Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-08-10
2002-06-18
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000
Reexamination Certificate
active
06407635
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, that employs a swept oscillator to locate and isolate the RF carrier component in the RF power amplifier output, so that distortion energy produced at the output of the amplifier may be detected. Once detected, the distortion energy may be controllably removed by a digital signal processor-controlled distortion cancellation device, such as pre-distortion unit installed in the input path of the RF power amplifier, or a gain/phase adjustment unit installed in the error path of a feed-forward RF amplifier.
BACKGROUND OF THE INVENTION
The 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.
With predistortion correction, a signal is modulated onto the RF input signal path upstream of the RF amplifier. The ideal predistortion signal has a characteristic, which 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 of the conventional mechanisms for extracting the error signal component is to inject 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.
SUMMARY OF THE INVENTION
In accordance with the present invention, RF power amplifier distortion is accurately measured, even in the presence of multi-frequency input signals, by using a swept local oscillator to tune respective RF input and output receivers. The power detected by the tuned input receiver is compared with a power reference to determine the presence of carrier at the amplifier's input. Whenever the power detected by the input receiver exceeds the power of the reference—indicating the presence of carrier energy within the tuned receiver's bandwidth—a similar signal path through the output tuned receiver may be controllably blanked with a high isolation switch. As a result, as the output receiver is swept across the bandwidth of the amplifier output signal, only distortion energy will be detected by the output receiver. The distortion energy detected by the output receiver may be digitized and processed to control pre-distortion correction circuitry upstream of the RF amplifier, or gain/phase adjustment circuitry in the error path of a feedforward error correction loop.
Pursuant to a first, dual (input-output) receiver-based embodiment of the present invention, an adaptive predistortion circuit is installed upstream of an RF power amplifier that has a relatively “low” carrier-to-interference distortion ratio (C/I) output signal. By relatively a low C/I ratio output signal is meant one in which the RF carrier level is effectively indistinguishable from that of intermodulation products, such as for the case of mixed modulation multicarrier signals and multicarrier signals having different power levels. (In contrast, a relatively ‘high’ C/I output signal, such as that produced at the output of a highly linear RF amplifier with equal power carriers, is one in which the level of the RF carrier is readily distinguishable from that of the IMDs.)
In the first, predistortion embodiment of the invention, the RF input signal to be amplified is coupled through a directional coupler to an input mixer and an IF bandpass filter used as part of a swept input receiver, which detects the presence of carrier energy at the input to the RF amplifier. Whenever the carrier energy detected by the input receiver exceeds a predefined threshold, a controllably swept output receiver coupled through a directional coupler to the output of the RF amplifier is blanked by a threshold detector. The output of the threshold detector is monitored by a digital signal processor (DSP) controller to keep track of where (in the swept spectrum) carrier energy is located.
A common sweep frequency for each of the input and output receivers is derived from the same local oscillator, that is controlled by a digital sweep-control signal generated by the DSP. The output of the swept oscillator is split and fed to respective mixers of the input and output receivers. The IF output
Bonds David Kent
Mucenieks Lance Todd
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Choe Henry
Pascal Robert
Spectrian Corporation
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