Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2001-11-30
2003-11-25
Tokar, Michael (Department: 2819)
Amplifiers
Modulator-demodulator-type amplifier
C330S12400D, C332S102000
Reexamination Certificate
active
06653896
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
Not applicable.
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to the amplification of communications signals, and more particularly to an improvement allowing practical processing of rapid phase changes in polar modulation signals.
Many signal modulation techniques require rapid or nearly instantaneous in the phase of signals. For example, a simple binary phase shift keying (BPSK) signal requires a 180 degree phase change of the carrier signal when the data used to modulate the carrier signal changes from one binary value to another, e.g., from “1” to “0.” As a practical matter, the ability to generate and process such rapid phase changes is constrained by the finite bandwidth limitations of physical devices. In the case of polar representation signals, that is, signals expressed in terms of magnitude and phase, the generation and processing of rapid phase changes is especially challenging.
FIG. 1A
illustrates an analog signal f(t) having a rapid phase change of about 180 degrees. As shown in the figure, the signal f(t) transitions from a first point
102
to a second point. The transition represents a phase change of almost 180 degrees in the signal f(t) and spans a very short period of time. Note that the phase change of nearly 180 degrees is shown here for clarity of illustration. The phase change can be less than, equal to, or greater than 180 degrees.
FIG. 1B
illustrates a corresponding digital signal f(n) having a rapid phase change of about 180 degrees. Here, the signal f(n) transitions from a first point
106
to a second point
108
. The transition represents a phase change of almost 180 degrees in the signal f(n) and spans very few samples of n. Specifically, only two samples, intermediate points
110
and
112
, exists between the first point
106
and the second point
108
. Again, the phase change of nearly 180 degrees is shown here for clarity of illustration. The phase change can be less than, equal to, or greater than 180 degrees.
FIG. 1C
is a complex vector plot of the nearly instantaneous phase change in f(t). The signal f(t) can be expressed as the real part of a complex vector rotating in the complex plane, according the equations:
sin
wt
=
ⅇ
j
wt
-
ⅇ
-
j
wt
2
j
cos
wt
=
ⅇ
j
wt
+
ⅇ
-
j
wt
2
where f(t) is sin wt, w is the instantaneous rate of phase change, and the complex vector e
jwt
rotates about the center of a real axis Re{e
jwt
} and imaginary axis Im{e
jwt
}. The center is also termed the origin of the complex plane. Note that for clarity of illustration, the phase change in f(t) of almost 180 degrees is not incorporated into the above equations. However, the phase change is illustrated in the complex vector plot of FIG.
1
C. Here, as the complex vector e
jwt
rotates in a counter-clockwise direction, it transitions nearly instantaneously from a first position
114
, corresponding to the first point
102
, to a second position
116
, corresponding to the second point
104
.
An angle
118
formed between the first position
114
and the second position
116
represents the phase change of almost 180 degrees. This change of phase occurs in a very short period of time. In a polar representation system, where a signal is expressed in terms of magnitude and phase, the expression of phase experiences a nearly instantaneous change corresponding to the angle
118
. Such a rapid change in the value of the phase expression is associated with a correspondingly wide bandwidth. The shorter the period over which the phase change occurs, the wider the associated bandwidth becomes.
In addition, the trajectory followed by the complex vector e
jwt
in its transition from the first position
114
to the second position
116
can dramatically increase the severity of rapid phase change even further. Under certain conditions, the trajectory followed by the complex vector e
jwt
as it transitions from the first position
114
to the second position
116
is one that passes near the origin of the complex plane. The phase change experienced by the complex vector e
jwt
increases sharply as the trajectory nears the origin of the complex plane. Although this situation can occur in both analog and digital signal, it is more easily illustrated in the context of a digital signal. Therefore, it is explained below using the example of the digital signal f(n).
FIG. 1D
is a complex vector plot of the nearly instantaneous phase change in f(n). The relationship of the signal f(n) to the complex vector plot shown in
FIG. 1D
is analogous to the relations already described between the signal f(t) and the complex vector plot shown in
1
C. Here, a complex vector e
jwn
, in the form of discrete samples, rotates in a counter-clockwise direction in the complex plane. The complex vector e
jwn
transitions nearly instantaneously from a first position
120
, corresponding to the first point
106
, to a second position
122
, corresponding to the second point
108
.
The trajectory traced by the endpoint of the complex vector e
jwn
(the signal point) over time is of considerable interest in communications engineering (one end of all vectors is at the origin). For bandlimited signals, which includes nearly all signals of practical interest, the speed of the signal point along its trajectory is upper bounded. Sampled points of this trajectory are represented in
FIG. 1D
by intermediate positions
124
and
126
, which correspond respectively to intermediate points
110
and
112
of FIG.
1
B.
Should this trajectory pass near the origin, the polar coordinates of the signal point can change quite rapidly indeed. As seen in
FIG. 1D
, even though the direct distances between the signal points of complex vectors
120
,
124
,
126
, and
122
respectively are nearly uniform, the angles subtended between adjacent vectors, and the magnitude changes between adjacent vectors, can change markedly. Note that the closer such a trajectory passes to the origin, the greater the associated phase change of the signal during this near approach.
FIG. 2A
is a vector transition diagram of a representative bandlimited signal. Note that there are numerous transitions near to the origin, some of which transition very close to the origin.
FIG. 2B
is a plot of the power spectral density (PSD) of the phase change of the signal shown in FIG.
2
A. Note that the PSD does not roll off very fast with increasing frequency, showing that there is a large amount of high frequency content in the phase changes of this signal. Such high frequency energy is due to the rapid phase changes whenever the signal trajectory passes nearby the origin. Representation of this type of signal directly using polar coordinates requires devices having correspondingly wide bandwidths. This ‘bandwidth expansion’ is a point of difficulty in the use of polar modulation. This can be contrasted with the signal shown in FIG.
6
. No trajectory of this signal passes near to the origin, and the corresponding PSD of the signal phase changes shows a marked rolloff with increasing frequency.
A need exists to provide an alternative approach to direct polar modulation, such that the large phase changes of signals having trajectories passing near to the origin do not require the application of devices with large bandwidth capability.
It is important to differentiate the present invention from earlier multiple amplifier approaches, such as LINC [ref: U.S. Pat. No. 4,178,557, entitled “Linear Amplification With Nonlinear Devices” (P. Henry), issued Dec. 11, 1979] and Doherty [ref: U.S. Pat. No. 5,420,541, entitled “Microwave Doherty Amplifier” (D. Upton, et.al.) issued May 30, 1995]. Specifically the LINC technique, gen
Sander Wendell B.
Schell Stephan V.
Sevic John F.
Krebs Robert E.
Nguyen Linh Van
Thelen Reid & Priest LLP
Tokar Michael
Tropian Inc.
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