Amplifiers – With pilot frequency control means
Reexamination Certificate
2001-05-16
2003-01-07
Nguyen, Patricia (Department: 2817)
Amplifiers
With pilot frequency control means
C330S149000, C330S151000
Reexamination Certificate
active
06504428
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is particularly directed to a hybrid RF power amplifier linearization mechanism that provides very high output distortion rejection and enhanced amplifier linearity. This hybrid architecture includes a main RF amplifier stage coupled with a carrier cancellation loop, the extracted residual distortion output of which feeds a feed-forward loop containing an auxiliary error amplifier stage. The main RF power amplifier stage is configured of a pair of parallel RF power amplifiers installed in intermod-complementing predistortion paths of the type disclosed in the U.S. Patent to Mucenieks, No. 6,111,462 (hereinafter referred to the '462 patent and the disclosure of which is incorporated herein). The outputs of the main and auxiliary amplifier stages are combined to produce a composite amplified RF output signal having very substantially reduced intermodulation products.
BACKGROUND OF THE INVENTION
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.
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.
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.
An inefficient approach to linearizing an RF power amplifier is to build the amplifier as a large, high power device, and then operate the amplifier at a low power level (namely, at only a small percentage of its rated output power), where the RF amplifier's transfer characteristic is relatively linear. An obvious drawback to this approach is the overkill penalty—a costly and large sized RF device.
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) 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.
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.
In accordance with the invention described in the above-referenced '462 Patent, and diagrammatically illustrated in
FIG. 1
, high efficiency RF power amplifier linearization is achieved by coupling a pair of effectively matched RF power amplifiers A
1
and A
2
in circuit with one another in a manner that causes one RF power amplifier to ‘pre-distort’ the other. For purposes of the present discussion, this parallel configured, effectively matched amplifier predistortion architecture will be referred to as an active cancellation technique (ACT) amplifier architecture. Being effectively matched implies that the two RF power amplifiers A
1
, A
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.
As shown in
FIG. 1
, an RF input signal to be amplified is split by a directional coupler CPL
1
into two paths. A first path includes an attenuator or scaling pad ATT and a controlled gain adjustment G
1
, which serve to adjust the RF input signal in amplitude prior to being amplified by the main amplifier A
1
. The output of the main path amplifier A
1
is coupled through a delay stage DL
2
to a first input of an output combining stage OCS (such as a quadrature hybrid).
A second split RF input signal path is used to construct a signal containing both of the original RF input signal to be amplified by the second amplifier A
2
, and a complementary version of the IMD products each of the two amplifiers inherently introduces. Distortion is extracted using carrier cancellation combiner circuitry very similar to that found in most conventional feed-forward RF power amplifiers. The extracted distortion products are adjusted in amplitude and phase and combined with an appropriately delayed sample of the RF input signal.
For this purpose, the second path from the directional coupler CPL
1
is coupled though a delay stage DL
1
to a first input of a Wilkinson splitter WS
1
, a first output of which is coupled to a first Wilkinson combiner WC
1
. A second output of Wilkinson splitter WS
1
is coupled through a variable gain stage G
2
to a first input of a second Wilkinson combiner WC
2
, a second input of which is coupled to the output of the first Wilkinson combiner WC
1
. A second input of the first Wilkinson combiner WC
1
is coupled through a variable phase adjustor &PHgr;
1
to a directional coupler CPL
2
installed in the output path of the main path amplifier A
1
.
The output of the second Wilkinson combiner WC
2
, which is a composite of the RF input signal and distortion products extracted from the first or main matched RF amplifier A
1
, is coupled through a variable gain stage G
3
and variable phase adjustor &PHgr;
2
to the second matched or error RF amplifier A
2
. Namely, the error amplifier A
2
is driven by the distortion products extracted from the main amplifier A
1
. The output of the second matched RF amplifier A
2
is coupled to a second input of the output combining stage OCS.
The amplitude of the RF input signal component of the composite RF signal driving the error amplifier A
2
is adjusted to be the same as the amplitude of the pure RF input signal driving the main amplifier A
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
2
, but also replace these distortion products with equal amplitude anti-phase replicas of these products. Thus, the delayed output of the main amplifier A
1
and the undelayed output of the matched error amplifier A
2
contain equal phase and amplitude amplified RF input signals and equal amplitude a
Chang Augustin P.
Cova Armando C.
Crescenzi, Jr. Emil J.
Foley Stephen J.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Nguyen Patricia
Spectrian Corporation
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