as) United States
`a2) Patent Application Publication co) Pub. No.: US 2009/0027119 Al
`(43) Pub. Date: Jan. 29, 2009
`
`Williamset al.
`
`US 20090027119A1
`
`(54) REDUCED DISTORTION RADIO
`FREQUENCY AMPLIFIERS
`
`(75)
`
`Inventors:
`
`Bertrand Jeffery Williams, Austin,
`TX (US); Kelly Mekechuk,Austin,
`TX (US); Thomas Johnson,Austin,
`TX (US); Dan Huslig, Dripping
`Springs, TX (US)
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`(2006.01)
`HO3F 1/26
`(52) US. CMe coeccccccecsssssssssssssssnseeeseeseees 330/149; 330/136
`
`(57)
`
`ABSTRACT
`
`A radio frequency power amplifier system includes a power
`amplifier coupled to an input signal and configured to provide
`an output signal at a radio frequency; a signal cancellation
`system coupledto the inputsignal, and a first feedback signal,
`whichis based on the output signal, and configuredto provide
`an error signal with a reduced level of the input signal; and a
`feedback control system coupledto the error signal and con-
`figured to provide a correction signal that is used to reduce
`distortion in the output signal. A corresponding method
`includes amplifying in a forward path an input signal to
`provide an output signal at a radio frequency; combining a
`reference andafirst feedback signal to provide an error signal
`(22)
`Filed:
`Jul. 25, 2008
`with a reducedlevel ofthe input signal; providing, responsive
`to the error signal and in a. feedback control system that is
`separate from the forward path, a correction signal; and then
`using the correction signal to reduce distortion in the output
`signal.
`
`Correspondence Address:
`LAW OFFICES OF CHARLES W. BETHARDS,
`LLP
`P.O. BOX 1622
`
`COLLEYVILLE, TX 76034 (US)
`
`(73) Assignee:
`
`PulseWave RF, Inc.
`
`(21) Appl. No.:
`
`12/220,594
`
`Related U.S. Application Data
`
`(60) Provisional application No. 60/961,945, filed on Jul.
`25, 2007.
`
` 120
`RADIO FREQUENCY
`POWER AMPLIFIER
`
`& OUTPUT
`
`102
`
`FEEDBACK CONTROL
`SYSTEM
`
`104
`
`ERROR SIGNAL
`
`107
`
`INPUT €
`100
`
`
`
`
`REFERENCE
`SIGNAL
`
`
`
`
`
`
`SIGNAL
`CANCELLATION SYSTEM
`
`
`
`110
`
`FEEDBACK
`SIGNAL
`
`APPLEET AL. EXHIBIT 1008
`
`APPLE ET AL. EXHIBIT 1008
`
`1
`
`

`

`Patent Application Publication
`
`Jan. 29,2009 Sheet 1 of 9
`
`US 2009/0027119 Al
`
`RADIO FREQUENCY
`
`
`
`120
`
`POWER AMPLIFIER
`
`101
`
`O OUTPUT
`
`102
`
`
`ERROR SIGNAL
`
`
`REFERENCE
`FEEDBACK
`SIGNAL
`SIGNAL
`
`0.4
`
`0.2
`
`AMPLITUDE
`(V)
`
`-0.2
`
`-0.4
`33.13
`
`0.4
`
`0.2
`
`9
`
`AMPLITUDE
`(vy)
`
`53.14
`
`35.16
`33.15
`TIME (us)
`
`33.17
`
`33.18
`
`Fig. 4A
`
`-0.2
`
`-0.4
`33.13
`
`33.14
`
`33.15
`
`33.16
`
`TIME (us)
`
`33.17
`
`35.18
`
`Fig. AB
`
`2
`
`

`

`Patent Application Publication
`
`Jan. 29, 2009 Sheet 2 of 9
`
`US 2009/0027119 Al
`
`
`
`
`
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`Patent Application Publication
`
`Jan. 29,2009 Sheet 3 of 9
`
`US 2009/0027119 Al
`
`T “SEQUENCER !
`
`
`FEEDBACK
`350
`
`
`| |
`
`338
`(dBm)
`
`300
`ERROR
`SIGNAL
`
`i
`
`COMPLEX
`LOOP
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`FILTER
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`i
`
`|
`DRIVER
`
`|
`354
`556
`|
`CORRECTION
`SIGNAL
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`
`
`POWER
`
`800
`
`900
`850
`FREQUENCY (MHz)
`
`950
`
`4
`
`

`

`Patent Application Publication
`
`Jan. 29, 2009 Sheet 4 of 9
`
`US 2009/0027119 Al
`
`-20
`
`POWER _49
`(dBm)
`
`-60
`
`~80
`
`0
`
`-20
`
`POWER _4p
`(dBm)
`
`-60
`
`-80
`
`0
`
`-20
`
`POWER _49
`(dBm)
`
`-60
`
`-80
`
`0
`
`-20
`
`POWER _49
`(dBm)
`
`-60
`
`-80
`
`800
`
`900
`850
`FREQUENCY (MHz)
`
`950
`
`800
`
`900
`850
`FREQUENCY (MHz)
`
`950
`
` 0
`
`
`800°
`
`|
`
`900
`850
`FREQUENCY (MHz)
`
`950
`
`800
`
`900
`890
`FREQUENCY (MHz)
`
`950
`
`30
`
`20
`
`10
`
`POWER -10
`(dBm) 5
`
`701
`
`-30
`
`40
`-50
`
`905
`
`702
`
`919
`910
`FREQUENCY (MHz)
`
`920
`
`925
`
`5
`
`5
`
`

`

`Patent Application Publication
`
`Jan. 29, 2009 Sheet 5 of 9
`
`US 2009/0027119 Al
`
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`Patent Application Publication
`
`Jan. 29, 2009 Sheet 6 of 9
`
`US 2009/0027119 Al
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`Patent Application Publication
`
`Jan. 29, 2009 Sheet 7 of 9
`
`US 2009/0027119 Al
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`

`Patent Application Publication
`
`Jan. 29,2009 Sheet 8 of 9
`
`US 2009/0027119 Al
`
`
` CONTROLLED POWER SUPPLY
`|
`1305
`1307
`1509
`
`
`
`ENVELOPE
`SIGNAL (LED
`
`
`DETECTOR||CONDITIONER ney
`
`
`RADIO FREQUENCY
`POWER AMPLIFIER
`
`
`1300
`
`SIGNAL ADJUSTER A
`
`PHASE
`
`
`
`
`
`GAIN
`CONTROL
`
`
`CONTROL
` SIGNAL
`ADJUSTER B
`
` DELAY LINE
`DRIVER FEEDBACK
`
`SIGNAL ADJUSTER C
`GAIN CONTROL
`
`
`
`
`
`
`
`1409
`
`|!
`
`oy
`ig
`
`
`~~SEQUENCER
`
`
`FEEDBACK
`
`1420
`DRIVER
`COMPLEX
`
`LOOP
`
`
`FILTER
`SIGNAL
`|
`|
`
`
`
`1450
`CORRECTION
`SIGNAL
`
`
`
`|
`
`9
`
`

`

`Patent Application Publication
`
`Jan. 29,2009 Sheet 9 of 9
`
`US 2009/0027119 Al
`
`
`
`
`INPUT
`
`SIGNAL
`ADJUSTER AJ
`
`
`CORRECTION
`SIGNALS
`
`OUTPUT
`
`POWER AMPLIFIER
`
`
`
`
`
`REFERENCE
`FEEDBACK
`SIGNAL
`SIGNAL
`
`
`
`
`
`
`
`
`
`AMPLIFYING IN A FORWARD PATH AN INPUT SIGNAL TO
`PROVIDE AN QUTPUT SIGNAL AT RADIO FREQUENCY
`
`COMBINING A REFERENCE SIGNAL AND FEEDBACK SIGNAL TO PROVIDE
`AN ERROR SIGNAL_HAVING REDUCED LEVELS OF THE INPUT SIGNAL
`
`
`PROVIDING IN A FEEDBACK CONTROL SYSTEM THAT IS SEPERATE FROM THE
`FORWARD PATH, A CORRECTION SIGNAL, RESPONSIVE TO THE ERROR SIGNAL
`
`
`
`
`
`
`USING THE CORRECTION SIGNAL TO REDUCE DISTORTION IN THE QUTPUT SIGNAL
`
`
`
`Fig. 76
`
`10
`
`10
`
`

`

`US 2009/0027119 Al
`
`Jan. 29, 2009
`
`REDUCED DISTORTION RADIO
`FREQUENCY AMPLIFIERS
`
`RELATED APPLICATIONS
`
`[0001] This application is related to and claimspriority
`from Provisional Application bearing Ser. No. 60/961,945,
`filed Jul. 25, 2007, titled REDUCED DISTORTION RADIO
`FREQUENCY AMPLIFIERS, by INVENTORS: BER-
`TRAND JEFFERY WILLIAMS; KELLY MEKECHUK;
`THOMAS JOHNSON; DAN HUSLIG,which application is
`incorporated hereinin its entirety by reference.
`
`FIELD OF THE INVENTION
`
`[0002] This invention relates in general to radio frequency
`power amplifier apparatus and methods and morespecifically
`to techniques and apparatus for reducing distortion in radio
`frequency power amplifiers.
`
`BACKGROUND OF THE INVENTION
`
`[0003] Radio frequency power amplifiers are known and
`variousclasses, from linear to hard switching classes, of such
`amplifiers have been utilized. All of these amplifiers have
`tradeoffs betweenlinearity, efficiency, and economic consid-
`erations. Generally, linearity comesat the cost ofefficiency.
`[0004] One recently develop power amplifier system is
`shownin FIG.8. In FIG. 8 a feedback approach is used, where
`the input signal and a feedback signal are combined with the
`feedback control system processing a combination of the
`input signal and the feedback signal. This can burden the
`feedback control system in terms of required dynamic range.
`[0005] Additionally, the quantization noise, largely out of
`bandfor the system of FIG. 8 can be higher than desirable and
`this can burden the power amplifier, since a portion ofits
`dynamic range is devoted to out of band power generation.
`[0006]
`Feed forward radio frequency power amplifiers,
`such as shown in FIG. 10 have been used wherein an error
`signal based on the difference between a power amplifier
`output signal and the input signal is fed forward and used to
`cancel the distortion. These systems are notoriously ineffi-
`cient, difficult to align so as to operate effectively, difficult to
`maintain alignment over environmental variables, and can be
`expensive.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0007] The accompanying figures where like reference
`numerals refer to identical or functionally similar elements
`throughout the separate views and which together with the
`detailed description below are incorporated in and form part
`ofthe specification, serve to furtherillustrate various embodi-
`ments and to explain various principles and advantagesall in
`accordance with the present invention.
`[0008]
`FIG. 1 depicts in a simplified and representative
`form,a high level diagram of a radio frequency power ampli-
`fier system with reduced distortion in accordance with one or
`more embodiments;
`[0009]
`FIG. 2 in a representative form, shows a more
`detailed embodimentof a reduced distortion radio frequency
`poweramplifier using principles in accordance with FIG.1;
`[0010]
`FIG. 3 depicts a representative diagram of a feed-
`back control system suitable for use in the FIG. 1 or FIG.2,
`among others, radio frequency power amplifiers in accor-
`dance with one or more embodiments;
`
`FIG. 10 showsa priorart feed forward power ampli-
`
`FIG. 4A-4B depicts diagrams of a representative
`[0011]
`correction signal from the FIG.3 feedback control system and
`an exemplary input signal;
`[0012]
`FIG. 5 depicts a representative power amplifier out-
`put spectrum with and withoutthe feedback control system of
`FIG.3;
`FIG. 6A-6D illustrates representative signal spec-
`[0013]
`trumsobserved in simulation results at various points in the
`FIG. 2 embodiment;
`[0014]
`FIG. 7 showsexperimentalresults for the radio fre-
`quency amplifier of FIG. 2 with and without the feedback
`control system of FIG.3;
`[0015]
`FIG. 8 shows a block diagram ofa prior art radio
`frequency power amplifier;
`[0016]
`FIG. 9 shows a comparison of output spectrums for
`simulations of the amplifier embodiment of FIG. 2 and FIG.
`8;
`[0017]
`fier;
`FIG. 11 showsa representative block diagram of a
`[0018]
`reduceddistortion radio frequency power amplifier similar to
`the system of FIG. 1 butillustrates one or more alternative
`embodiments to that shown in FIG.2;
`[0019]
`FIG. 12 illustrates a representative block diagram of
`reduced distortion radio frequency power amplifier employ-
`ing a predistorter and the feedback control system in accor-
`dance with one or more embodiments;
`[0020]
`FIG. 13 depicts a representative block diagram of
`reduced distortion radio frequency power amplifier employ-
`ing a controlled supply and the feedback control system in
`accordance with one or more embodiments;
`[0021]
`FIG. 14 depicts an alternative feedback control sys-
`tem employing a synchronous sequencerin accordance with
`one or more embodiments;
`[0022] FIG.15 showsa representative block diagram show-
`ing additional power amplifiers and feedback control systems
`in accordance with one or more embodiments; and
`[0023]
`FIG. 16 showsa flow chart illustrating representa-
`tive embodiments ofmethods ofreducing distortion ina radio
`frequency power amplifier system.
`
`DETAILED DESCRIPTION
`
`In overview, the present disclosure concerns radio
`[0024]
`frequency poweramplifiers and morespecifically techniques
`and apparatus, in radio frequency power amplifiers, that are
`arranged and constructed for separating an input signal from
`a feedback signal to provide a correction signalthat is used to
`reduce distortion generated by the radio frequency power
`amplifier will be discussed and disclosed.
`[0025] The radio frequency power amplifiers that are of
`interest may vary widely but include such amplifiers used for
`wireless transmitters, e.g., Base station Transmitter Systems,
`in Cellular Phone Systems, dispatch systems, broadcast sys-
`tems, or other public or private radio access networks and the
`like.
`
`Theinstant disclosure is providedto further explain
`[0026]
`in an enabling fashion the best modes, at the time of the
`application, of making and using various embodiments in
`accordance with the present invention. The disclosureis fur-
`ther offered to enhance an understanding of and appreciation
`for the inventive principles and advantages thereof, rather
`than to limit in any mannerthe invention. The invention is
`defined solely by the appended claims including any amend-
`
`11
`
`11
`
`

`

`US 2009/0027119 Al
`
`Jan. 29, 2009
`
`ments made during the pendencyofthis application andall
`equivalents of those claimsas issued.
`[0027]
`It is further understood that the use of relational
`terms, if any, such asfirst and second, top and bottom, and the
`like are used solely to distinguish one from another entity or
`action without necessarily requiring or implying any actual
`such relationship or order between such entities or actions.
`[0028] Muchof the inventive functionality and manyofthe
`inventive principles are best implemented with or in inte-
`grated circuits (ICs) including possibly application specific
`ICs or ICs with integrated processing controlled by embed-
`ded software or firmware. It is expected that one of ordinary
`skill, notwithstanding possibly significant effort and many
`design choices motivated by, for example, available time,
`current
`technology, and economic considerations, when
`guided by the concepts andprinciples disclosed herein will be
`readily capable of generating such software instructions and
`programsand ICs with minimal experimentation. Therefore,
`in the interest of brevity and minimization of any risk of
`obscuring the principles and concepts according to the
`present invention, further discussion of such software and
`ICs, if any, will be limited to the essentials with respect to the
`principles and concepts of the various embodiments.
`[0029] Referring to FIG. 1, a simplified and representative
`high level diagram of a radio frequency power amplifier sys-
`tem with reduced distortion in accordance with one or more
`
`embodiments will be briefly discussed and described. The
`FIG.1 diagram will be used to discuss and describe various
`methods and apparatus to reduce distortion in a radio fre-
`quency power amplifier. Throughout the following discus-
`sions reference numerals maybe usedto refer to a physical
`elementas well as to signals andthe like that can be presentat
`that element, e.g., an output 102 mayrefer to an output signal
`at that output or to the actual physical output, where the
`context can generally be used to distinguish one from the
`other.
`
`[0030] The general block diagram of a power amplifier
`system with reduced distortion of FIG. 1 includes three main
`elements: 1) a radio frequency power amplifier 120, 2) a
`feedback control system 122, and 3) a signal cancellation
`system 124all inter coupled asillustrated. The power ampli-
`fier 120 or radio frequency power amplifier has an output
`signal 102 greater than its input signal 100, specifically 101,
`and, during the amplification process, the power amplifier
`typically addsor creates distortion (often referredto as native
`distortion) due to various non-linearity’s of the power ampli-
`fier, which appears in the output signal. The power amplifier
`system of FIG. 1 can and is arranged and configured to reduce
`the native distortion which can be generated by the power
`amplifier and which would otherwise be presentin the output
`signal 102.
`[0031] The distortion in or generated by the power ampli-
`fier 120 is reduced by injecting a correction signal 106 gen-
`erated by the feedback control system 122 intothe signal path
`of the power amplifier 120. A feedback loop is created by
`signal flow from the output signal 102, through the signal
`cancellation system 124, through the feedback control system
`122, and then back to the output 102 through the power
`amplifier 120. One or more embodiments of the feedback
`control system 122, or portions thereof, are similar to various
`systemsandthe like described in pending patentapplications
`with Ser. No. 11/089,834, filed Mar. 25, 2005, now U.S. Pat.
`No. 7,352,237,
`issued Apr. 1, 2008, titled RADIO FRE-
`QUENCY POWER AMPLIFIER AND CORRESPONDING
`
`METHOD by inventors, WILLIAM MARTIN SNEL-
`GROVE, KELLY MEKECHUK, DAVID KELLY, RICH-
`ARD WILSON;Ser. No. 11/413,998, filed Apr. 28, 2006,
`titled RADIO FREQUENCY POWER AMPLIFIER AND
`METHOD USING A PLURALITY OF FEEDBACKSYS-
`
`TEMS by inventors, KELLY MEKECHUK, WILLIAM
`MARTIN SNELGROVE; Ser. No. 11/413,999, filed Apr. 28,
`2006, titled RADIO FREQUENCY POWER AMPLIFIER
`AND METHOD USING AN AMPLITUDE CONTROL
`
`SYSTEM by inventors, WILLIAM MARTIN SNEL-
`GROVE, DAVID LOVELACE, RICHARD WILSON,
`KELLY MEKECHUN, THOMASA. BLEASE,Jr; and Ser.
`No. 11/818,925, filed Jun. 15, 2007, titled RADIO FRE-
`QUENCY POWER AMPLIFIER AND METHOD USINGA
`CONTROLLED SUPPLYby inventors, WILLIAM MAR-
`TIN SNELGROVE, KELLY MEKECHUK,each ofwhichis
`hereby includedin their entirety by reference.
`[0032] The input to the feedback control system is an error
`signal 104, which is the output of the signal cancellation
`system 124. The signal cancellation system will control the
`level of the desired or input signal, i.e., reference signal 108
`(correspondsto or is based on the input signal 100), which is
`removed from the feedback signal 110. In one or more
`embodiments of the signal cancellation system 124,
`the
`amplitude and phase ofthe feedback signal 110 and reference
`signal 108 are appropriately conditioned to generate the
`required input signal attenuation or level reduction in the
`error signal 104.
`[0033] The signal cancellation system uses two signals at
`its inputs; the reference signal 108 whichis or correspondsto
`or is based on the input signal 100 and the feedback signal
`110, which is or corresponds to or is based on the output
`signal 102 from the power amplifier system, to control the
`cancellation or reduction in level of the desired or reference
`
`signal or input signal componentin the feedback signal 110.
`The output error signal 104 from the signal cancellation sys-
`tem is therefore similar to the feedback input signal 110
`except for a reduction in amplitude of the desired or input
`signal components. Usually the signal cancellation system is
`aligned to generate an error signal 104 which primarily con-
`sists ofresidualdistortion in the amplifier output signal. How-
`ever,
`it may in certain embodiments be advantageous or
`appropriate to adjust the signal cancellation system to reduce,
`e.g., to a predeterminedlevel, rather than fully attenuate, the
`desired signal components in the feedback signal. For
`example, in some embodimentsit is appropriate to reduce the
`level of the input or desired signal to a predeterminedlevel
`and then use the feedback control system to add or subtract
`the amountof desired or input signalin the correction signal.
`This helps to compensate for imperfect signal cancelation
`systemsor allowsfor the use ofmore economically attractive
`cancellation systems and devices.
`[0034]
`FIG. 1 and these discussions illustrate and have
`described a radio frequency power amplifier system compris-
`ing a radio frequency power amplifier 120 having an input
`101 coupled to an input signal 100 and configuredto provide
`an output signal at 102, which is at a radio frequency and
`reasonably high powerlevels. The power amplifier 120 as
`suggested by FIG. 1 can have a multiplicity of gain stages and
`some of these can include parallel gain stages as known.
`Further shown is a signal cancellation system which is
`coupledto a reference signal 108, where the reference signal
`correspondsto or is based on or is possibly equivalent to the
`input signal 100. The signal cancellation system is also
`
`12
`
`12
`
`

`

`US 2009/0027119 Al
`
`Jan. 29, 2009
`
`coupledto a first feedback signal 110, which is based on or
`correspondsto the output signal. The signal cancellation sys-
`tem is arranged and configuredto provide an error signal 104,
`wherethe error signal has a reducedlevel ofthe input signal.
`The signal cancellation system essentially reducesthe level of
`the reference signal,i.e., input signal 100, whichis present in
`the output signal and provides the result as the error signal
`104. The radio frequency power amplifier further includes a
`feedback control system 122 that is coupled to the error signal
`and configured to provide a correction signal 106, where the
`correction signal is used to reduce distortion in the output
`signal. Various embodiments of the feedback control system
`and their uses to reduce distortion will be described herein
`below.
`
`[0035] Referring to FIG. 2, a representative but more
`detailed embodimentof a reduced distortion radio frequency
`power amplifier system using principles in accordance with
`FIG. 1 will be discussed and described. In the exemplary
`embodiment of FIG. 2, the signal cancellation system is
`implemented with three signal adjuster functions or blocks
`(signal adjusters 202, 204, 206) and a signal combiner 212.
`Signal adjuster A 202 adjusts the input signal (level and
`phase) and signal adjuster C 206 adjusts the level of the
`feedback signal. These signal adjusters 202, 204, 206 can be
`usedto align the phase and amplitude of the feedback signal
`230 such that the signal componentor desired component in
`the feedback signal 230 (output signal 232 as adjusted by 206)
`is reducedat the output 214 of the signal combiner 212 (1.e.,
`in the error signal 214). This can be done in an experimental
`manneror otherwise. The combiner 212 cancels essentially
`all or a predetermined portion of the input signal component
`that would otherwise be in the error signal. In some embodi-
`ments the signal cancellation system is configured to provide
`the error signal with a predeterminedlevel ofthe input signal.
`Thus, the signal cancellation system is comprised of signal
`adjusters that adjust the amplitude and phase of the feedback
`signal or the amplitude or phase ofthe reference signal, such
`that the level of the input signalin the error signal is reduced
`appropriately.
`[0036] After signal cancellation, the error signal 214 is
`primarily residual distortion which the feedback control sys-
`tem 238 uses to synthesize a correction signal 240 for the
`power amplifier 220. The correction signal 240 is summed
`with the signal adjuster A output signal 241 to form an ampli-
`fier input signal 244 to the power amplifier 220. After ampli-
`fication this is provided at output 102 and as an input signal
`232 to adjuster 206. Thus FIG. 2 illustrates an embodiment
`where the correction signal and the input signal as adjusted
`are coupled to the input 244 of the radio frequency power
`amplifier 220.
`[0037]
`Signal adjuster B (204), e.g., a delay line, is used to
`improve the bandwidth ofthe signal cancellation in the feed-
`back loop. The delay line compensates for the delay of the
`desired signal componentasit passes through signal adjuster
`A (202), the power amplifier (220), and signal adjuster C
`(206) connecting to the feedback input port 230 of the signal
`combiner 212. Signal adjusters A, B, C and there respective
`functions are generally known.
`[0038] Referring to FIG. 3, a representative diagram of a
`feedback control system suitable for use in the FIG.1 or FIG.
`2 radio frequency power amplifier systems, among others, in
`accordance with one or more embodiments will be discussed
`and described. The feedback control system in FIG. 3
`includes, amongothers, a loop filter 312 and a sequencer 338
`
`and is arranged and constructed to synthesize a quantized,
`e.g., two level quantized amplitude, correction signal 356
`(supplied by feedback driver 354) with, in some embodi-
`ments variable pulse widths from an input error signal 300.
`The amplitude quantized correction signal leads to a robust
`feedback system that may otherwise be more difficult to
`implement. An example of a typical correction signal gener-
`ated by the feedback control system is shown in FIG.4A.It
`will be observedthat the correction signal comprises a wave-
`form with quantized amplitude occurring at an average fre-
`quency in accordance with or equivalent to the radio fre-
`quencyof the output signal with timing (pulse widths or zero
`crossings) determined by the feedback control system.
`[0039] With reference again to FIG.2, the amplitude quan-
`tized correction signal 240 (analogous to correction signal
`356) is combined with a signal 241, i.e., the input signal 100,
`as adjusted, from the output of signal adjuster A, to synthesize
`the amplifier input signal 244. FIG. 4B shows an example of
`signal 241 at the output of signal adjuster A.
`[0040] Within the feedback control system of FIG. 3 there
`is a pair of down convert mixers 310 (complex mixers) which
`using local oscillators, LO1 320, LO2 321 translate the carrier
`frequency of the inputerror signal 300 to a complex interme-
`diate frequency with in-phase and quadrature components.
`The intermediate frequency facilitates use of lower cost
`implementations ofa highly selective complex bandpassfilter
`or loop filter 312 (e.g., in one or more embodiment a fourth
`order bandpassfilter with a bandwidth of 15 MHz and center
`frequency ofapproximately 40 MHz) to shapethe error signal
`spectrum.Afterfiltering, the signalis up converted using LO3
`324, LO4 325, through a pair of mixers 314 and summedvia
`combiner 330 to synthesize and provide a sequencer input
`signal 332.
`[0041] The FIG.2 radio frequency power amplifier system
`and feedback control system of FIG. 3 illustrates a system
`where the input signal 100 is a radio frequency signal at the
`radio frequency. It will be appreciated that the input signal
`could be a complex signal at a frequency whichis different
`thanthe radio frequency, i.e. a complex basebandsignal at a
`zero frequency carrier or a complex basebandsignal with an
`intermediate frequency(IF) carrier. For ease of reference this
`will be referred to as a baseband signal or input baseband
`signal. In such cases, the feedback signal 230 would need to
`be downconverted via complex mixers, similar to mixers 310,
`to form an intermediate complex feedback signalat the carrier
`frequency of the basebandsignal, and combined with a com-
`plex combinerwith the basebandsignal with the output from
`the complex combiner applied directly to the complex loop
`filter 312. Similarly the baseband input signal would need to
`be up converted with complex mixers, similar to mixers 314,
`either before or after signal adjuster A 202. Thus, the radio
`frequency power amplifier system can operate with baseband
`input signals, e.g., by including complex mixers for fre-
`quency conversion of the input signal from a basebandfre-
`quency to the radio frequency and for frequency conversion
`of the feedback signal from the radio frequencyto the base-
`band frequency.
`[0042] An embodiment of a sequencer 338 as shown in
`FIG. 3 consists of a D flip-flop 340 and a timer 342. The
`sequencer embodiment of FIG. 3 is self clocked, i.e., is an
`asynchronous sequencer. The sequencer outputstate at 350 is
`normally high, and a rising edge zero-crossing on the
`sequencer input signal 332 changes the output state of the
`flip-flop or sequencer 350 from a high to low voltage, where
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`US 2009/0027119 Al
`
`Jan. 29, 2009
`
`the low output voltage condition can also be referred to as an
`off state. In some embodiments, the sequencer input signal
`332 also connects to a timer 342. The timer is normally
`triggered by the highto low transition of the outputstate 350
`and controls the reset input 351 ofthe flip-flop. The timer is
`typically set to generate an off state time for a preset time
`interval. The off state time interval is usually selected to(be
`approximately half the period of the carrier frequency of the
`radio frequency output signal from the power amplifier. After
`the timerexpires, it generates an output signal 351to reset the
`flip-flop whichreturnstheflip-flop output 350 to a high state.
`The timercanberetriggered by subsequent rising edge zero-
`crossings at the sequencer input signal 332 which extend the
`off time to an interval longer than half the radio frequency
`carrier period.
`[0043] The feedback control system may also include a
`sequencer feedback network or system 352 which can
`improve the performanceoferror correction depending,e.g.,
`on the bandwidth and nonlinearity ofthe power amplifier. The
`sequencer feedback system or network 352 is arranged and
`configuredto or for coupling a portion ofthe sequenceroutput
`350 to the sequencer input 332 via the combiner 330. In the
`embodiment shownin FIG.3, a transversal filter structure is
`employed in the sequencer feedback network 352 which con-
`sists of four branches with delays of T/2, T, 3T/2, and 2T,
`respectively, where T is the period of the radio frequency
`carrier. Each branch has an output weight 353 that determines
`the response of the sequencer feedback system.
`[0044]
`Simulation and experimentalresults of the embodi-
`ment shown in FIG. 2 have been obtained for the feedback
`control system shownin FIG.3. Simulations and correspond-
`ing results for the embodiment shownin FIG. 2 employing the
`feedback control system of FIG. 3 will be discussed.
`[0045] A behavioral model of a power amplifier is used for
`the simulations which includes a nonlinear gain response.
`The input source signal 100 is two sinusoidal tones with
`frequencies of 873.15 MHz and 877.78 MHz. FIG. 5 depicts
`a representative power amplifier output spectrum illustrating
`the impact of the feedback control system of FIG. 3. The
`native distortion 501 of the power amplifier without any dis-
`tortion correction is shown in FIG. 5 for the two tone source
`
`and the amplifier has a total output powerof about 58 W. The
`figure shows the amplifier output signal in the frequency
`domain, and the native distortion, e.g., third order intermodu-
`lation products 501 are approximately —31 dBc (dBrelative to
`carrier) relative to the desired signals. With the proposed
`distortion reduction apparatus and method, i.e., signal can-
`cellation and feedback control systems of FIG. 2 and FIG.3,
`which are configured to linearize the power amplifier, third
`order intermodulation products 502 are reduced to -73 dBc
`demonstrating a reduction in third order intermodulation of
`more than 40 dB.
`
`[0046] Examples of the corresponding reference signal
`210, feedback signal 230, error signal 214 and correction
`signal 240 are shown in FIG. 6A-6D for the aforementioned
`simulation ofthe embodiment shown in FIG. 2 employing the
`feedback control system ofFIG.3. The signals in FIG. 6A-6D
`are shownin the frequency domain wherethe input signalis
`two sinusoidal tones with frequencies of 873.15 MHz and
`877.78 MHz. Thereference or input signal spectrum in FIG.
`6A correspondsto the reference signal 210 or input signal 100
`in FIG. 2. The spectrum consists oftwo tonesfree ofany noise
`and distortion products over a dynamic range that exceeds 90
`dB. The feedback signal spectrum in FIG. 6B corresponds to
`
`signal 230 in FIG. 2, and is similar to the amplifier output
`signal 102 in FIG. 2, except scaled in amplitude. The feed-
`back spectrum showsthird order intermodulation products
`whichare less than —-73 dBcrelative to the desired signals and
`is similar to the amplifier output spectrum shownin FIG.5.
`After the reference signal 210 and the feedback signal 230 are
`combined with appropriate signal adjuster settings to gener-
`ate an error signal 214, the desired signal componentin the
`error spectrum is reduced by signal cancellation. FIG. 6C
`showsthe error signal spectrum, and compared with the feed-
`back signal spectrum in FIG.6B, the desired signal is reduced
`by more than 40 dB. The spectrum ofthe correction signal
`240 generated by the feedback control system is shown in
`FIG. 6D. The correction signal 240 is then summed with the
`signal 241 in FIG.2 to form the input signal 244 for the power
`amplifier.
`[0047] Experimental results for a circuit employing the
`feedback control system in FIG.3 in the embodimentof FIG.
`2 are shownin FIG.7. Forthis experiment, the source signal
`is a WCDMA(Wideband Code Division Multiple Access)
`modulated radio frequencycarrier at a frequency of 915 MHz
`with a peak to average ratio of approximately 7.6 dB. The
`measured output power is 20 W for a power amplifier with a
`1 dB compression point of approximately 110 W. The pro-
`posed method reduces signal distortion in the adjacent signal
`band by more than 15 dB comparedto the spectrum of the
`amplifier without linearization. Trace 701 in FIG. 7 corre-
`spondsto the nativedistortion in the amplifier, while trace 702
`corresponds to the amplifier response with linearization or
`distortion reduction.
`
`[0048] One distinction between the present approach for
`distortion reduction compared to other approaches is the
`addition of a signal cancellation system. The signal cancela-
`tion system operates to improve the dynamic range of error
`correction in the power amplifier system, specifically, the
`feedback control system. The difference between the pro-
`posed method and one other approachis illustrated by com-
`paring an example approach, shownin FIG.8 (also see one or
`more of the co-pending applications identified above), with
`the embodiment of the proposed method, in FIG. 2. In the
`FIG.8 approach, the feedback control system 804 synthesizes
`a power amplifier input signal 806 which consists of