WO2001056147A1 - Transmission feedback system - Google Patents

Transmission feedback system Download PDF

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Publication number
WO2001056147A1
WO2001056147A1 PCT/GB2001/000307 GB0100307W WO0156147A1 WO 2001056147 A1 WO2001056147 A1 WO 2001056147A1 GB 0100307 W GB0100307 W GB 0100307W WO 0156147 A1 WO0156147 A1 WO 0156147A1
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WO
WIPO (PCT)
Prior art keywords
output
amplitude
signal
power amplifier
modulation
Prior art date
Application number
PCT/GB2001/000307
Other languages
French (fr)
Inventor
Julian Hildersley
Patrick Wurm
Original Assignee
Ttpcom Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from EP00300612A external-priority patent/EP1120903A1/en
Priority claimed from GB0013680A external-priority patent/GB0013680D0/en
Application filed by Ttpcom Ltd. filed Critical Ttpcom Ltd.
Priority to AU30337/01A priority Critical patent/AU3033701A/en
Publication of WO2001056147A1 publication Critical patent/WO2001056147A1/en

Links

Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03G—CONTROL OF AMPLIFICATION
    • H03G3/00—Gain control in amplifiers or frequency changers without distortion of the input signal
    • H03G3/20—Automatic control
    • H03G3/30—Automatic control in amplifiers having semiconductor devices
    • H03G3/3036—Automatic control in amplifiers having semiconductor devices in high-frequency amplifiers or in frequency-changers
    • H03G3/3042—Automatic control in amplifiers having semiconductor devices in high-frequency amplifiers or in frequency-changers in modulators, frequency-changers, transmitters or power amplifiers
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
    • H03F1/3241—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits

Definitions

  • This invention relates to feedback control systems for employment in the generation of amplitude and phase modulated radio frequency carrier waves of high power.
  • Some known systems use modulation feedback to control a power amplifier but suffer from a number of disadvantages and are therefore not widely used.
  • Cartesian feedback the wide band noise floor that is produced by any vector modulator which is employed results in a requirement for complex filtering to ensure duplex operation.
  • polar feedback the amplitude and phase components suffer from differential delays if the loops associated with each are not matched, which can result in inaccurate modulation and can create spectral distortion.
  • the present invention seeks to overcome some of the above problems.
  • a modulation feedback system for use with a transmitter power amplifier, the system comprising: means for sampling, in use, the output of a power amplifier; vector modulator means for receiving the output from the sampling means and applying a Cartesian representation of the desired modulation thereto; and means for receiving the output of the vector modulator means and means for employing the output to modulate, both in phase and amplitude terms, the input to the power amplifier in use.
  • the system may further comprise a frequency convertor for mixing the output of a reference frequency oscillator with the signal from the sampling means.
  • the system may further comprise a variable gain amplifier for scaling the output of the sampling means.
  • the phase modulation may be performed by provision of a limiter for limiting a signal from the vector modulator; a frequency mixer for mixing a reference with the output of the limiter; and a phase loop filter for filtering the output of the frequency mixer and applying an input to a frequency controlled oscillator.
  • the amplitude modulation may be performed by provision of an envelope detector; a comparator for comparing the output of the envelope detect with a reference amplitude; and a amplitude loop filter.
  • Figure 1 is a schematic diagram of a first example of the present invention
  • Figure 2 is a schematic diagram of a second example of the present invention.
  • Figure 3 is a schematic diagram of a third example of the present invention.
  • Figure 4 is a schematic diagram of a further example of the present invention.
  • a power amplifier 1 provides an amplified signal to an antenna 21 at a level sufficient to ensure that the transmitted RF power from the transmitter is sufficient for the application for which the antenna 21 is being employed.
  • Sampling means 2 samples the output of the power amplifier 1 and provides it to a vector modulator 3, which is used to provide an error signal based upon the sampled output and a reference quadrature representation of the modulation that it is desired for the output of the power amplifier 1 to have .
  • a filter 4 is provided to remove harmonics from the output from the vector modulator means 3.
  • a splitter 5 is provided to split the output from the filter 4 and provide it to two feedback paths A and B.
  • Path A comprises an envelope detector 6 which extracts amplitude information from the signal from the filter 4. It further comprises an amplitude reference generator and a comparator 7 for comparing the output of the envelope detector 6 with the generated referenced amplitude. The output from the comparator 7 is filtered by a further filter 8 and provided to an amplitude modulator 9.
  • Path B comprises a limiter 10 which extracts phase information from the output of the filter 4 and a phase comparator 11 which compares the output of the limiter 10 with a reference frequency.
  • a filter 12 is provided to filter the output of the phase comparator 11 and the output of the filter 12 is then forwarded to a voltage controlled oscillator 13. The output of the voltage controlled oscillator 13 is then forwarded to the amplitude modulator 9, the output of which is provided to the amplifier 1 in order to close the feedback loops.
  • Signal path A has a baseband representation of the amplitude of the signal generated by the envelope detector 6 in a manner which is independent of any phase modulation. After comparison with a reference amplitude by the comparator 7 the resulting signal represents an error in amplitude between ideal modulation and the modulation present at the output of the power amplifier 1. After further filtering this signal is applied to the amplitude modulator 9.
  • the other portion of the split signal from the vector modulator output is sent via path B and applied to a limiter 10 which produces a phase-modulated carrier wave independent of any amplitude modulation.
  • the output of the limiter 10 is applied to a phase comparator 11, where it is compared with a stable phase/frequency reference 20.
  • the resulting base band signal represents the error in phase between an ideal modulation and the modulation at the output of the power amplifier 1.
  • phase error signal thus generated is filtered by a filter 12 and used to control the frequency of a variable frequency oscillator 13.
  • the output of the variable frequency oscillator 13 is a phase-modulated carrier wave of constant amplitude. This output is then applied to the amplitude modulator 9 and amplitude modulated from the signal from path A such that the resulting signal carrier's complex modulation closely related to the ideal modulation represented by the quadrature baseband components applied to the vector modulator 3, but predistorted to compensate for the non- linearity of the power amplifier 1.
  • the complex modulator signal is applied to the input of the nonlinear power amplifier 1, resulting in an undistorted, high power, complex modulated signal at its output. Thus, both the amplitude and phase modulation loops are closed.
  • Fig. 2 shows a further example of the present invention in which there is further provided a frequency convertor 14 which mixes the output of the sampling means 2 with the output of a reference local oscillator 15.
  • the spectrum output from the frequency convertor 14 will have a centre frequency equal to that of the stable phase frequency reference 20.
  • the loop operates as per the example shown in Fig. 1, except that the output frequency and the reference frequency differ by the local oscillator frequency.
  • the output from the local oscillator 15 is variable then the output frequency will also vary, but the vector modulator 3, together with all of the other signal processing opponents, may operate at a fixed frequency with narrow bandwidth by appropriate control of variations in the local oscillator frequency.
  • Fig. 3 shows a further example of the present invention in which a variable gain amplifier 16 is provided in the sampling path in combination with the additional features of Fig. 2.
  • the amplitude output signal from the power amplifier 1 is sampled by the sampling means 2, but is scaled by the variable gain amplifier 16 before it is applied to the vector modulator 3.
  • the gain of the variable gain amplifier 16 it is possible to reduce the dynamic range of the remaining processing components. This provides particular benefits, in that it allows the invention to support shaped pulses in TDMA systems without increasing the resolution required in the quadrature base band representation.
  • Figure 4 shows yet another example of the present invention, which is similar to example of Figure 3, although with a number of modifications.
  • (I,-Q) signals contain both the amplitude and phase content of the modulation. They are mixed with a carrier that contains only the phase content of the feedback signal.
  • the output signal of the complex modulator carries the amplitude content of the modulation and the phase difference between the modulation and the feedback signal. Therefore, in the example of Figure 4 the complex error signal is obtained by mixing I and -Q signals with a constant envelope version of the feedback signal.
  • a limiter 19 is inserted into the feedback path before the modulator and removes the amplitude content of the feedback signal fed through the modulator.
  • the feedback signal is also fed through an envelope detector (6a) corresponding to that used in previous examples, the output of which is fed to the comparator 7 , and compared to the output of the envelope detector (6b) already included in path A allowing a straightforward subtraction of amplitude to be performed in the feedback loop.
  • example of Figure 4 provides a linear behaviour for the amplitude loop as long as the power amplifier operates in its linear region, rather than the non-linear comparison of inverses of the earlier examples.
  • the linearity feature allows the amplitude loop bandwidth to be reduced as compared to the previous examples. This is a significant improvement as the amplitude loop bandwidth is the result of a compromise: the bandwidth must be made much wider than the modulation bandwidth in order to apply the linearisation effect properly but is restricted by the limitations of the electronic components. So an easier compromise may be found and better performance should be obtained.
  • the limiter 19 removes the amplitude content of the feedback signal. Therefore the I and Q modulator operates with a constant envelope signal on its carrier input allowing the mixers associated with the I and Q signal to operate at a higher carrier input level, relaxing their operational requirements and the specification placed on the devices providing their function.

Abstract

A modulation feedback system for use with a transmitter power amplifier. The system comprises means for sampling, in use, the output of a power amplifier. Vector modulator means receives the output from the sampling means and applies a Cartesian representation of the desired modulation thereto. Means receive the output of the vector modulator means and employs the output to modulate, both in phase and amplitude terms, the input to the power amplifier in use.

Description

Transmission Feedback System
This invention relates to feedback control systems for employment in the generation of amplitude and phase modulated radio frequency carrier waves of high power.
There are currently many types of feedback systems employed to control the output of high power amplifier associated with radio frequency carrier wave transmission.
Some known systems use modulation feedback to control a power amplifier but suffer from a number of disadvantages and are therefore not widely used. In systems which use Cartesian feedback, the wide band noise floor that is produced by any vector modulator which is employed results in a requirement for complex filtering to ensure duplex operation. In systems which use polar feedback, the amplitude and phase components suffer from differential delays if the loops associated with each are not matched, which can result in inaccurate modulation and can create spectral distortion.
The present invention seeks to overcome some of the above problems.
According to the present invention there is provided a modulation feedback system for use with a transmitter power amplifier, the system comprising: means for sampling, in use, the output of a power amplifier; vector modulator means for receiving the output from the sampling means and applying a Cartesian representation of the desired modulation thereto; and means for receiving the output of the vector modulator means and means for employing the output to modulate, both in phase and amplitude terms, the input to the power amplifier in use.
The system may further comprise a frequency convertor for mixing the output of a reference frequency oscillator with the signal from the sampling means.
The system may further comprise a variable gain amplifier for scaling the output of the sampling means.
The phase modulation may be performed by provision of a limiter for limiting a signal from the vector modulator; a frequency mixer for mixing a reference with the output of the limiter; and a phase loop filter for filtering the output of the frequency mixer and applying an input to a frequency controlled oscillator.
The amplitude modulation may be performed by provision of an envelope detector; a comparator for comparing the output of the envelope detect with a reference amplitude; and a amplitude loop filter.
One example of the present invention will now be described with reference to the accompanying drawings, in which: -
Figure 1 is a schematic diagram of a first example of the present invention;
Figure 2 is a schematic diagram of a second example of the present invention;
Figure 3 is a schematic diagram of a third example of the present invention; and
Figure 4 is a schematic diagram of a further example of the present invention.
Referring to figure 1, a power amplifier 1 provides an amplified signal to an antenna 21 at a level sufficient to ensure that the transmitted RF power from the transmitter is sufficient for the application for which the antenna 21 is being employed. Sampling means 2 samples the output of the power amplifier 1 and provides it to a vector modulator 3, which is used to provide an error signal based upon the sampled output and a reference quadrature representation of the modulation that it is desired for the output of the power amplifier 1 to have .
A filter 4 is provided to remove harmonics from the output from the vector modulator means 3. A splitter 5 is provided to split the output from the filter 4 and provide it to two feedback paths A and B.
Path A comprises an envelope detector 6 which extracts amplitude information from the signal from the filter 4. It further comprises an amplitude reference generator and a comparator 7 for comparing the output of the envelope detector 6 with the generated referenced amplitude. The output from the comparator 7 is filtered by a further filter 8 and provided to an amplitude modulator 9.
Path B comprises a limiter 10 which extracts phase information from the output of the filter 4 and a phase comparator 11 which compares the output of the limiter 10 with a reference frequency. A filter 12 is provided to filter the output of the phase comparator 11 and the output of the filter 12 is then forwarded to a voltage controlled oscillator 13. The output of the voltage controlled oscillator 13 is then forwarded to the amplitude modulator 9, the output of which is provided to the amplifier 1 in order to close the feedback loops.
The operation of the invention will now be described. If we consider signal flow starting at the output of the power amplifier 1. At this point, for ease of understanding it should be assumed that there is a RF carrier with near-ideal complex modulation applied to it. A sample of this RF signal is collected by the sampling means 2 and used as a feedback signal . The feedback signal is directly used as the RF input of the modulator. This RF signal is complex modulated by quadrature components derived from the I&Q signals so as to cancel the amplitude modulation on the feedback signal. Thus, at the output of the vector modulator 3, there is provided a RF carrier wave with complex error modulation applied. The complex error modulation results from the difference between the ideal modulation represented by the quadrature based band signals applied to the vector modulator 3 and the actual complex modulation on the sampled signal.
After filtering to remove harmonics the signal output from the vector modulator 3 is split. Signal path A has a baseband representation of the amplitude of the signal generated by the envelope detector 6 in a manner which is independent of any phase modulation. After comparison with a reference amplitude by the comparator 7 the resulting signal represents an error in amplitude between ideal modulation and the modulation present at the output of the power amplifier 1. After further filtering this signal is applied to the amplitude modulator 9.
The other portion of the split signal from the vector modulator output is sent via path B and applied to a limiter 10 which produces a phase-modulated carrier wave independent of any amplitude modulation. The output of the limiter 10 is applied to a phase comparator 11, where it is compared with a stable phase/frequency reference 20. The resulting base band signal represents the error in phase between an ideal modulation and the modulation at the output of the power amplifier 1.
The phase error signal thus generated is filtered by a filter 12 and used to control the frequency of a variable frequency oscillator 13.
The output of the variable frequency oscillator 13 is a phase-modulated carrier wave of constant amplitude. This output is then applied to the amplitude modulator 9 and amplitude modulated from the signal from path A such that the resulting signal carrier's complex modulation closely related to the ideal modulation represented by the quadrature baseband components applied to the vector modulator 3, but predistorted to compensate for the non- linearity of the power amplifier 1. The complex modulator signal is applied to the input of the nonlinear power amplifier 1, resulting in an undistorted, high power, complex modulated signal at its output. Thus, both the amplitude and phase modulation loops are closed.
Fig. 2 shows a further example of the present invention in which there is further provided a frequency convertor 14 which mixes the output of the sampling means 2 with the output of a reference local oscillator 15. The spectrum output from the frequency convertor 14 will have a centre frequency equal to that of the stable phase frequency reference 20. Thus, the loop operates as per the example shown in Fig. 1, except that the output frequency and the reference frequency differ by the local oscillator frequency. Thus, if the output from the local oscillator 15 is variable then the output frequency will also vary, but the vector modulator 3, together with all of the other signal processing opponents, may operate at a fixed frequency with narrow bandwidth by appropriate control of variations in the local oscillator frequency.
Fig. 3 shows a further example of the present invention in which a variable gain amplifier 16 is provided in the sampling path in combination with the additional features of Fig. 2. In this case, the amplitude output signal from the power amplifier 1 is sampled by the sampling means 2, but is scaled by the variable gain amplifier 16 before it is applied to the vector modulator 3. Thus, by appropriate control of the gain of the variable gain amplifier 16 it is possible to reduce the dynamic range of the remaining processing components. This provides particular benefits, in that it allows the invention to support shaped pulses in TDMA systems without increasing the resolution required in the quadrature base band representation. Figure 4 shows yet another example of the present invention, which is similar to example of Figure 3, although with a number of modifications. (I,-Q) signals contain both the amplitude and phase content of the modulation. They are mixed with a carrier that contains only the phase content of the feedback signal. The output signal of the complex modulator carries the amplitude content of the modulation and the phase difference between the modulation and the feedback signal. Therefore, in the example of Figure 4 the complex error signal is obtained by mixing I and -Q signals with a constant envelope version of the feedback signal. For this purpose, a limiter 19 is inserted into the feedback path before the modulator and removes the amplitude content of the feedback signal fed through the modulator. In addition, the feedback signal is also fed through an envelope detector (6a) corresponding to that used in previous examples, the output of which is fed to the comparator 7 , and compared to the output of the envelope detector (6b) already included in path A allowing a straightforward subtraction of amplitude to be performed in the feedback loop.
This particular configuration has further advantages over the earlier examples. Firstly, there is no need to calculate the inverse of the I and Q signals. Secondly, example of Figure 4 provides a linear behaviour for the amplitude loop as long as the power amplifier operates in its linear region, rather than the non-linear comparison of inverses of the earlier examples. The linearity feature allows the amplitude loop bandwidth to be reduced as compared to the previous examples. This is a significant improvement as the amplitude loop bandwidth is the result of a compromise: the bandwidth must be made much wider than the modulation bandwidth in order to apply the linearisation effect properly but is restricted by the limitations of the electronic components. So an easier compromise may be found and better performance should be obtained. Furthermore, by providing two envelope detectors which can be provided by a single semiconductor device or on very similar devices they will operate in a similar fashion over varying temperatures and auto-conversation of temperature related affects can be obtained. Additionally, the limiter 19 removes the amplitude content of the feedback signal. Therefore the I and Q modulator operates with a constant envelope signal on its carrier input allowing the mixers associated with the I and Q signal to operate at a higher carrier input level, relaxing their operational requirements and the specification placed on the devices providing their function.

Claims

1. A modulation feedback system for use with a transmitter power amplifier, the system comprising: means for sampling, in use, the output of a power amplifier; vector modulator means for receiving the output from the sampling means and applying a Cartesian representation of the desired modulation thereto; and means for receiving the output of the vector modulator means and means for employing the output to modulate, both in phase and amplitude terms, the input to the power amplifier in use.
2. A system according to claim 1, further comprising a frequency convertor for mixing the output of a reference frequency oscillator with the signal from the sampling means .
3. A system according to claim 1 or claim 2, further comprising a variable gain amplifier or variable attenuator for scaling the output of the sampling means.
4. A system according to any preceding claim further comprising a phase modulator comprising a limiter for limiting a signal from the vector modulator; a frequency mixer for mixing a reference with the output of the limiter; and a phase loop filter for filtering the output of the frequency mixer and applying an input to a frequency controlled oscillator.
5. A system according to any preceding claim further comprising an amplitude modulator comprising: an envelope detector; a comparator for comparing the output of the envelope detect or with a reference amplitude; and a amplitude loop filter.
6. A system according to claim 4, further comprising: a first envelope detector for receiving the output of the vector modulator means and providing it to a comparator; an amplitude limiter placed at the input of the vector modulator for removing the amplitude of the input thereto; and a second envelope detector for receiving the mixed feedback signal from the output of the variable gain amplifier and forwarding it to the comparator.
PCT/GB2001/000307 2000-01-27 2001-01-26 Transmission feedback system WO2001056147A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU30337/01A AU3033701A (en) 2000-01-27 2001-01-26 Transmission feedback system

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
EP00300612A EP1120903A1 (en) 2000-01-27 2000-01-27 Transmission feedback system
EP00300612.9 2000-01-27
GB0013680A GB0013680D0 (en) 2000-06-05 2000-06-05 Transmission feedback system
GB0013680.4 2000-06-05

Publications (1)

Publication Number Publication Date
WO2001056147A1 true WO2001056147A1 (en) 2001-08-02

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WO (1) WO2001056147A1 (en)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7076698B2 (en) 2003-05-21 2006-07-11 Agere Systems, Inc. Vector monitor, related method of controlling a transmitter and transmitter employing the same
US7345531B2 (en) 2003-05-21 2008-03-18 Agere Systems Inc. Comparator circuit, related method of comparing a vector to comparison boundaries and transmitter employing the same
CN100477511C (en) * 2004-01-22 2009-04-08 美国博通公司 System and method for adjusting output power of power amplifier
US8290527B2 (en) 2004-07-30 2012-10-16 Airvana, Corp. Power control in a local network node (LNN)
US8503342B2 (en) 2004-07-30 2013-08-06 Airvana Llc Signal transmission method from a local network node
US9876670B2 (en) 2004-07-30 2018-01-23 Commscope Technologies Llc Local network node

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2532491A1 (en) * 1982-08-24 1984-03-02 Thomson Csf Linearising device for high-frequency amplifier.
US5101172A (en) * 1989-12-26 1992-03-31 Mitsubishi Denki Kabushiki Kaisha Linear amplifier
US5175879A (en) * 1991-04-25 1992-12-29 Motorola, Inc. Linear amplifier with feedback path and phase error compensation
US5182524A (en) * 1992-03-10 1993-01-26 The Regents Of The University Of Calif. Method and apparatus for stabilizing pulsed microwave amplifiers
US5420536A (en) * 1993-03-16 1995-05-30 Victoria University Of Technology Linearized power amplifier
US5675288A (en) * 1994-07-08 1997-10-07 Alcatel Espace Method of linearizing a non-linear amplifier, linearization circuit and amplifier including a circuit of this kind

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2532491A1 (en) * 1982-08-24 1984-03-02 Thomson Csf Linearising device for high-frequency amplifier.
US5101172A (en) * 1989-12-26 1992-03-31 Mitsubishi Denki Kabushiki Kaisha Linear amplifier
US5175879A (en) * 1991-04-25 1992-12-29 Motorola, Inc. Linear amplifier with feedback path and phase error compensation
US5182524A (en) * 1992-03-10 1993-01-26 The Regents Of The University Of Calif. Method and apparatus for stabilizing pulsed microwave amplifiers
US5420536A (en) * 1993-03-16 1995-05-30 Victoria University Of Technology Linearized power amplifier
US5675288A (en) * 1994-07-08 1997-10-07 Alcatel Espace Method of linearizing a non-linear amplifier, linearization circuit and amplifier including a circuit of this kind

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7076698B2 (en) 2003-05-21 2006-07-11 Agere Systems, Inc. Vector monitor, related method of controlling a transmitter and transmitter employing the same
US7345531B2 (en) 2003-05-21 2008-03-18 Agere Systems Inc. Comparator circuit, related method of comparing a vector to comparison boundaries and transmitter employing the same
CN100477511C (en) * 2004-01-22 2009-04-08 美国博通公司 System and method for adjusting output power of power amplifier
US8290527B2 (en) 2004-07-30 2012-10-16 Airvana, Corp. Power control in a local network node (LNN)
US8311570B2 (en) 2004-07-30 2012-11-13 Airvana Llc Method and system of setting transmitter power levels
US8503342B2 (en) 2004-07-30 2013-08-06 Airvana Llc Signal transmission method from a local network node
US8886249B2 (en) 2004-07-30 2014-11-11 Airvana Lp Method and system of setting transmitter power levels
US9876670B2 (en) 2004-07-30 2018-01-23 Commscope Technologies Llc Local network node

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