Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-04-19
2004-11-16
Nguyen, Khanh Van (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S151000
Reexamination Certificate
active
06819173
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication devices, and in particular to a transmitter portion of a wireless communication device.
BACKGROUND OF THE INVENTION
The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, signal distortion components that include an amplitude component and a phase component are added to the original signal. These additional components are due to the amplitude compression characteristics (AM/AM) and the phase distortion (AM/PM) characteristics of the RF power amplifier when it is driven over a range of amplitudes. If these distortion components are not compensated they will cause spreading of the spectrum into the adjacent channels and thus interfere with communication devices using adjacent channels.
A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of RF power amplifiers. One such technique involves the use of a feed forward correction circuit in a feed forward amplifier. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier to create an error signal. The error signal is then amplified and added to the RF power amplifier's output with an amplitude, phase, and delay adjusted for maximum cancellation of the distortion components. However, the amount of distortion reduction available in a feed forward amplifier is limited by the distortion introduced into the error signal when the error signal is amplified by an error amplifier.
For example,
FIG. 1
is a block diagram of an exemplary feed forward amplifier
100
of the prior art. Feed forward amplifier
100
includes a main signal path
102
, a feed forward correction circuit
104
, and a control circuit
106
. An input signal
101
having carrier components is sourced to main signal path
102
, where the signal is routed to a gain and phase adjuster
110
via an input signal coupler
108
. Gain and phase adjuster
110
adjusts the amplitude and phase of input signal
101
based on a control signal received from control circuit
106
. Gain and phase adjuster
110
conveys the amplitude and phase adjusted input signal to a radio frequency (RF) power amplifier
112
that amplifies the signal to produce an amplified signal
113
. RF power amplifier
112
then conveys amplified signal
113
to a first output signal coupler
120
via a signal coupler
114
and a delay circuit
116
. As mentioned above, RF power amplifier
112
introduces distortion components to the amplified signal, which distortion components are partially cancelled by an error signal output by feed forward correction circuit
104
.
Feed forward correction circuit
104
produces the error signal based on input signal
101
and amplified signal
113
. A summation junction
124
included in feed forward correction circuit
104
receives a portion of input signal
101
via input signal coupler
108
and delay circuit
122
and further receives a portion of amplified signal
113
via signal coupler
114
. Summation junction
124
subtracts the received portion of the amplified signal from the received portion of the input signal to produce an error signal
125
. The subtraction results in a partial cancellation of the carrier components of the received portion of amplified signal by the carrier components of the received portion of the input signal. As a result, error signal
125
primarily contains the distortion components of the received portion of the amplified signal.
Summation junction
124
conveys error signal
125
to a feed forward correction circuit error amplifier
130
via a feed forward signal coupler
126
and a feed forward gain and phase adjuster
128
. Error amplifier
130
amplifies the received error signal to produce an amplified error signal
131
and conveys the amplified error signal to first output signal coupler
118
. First output signal coupler
118
combines amplified error signal
131
with amplified signal
113
to partially cancel the distortion components of amplified signal
113
and produce a distortion reduced output signal
121
.
Amplification of error signal
125
by error amplifier
130
may result in an introduction of distortion components to the error signal due to the amplitude compression and the phase distortion characteristics of the error amplifier. Since amplified error signal
131
is combined with amplified signal
113
at output signal coupler
120
, it is desirable to minimize the added distortion. In order to reduce the distortion introduced into amplified error signal
131
by error amplifier
130
, control circuit
106
controls an average power of an error amplifier drive signal, that is, error signal
125
.
Control circuit
106
receives a portion of error signal
125
, that is, attenuated error signal
127
, from feed forward signal coupler
126
. Control circuit
106
further receives a portion of output signal
121
, that is, attenuated output signal
132
, from a second main signal path output signal coupler
120
that receives output signal
121
from first main signal path output signal coupler
118
. Control circuit
106
routes each of attenuated error signal
127
and attenuated output signal
132
to a switch
140
, typically a multiplexer.
Switch
140
is controlled by a controller
148
coupled to the switch. When switch
140
receives a first switch control signal
152
from controller
148
, the switch routes attenuated error signal
127
to a mixer
142
. A local oscillator
150
coupled to mixer
142
and controller
148
sources a reference signal to the mixer. In response to a first local oscillator control signal
149
sourced by controller
148
to local oscillator
150
, the local oscillator adjusts a frequency of the reference signal such that mixer
142
downconverts attenuated error signal
127
to baseband to produce a baseband error signal. Mixer
142
then conveys the baseband error signal to an average power detector
146
via a band pass filter
144
coupled to the mixer. Average power detector
146
determines an average power of the baseband error signal. Controller
148
reads the average power determined by average power detector
146
and, based on the average power, conveys a control signal
156
to gain and phase adjuster
110
that is designed to minimize the average power of the baseband error signal. In response to receiving control signal
156
, gain and phase adjuster
110
adjusts an amplitude and phase of input signal
101
, thereby adjusting an average power of error signal
125
and adjusting the average power of the baseband error signal.
When switch
140
receives a second switch control signal
154
from controller
148
, the switch routes attenuated output signal
132
to mixer
142
. When attenuated output signal
132
is routed to mixer
142
, controller
148
sources a second local oscillator control signal
151
to local oscillator
150
. In response to the second local oscillator control signal
151
, local oscillator
150
adjusts the frequency of the reference signal such that mixer
142
downconverts attenuated output signal
1
Leffel Michael David
Louis Edward Vincent
May Steven A.
Motorola Inc.
Nguyen Khanh Van
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