Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-05-24
2004-06-22
Nguyen, Khanh Van (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S302000
Reexamination Certificate
active
06753728
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a distortion compensation apparatus; and, more particularly, to a distortion compensation apparatus capable of effectively canceling out upper and lower side third-order distortions produced by an amplifier.
BACKGROUND OF THE INVENTION
Distortions are produced when baseband signals having multiple frequency components, e.g., a WCDMA (Wide-Band Code Division Multiple Access) signal, a multi-carrier signal and the like are amplified by an amplifier in a communications system for amplifying a communications signal; and such distortions need to be compensated. Conventionally, in order to amplify these signals while suppressing distortions, a sufficiently large backoff is set in the amplifier or a distortion compensation process by way of a feed forward scheme or a pre-distortion scheme is carried out.
Referring to
FIG. 6
, there is illustrated a block diagram of a conventional amplification apparatus including a distortion compensation circuit employing the feed forward scheme.
In the amplification apparatus shown in
FIG. 6
, an input signal (a main signal) is divided into a primary signal and a subsidiary signal by a divider
1
. The primary signal is amplified by a main amplifier
2
and transferred to a subtractor
4
. The subsidiary signal is provided to the subtractor
4
through a delay line
3
. The subtactor
4
subtracts the subsidiary signal inputted through the delay line
3
from a portion of the amplified primary signal inputted from the main amplifier
2
to extract distortion components. The extracted distortion components are fed to a distortion amplifier
5
and the amplified primary signal with distortions is transferred into a subtractor
7
through a delay line
6
. The distortion components fed into the distortion amplifier
5
are amplified by the distortion amplifier
5
and then provided to the subtractor
7
. The subtractor
7
subtracts the amplified distortion component provided from the distortion amplifier
5
from the amplified primary signal provided through the delay line
6
, thereby outputting an amplified and compensated signal without distortion.
The signal inputted into the subtractor
7
through the delay line
6
includes distortions produced by the main amplifier
2
and the amplified distortion components inputted into the subtractor
7
from the distortion amplifier
5
corresponds to the distortions produced by the main amplifier
2
. Therefore, the output signal of the subtractor
7
corresponds to a signal in which the distortions produced by the main amplifier
2
are removed from the primary signal amplified by the main amplifier
2
. The divider
1
and the subtractors
4
and
7
are respectively implemented by, e.g., directional couplers.
Referring to
FIG. 7
, there is provided a block diagram of a conventional amplification apparatus including a distortion compensation circuit employing the pre-distortion scheme.
In the amplification apparatus shown in
FIG. 7
, a pre-distortion circuit
11
is installed at a front stage of a main amplifier
12
. The pre-distortion circuit
11
introduces in advance pre-distortions having same amplitudes but differing in phase by 180 degrees (i.e., being in opposite phases) compared with actual distortions to be produced in the main signal by the main amplifier
12
. Subsequently, the pre-distortion circuit
11
outputs the main signal containing the pre-distortions. The actual distortions produced by the main amplifier
12
and the pre-distortions introduced by the pre-distortion circuit
11
are cancelled out.
In such amplification apparatus, the pre-distortions introduced by the pre-distortion circuit
11
and the actual distortions produced by the main amplifier
12
should be wholly matched to each other in relation to the input variation of the main signal and frequency characteristics of distortions. The distortions produced in the amplified signal are caused by AM (Amplitude Modulation)-AM conversion and AM-PM (Phase Modulation) conversion.
The gain and phase characteristics of the pre-distortion circuit should be set to be ideal with respect to those of the amplifier. Since, however, the characteristics of AM-AM and AM-PM conversions are very complicated, the characteristics of such an ideal pre-distortion circuit only can be expressed by a complicated function, rendering it virtually impossible to analytically or computationally obtain the coefficients of characteristics curve.
Thus, an alternative amplification apparatus including a distortion compensation circuit employing the pre-distortion scheme has been contemplated as shown in FIG.
8
.
In the amplification apparatus shown in
FIG. 8
, an input signal, e.g., an RF (Radio Frequency) signal, is divided into a primary signal and a subsidiary signal by a divider
21
, wherein the primary signal is transferred to an amplitude/phase circuit
27
via a delay circuit
22
and the subsidiary signal is fed to an amplitude detector (envelope detector)
23
.
The amplitude detector
23
detects an amplitude level (envelope level) of the subsidiary signal. The detection result of an analog signal is converted into a digital signal by an A/D (Analog to Digital) converter
24
and then provided to a table section
25
.
In the table section
25
, amplitude correction data and phase correction data are stored as a table for various amplitude levels in a memory (not shown) of the table section
25
. The amplitude correction data and the phase correction data in the table corresponding to the detection result of the amplitude level inputted from the A/D converter
24
are read and loaded to a D/A converter
26
. In the D/A converter
26
, the amplitude correction data and the phase correction data loaded from the table section
25
are converted from a digital signal into an analog signal and provided to the amplitude/phase circuit
27
.
The primary signal outputted from the divider
21
is delayed by the delay circuit
22
such that the input timing of the delayed primary signal to the amplitude/phase circuit
27
is synchronized with that of the amplitude correction data and the phase correction data from the D/A converter
26
.
By means of such delay, amplitude distortion is produced based on the amplitude correction data corresponding to the amplitude level of the subsidiary signal and then introduced into the primary signal at the amplitude/phase circuit
27
. At the same time, phase distortion is generated in response to the phase correction data corresponding to the amplitude level of the subsidiary signal and then added to the primary signal at the amplitude/phase circuit
27
. Herein, the amplitude distortion and the phase distortion are provided by the amplitude/phase circuit
27
such that they can cancel out actual amplitude and phase distortions to be produced by a main amplifier
28
.
That is, the table section
25
stores such amplitude and phase correction data that have been provided in consideration of the characteristics of the main amplifier
28
in terms of the AM-AM and AM-PM conversion thereof as a function of input level. Therefore, the amplitude/phase circuit
27
can produce, in response to the amplitude and phase correction data provided from the D/A converter
26
, the predistorted primary signal having opposite characteristics to those to be produced by the main amplifier
28
. As a result, ideal distortionless amplification can be realized over the whole amplification apparatus.
In other words, an output signal of the amplitude/phase circuit
27
is amplified by the main amplifier
28
, and the actual amplitude distortion and the actual phase distortion produced by the main amplifier
28
are canceled out by the amplitude distortion and the phase distortion introduced by the amplitude/phase circuit
27
. Consequently, an amplified signal without distortion is outputted from the main amplifier
28
via a divider
29
.
The divider
29
divides the amplified signal inputted from the main amplifier
28
and dispatches a portion of the amplified signal to a distortio
Funada Kiyoshi
Hongo Naoki
Horaguchi Masato
Okubo Yoichi
Suto Masaki
Bacon & Thomas PLLC
Hitachi Kokusai Electric Inc.
Nguyen Khanh Van
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