Amplifiers – With amplifier bypass means
Reexamination Certificate
2000-02-25
2001-08-14
Pascal, Robert (Department: 2817)
Amplifiers
With amplifier bypass means
C330S149000
Reexamination Certificate
active
06275106
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 linearization mechanism, that employs a spectral distortion measurement and differential combining scheme, which is operative to optimize the operation of a carrier cancellation combiner, and pre-distortion and feed-forward loops of the RF power amplifier, so that intermodulation distortion produced at the output of the RF amplifier may be minimized.
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 relatively low bandwidth (baseband) telephone networks, linearizing components such as RF power amplifiers 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 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 predistortion correction. Feedforward and predistortion correction, however, are not limited in this regard.
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.
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 that is the inverse or complement 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 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. In addition, pilot tone injection causes the generation of an unwanted spur; also, a piloted system is open-loop in the sense that the controller operates on the pilot and not the IMDs. Hence, the system only assumes that IMDs are being properly cancelled.
Other approaches include the use of a high intercept receiver to detect low level distortion in the presence of high power carriers, which adds substantial complexity and cost, or the use of a wideband correlator. The latter mechanism suffers from the fact that it relies on measurement of wideband energy, rather than on the spectral distortion components.
SUMMARY OF THE INVENTION
In accordance with the present invention, the operation of a carrier cancellation combiner, and predistortion and feed-forward loops of an RF power amplifier are controlled to minimize IMD components at the output of the RF amplifier, by a spectral distortion measurement scheme that performs (Fast Fourier Transform (FFT)-based) spectral power measurements at a ‘reference’ signal port (associated with the RF input signal), and a plurality of ‘test’ signal ports (associated with various parameter adjustment locations of the amplifier). Signals extracted from the control and test ports are downconverted to baseband, bandpass filtered, sampled and then stored as a spectral ‘snapshot’ of the entire band of the signal infrastructure of the amplifier.
Averaged FFT's of the data extracted from the reference port and data from the test ports by a digital signal processor based controller provide spectral information for each sampled signal set. The control data FFT is processed to establish a baseline, with which the test data FFTs are compared to generate adjustment signals for various control parameters, through which IMDs introduced by the amplifier are minimized. The performance of the RF amplifier is continuously monitored and the control parameters modified as necessary to compensate for drift in the amplifier's characteristics.
In accordance with a preferred, but non-limiting embodiment, the invention includes digitally controlled gain and phase adjustment circuits and a digitally controlled predistortion unit incorporated in the RF input signal path to a main RF amplifier. The predistortion unit may contain a work function-based vector modulator that is coupled to receive weighting coefficients from the controller. Since it contains any intermodulation (spectral regrowth) distortion products (IMDs) introduced by the RF amplifier, the output of the amplifier is monitored as one of the test inputs to the controller.
The output of the main RF amplifier is further coupled to a carrier cancellation combiner, a second input of which is coupled to a feed forward path from the RF input signal port. The feed forward path from the RF input signal port includes a fixed delay and a variable delay unit, that serve to substantially equalize the propagation delay of the signal path through the main RF amplifier, and thereby provide proper phase alignment of the signals applied to the carrier cancellation combiner. The output of the carrier cancellation combiner is coupled to gain and phase adjustment circuits of a feed forward error amplifier, the output of which is reinjected into the output path of t
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Nguyen Patricia
Pascal Robert
Spectrian Corporation
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