Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-08-10
2002-05-07
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000, C375S297000
Reexamination Certificate
active
06384681
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to radio frequency (RF) communication systems, and is particularly directed to an RF power amplifier distortion correction mechanism for controlling an adaptive digital signal processor-controlled pre-distortion circuit installed in the input path to an RF amplifier having a relatively “low” carrier to intermod ratio (C/I). The invention employs a swept oscillator to sweep input and output receivers, and locate and isolate the RF carrier component in the amplifier output, so that distortion energy produced at the output of the RF amplifier may be detected. Once detected, distortion energy may be controllably removed by the pre-distortion unit.
BACKGROUND OF THE INVENTION
As described in the above-referenced co-pending '723 application, specifications and regulations of the Federal Communications Commission (FCC) mandate that communication service providers comply with very strict bandwidth constraints, including the requirement that the amount of energy spillover outside a licensed channel or band of interest, be sharply attenuated (e.g., on the order of 50 dB). Although such limitations may be readily overcome 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 sidebands sufficiently to meet industry or regulatory-based standards using such modulation techniques requires very linear signal processing systems and components. Although relatively linear components can be obtained at a reasonable cost for the relatively low bandwidths (baseband) of telephone networks, linearizing components such as power amplifiers at RF frequencies can be prohibitively expensive.
A fundamental difficulty in linearizing an RF power amplifier is the fact that it is an inherently non-linear device, and generates unwanted intermodulation distortion products (IMDs). IMDs manifest themselves as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. A further manifestation of IMD is spectral regrowth or spreading of a compact spectrum into spectral regions that were not occupied by 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 straightforward way to implement a linear RF power amplifier is to build it as a large, high power device, but operate the amplifier at a only a low power level (namely, at a small percentage of its rated output power), where the RF amplifier s transfer function is relatively linear. An obvious drawback to this approach is the overkill penalty—a costly and large sized RF device. Other prior art techniques which overcome this penalty include feedback correction techniques, feedforward correction, and pre-distortion correction.
Feedback correction techniques include polar envelope correction (such as described in U.S. Pat. No. 5,742,201), and Cartesian feedback, where the distortion component at the output of the RF amplifier is used to directly modulate the input signal to the amplifier in real time. Feedback techniques possess the advantage of self-convergence, as do negative feedback techniques in other fields of design. However, systems which employ negative feedback remain stable over a limited bandwidth, which prevents their application in wide-bandwidth environments, such as multi-carrier or W-CDMA. Feedforward and predistortion correction, however, are not limited in this regard. In the feedforward approach, error (distortion) present in the RF amplifier's output signal is extracted, amplified to the proper level, and then reinjected with equal amplitude but opposite phase into the output path of the amplifier, so that (ideally) the RF amplifier's distortion is effectively canceled.
When predistortion correction is used, a signal is modulated onto the RF input signal path upstream of the RF amplifier. The characteristic of an ideal predistortion signal is the inverse of the distortion expected at the output of the high power RF amplifier, so that when subjected to the distorting transfer function of the RF amplifier, it effectively cancels the distortion behavior.
Either predistortion or feedforward may be made adaptive by extracting an error signal component in the output of the RF amplifier and then adjusting the control signal(s), in accordance with the extracted error behavior of the RF amplifier, so as to effectively continuously minimize distortion in the amplifier's output.
One conventional mechanism for extracting the error signal component involves injecting a pilot (tone) signal into the signal flow path through the amplifier and measure the amplifier's response. A fundamental drawback to the use of a pilot tone is the need for dedicated pilot generation circuitry and the difficulty of placing the pilot tone within the signal bandwidth of the amplifier. Other approaches employ a high intercept receiver to detect low level distortion in the presence of high power carriers, which adds substantial complexity and cost.
Pursuant to the invention described in the '723 application, RF power amplifier distortion in the presence of multi-frequency input signals is accurately measured by using a swept local oscillator to tune RF input and output receivers. Where that distortion is corrected by means of an adaptive predistortion circuit installed in the input path to an RF amplifier having a relatively “low” carrier to intermod ratio (C/I), the swept local oscillator scheme may be configured as diagrammatically shown in
FIG. 1
(which corresponds to
FIG. 1
of the '723 application). By relatively low C/I ratio RF amplifier is meant one whose RF carrier level is effectively indistinguishable from that of intermodulation products. As a non-limiting example, the term low C/I ratio may be considered to apply to those RF amplifiers having intermodulation products above −50 dBC.
In the architecture of
FIG. 1
, an RF input signal RF
in
to be amplified is coupled to an input port
11
of a signal input path to RF power amplifier
10
, the distortion characteristic of which is to be measured by a controllably blanked distortion energy detector subsection
100
. In order to monitor the RF input signal RF
in
for the presence of carrier energy, the RF input port
11
is coupled through a first directional coupler
13
to a first input
21
of a mixer
22
of a controllably tuned or swept input receiver
20
, and to a digitally controlled predistortion unit
14
installed in the signal input path to RF power amplifier
10
.
The predistortion unit
14
is operative to dynamically adjust the amplitude and phase of the RF input signal to the RF amplifier
10
, and may contain a vector modulator driven by a complex polynomial work function. Predistortion unit
14
is coupled to receive weighting coefficients w
0
, w
1
, w
2
, . . . , w
N
, supplied over a multi-link
15
by a performance monitoring and parameter updating digital signal processor (DSP)
16
. DSP executes
16
one or more error minimization algorithms (e.g., power or least mean square) for controllably adjusting the distortion introduced into the RF signal input path through the predistortion unit
14
.
The output of RF power amplifier
10
is coupled to an RF output port RF
out
, and through a second directional coupler
17
to a first input
31
of a mixer
32
within a controllably tuned or swept output receiver
30
. The output of the directional coupler
17
is representative of the amplified original RF input signal and any intermodulation (spectral regrowth) distortion products (IMDs) introduced by the RF amplifier.
Each of the input and output receivers
20
,
30
is controlled by a digital sweep-control signal generated by the DSP
16
. For this purpose, digital sweep-control signal lines
17
are coupled to a digital-to-analog co
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Choe Henry
Pascal Robert
Spectrian Corporation
LandOfFree
Swept performance monitor for measuring and correcting RF... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Swept performance monitor for measuring and correcting RF..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Swept performance monitor for measuring and correcting RF... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2834225