Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-12-31
2004-01-13
Nguyen, Khanh Van (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S107000
Reexamination Certificate
active
06677820
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion compensation apparatus to compensate for distortion occurring in an amplifier, and more particularly to a distortion compensation apparatus for realizing distortion compensation of high precision, by adjusting the timing for controlling the amount of distortion generated on a signal to be provided for the amplifier.
2. Description of the Related Art
For example, in a mobile wireless communication system such as cellular phone system, in order to assure wireless communication with a mobile station existing at an end of an area (cell) covered by a base station, and in order to realize wireless transmission of signals simultaneously to plural mobile stations (plural users) from the base station depending on the status of communication, the base station is required to send signals with a large power. Similarly, in a repeater station (repeating amplifier) for receiving wireless transmitted signals from the base station, amplifying these signals, and sending the amplified signals to mobile stations by wireless transmission, it is also required to send signals with a large power.
Accordingly, in such a base station or a repeater station, signals to be transmitted (for example, modulated waves) are amplified to a desired level by means of a (large) power amplifier (PA) capable of covering a physical distance up to the end of the cell. In such an amplifier, however, a nonlinear response (AM-AM conversion or AM-PM conversion) characteristic may take place near the critical point (saturation point) of an element, and nonlinear distortion may occur.
Radio Law demands wireless communication service providers to strictly regulate their band limit in order to eliminate effects between different services of wireless communication offered by using adjacent bands.
As a method of compensating for nonlinear distortion occurring in the amplifier, hitherto, it was proposed to use a predistorter type compensation system to compensate for the nonlinear distortion by generating distortion having a reverse characteristic to the nonlinear distortion occurring in the amplifier (that is, the distortion to cancel the nonlinear distortion) in a prior stage.
Other methods of compensating for nonlinear distortion include, for example, a feed-forward type distortion compensation system and negative feedback type distortion compensation system. In the feed-forward type distortion compensation system, the operation is advantageously stable, but it is required to extract a distortion component (occurring in the main amplifier) in the distortion detecting loop, and to amplify the distortion component with a sub amplifier in the distortion compensation loop and subtract the amplified signal from the output signal of the main amplifier. Hence, there are problems in that the circuit is complicated, and the power source efficiency is lowered by the sub amplifier. In contrast, in the predistorter type distortion compensation system, the structure is relatively simple, and a sub amplifier is not needed, and it is, hence, advantageous in both circuit scale and power source efficiency.
An example of a (distortion compensation) amplifying device having a predistorter for compensating for distortion by such a predistorter type distortion compensation system is explained below.
FIG. 11
shows an example of circuitry of an amplifying device with a predistorter (amplifier with predistortion function), and the operation of this amplifying device is explained below by referring to the diagram. This amplifying device is installed in the transmission section of a base station or repeater station, and the signal to be transmitted (transmission signal) is entered from a transmitter. This signal is amplified in the amplifier, and sent out to an antenna.
First, the signal to be transmitted which is issued from the transmitter is put into this predistortion circuit, and the signal is distributed into two, and one distribution signal is fed into delay means
81
, and other distribution signal is fed into a level detector
85
.
FIG. 12
shows an example of spectrum of a signal in an input stage from the transmitter to the amplifying device, in which the axis of abscissas denotes the signal frequency [kHz], and the axis of ordinates indicates the signal level by power ratio [dB]. As shown in the diagram, at this stage, there is no distortion by a predistorter (a variable attenuator
82
and a variable phase shifter
83
) and no distortion by an amplifier
84
, and hence the spectrum shows a low level of an unnecessary signal out of the band of use.
The delay means
81
delays the input signal (one distribution signal), and it sends it to the variable attenuator
82
.
The variable attenuator
82
changes (attenuates) the amplitude of the signal entered from the delay means
81
, depending on the (analog) control signal entered from a D/A converter
88
, described below, to generate an amplitude distortion in an amount corresponding to the control signal to the input signal, and sends this signal (including amplitude distortion) to the variable phase shifter
83
.
The variable phase shifter
83
changes the phase of the signal entered from the variable attenuator
82
depending on the (analog) control signal entered from a D/A converter
89
, described below, to generate a phase distortion in an amount corresponding to the control signal to the input signal, and sends this signal (including phase distortion) to the amplifier
84
.
In this example, the predistorter (predistortion generator) is composed of the variable attenuator
82
and variable phase shifter
83
connected in series and control systems
81
,
85
to
90
for controlling them.
The amplifier
84
amplifies the input signal from the variable phase shifter
83
to a desired level, and sends out the amplified signal (from the amplifying device) to the antenna.
FIG. 13
shows an example of a spectrum of a signal issued from the amplifier
84
when the distortion is not compensated, in which the axis of abscissas denotes the signal frequency [kHz], and the axis of ordinates indicates the signal level by power ratio [dB]. As shown in the diagram, in this case, the spectrum shows there is a distortion component (leak power to adjacent channels) out of the band of use due to distortion occurring in the amplifier
84
.
Such a distortion component can be compensated for by generating distortion of a reverse characteristic to the distortion occurring in the amplifier
84
(amplitude distortion or phase distortion) by the predistorter (variable attenuator
82
and variable phase shifter
83
).
FIG. 14
shows an example of a spectrum of a signal issued from the amplifier
84
when the distortion is compensated for by the predistorter, in which the axis of abscissas denotes the signal frequency [kHz], and the axis of ordinates indicates the signal level by power ratio [dB]. As shown in the diagram, in this case, the spectrum shows a decrease of the distortion component (leak power to adjacent channels) out of the band of use occurring in the amplifier
84
.
The level detector
85
is composed of, for example, an envelope detector for detecting the envelope of a signal, a low pass filter (LPF) for extracting a specific frequency component only about the detected envelope, and an A/D (analog/digital) converter for converting the detected envelope component from analog to digital signal. The level detector
85
, having such a structure, detects the level (for example, power level) of the input signal (other distribution signal), and issues the result of detection to a controller
90
by a digital value.
Distortion extracting means
86
is composed of, for example, a directional coupler, and extracts distortion (for example, part of the amplified signal) included in the amplified signal issued from the amplifier
84
, and sends out to the controller
90
.
A clock source
87
generates a clock signal of a specified period, and issues a
Hitachi Kokusai Electric Inc.
Nguyen Khanh Van
Wenderoth , Lind & Ponack, L.L.P.
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