Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-03-11
2003-03-25
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S12400D
Reexamination Certificate
active
06538509
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a linearizer for a power amplifier and in particular but not limited to a linearizer for a microwave power amplifier,
BACKGROUND OF THE INVENTION
In modern communication systems, linearized power amplifiers have found wide use. This is mainly due to the availability of highly efficient power amplifiers (PA), spectral efficient digital multi-state modulation techniques, and the requirement fox multi-carrier operation. Digital multi-state signals and multi-carrier signals are very sensitive to non-linearities of the PA, which cause unwanted interference to be produced. The unwanted interference is mainly produced by Amplitude Modulation to Amplitude Modulation (AM-AM) conversion and Amplitude Modulation to Phase Modulation (AM-PM) conversion in the power amplifier. A traditional way to reduce the unwanted interference is to back-off the PA from its saturated output power. Back-offs of 10 to 20 dB are common which causes the power efficiency of whole system to be greatly reduced. The unwanted interference generated by the amplifier non-linearities can be especially serious if multiple carriers or multi-state digitally modulated signals are applied to the PA. This is because the peak power of multiple carriers and the multi-state digitally modulated signals are much higher than their average powers. Commonly, the peak power can range from 3 to 20 dB or higher than the average power depending on the numbers of carriers and modulation technique. These higher peak powers cause the signals to enter the saturated regions of the PA sooner and increase their sensitivity to the non-linearities of the PA. In order to increase power efficiency of the system, linearization techniques must be applied in the PA. Solutions so do this include feedback techniques, Cartesian feedback, feedforward, predistortion, LINC linearized transmitter, Kahn envelope elimination and restoration (EER) techniques and many other techniques.
Among these techniques, predistortion is the most popular, especially the RF analogue predistorter that is suitable for application to the high frequency and millimeter wave bands. Many different types of predistorters have been proposed. One implementation employs two variable gain amplifiers as described in M. Kumar, J. C. Whartenby and H. J. Wolkstein, “Predistortion Linearizer Using GaAs Dual-Gate MESFET for TWTA and SSPA Used in Satellite Transponders”, IEEE Trans. On MTT, Vol. MTT-33, pp. 1479-1488, No. 12, 1985, and U.S. Pat. No. 5,576,660. However, there are still some problems concerning bandwidth and cost because hybrids limit the bandwidth and many components are used to adjust the predistorter. All of these can increase the cost. Another problem that limits the bandwidth is the memory effect caused by the predistorter. Even though two ideal flat variable gain amplifiers are applied, the predistorter can still have a slope over the bandwidth. The slope will significantly affect linearization in wideband applications.
In the implementation described by M. Kumar, J. C. Whartenby and H. J. Wolkstein, “Predistortion Linearizer Using GaAs Dual-Gate MESFET for TWTA and SSPA Used in Satellite Transponders”, IEEE Trans. On MTT, Vol. MTT-33, pp. 1479-1488, No. 12, 1985, two dual gate MESFET variable gain amplifiers are adopted. It was realized in a hybrid Microwave Integrated Circuit (MIC) technique, that many tunings are needed to ensure that the predistorter produces proper distorted signals, especially for the phase difference between the linear path and non-linear path. It is a difficult thing to choose two amplifiers that can satisfy the phase difference requirement. Another problem concerning the implementation is the bandwidth. Normally, a 90° hybrid has limited bandwidth of only 10%. If 20% bandwidth or more is needed, a new design is required and this will increase design costs.
In the implementation described in U.S. Pat. No. 5,576,660, two identical voltage controlled phase shifters, two identical voltage controlled attenuators and two identical amplifiers are used. Two voltage controlled phase shifters make it relatively easy to control the phase difference between the linear path and the nonlinear path. But they are costly to manufacture, especially for the application to mm-wave band. The predistorter is the same as the first realization in that a 90° hybrid is adopted This hybrid limits the bandwidth if wideband is needed.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a predistortion linearizer comprising a signal separator for receiving a signal and for dividing the signal into a first signal and a second signal, a first path for caring the first signal, a second path for carrying the second signal, a signal combiner for combining the signals from the first and second paths, the second path between the separator and combiner including a non-linear device for receiving the second signal and generating distortion therefrom, and a vector modulator in one of the first and second paths for controlling at least one of the amplitude and phase of the signal carried by the respective path, the vector modulator comprising a second signal separator for dividing the signal carried by the respective path into a first modulator signal and a second modulator signal, a first modulator path for carrying the first modulator signal, a second modulator path for carrying the second modulator signal, and a second signal combiner for combining the first and second modulator signals received from the first and second modulator paths, the vector modulator including means for producing a phase difference between the first modulator signal and the second modulator signal, and wherein the first modulator path includes amplitude controlling means for controlling the amplitude of the first modulator signal, and the second modulator path includes amplitude controlling means for controlling the amplitude of the second modulator signal.
Advantageously, in this arrangement of a predistortion linearizer, the means for controlling the amplitude and chase of the distorted signal is provided by vector modulator means which allows the predistortion linearizer circuit to be considerably simplified and the number of components and required control signals to be reduced over known predistortion linearizers.
In one embodiment, the vector modulator may include means for dividing the distorted signal into first and second modulator signals, means for providing a phase difference between the two signals, and means for varying the amplitude of each signal independently of the other, and a signal combiner to combine the first and second modulator signals. Advantageously, this arrangement enables the phase and amplitude of the distorted signal to be varied independently of one another using just two control signals, each of which is used to control the amplitude of the modulator signals.
In one embodiment, the signal separator may comprise a wideband separator, for example having a bandwidth of at least 20% of the center frequency of the input signal. The signal combiner may also be a wideband signal combiner having a bandwidth of for example at least 20% of the center frequency. Advantageously, this enables the predistortion linearizer to operate over a wide frequency range, enabling a single design of device to be used in a wide range of different applications, thereby enabling the design and manufacturing costs to be reduced.
A delay line may be incorporated in one of the signal paths in order to provide a phase difference between the first and second signals, in addition to that that can be provided by the vector modulator.
According to another aspect of the present invention, there is provided a vector modulator comprising a signal separator for dividing a signal into a first modulator signal and a second modulator signal, a first path for carrying the first modulator signal, a second path for carrying the second modulator signal and a signal combiner for combining the first and second modulator sign
Choe Henry
Dragonwave Inc.
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