Closed loop active cancellation technique (ACT)-based RF...

Details Closed loop active cancellation technique (ACT)-based RF... Closed loop active cancellation technique (ACT)-based RF...

2001-12-20

2002-09-17

Nguyen, Patricia (Department: 2836)

Amplifiers

With pilot frequency control means

C330S149000, C330S151000

Reexamination Certificate

active

06452446

ABSTRACT:

FIELD OF THE INVENTION
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
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.
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.
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 ‘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
1
and A
2
, such that one RF power amplifier ‘pre-distorts’ the other. Being matched implies that the two 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.
More particularly, 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
and a phase adjustment element &PHgr;
1
, which adjust the amplitude and phase of the RF input signal 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 derive a signal containing both the original RF input signal to be amplified by the second ‘matched’ amplifier A
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 WS
1
-WC
1
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 G
1
and &PHgr;
1
and combined with an appropriately delayed and scaled sample of the RF input signal at WC
2
.
For this purpose, the second path from the directional coupler CPL
1
is coupled through a delay stage DL
1
to a first input of (Wilkinson) splitter WS
1
, a first output of which is coupled to (Wilkinson) combiner WC
1
. A second output of splitter WS
1
is coupled through a variable gain stage G
2
to a first input of further (Wilkinson) combiner WC
2
, a second input of which is coupled to the output of the combiner WC
1
. A second input of combiner WC
1
is coupled to a directional coupler CPL
2
installed in the output path of main path amplifier A
1
.
The output of combiner WC
2
, which is a composite of the RF input signal and complementary distortion products extracted from the RF amplifier A
1
, is coupled through a variable gain stage G
3
and variable phase adjustor &PHgr;
2
to the matched RF amplifier A
2
. The output of RF amplifier A
2
is coupled to a second input of output combining stage OCS.
The amplitude of the RF input signal component of the composite RF signal driving the amplifier A
2
is adjusted to be the same as the amplitude of the pure RF input signal driving 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 amplifier A
1
and the undelayed output of the amplifier A
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.
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 dist

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