Amplifiers – With pilot frequency control means
Reexamination Certificate
2002-09-17
2004-03-02
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With pilot frequency control means
C330S151000
Reexamination Certificate
active
06700441
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication devices, and in particular to a feed forward linear amplifier 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 that typically includes high order non-linearities (e.g., third order, fifth order, seventh order, and ninth order intermodulation products) is introduced to an amplified output signal. When the output signal is transmitted, the distortion creates undesirable interference, for example at frequencies corresponding to the third order, fifth, seventh, and ninth order intermodulation distortion (IM) products. 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 carrier cancellation circuit and an IM reduction circuit in a feed forward amplifier. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier and reference signal modulation products introduced to an input signal 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 adjustment for cancellation of distortion components and reduction of IM products.
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
having two low frequency modulators
104
,
110
and an RF power amplifier
116
. An input signal
101
having a carrier component is sourced to main signal path
102
, where the signal is routed to an amplitude modulator
104
and a phase modulator
110
that respectively modulate an amplitude and a phase of the input signal. Amplitude modulator
104
includes an amplitude reference signal generator
106
coupled to an amplitude adjuster
108
. Amplitude reference signal generator
106
produces a first low frequency pilot tone
107
, such as a 13 KHz tone, that is conveyed to amplitude adjuster
108
. Amplitude adjuster
108
then varies an amplitude of input signal
101
in response to pilot tone
107
to impose a reference amplitude modulation upon input signal
101
. Phase modulator
110
includes a phase reference signal generator
112
coupled to a phase adjuster
114
. Phase reference signal generator
112
produces a second low frequency pilot tone
113
, such as an 11 KHz tone, that is conveyed to a phase adjuster
114
. Phase adjuster
114
then varies a phase of input signal
101
in response to pilot tone
113
to impose a reference phase modulation upon input signal
101
. Amplitude adjuster
108
and phase adjuster
114
respectively also vary an amplitude and a phase of input signal
101
in response to respective control signals
156
,
158
received from a carrier cancellation controller
154
.
The amplitude and phase modulated input signal is applied to an RF power amplifier
116
, which amplifies the modulated input signal to produce an amplified signal
118
. Amplified signal
118
includes a carrier component, distortion components introduced to the amplified signal due to the nonlinearities of amplifier
116
, and reference modulation components resulting from the introduction of pilot tones
107
and
113
to input signal
101
by amplitude and phase adjusters
108
and
114
. Amplified signal
118
is sampled by a first output signal coupler
120
to produce a first sampled amplified signal
121
. Sampled amplified signal
121
is then supplied as one input to a signal combiner
132
. Also routed to combiner
132
is a time-delayed, sampled version
131
of input signal
101
, which is routed to the combiner via a delay circuit
130
. Signal combiner
132
subtracts sampled input signal
131
from sampled amplified signal
121
to provide carrier cancellation and to produce an error signal
133
comprising the distortion and reference modulation components contained in sampled amplified signal
121
.
Error signal
133
is then supplied to a amplitude adjuster
134
via a signal coupler
150
. Amplitude adjuster
134
varies an amplitude of error signal
133
, and thereby of the modulation component contained in the error signal, in response to an amplitude control signal
137
provided by an amplitude reference signal receiver
136
. The amplitude adjusted error signal is then applied to a phase adjuster
138
, which varies a phase of the amplitude adjusted error signal, and thereby of the modulation component contained in the amplitude adjusted error signal, in response to a phase control signal
141
provided by a phase reference signal receiver
140
. The amplitude and phase adjusted error signal is then applied to an error amplifier
142
. Error amplifier
142
amplifies the amplitude and phase adjusted error signal and provides the amplified amplitude and phase adjusted error signal to a second output signal coupler
124
.
Second output signal coupler
124
also receives a time-delayed version of amplified signal
118
from RF power device
116
via a delay circuit
122
. Signal coupler
124
combines the amplified amplitude and phase adjusted error signal with the time-delayed amplified signal to effectively subtract the amplified amplitude and phase adjusted error signal from the time-delayed amplified signal and to produce a corrected output signal
128
. Corrected output signal
128
is then sampled by a third output signal coupler
126
to produce a sampled corrected output signal
127
. Sampled corrected output signal
127
is conveyed to a first amplitude detector
144
that detects an amplitude of any residual modulation component remaining in the sampled corrected output signal.
Since both amplitude and phase modulation components may be present in sampled corrected output signal
127
, amplitude detector
144
provides the detected residual modulation to each of phase reference signal receiver
140
and amplitude reference signal receiver
136
via a band pass filter
146
. Band pass filter
146
is designed to filter out any non-residual reference modulation component of the signal conveyed by detector
144
. Based on an amount of detected residual modulation, amplitude reference signal receiver
136
and phase reference signal receiver
140
then each adjust their respective control signals
137
and
141
in order to minimize the amount of detected residual reference modulation. By adjusting control signals
137
and
141
such that the amount of detected residual reference modulation is minimized, reference signal receivers
136
and
140
minimize the amount of IM included in corrected output signal
128
.
The carrier cancellation performed by signal combiner
132
is not perfect, and as a result error signal
133
further includes a residual carrier component. Signal coupler
150
samples error signal
133
to produce a sampled error signal that the signal coupler provides to a second amplitude detector
152
. Amplitude detector
152
detects an amplitude of the sampled error signal and pro
Irons Bryan Todd
Van Horn Mark Ian
Zhang Wentian
May Steven A.
Motorola Inc.
Nguyen Khanh Van
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