Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-11-30
2004-09-21
Tokar, Michael (Department: 2819)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S010000, C330S158000
Reexamination Certificate
active
06794937
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the reduction of distortion that is added to a signal by a nonlinear device (e.g., power amplifier, mixer) in a transmitter.
2. Description of Related Art
Transmitters use complex modulation schemes (e.g., 16QAM, 64 QAM, 32TCM, 128TCM) to make efficient use of the available spectrum. However, these complex modulation schemes are very sensitive to all types of distortion and, in particular, to nonlinear types of distortion caused by nonlinear devices (e.g., power amplifiers, mixer) in the transmission chain. Therefore, it is important to reduce the distortion added to a signal by nonlinear devices so that transmitters can use complex modulation schemes and make efficient use of the available spectrum.
It is well known that power amplifiers have nonlinear characteristics, see
FIG. 1
which illustrates a Power
IN
-Power
OUT
graph
100
that shows a linear zone
102
and a saturation zone
104
of a power amplifier. As shown, if the power amplifier is used in the linear zone
102
, then the full power of the power amplifier may not be utilized. And, if the power amplifier is used in the saturation zone
104
, then the power amplifier unacceptably distorts an input signal. As such, the power amplifier should operate near but not to close to the saturation zone
104
. The efficiency of the power amplifier can be increased by extending the linear range of the linear zone
102
into and above the saturation zone
104
(see dashed line). A nonlinear correction device known as a linearizer can be used to increase the efficiency of the power amplifier. There are many different types of linearizers used today including feedback linearizers and feedforward linearizers. Examples of a traditional feedback linearizer and a traditional feedforward linearizer are briefly discussed below with respect to
FIGS. 2 and 3
.
Referring to
FIG. 2
(PRIOR ART), there is shown a block diagram of a transmitter
200
incorporating a traditional feedback linearizer
202
. Certain details associated with the transmitter
200
such as a modulator, filter and mixer are well known in the industry and as such need not be described herein. Therefore, for clarity, the description provided below in relation to the transmitter
200
omits the components not necessary to understand the invention.
The transmitter
200
receives an undistorted input signal
204
at an input terminal
206
that is processed by a subtractor
208
and amplified to a higher power level by a power amplifier
210
which generates an output signal
212
that is transmitted by an antenna
214
. As described above, the power amplifier
210
imparts some distortion (e.g., nonlinear components, nonlinear spurs) to the input signal
204
which without the presence of the feedback linearizer
202
would be present in the output signal
212
. To help compensate for the distortion, a portion of the output signal
212
is applied to a voltage divider
216
which outputs a lower powered signal
216
. The lower powered signal
218
is applied to the subtractor
208
. In the subtractor
208
, the lower powered signal
218
which is 180° out of phase with the input signal
204
is added to the input signal
204
which helps to compensate for the nonlinearities that are going to be added by the power amplifier
210
to the input signal
204
. As such, the feedback linearizer
202
including the subtractor
208
and the voltage divider
216
helps to compensate for at least some of the distortion that is going to be added to the input signal
204
by the power amplifier
210
. Essentially, the traditional feedback linearizer
202
relies on a feedback loop to compensate for the nonlinearities produced by the power amplifier
210
.
A main drawback of the traditional feedback linearizer
202
is that only a limited amount of correction can be achieved with this arrangement. Because, the delay associated with the feedback loop limits the effectiveness of the feedback linearizer
202
. In practice, the traditional feedback linearizer
202
is difficult to implement and often become unstable in transmitters that operate at higher frequencies (e.g., 2 GHz-11 GHz).
Referring to
FIG. 3
(PRIOR ART), there is shown a block diagram of a transmitter
300
incorporating a traditional feedforward linearizer
302
. Again, certain details associated with the transmitter
300
such as a modulator, filter and mixer are well known in the industry and as such need not be described herein. Therefore, for clarity, the description provided below in relation to the transmitter
300
omits the components not necessary to understand the invention.
The transmitter
300
receives an undistorted input signal
304
(see exploded view where signal
304
includes two tones) at an input terminal
306
which is split by a splitter
308
and input into a power amplifier
310
and a time delay circuit
312
. The power amplifier
310
amplifies one portion of the input signal
304
and outputs a distorted signal
314
. The time delay circuit
312
delays the other portion of the input signal
304
and outputs a delayed signal
316
. The delayed signal
316
has the same phase as the distorted signal
314
. The distorted signal
314
is then stepped downed and forwarded by a coupler
318
to an input of a subtractor
320
that also receives the delayed signal
316
and outputs a nonlinear signal
322
. In effect, the subtractor
320
cancels the main signals
324
of the distorted signal
314
and the delayed signal
316
such that only the nonlinear spurs
326
remain which form the nonlinear signal
322
. The nonlinear signal
322
is amplified by an error power amplifier
328
which outputs a distorted nonlinear signal
330
. It should be noted that the error power amplifier
328
has the same characteristics as power amplifier
310
. At the same time, another time delay circuit
332
delays the distorted signal
314
and outputs a delayed distorted signal
334
. The delayed distorted signal
334
has the same phase as the distorted nonlinear signal
330
. The nonlinear distorted signal
330
is then stepped up and forwarded by a coupler
336
to an input of another subtractor
338
that also receives the delayed distorted signal
334
and outputs a compensated signal
340
. The compensated signal
340
has the same main signals
324
and smaller nonlinear spurs
326
when compared to the distorted signal
314
. The compensated signal
340
is transmitted by an antenna
342
.
A main drawback of the traditional feedforward linearizer
302
is that it is a complex system in that it is difficult to tune the time delay circuits
312
and
332
, the couplers
318
and
336
, the power amplifiers
310
and
328
and the subtractors
320
and
338
. Another drawback of the traditional feedforward linearizer
302
is that it is expensive to make and difficult to implement in transmitters that operate at higher frequencies (e.g., 2 GHz-11 GHz). Accordingly, there has been a need for a linearizer that can address the aforementioned problems and other problems associated with traditional linearizers. These needs and other needs are addressed by the predistortion linearizer and method of the present invention.
BRIEF DESCRIPTION OF THE INVENTION
The present invention includes a predistortion linearizer, transmitter and method for linearizing a nonlinear device (e.g., power amplifier, mixer). Basically, the predistortion linearizer includes a coupling circuit, a diode and a direct current adjusting circuit that work together to generate a distorted signal which is reflected onto a signal path and inputted into the nonlinear device. The distorted signal compensates for at least some of the nonlinear spurs introduced by the nonlinear device to an input signal which was also applied to the signal path and inputted into the nonlinear device. As a result, the nonlinear device outputs a compensated output signal.
REFERENCES:
patent: 5049832 (1991-09-01), Cavers
patent: 5148448 (1992-09-01), Karam et al.
patent: 51
Burchfield Kenneth E.
Timaru Theodore
Alcatel
Nguyen Linh V
Sewell V. Lawrence
Smith Jessica W.
Tucker William J.
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