U.S. patent application number 10/034016 was filed with the patent office on 2002-07-04 for closed loop active cancellation technique (act) - based rf power amplifier linearization architecture.
This patent application is currently assigned to SPECTRIAN CORPORATION. Invention is credited to Avis, Steve, Eisenberg, John.
Application Number | 20020084845 10/034016 |
Document ID | / |
Family ID | 26710445 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020084845 |
Kind Code |
A1 |
Eisenberg, John ; et
al. |
July 4, 2002 |
Closed loop active cancellation technique (ACT) - based RF power
amplifier linearization architecture
Abstract
A closed loop, active cancellation technique (ACT)-based RF
power amplifier linearization architecture injects a pilot tone as
a `pseudo distortion` signal into signal paths through first and
second matched RF amplifiers, and a set of power minimization loops
are closed around the RF amplifier pair. The power minimizing
control loops control a set of vector modulators such that both the
injected pilot tone and intermodulation distortion products are
canceled, while RF carrier components constructively sum in the
composite output of the two RF amplifiers.
Inventors: |
Eisenberg, John; (Los Altos,
CA) ; Avis, Steve; (Pleasanton, CA) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Assignee: |
SPECTRIAN CORPORATION
Sunnyvale
CA
|
Family ID: |
26710445 |
Appl. No.: |
10/034016 |
Filed: |
December 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60259012 |
Dec 29, 2000 |
|
|
|
Current U.S.
Class: |
330/52 ;
330/151 |
Current CPC
Class: |
H03F 1/3235
20130101 |
Class at
Publication: |
330/52 ;
330/151 |
International
Class: |
H03F 003/66 |
Claims
What is claimed:
1. An RF power amplifier apparatus comprising: an RF input port to
which an RF input signal is applied; an RF output port from which
an amplified RF output signal is derived; first and second RF
signal processing paths coupled between said input and output ports
and containing first and second RF power amplifiers; said first RF
signal processing path including a first RF signal vector
modulator, that is adapted to controllably adjust a first RF signal
component of said RF input signal applied to said first RF power
amplifier; said second RF signal processing path including an
intermodulation distortion (IMD) extraction circuit coupled to an
output of said first RF power amplifier and to said RF input port,
and including a second RF signal vector modulator adapted to
controllably adjust said RF input signal, and a third RF signal
vector modulator adapted to controllably adjust an IMD component
present in an amplified RF output signal from said first RF power
amplifier, and to generate a second RF signal component as a
combination of a controllably adjusted RF input signal and said
controllably adjusted IMD component; an output signal combiner
coupled to outputs of said first and second RF power amplifiers and
to said output port, and being operative to produce said RF output
signal as a composite of said first and second RF signal
components, in which RF carrier components produced by said first
and second RF power amplifiers constructively sum and IMD
components produced thereby destructively combine; and a vector
modulator controller, which is adapted to control said first,
second and third vector modulators independently of one
another.
2. The RF power amplifier apparatus according to claim 1, wherein
said first and second RF power amplifiers are matched RF power
amplifiers.
3. The RF power amplifier apparatus according to claim 1, wherein
said vector modulator controller is adapted to control one of said
vector modulators based upon power minimization at a signal port of
said output signal combiner.
4. The RF power amplifier apparatus according to claim 3, wherein
said vector modulator controller is adapted to control said first
vector modulator based upon power minimization of carrier energy
leakage in said output signal combiner.
5. The RF power amplifier apparatus according to claim 1, wherein
said IMD extraction circuit includes a cancellation combiner that
is operative to differentially combine a sample of the amplified RF
output signal produced by said first RF power amplifier with a
portion of said RF input signal to derive said IMD component, and
to combine said IMD component with said RF input signal applied to
said second signal processing path, to produce said second RF
signal component as a combination of said RF input signal and said
IMD component, where said IMD component is phase-inverted with
respect to a carrier component at the output of the first RF power
amplifier.
6. The RF power amplifier apparatus according to claim 5, wherein
said vector modulator controller is adapted to control said third
vector modulator based upon power minimization of monitored
distortion energy the output of said carrier cancellation
combiner.
7. The RF power amplifier apparatus according to claim 1, further
including a pilot tone generator, which is operative to inject a
first pilot tone into the signal path through said first RF power
amplifier, and a second pilot tone into the signal path through
said second RF power amplifier, and wherein said vector modulator
controller includes a pilot tone receiver coupled to said output
port of said output signal combiner, and being adapted to control
said third vector modulator based upon pilot tone energy in said
output port of said output signal combiner.
8. The RF power amplifier apparatus according to claim 7, wherein
said vector modulator controller is adapted to control said second
vector modulator based upon pilot tone energy in the composite
output of said output signal combiner.
9. The RF power amplifier apparatus according to claim 8, wherein
said second pilot tone has the same frequency as, but complementary
phase with respect to, said first pilot tone.
10. The RF power amplifier apparatus according to claim 1, further
including a feed-forward RF amplifier loop coupled to said first RF
signal processing path and said output port, and being operative to
provide an amplified RF distortion output signal containing
distortion introduced by said first and second RF amplifiers, but
effectively excluding said RF input signal, and wherein said output
signal combiner is coupled to said feed-forward amplifier loop and
is operative to combine said amplified RF distortion output signal
with said RF output signal to produce a refined RF output
signal.
11. The RF power amplifier apparatus according to claim 10, further
including a fourth vector modulator coupled with said first RF
signal processing path and being adapted to controllably adjust
said first RF signal component of said RF input signal applied to
said first RF power amplifier, and wherein said feed-forward RF
amplifier loop includes a fifth vector modulator adapted to
controllably adjust said amplified RF distortion output signal, and
wherein said vector modulator controller is adapted to control said
fourth and fifth vector modulators independently of one another and
independently of said first, second and third vector
modulators.
12. The RF power amplifier apparatus according to claim 11, wherein
said vector modulator controller is adapted to control said fourth
vector modulator based upon power minimization at a signal port of
said signal combiner.
13. The RF power amplifier apparatus according to claim 12, wherein
said vector modulator controller is adapted to control said fifth
vector modulator based upon pilot tone energy in the composite
output of said output signal combiner.
14. An RF power amplifier apparatus comprising: an RF input port;
an RF output port; a first RF signal processing path coupled
between said RF input and output ports and containing a first RF
power amplifier, and a first RF signal vector modulator, that is
adapted to controllably adjust a first RF signal component of an RF
input signal coupled to said RF input port; a second RF signal
processing path coupled between said input and output ports and
containing a second RF power amplifier, that is effectively matched
with said first RF power amplifier, and an intermodulation
distortion (IMD) extraction circuit coupled to an output of said
first RF power amplifier and to said RF input port, said second RF
signal processing path including a second RF signal vector
modulator, that is adapted to controllably adjust said RF input
signal, and a third RF signal vector modulator, that is adapted to
controllably adjust said IMD component, and to generate a second RF
signal component as a combination of a controllably adjusted RF
input signal and a controllably adjusted IMD component present in
an amplified RF output signal from said first RF power amplifier;
an output signal combiner coupled to outputs of said first and
second RF power amplifiers and to said output port, and being
adapted to produce said RF output signal as a composite of said
first and second RF signal components, in which RF carrier
components produced by said first and second RF power amplifiers
constructively sum and IMD components destructively combine; a
pilot tone generator, which is operative to inject a first pilot
tone into the signal path through said first RF power amplifier and
a second pilot tone into the signal path through said second RF
power amplifier; and a vector modulator controller, which is
adapted to control said first vector modulator based upon power
minimization of carrier energy in a carrier cancellation path of
said intermodulation distortion (IMD) extraction circuit, said
second vector modulator based upon leakage energy in a terminated
output port of said output signal combiner, and said third vector
modulator based upon power minimization of monitored pilot tone
energy at the output of said carrier cancellation combiner.
15. The RF power amplifier apparatus according to claim 14, wherein
said first pilot tone has the same frequency as, but complementary
phase with respect to, said second pilot tone.
16. The RF power amplifier apparatus according to claim 14, further
including a feed-forward RF amplifier loop coupled to said first RF
signal processing path and said output port, and being operative to
provide an amplified RF distortion output signal containing
distortion introduced by said first and second RF amplifiers, but
effectively excluding said RF input signal, and wherein said output
signal combiner is coupled to said feed-forward amplifier loop and
is operative to combine said amplified RF distortion output signal
with said RF output signal to produce a refined RF output
signal.
17. The RF power amplifier apparatus according to claim 16, further
including a fourth vector modulator coupled with said first RF
signal processing path and being adapted to controllably adjust
said first RF signal component of said RF input signal applied to
said first RF power amplifier, and wherein said feed-forward RF
amplifier loop includes a fifth vector modulator adapted to
controllably adjust said amplified RF distortion output signal, and
wherein said vector modulator controller is adapted to control said
fourth and fifth vector modulators independently of one another and
independently of said first, second and third vector
modulators.
18. The RF power amplifier apparatus according to claim 17, wherein
said vector modulator controller is adapted to control said fourth
vector modulator based upon power minimization of a signal port of
said output signal combiner, and said fifth vector modulator based
upon pilot tone energy in the composite output of said output
signal combiner.
19. A method of amplifying an RF signal comprising the steps of:
(a) applying said RF signal to a first RF signal processing path
containing a first RF power amplifier; (b) controllably adjusting
said RF signal applied to said first RF power amplifier; (c)
combining said RF signal and a first amplified RF output signal
from said first RF signal amplifier in a carrier cancellation path
to produce a distortion signal containing distortion in said first
amplified RF output signal; (d) controllably adjusting said
distortion signal to produce an adjusted distortion signal; (e)
controllably adjusting said RF signal to produce a further adjusted
RF signal; (f) combining said adjusted distortion signal with said
further adjusted RF signal to produce a distorted RF signal; (g)
coupling said distorted RF signal to a second signal processing
path containing a second RF power amplifier, that is effectively
matched with said first RF power amplifier; (h) combining said
first amplified RF output signal from said first RF power amplifier
with a second amplified RF output signal from said second RF power
amplifier to produce at an output port an RF output signal in which
RF carrier components produced by said first and second RF power
amplifiers constructively sum and distortion components
destructively combine; and (i) injecting a first pilot tone into
said first signal path through said first RF power amplifier and a
second pilot tone into said second signal path through said second
RF power amplifier; and wherein step (b) comprises controllably
adjusting said RF signal applied to said first RF power amplifier
based upon power minimization of carrier energy in said carrier
cancellation path, step (d) comprises controllably adjusting said
distortion signal based upon power minimization of monitored pilot
tone energy in said RF output signal produced in step (h), and step
(e) comprises controllably adjusting said RF signal to produce said
further adjusted RF signal based upon leakage energy produced in
step (h).
20. The method according to claim 19, further including the step
(j) of coupling a feed-forward RF amplifier loop coupled to said
first RF signal processing path and said output port, and being
operative to provide an amplified RF distortion output signal
containing distortion introduced by said first and second RF
amplifiers, but effectively excluding said RF signal, and wherein
said step (h) further includes combining said amplified RF
distortion output signal with said RF output signal to produce a
refined RF output signal.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims the benefit of now-abandoned
U.S. Provisional Patent Application Serial No. 60/259,012, filed
Dec. 29, 2000, by S. Avis et al, entitled: "Improved ACT
Pre-Distortion Linearized Power Amplifier," assigned to the
assignee of the present application and the disclosure of which is
incorporated herein.
FIELD OF THE INVENTION
[0002] The present invention relates in general to communication
systems, and is particularly directed to a new and improved, closed
loop, Active Cancellation Technique (ACT)-based RF power amplifier
linearization architecture having parallel RF amplifiers coupled in
intermod-complementing predistortion paths of the type disclosed in
the U.S. patent to Mucinieks, U.S. Pat. No. 6,111,462, (hereinafter
referred to as the '462 patent and the disclosure of which is
incorporated herein). By injecting a pilot tone into the signal
transport paths of each of the RF power amplifiers, the invention
is able to close a set of vector modulation control loops and
thereby track and cancel intermodulation distortion products from
the composite output of the pair of RF amplifiers.
BACKGROUND OF THE INVENTION
[0003] Communication service providers are subject to very strict
bandwidth usage spectrum constraints, including technically
mandated specifications and regulations imposed by the Federal
Communications Commission (FCC). These rules require that sideband
spillage, namely the amount of energy spillover outside a licensed
band of interest, be sharply attenuated (e.g., on the order of 50
dB). Although these regulations may be easily met for traditional
forms of modulation, such as FM, they are difficult to achieve
using more contemporary, digitally based modulation formats, such
as M-ary modulation.
[0004] Attenuating the sidebands sufficiently to meet industry and
regulatory-based requirements by such modulation techniques
requires very linear signal processing systems and components.
Although linear components can be produced at a reasonable cost at
the relatively narrow bandwidths (baseband) of telephone networks,
linearizing inherently non-linear components such as RF power
amplifiers can be prohibitively expensive.
[0005] A fundamental difficulty in linearizing RF power amplifiers
is the fact that they generate unwanted intermodulation distortion
products (IMDs) which manifest themselves as spurious signals in
the amplified RF output signal, such as spectral regrowth or
spreading of a compact spectrum into spectral regions that do not
appear in the RF input signal. This distortion causes the
phase/amplitude of the amplified output signal to depart from the
phase/amplitude of the input signal, and may be considered as an
incidental (and undesired) amplifier-sourced modulation of the RF
input signal.
[0006] A brute force and relatively inefficient approach to
linearize an RF power amplifier is to build the RF amplifier as a
large, high power device, and then operate the amplifier at a very
low power level (namely, at only a small percentage of its rated
output power), where the RF amplifier's transfer characteristic is
substantially linear. An obvious drawback to this approach is the
overkill penalty--a costly, inefficient and large sized RF
device.
[0007] Other prior art linearization techniques include baseband
polar (or Cartesian) feedback, post-amplification, feed-forward
correction, and pre-amplification, pre-distortion correction. In
the first approach, the output of the RF power amplifier is
compared to the input, and a baseband error signal is used to
directly modulate the signal which enters the amplifier. In the
second approach, error (distortion)
[0008] present in the RF amplifier's output signal is extracted,
amplified to the proper level, and then reinjected (as a complement
of the error signal back) into the output path of the amplifier, so
that (ideally) the RF amplifier's distortion is effectively
canceled.
[0009] Pursuant to a third approach, a predistortion signal is
injected into the RF input signal path upstream of the RF
amplifier. Ideally, the predistortion signal has a characteristic
equal and opposite to the distortion expected at the output of the
RF amplifier. As a result, when subjected to the (distorting)
transfer characteristic of the RF amplifier, it effectively cancels
the distortion in the output. Predistortion may be made adaptive by
measuring the distortion at the output of the RF amplifier and
adjusting the predistortion control signal to minimize the
distortion of the output signal of the power amplifier during real
time operation.
[0010] In accordance with the `Active Cancellation Technique` (ACT)
RF power amplifier linearization scheme described in the '462
Patent, and shown diagrammatically in FIG. 1, high efficiency RF
power amplifier linearization is achieved by an open loop technique
that adjusts signal components driving a pair of effectively
`matched` RF power amplifiers A.sub.1 and A.sub.2, such that one RF
power amplifier `pre-distorts` the other. Being matched implies
that the two amplifiers A.sub.1, A.sub.2 have essentially the same
transfer characteristics--both in terms of their intended RF
performance and unwanted IMD components they inherently introduce
into their amplified outputs.
[0011] More particularly, an RF input signal to be amplified is
split by a directional coupler CPL1 into two paths. A first path
includes an attenuator or scaling pad ATT and a controlled gain
adjustment G1 and a phase adjustment element .PHI.1, which adjust
the amplitude and phase of the RF input signal prior to being
amplified by the main amplifier A.sub.1. The output of the main
path amplifier A.sub.1 is coupled through a delay stage DL2 to a
first input of an output combining stage OCS (such as a quadrature
hybrid).
[0012] A second split RF input signal path is used to derive a
signal containing both the original RF input signal to be amplified
by the second `matched` amplifier A.sub.2, and a complementary
version of the IMD products which each of the two amplifiers
inherently introduces. IMD products are extracted using carrier
cancellation circuitry WS1-WC1 similar to that found in most
conventional feed-forward RF power amplifiers. The extracted
distortion products are adjusted in amplitude and phase by gain and
phase control elements G1 and .PHI.1 and combined with an
appropriately delayed and scaled sample of the RF input signal at
WC2.
[0013] For this purpose, the second path from the directional
coupler CPL1 is coupled through a delay stage DL1 to a first input
of (Wilkinson) splitter WS1, a first output of which is coupled to
(Wilkinson) combiner WC1. A second output of splitter WS1 is
coupled through a variable gain stage G2 to a first input of
further (Wilkinson) combiner WC2, a second input of which is
coupled to the output of the combiner WC1. A second input of
combiner WC1 is coupled to a directional coupler CPL2 installed in
the output path of main path amplifier A.sub.1.
[0014] The output of combiner WC2, which is a composite of the RF
input signal and complementary distortion products extracted from
the RF amplifier A.sub.1, is coupled through a variable gain stage
G3 and variable phase adjustor .PHI.2 to the matched RF amplifier
A.sub.2. The output of RF amplifier A.sub.2 is coupled to a second
input of output combining stage OCS.
[0015] The amplitude of the RF input signal component of the
composite RF signal driving the amplifier A.sub.2 is adjusted to be
the same as the amplitude of the pure RF input signal driving
amplifier A.sub.1. Namely, the phase and amplitude of the
distortion products are adjusted so that they not only cancel the
distortion products generated by the input signals applied to the
error amplifier A.sub.2, but also replace these distortion products
with equal amplitude anti-phase replicas of these products. Thus,
the delayed output of amplifier A.sub.1 and the undelayed output of
the amplifier A.sub.2 contain equal phase and amplitude amplified
RF input signals and equal amplitude anti-phase distortion
products. Thus, distortion components resulting from the RF input
signal components driving both amplifiers are essentially the
same.
[0016] In the output combining stage OCS, these signals are summed,
so that (desired) amplified RF (carrier) signals add and (unwanted)
distortion products cancel. The output from the combining stage OCS
is therefore an amplified version of the RF input signal, that is
substantially free of distortion, even though both amplifiers
contain distortion products at their outputs. Both amplifiers
contribute essentially equal amounts of amplification power to the
output of the overall system. Operating efficiency is better than
that of a conventional feed forward amplifier because essentially
the entirety of both amplifiers' output power appears at the output
of the combining stage.
[0017] It should be noted that the ACT architecture of FIG. 1 is
not a classic feed-forward architecture. Rather, it is a very
effective type of dual amplifier-based, RF pre-distortion amplifier
structure, in which the source of the energy used to pre-distort
the matched amplifier A.sub.2 is produced by an identical (main)
amplifier A.sub.1, driven by essentially the same input signals as
its matched counterpart. The level of distortion components in the
energy driving the matched amplifier A.sub.2 is on the order of 30
dB below the RF input signal component. Thus, the dynamics of both
amplifiers is controlled by the dominant input signal energy.
[0018] Now although the ACT amplifier-based linearization scheme
described in the '462 Patent is very effective for achieving a
level of non-linear distortion correction at least on the order of
20 dB and greater, a given production device may not be capable of
maintaining this level of performance over a wide range of ambient
temperature and varying power supply voltage.
[0019] One of the reasons for this potential performance
shortcoming of the linearization scheme of the '462 patent is the
fact that it is an open loop architecture, and operates on the
assumption that since the attenuators which adjust the power to the
`matched` main and power amplifiers are slaved together, it can be
reasonably inferred that the resulting output signal and distortion
energy delivered by each amplifier will be exactly the same.
However, investigation by the present inventors on substantial
numbers of practical production power amplifiers linearized in
accordance with the '462 Patent approach has shown this not to be
the case.
SUMMARY OF THE INVENTION
[0020] Pursuant to a first embodiment of the invention, this
amplifier output signal and distortion energy inequality problem is
effectively remedied by injecting a pilot tone as a `pseudo
distortion` signal into the signal transport paths of each of the
pair of RF power amplifiers, in order to track intermodulation
distortion products produced by each amplifier. Prescribed signal
transport paths of the dual amplifier architecture are monitored by
a set of minimization control loops, which control associated
vector modulators, such that both the injected pilot tones and
intermodulation distortion products are canceled, while RF carrier
components are mutually reinforced or constructively sum in the
composite output of the pair of RF amplifiers.
[0021] To this end, the signal flow path to one of the two
`matched` RF amplifiers includes a first vector modulator, which is
controlled by a first digital signal processor-executed carrier
power control mechanism. This first carrier power control mechanism
monitors carrier power measured by a detector at the output of the
carrier cancellation loop, and adjusts the operation of the first
vector modulator, so as to effectively minimize carrier energy,
leaving only an injected pilot tone and amplifier distortion energy
at the output of the carrier cancellation loop.
[0022] A relatively low level, out-of-band pilot tone, which serves
as a `pseudo distortion` signal, is injected into the signal
transport paths of the RF amplifiers, and is used to track and
cancel intermodulation distortion products produced by each
amplifier. The amplifier outputs thus contain a pilot tone
component in addition to the desired RF signal and undesired IMDs.
Since neither IMDs nor the pilot tone are part of the desired
modulated carrier signal being amplified, they constitute unwanted
distortion. The use of the pilot tone as a `pseudo distortion`,
signal allows the pilot to be treated as representative of whatever
noise or distortion is produced by the amplifier pair. By
minimizing the contribution of the pilot tone to the composite
output signal produced by the amplifier pair, IMDs are also
minimized.
[0023] A further vector modulator is also installed in the first
signal transport path feeding the second amplifier, and is
controlled in accordance with a monitored pilot tone-based control
loop, so that the injected pilot tone (and therefore any
intermodulation distortion products) cancel, at the composite
output of the two RF amplifiers.
[0024] A second vector modulator is further installed in the second
signal transport path feeding the second amplifier, and is
controlled in accordance with a power detector coupled to a
terminated port of an output combiner. When the desired RF carrier
components at the terminated port of the output combiner null, the
carrier power levels at first and second input ports of the output
combiner are essentially equal and sum constructively at the output
signal port of the combiner.
[0025] The second vector modulator adjusts a component of input RF
carrier energy so as to ensure that the RF carrier energy applied
to the second amplifier is the same as that applied to the one
amplifier. For this purpose, the output of the power detector
monitoring the terminated output of the output combiner is applied
to a second carrier power minimization-based control mechanism
within the digital signal processor. This second carrier power
control mechanism controls the second vector modulator so as to
minimize any carrier leakage energy at the output combiner's
terminated port, and thereby equalize the carrier inputs to the two
amplifiers.
[0026] The amplitude and phase of the pilot energy injected into
the second amplifier is adjusted by a gain/phase adjustor, so that
when distortion is minimized at the output of the composite
amplifier, the pilot energy is also minimized. The vector modulator
installed in the first signal transport path is controlled so as to
minimize both pilot energy and distortion at the signal output port
of the output combiner. This ensures that the total contribution of
the pilot tone component from the output of the first amplifier is
exactly the opposite of the pilot tone component in the output of
the second amplifier.
[0027] In accordance with a second embodiment, a feed-forward loop
is wrapped around the closed loop, pre-distortion architecture of
the first embodiment. The addition of the feed forward stage
enables the integrated amplifier architecture containing the RF
amplifier linearization stage and the feed forward stage to deliver
extremely high linearity, including the ability to routinely
achieve carrier-to-distortion ratios as high as 85 dB. Efficiency
is significantly better than that which is typically seen in
competitive dual loop feed forward power amplifiers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 diagrammatically illustrates an ACT-stage based RF
power amplifier linearization architecture of the type disclosed in
the above-referenced '462 Patent;
[0029] FIG. 2 diagrammatically illustrates a closed loop ACT
stage-based RF power amplifier linearization mechanism in
accordance with a first embodiment of the present invention;
and
[0030] FIG. 3 diagrammatically illustrates a second embodiment of
the present invention in which a feed forward loop is wrapped
around the enhanced ACT-based RF power amplifier linearization
architecture of FIG. 2.
DETAILED DESCRIPTION
[0031] Before detailing the closed loop ACT stage-based RF power
amplifier linearization architecture of the present invention, it
should be observed that the invention resides primarily in an
arrangement of conventional RF communication circuits and
associated digital signal processing components and attendant
supervisory control circuitry, that controls the operations of such
circuits and components. Consequently, the configuration of these
circuits and components and the manner in which they are interfaced
with other communication system equipment have, for the most part,
been illustrated in the drawings by readily understandable block
diagrams, which show only those specific details that are pertinent
to the present invention, so as not to obscure the disclosure with
details which will be readily apparent to those skilled in the art
having the benefit of the description herein.
[0032] Thus, the block diagram illustrations are primarily intended
to show its major components in a convenient functional grouping,
whereby the present invention may be more readily understood. In
addition, to facilitate an understanding of signal processing flow
through the respective paths of the linearization system of the
various embodiments of the invention, reduced complexity spectral
diagrams of the desired modulated RF carrier and the unwanted IMD
components have been placed throughout the Figures in association
with respective transfer functions of the system components.
[0033] Referring now to FIG. 2, a first embodiment of a closed loop
ACT stage-based RF power amplifier linearization mechanism of the
present invention is diagrammatically illustrated as comprising an
input terminal 10 to which an RF input signal RF.sub.IN (such as a
multicarrier RF signal pictorially represented by a carrier
frequency pair 12) is applied. The RF input terminal 10 is coupled
through a preamplifier 13 to a directional coupler (CPLR 1) 14
installed in a main signal flow path (designated by arrow MP) of a
first, main RF amplifier 20.
[0034] The main signal flow path MP contains a first vector
modulator 16 cascaded with a buffer amplifier 19 feeding the main
RF amplifier 20. As a non-limiting example, the vector modulator
unit 16 may be implemented as a variable gain (attenuator) stage 17
series coupled with a variable phase shifting stage 18. The
variable gain stage 17 and phase shifting stage 18 are controlled
by control signals supplied over a control link 15 from a first
carrier power control mechanism 116 executed within a digital
signal processor (DSP) 100.
[0035] As will be described, via an input 101, this first carrier
power control mechanism 116 monitors the output of a power detector
37, which is coupled to a directional coupler 35 in the output path
of a (pilot tone and distortion signal extracting) carrier
cancellation (Wilkinson) combiner 30 of a carrier cancellation loop
(loop 1). The carrier power control mechanism 116 executes a
standard error minimization (e.g., power or least mean squared
minimization), and adjusts the vector modulator 16 input to the RF
amplifier 20, so that its contribution at the first input 31 of the
carrier cancellation combiner 30 is the same amplitude and opposite
phase as that of the RF carrier at the second input of the combiner
30. This effectively minimizes carrier energy, leaving only an
injected pilot tone (injected via a directional coupler 22 at the
input of amplifier 20) and amplifier distortion energy at the
carrier cancellation combiner output 33.
[0036] The directional coupler 22 in the input path MP to main RF
power amplifier 20 is coupled to one output of a (Wilkinson)
splitter 23, which is coupled to receive a first, relatively low
level, out-of-band pilot tone generated by a pilot tone generator
24. This low level pilot tone is injected into the signal transport
paths of each of the RF power amplifier 20 and a matched companion
RF power amplifier 80, and is used to track and cancel
intermodulation distortion products produced by each amplifier. The
vector modulator 25 is used to adjust the amplitude and phase of
pilot energy incident upon the RF power amplifier 80, so that when
vector modulator 50 is adjusted for optimum distortion cancellation
at an output 93 of an output combiner 90, the pilot energy is
simultaneously nulled at output 93 as well.
[0037] Because the pilot tone is injected immediately upstream of
each RF amplifier 20/80, each amplifier output will contain a pilot
tone component in addition to the desired RF signal and undesired
IMDs. Since neither IMDs nor the pilot tone are part of the desired
modulated carrier signal being amplified, they constitute unwanted
distortion. The use of the pilot tone as a `pseudo noise` signal
allows the pilot to be treated as representative of whatever noise
or distortion is produced by the amplifier pair. Therefore, by
minimizing the contribution of the pilot tone to the composite
output signal produced by the amplifier pair, IMDs are also
minimized. As pointed out above, the distortion minimization scheme
of the invention controls vector modulators for signal paths
feeding the matched amplifier 80 by a pair of control loops, one
monitoring pilot energy at the output of the composite ACT
amplifier, and the other acting to equalize the desired carrier
power at the outputs of the RF power amplifiers 20 and 80, so that
the two injected pilot tones (and therefore any intermodulation
distortion products) cancel, while the desired RF carrier
components constructively sum in the composite output of RF
amplifiers 20/80.
[0038] A second output of splitter 23 is applied to an
amplitude/phase adjustor 25, which sets the amplitude of the pilot
tone and adjusts its phase, for application via a directional
coupler 65 to the input signal path to RF amplifier 80. The
amplitude adjustment is set such that the pilot signal produced at
the output of amplifier 80 and applied to an input 92 of an output
combiner 90, shown configured as a four port combiner hybrid, will
have exactly the same amplitude as the pilot signal produced at the
output of amplifier 20 and applied to input 91 of output combiner
90. Since these two injected pilot signals are mutually antiphase
(owing to the proper phase adjustment imparted by amplitude/phase
adjustor 25), for equal amplitude pilot components at the output of
each amplifier, the two pilots subtract from one another and are
canceled at an output 93 of output combiner 90.
[0039] Because the input path to amplifier 80 also includes a
component of the pilot tone that has been injected into the other
amplifier 20 and extracted via the carrier cancellation combiner
30, the combiner's output port 93 is monitored via a directional
coupler 95 feeding an input port 103 of a pilot tone receiver,
which is coupled to an energy minimization mechanism 110 in the DSP
100. The pilot energy minimization mechanism controls the vector
modulator 50, so as to minimize (drive to zero) any pilot tone
energy at the output combiner's output port 93. This serves to
ensure that the total contribution of the pilot tone component from
the output of amplifier 20 (including the direct component applied
to combiner input 91 and carrier cancellation component applied to
combiner input 92) is exactly the opposite of the pilot tone
component injected via the directional coupler 65 and amplified by
amplifier 80 and applied to the combiner input 92.
[0040] For carrier cancellation, a portion of the amplified output
signal from the main path RF power amplifier 20 is extracted via a
further directional coupler 26 and coupled through an
(output-scaling) attenuator pad 28 to input port 31 of the carrier
cancellation combiner 30. The second input port 32 of combiner 30
is coupled to a first output 42 of a (Wilkinson) splitter 40.
Wilkinson splitter 40 has its input 41 coupled through a delay line
34 in a signal flow path from directional coupler 14. The delay
imparted by the delay line 34 corresponds to the delay in the main
signal path MP through the power amplifier 20, thereby providing
time alignment between the signals applied to carrier cancellation
combiner 30.
[0041] The carrier components at inputs 31 and 32 of the carrier
cancellation combiner 30 are adjusted by the vector modulator to be
of equal carrier amplitude and anti-phase, so that carrier
cancellation occurs at output 33. By subtracting a scaled version
of the amplified output from the main path amplifier 20 from the RF
input signal RF.sub.IN delayed through the first delay line 34, the
carrier cancellation combiner 30 provides an output (absent carrier
energy) representative of an estimate of the residual distortion
produced by the main power amplifier 20 (which includes the `pseudo
distortion` pilot tone produced by the pilot tone generator 24 and
injected by directional coupler 22.
[0042] Carrier cancellation combiner 30 has its output 33 coupled
through a directional coupler 35 to the vector modulator 50
installed in a distortion energy transport path feeding a first
input 61 of a distortion and carrier energy combiner 60. As noted
earlier, the output of the directional coupler 35 is detected by
power detector 37 and represents the residual carrier energy left
over after carrier cancellation in combiner 30. The carrier power
control mechanism 116 minimizes detected carrier energy, by
adjusting the vector modulator 16 input to the RF amplifier 20, so
that its contribution at the first input 31 of the carrier
cancellation combiner 30 is the same as that of the RF carrier at
the second input of the combiner 30. This leaves only the pilot
tone injected at the input of RF amplifier 20 and amplifier
distortion energy at the carrier cancellation combiner output
33.
[0043] Wilkinson splitter 40 has a second output 43 coupled to a
variable gain stage 71 and variable phase shifter 73 of a vector
modulator 70 installed in an input path to a second input 62 of the
pilot/distortion and carrier energy combiner 60. Vector modulator
70 is operative to control the RF carrier energy therethrough so as
to ensure that the RF carrier energy output from amplifier 80 is
the same as that delivered by RF amplifier 20. For this purpose,
the residual power at port 94 of output combiner 90 is monitored by
power detector 97 which is coupled to input port 102 of DSP 100.
The output of power detector 97 is applied to a second carrier
power minimization-based control mechanism 117 within the DSP 100.
This second carrier power control mechanism controls the vector
modulator 70, so as to minimize (drive to zero) any carrier leakage
energy at the output combiner's terminated port 94, and thereby
maximizing the carrier power delivered at port 93 of the output
combiner 90.
[0044] The output 63 of the distortion and carrier energy combiner
60 is coupled through a buffer amplifier 64 and antiphase pilot
tone-injecting directional coupler 65 to RF amplifier 80. Thus, the
input to the RF amplifier 80 includes three components. A first is
RF carrier energy extracted from the RF input 10 via splitter 40
and adjusted by the vector modulator 70 to match the RF carrier
energy applied to the companion RF amplifier 20. A second contains
two distortion energy components--antiphase pilot tone and
IMDs--which, when amplified by the RF amplifier 80, will exactly
oppositely match the pilot and IMD components produced at the
output of RF amplifier 20.
[0045] As in the open loop architecture of FIG. 1, the output
combiner 90 of the multi vector modulator controlling, closed loop
architecture of FIG. 2 produces a summation of the signals applied
to its inputs 91 and 92, so that what is produced at output port 93
is a composite RF signal, in which the desired amplified RF carrier
signals outputs produced by the two RF amplifiers 20 and 80 sum or
constructively combine, and unwanted distortion products (including
the injected pilot tones) destructively combine, or cancel.
[0046] FIG. 3 diagrammatically illustrates a second embodiment of
the invention, having feed forward loops shown in broken lines 300,
that are `wrapped around` the closed loop ACT-based RF power
amplifier linearization architecture of FIG. 2, described above,
and shown in broken lines as ACT RF amplifier linearization stage
200. The addition of the feed forward loops 300 enables the
integrated amplifier architecture (containing the RF amplifier
linearization stage 200 and the feed forward loops 300) to deliver
extremely high linearity, including the ability to routinely
achieve carrier-to-distortion ratios as high as 85 dB. Efficiency
is significantly better than that which is typically seen in
competitive dual loop feed forward power amplifiers.
[0047] For this purpose, the `wrapped around` feed forward loops
300 of the embodiment of FIG. 3 include a buffer amplifier 313 and
a further directional coupler 314 coupled between the RF input
terminal 10 and the main signal flow path of the ACT RF amplifier
linearization stage 200. A first main path through the directional
coupler 314 is coupled to a vector modulator 316, having a
processor-controlled variable gain (attenuator) stage 317 and a
processor-controlled variable phase shifting stage 318 cascaded
with variable gain stage 317.
[0048] The output port 93 of the output combiner 90 of the
amplifier linearization stage 200 is coupled through a circulator
302 to a directional coupler .sup.328, used to extract a portion of
the amplified output signal from the output combiner's output port
93. This extracted output signal is coupled through an
(output-scaling) attenuator pad 329 to a first input 331 of a
carrier cancellation (Wilkinson) combiner 330 within a further
carrier cancellation loop (loop 3). A second input 332 of the
carrier cancellation combiner 330 is coupled to a first delay line
336 installed in a second signal flow path from the directional
coupler 314. The delay imparted by the delay line 336 corresponds
to the delay in the signal path through the amplifier stage 200,
and serves to time-align the two signals applied to the carrier
cancellation combiner 330. By subtracting a scaled and antiphase
version of the output of amplifier stage 200 from the RF input
signal delayed through the first delay line 336, the carrier
cancellation combiner 330 provides a `further refined` estimate of
the residual distortion produced by the ACT RF amplifier
linearization stage 200.
[0049] Carrier rejection within the feed forward stage 300 is
optimized by carrier power minimization mechanism 136 within DSP
100, which is coupled via a DSP input port 104 to a power detector
337, coupled to a directional coupler 338 at the output of the
carrier cancellation combiner 330. The carrier power minimization
mechanism 136 executes a detected power minimization algorithm, and
outputs signals for controlling the attenuator 317 and the phase
shifter 318 of the vector modulator 316 in the RF input path of the
amplifier stage 200, so as to set the input RF carrier energy at a
value that will cause the RF carrier power in the composite output
signal at combiner output port 93, when scaled by attenuator 329,
to cancel the RF input energy applied to the second input 332 of
carrier cancellation combiner 330.
[0050] The distortion energy-representative output 333 of carrier
cancellation combiner 330 is coupled to a vector modulator 346 of a
feed-forward distortion cancellation loop (loop 4). Like the vector
modulator 316, vector modulator 346 contains a processor-controlled
variable gain stage 347 cascaded with a processor-controlled
variable phase shifting stage 348.
[0051] The output of the vector modulator 346 is coupled through a
buffer amplifier 351 and a directional coupler 353 to a
feed-forward error amplifier 360 of loop 4. The feed-forward error
amplifier 360 amplifies the gain and phased adjusted residual power
amplifier distortion derived from the carrier cancellation combiner
330 of loop 3 and injects the error signal to the amplifier's
output signal path via directional coupler 371.
[0052] The directional coupler 666 at the output of the composite
ACT/Feedforward amplifier couples a portion of the distortion
signal to an input port 105 of a further pilot tone receiver 667,
which drives a further pilot energy minimization mechanism 668
within DSP 100. This auxiliary pilot tone minimization mechanism
668 adjusts the parameters of the vector modulator 346 based upon
detected pilot tone distortion energy, to produce auxiliary
distortion signal products that are equal in amplitude and opposite
in phase from the original amplifier distortion of ACT main RF
amplifier stage 200. These distortion products provide further
cancellation of any remaining distortion generated by the main RF
amplifier stage 200 when the feed forward error path is reinjected
into the output of the output combiner 90 at directional coupler
371.
[0053] Directional coupler 371 is installed between a delay line
373 in the output path of the output combiner 90 and the output
directional coupler 95. Similar to the use of delay line 336, the
delay imparted by the delay line 373 equals the signal propagation
delay through in error amplifier path and serves to ensure there is
adequate phase matching at the feed forward directional coupler 371
over the operating frequency range. The output of directional
coupler 371 corresponds to a recombination of the amplified
feed-forward signal anti-phase with a delayed version of the output
signal from the main path amplifier stage 200, and achieves very
high distortion suppression so that the linearity of the amplifier
is significantly enhanced.
[0054] Directional coupler 353 samples distortion and residual
pilot energy due to imperfect adjustment of the ACT amplifier stage
200. Pilot energy at the output of direction coupler 353 is sensed
by pilot receiver 114 and used to control ACT vector modulator 50
via a power minimization loop 110 to minimize distortion at the
output of the ACT amplifier stage 200.
[0055] In the embodiment of FIG. 2, the output direction coupler 95
samples desired carrier, pilot and distortion energy at the output
of the ACT power amplifier. In FIG. 3, the directional coupler 353
samples the same energy, with the carrier power substantially
eliminated by the carrier cancellation process of loop 3 of the
feedforward process. As a result, the pilot signal to carrier ratio
at the coupled port of directional coupler 353 is enhanced over
that of the coupled port of the directional coupler 95 of the
embodiment of FIG. 2.
[0056] For this reason, the pilot energy at the coupled port of
directional coupler 353 is used to control the ACT vector modulator
50 via the pilot receiver 114 and energy minimization mechanism
110.
[0057] Thus, the power minimization and performance monitor
controllers and associated pilot generator and pilot receiver
circuits of the ACT RF amplifier linearization stage 200 maintain
distortion cancellation in the pre-distortion loop, while the feed
forward loop of the feed-forward stage 300 results in a
simplification in circuit implementation that reduces amplifier
size, complexity and cost.
[0058] As will be appreciated from the foregoing description, the
potential amplifier output signal and distortion energy inequality
problem of an open loop ACT-based RF power amplifier linearization
architecture is substantially minimized, by injecting a pilot tone
as a `pseudo distortion`, signal and using a set of power
minimization loops that are closed around the RF amplifier pair.
The power minimizing control loops control a set of vector
modulators such that both the injected pilot tone and
intermodulation distortion products are canceled, while RF carrier
components constructively sum in the composite output of the
amplifiers. Both signal summation adjustment and distortion
cancellation adjustment may be independently maintained to provide
more than 20 dB of distortion reduction over a wide range of supply
voltage, temperature and different input signal
characteristics.
[0059] In addition, by wrapping a feed forward loop around this
closed loop pre-distortion architecture, the amplitude of the
distortion products may be decreased by up to another 35 dB. The
addition of the feed-forward loop permits an overall
signal-to-distortion ratio as high as 85 dBc to be achieved.
Because the closed loop ACT pre-distortion architecture is more
efficient than a equivalent single loop feed forward power
amplifier, the overall system is several percent more efficient
than a dual loop feed forward design with similar performance.
[0060] While we have shown and described a number embodiments in
accordance with the present invention, it is to be understood that
the same is not limited thereto but is susceptible to numerous
changes and modifications as are known to a person skilled in the
art, and we therefore do not wish to be limited to the details
shown and described herein, but intend to cover all such changes
and modifications as are obvious to one of ordinary skill in the
art.
* * * * *