circuit, wherein the input phase modulator is capable of generating a radio frequency input signal and phase modulating the radio frequency input signal to produce a phase modulated input signal, wherein the impedance modulator is responsive to the first control signal to reduce the amplitude modulation error, and wherein the input phase modulator is responsive to the second control signal to reduce the phase modulation error.

8. The apparatus of claim 1, wherein the baseband input signal comprises a quadrature modulation information signal, and wherein the feedback circuit comprises:

a quadrature modulator that is effective to upconvert the quadrature modulation information signal to a frequency of the output signal to produce a quadrature modulated reference signal;

an amplitude suppressor, coupled to the quadrature modulator, that is effective to produce a phase modulated reference signal at the frequency of the output signal;

a comparator, coupled to the quadrature modulator and the signal coupler, that is effective to produce an error signal at approximately the frequency of the output signal based on a difference between the attenuated output signal and the quadrature modulated reference signal;

a first mixer, coupled to the comparator and the amplitude suppressor, that is effective to produce a first control signal of the at least one control signal representing an amplitude modulation error;

a phase-shifting circuit, coupled to the amplitude suppressor, that is effective to introduce an approximately ninety degree phase offset to the phase modulated reference signal; and

a second mixer, coupled to the phase-shifting circuit and the subtractor, that is effective to produce a second control signal of the at least one control signal representing a phase modulation error.

9. The apparatus of claim 8, further comprising a phase modulator coupled to the input port of the amplifier and the feedback circuit, wherein the phase modulator is capable of generating a radio frequency input signal and phase modulating the radio frequency input signal to produce a phase modulated input signal, wherein the impedance modulator is responsive to the first control signal to reduce the amplitude modulation error, and wherein the phase modulator is responsive to the second control signal to reduce the phase modulation error.

10. The apparatus of claim 9, further comprising:

a third mixer, coupled to the signal coupler and to the comparator;

a local oscillator circuit, coupled to the third mixer; and

wherein the third mixer is effective to downconvert the attenuated output signal to an intermediate frequency to produce an intermediate frequency attenuated output signal, the quadrature modulator is effective to upconvert the quadrature modulated signal to the intermediate frequency to produce a quadrature modulated reference signal, and the comparator is effective to produce an error signal at approximately the intermediate frequency based on a difference between the intermediate frequency attenuated output signal and the quadrature modulated reference signal.

11. An apparatus for the linear amplification of a radio frequency signal, the apparatus comprising:

an amplifier having at least one input port coupled to receive a modulated input signal and an output port that is effective to produce an output signal based on the modulated input signal;

a first modulator, coupled to the at least one input port of the amplifier, that is effective to generate a modulated input signal responsive to at least one control signal;

a signal coupler, coupled to the output port of the amplifier, that is effective to sample a portion of the output signal to produce an attenuated output signal;

a polar feedback circuit, coupled to the signal coupler and at least the first modulator, that is effective to receive a baseband input signal, to determine a modulation error between the baseband input signal and the attenuated output signal, and to generate the at least one control signal to reduce the modulation error, wherein the polar feedback circuit comprises;

a second modulator that is effective to produce a phase modulated reference signal corresponding to a phase variation of the baseband input signal;

a phase-shifting circuit, coupled to the second modulator, that is effective to introduce an approximately ninety degree phase offset to the phase modulated reference signal to produce a phase-shifted reference signal; and

a first mixer, coupled to the phase-shifting circuit, the signal coupler, and the first modulator, that is effective to produce at least one control signal representing a phase modulation error based on a difference between the attenuated output signal and the phase-shifted reference signal.

12. The apparatus of claim 11, wherein the first modulator comprises a phase modulator, coupled to the at least one input port of the amplifier, capable of generating a radio frequency input signal and phase modulating the radio frequency input signal to produce a phase modulated input signal.

13. The apparatus of claim 11, wherein the first modulator comprises an amplitude modulator, coupled to the at least one input port of the amplifier, capable of generating an amplitude modulated input signal.

14. The apparatus of claim 13, wherein the amplitude modulator comprises a switching power supply.

15. The apparatus of claim 13, wherein the polar feedback circuit further comprises:

a second mixer, coupled to the second modulator, that receives the attenuated output signal and is effective to produce a downconverted signal;

a subtractor, coupled to the second mixer, that is effective to produce a first control signal of the at least one control signal representing an amplitude modulation error based on a difference between the downconverted signal and the baseband input signal; and

wherein the first mixer produces a second control signal of the at least one control signal representing an phase modulation error based on the attenuated output signal and the phase-shifted reference signal.

16. The apparatus of claim 15, further comprising:

a third mixer, coupled to the signal coupler, the first mixer, and the second mixer;

a local oscillator circuit, coupled to the third mixer; and

wherein the third mixer is effective to downconvert the attenuated output signal to an intermediate frequency to produce an intermediate frequency attenuated output signal, and wherein at least a portion of the intermediate frequency attenuated output signal is conveyed to each of the first mixer and the second mixer.

17. The apparatus of claim 16, wherein the amplitude modulator is coupled to the feedback path and is responsive to the first control signal to reduce the amplitude modulation error and wherein the first modulator is responsive to the second control signal to reduce the phase modulation error.

18. The apparatus of claim 11, wherein the baseband input signal comprises a quadrature modulation information signal, wherein the second modulator is effective to modulate the quadrature modulation information signal to a frequency of the output signal to produce a quadrature modulated reference signal, wherein the phase-shifting circuit is effective to introduce an approximately ninety degree phase offset to the quadrature modulated reference signal to produce a phase-shifted reference signal, wherein the first mixer is effective to produce at least one control signal representing a phase modulation error based on an error signal and the phase-shifted reference signal, and wherein the polar feed back circuit further comprises a comparator, coupled to the second modulator, that receives the attenuated output signal and is effective to produce the error signal at approximately the frequency of the output signal based on a difference between the attenuated output signal and the quadrature modulated reference signal.

19. The apparatus of claim 18, wherein the first modulator comprises a phase modulator, coupled to the at least one input port of the amplifier, capable of generating a radio frequency input signal and phase modulating the radio frequency input signal to produce a phase modulated input signal.

20. The apparatus of claim 19, wherein the linear transmitter further comprises a third modulator, coupled to the at least one input port of the amplifier, capable of generating an amplitude modulated input signal.

21. The apparatus of claim 20, wherein the third modulator comprises a switching power supply.

22. The apparatus of claim 20, wherein the polar feedback circuit further comprises:

a second mixer, coupled to the comparator, the second modulator, and the third modulator, that is effective to produce a first control signal of the at least one control signal representing an amplitude modulation error; and

wherein the first mixer is effective to produce a second control signal of the at least one control signal representing a phase modulation error.

23. The apparatus of claim 22, wherein the third modulator is responsive to the first control signal to reduce the amplitude modulation error and wherein the first modulator is responsive to the second control signal to reduce the phase modulation error.

24. The apparatus of claim 22, further comprising a scalar modulator, coupled to the output port of the amplifier, to the signal coupler, and to the polar feedback circuit, wherein the scalar modulator is responsive to the first control signal to reduce the amplitude modulation error and wherein the first modulator is responsive to the second control signal to reduce the phase modulation error.

25. The apparatus of claim 24, wherein the first control signal comprises a high frequency component, a low frequency component, or both a high frequency component and a low frequency component, and wherein the polar feedback circuit further comprises:

a low pass filter coupled to the second mixer and to the third modulator, wherein the low pass filter is effective to receive at least a portion of the first control signal, filter out the high frequency component, and convey the low frequency component to the third modulator; and

a high pass filter coupled to the second mixer and to the scalar modulator, wherein the high pass filter is effective to receive at least a portion of the first control signal, filter out the low frequency component, and convey the high frequency component to the scalar modulator.

26. The apparatus of claim 22, wherein the polar feedback circuit further comprises an amplitude suppressor, coupled between the second modulator, the phase-shifting circuit, and the second mixer, that is effective to produce a phase modulated reference signal at approximately the frequency of the output signal, and wherein the phase-shifting circuit introduces an approximately ninety degree phase offset to the carrier reference signal to produce a phase-shifted reference signal.

27. The apparatus of claim 26, further comprising:

a third mixer, coupled to the signal coupler and to the subtractor;

a local oscillator circuit, coupled to the third mixer; and

wherein the third mixer is effective to downconvert the attenuated output signal to an intermediate frequency to produce an intermediate frequency attenuated output signal, the second modulator is effective to upconvert the quadrature modulated signal to the intermediate frequency to produce a quadrature modulated reference signal, and the subtractor is effective to produce an error signal at approximately the intermediate frequency based on a difference between the intermediate frequency attenuated output signal and the quadrature modulated reference signal.

28. A method for amplifying a radio frequency signal comprising the steps of:

receiving a composite baseband signal, wherein the phase modulated baseband signal comprises an amplitude component;

amplifying the radio frequency signal;

amplitude modulating the radio frequency signal based on the amplitude component of the composite baseband signal;

modulating the amplified and amplitude modulated radio frequency signal to produce an output signal;

sampling a portion of the output signal to produce an attenuated output signal;

determining a desired attenuated output signal based on the composite baseband signal;

comparing the desired attenuated output signal to the attenuated output signal;

determining a modulation error based on a difference between the attenuated output signal and the desired attenuated output signal;

producing at least one control signal based on the modulation error; and

adjusting the amplified and amplitude modulated radio frequency signal based on the at least one control signal to produce a corrected output signal.

29. The method of claim 28, wherein the step of determining a desired attenuated output signal based on the composite baseband signal comprises the step of modulating the composite baseband signal to a frequency of the attenuated output signal to produce a desired attenuated output signal.

30. The method of claim 28, wherein the modulation error comprises an error signal and wherein the step of producing at least one control signal based on the modulation error comprises the following steps:

producing a reference signal based on the desired attenuated output signal; and

determining at least one control signal based on the reference signal and the error signal.

31. The method of claim 30, wherein the at least one control signal comprises a first control signal, comprising an amplitude modulation error signal, and a second control signal, comprising a phase modulation error signal, and wherein the step of determining at least one control signal based on the reference signal and the error signal comprises the following steps:

splitting the error signal into a first portion and a second portion;

multiplying the first portion of the error signal with the reference signal to produce an amplitude modulation error signal;

phase shifting the reference signal to produce a phase shifted reference signal; and

multiplying the second portion of the amplified error signal with the phase shifted reference signal to produce a phase modulation error signal.

32. The method of claim 28, wherein the at least one control signal comprises an amplitude modulation error signal and wherein the step of adjusting the amplified and amplitude modulated radio frequency signal based on the at least one control comprises the step of amplitude modulating the amplified and amplitude modulated phase modulated RF signal in response to the amplitude modulation error signal to reduce the amplitude modulation error.

33. The method of claim 28, wherein the at least one control signal comprises a phase modulation error signal and wherein the step of adjusting the amplified and amplitude modulated radio frequency signal based on the at least one control signal comprises the step of phase modulating the amplified and amplitude modulated phase modulated RF signal in response to the phase modulation error signal to reduce the phase modulation error.

34. The method of claim 28, wherein the step of determining a modulation error comprises the steps of:

modulating the composite baseband signal to an intermediate frequency (IF) to produce a desired IF attenuated output signal;

generating a reference signal at the IF to produce an IF reference signal;

multiplying the attenuated output signal with the IF reference signal to produce an IF attenuated output signal;

comparing the desired IF attenuated output signal to the IF attenuated output signal;

determining a modulation error based on a difference between the desired IF attenuated output signal and the IF attenuated output signal, wherein the modulation error comprises an error signal.

35. The method of claim 28, wherein the modulation error comprises an amplitude modulation error and wherein the step producing at least one control signal based on the modulation error comprises the steps of:

producing a control signal based on the amplitude modulation error;

filtering the control signal to produce a filtered control signal; and

wherein the step of adjusting the amplified and amplitude modulated radio frequency signal based on the at least one control signal to produce a corrected output signal comprises amplitude modulating the amplified and amplitude modulated radio frequency signal in response to the filtered control signal to reduce the amplitude modulation error.

36. A method for amplifying a radio frequency signal comprising the steps of:

receiving a composite baseband signal, wherein the composite baseband signal comprises a phase component and an amplitude component;

phase modulating the radio frequency (RF) signal based on the phase component of the composite baseband signal to produce a phase modulated RF signal;

amplifying the phase modulated RF signal;

modulating the amplitude of the phase modulated RF signal based on the amplitude component of the composite baseband signal;

modulating the amplified and amplitude modulated phase modulated RF signal to produce an output signal;

sampling a portion of the output signal to produce an attenuated output signal;

downconverting the attenuated output signal to produce a downconverted attenuated output signal;

comparing the composite baseband signal and the downconverted attenuated output signal;

determining a modulation error based on a difference between the downconverted attenuated output signal and the composite baseband signal;

producing at least one control signal based on the modulation error; and

adjusting the amplified and amplitude modulated phase modulated RF signal based on the at least one control signal to produce a corrected output signal.

37. The method of claim 36, wherein the modulation error comprises an amplitude modulation error.

38. The method of claim 37, wherein the step of adjusting the amplified and amplitude modulated phase modulated radio frequency signal comprises modulating the amplified and amplitude modulated phase modulated radio frequency signal in response to the at least one control signal to reduce the amplitude modulation error.

39. The method of claim 36, wherein the step of downconverting comprises the steps of:

generating a reference signal that is based on the phase component of the composite baseband signal; and

multiplying the reference signal and at least a portion of the attenuated output signal to produce a downconverted attenuated output signal.

40. The method of claim 36, wherein the step of downconverting comprises the steps of:

generating a reference signal that is based on the phase component of the composite baseband signal;

splitting the attenuated output signal into a first portion and a second portion;

multiplying the reference signal and the first portion of the attenuated output signal to produce an amplitude modulated downconverted version of the attenuated output signal;

phase shifting the reference signal to produce a phase shifted reference signal;

multiplying the phase shifted reference signal and the second portion of the attenuated output signal to produce a phase modulated downconverted version of the attenuated output signal.

41. The method of claim 40, wherein the step of determining a modulation error comprises the steps of:

determining an amplitude modulation error based on a difference between the downconverted attenuated output signal and the composite baseband signal; and

determining a phase modulation error based on the phase modulated downconverted version of the attenuated output signal.

42. The method of claim 41, wherein the step of producing at least one control signal comprises producing a first control signal based on the amplitude modulation error and a second control signal based on the phase modulation error.

43. The method of claim 42, wherein the step of adjusting the amplified and amplitude modulated phase modulated radio frequency signal comprises modulating the amplified and amplitude modulated phase modulated radio frequency signal in response to the second control signal to reduce the phase modulation error.

44. A communication device comprising:

a receiver;

a linear transmitter comprising;

an amplifier having at least one input port coupled to receive a modulated input signal and an output port that produces an amplified modulated signal;

a complex modulator, coupled to the output port of the amplifier, that further modulates the amplified modulated signal responsive to at least one control signal to produce an output signal;

a signal coupler, coupled to the complex modulator, that samples a portion of the output signal to produce an attenuated output signal; and

a feedback circuit, coupled to the signal coupler and at least the complex modulator, that receives a quadrature modulation information signal from an information source, receives the attenuated output signal from the signal coupler, determines a modulation error based on quadrature modulation information signal and the attenuated output signal, and generates the at least one control signal to reduce the modulation error.