Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-03-19
2004-09-21
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S12400D, C333S117000
Reexamination Certificate
active
06794938
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronic circuits, devices, and systems and the linearization of nonlinearites inherent in such devices and, more specifically, the present invention relates to methods and apparatus for reduction, cancellation, and enhancement of nonlinearities of electronic devices.
BACKGROUND OF THE INVENTION
Circuit and device nonlinearities are well known in the art to create undesired intermodulation distortion. In many applications, these nonlinearities and associated intermodulation distortion of circuits and devices limit the performance of systems and often lead to designs with increased power consumption in efforts to avoid intermodulation distortion. Example applications include the receiver and transmitter portions of cellular phone handsets, base stations, cable television head-ends, cable television amplifiers, and general purpose amplifiers. In receivers, the presence of strong undesired signals, typically at nearby frequencies, can produce intermodulation products that interfere with the reception of weak desired signals. In transmitters, intermodulation distortion can lead to the generation of undesired frequency emissions that violate regulatory requirements and interfere with other services, typically at nearby frequencies.
For example, in cellular phones the problem of receiving weak signals in the presence of strong signals is of considerable interest. It is common for a cellular phone to be situated far from a base-station antenna tower (leading to a weak desired signal from the tower) while other strong signals such as nearby cellular phones, television transmitters, radar and other radio signals interfere with the reception of the desired weak signal. This interference is further exacerbated by nonlinearities within electronic circuits, including third order nonlinearities that are well known in the art to limit the performance of circuits and devices.
In addition to the problems created by nonlinearities in radio receivers, radio transmitters are similarly affected by third-order and other nonlinearities. Such nonlinearities in transmitters lead to undesired transmitter power in frequency bands outside the desired transmission frequency bands, such effects being commonly referred to as spectral re-growth in the art. These out-of-band signals in radio transmitters can violate regulatory emission requirements and cause interference with other users operating at nearby frequencies.
In broadband systems, such as cable television, nonlinearities present particular problems since such systems have a plurality of signals (i.e., television signals) at relatively high power levels. This plurality of signals, combined with relatively high power levels, can lead to particular sensitivity to channel-to-channel interference problems induced by nonlinearities in broadband and cable television applications.
In addition, it is well known in the art that nonlinearities are also used in a beneficial manner to achieve desired effects, and in such cases enhancement of the nonlinearities is the desired outcome. Example applications where such enhancement of nonlinearities is desirable are harmonic mixers and frequency multipliers.
The nonlinearities of devices, such as amplifiers, are commonly modeled as Taylor series expansions, i.e., power series expansions or polynomial expansions, of an input signal. For example, the output voltage y of a device may be described as a Taylor series, or polynomial, expansion of the input voltage x:
y=a
0
+a
1
x+a
2
x
2
+a
3
x
3
+ . . .
where a
0
, a
1
, a
2
, a
3
. . . are constants representative of the behavior of the particular device being modeled, and the order of the polynomial is determined by the highest power of x in the polynomial expansion. In most situations, the linear term a
1
x is the desired linear signal, and the terms a
n
x
n
, with n≠1, are undesired. The term a
0
represents a constant, or DC (direct current), offset that is easily removed in most applications.
In radio applications, the term a
3
x
3
is particularly problematic when an input signal such as x=A cos(&ohgr;
1
t)+A cos (&ohgr;
2
t) is considered. In this case the cubic term of the Taylor series is defined as:
a
3
x
3
=a
3
A
3
[cos
3
(&ohgr;
1
t
)+3 cos
2
(&ohgr;
1
t
)cos(&ohgr;
2
t
)+3 cos(&ohgr;
1
t
)cos
2
(&ohgr;
2
t
)+
cos
3
(&ohgr;
2
t
)]
where the terms cos
2
(&ohgr;
1
t)cos(&ohgr;
2
t) and cos(&ohgr;
1
t)cos
2
(&ohgr;
2
t) can be further expanded to:
cos
2
(
ω
1
t
)
cos
(
ω
2
t
)
=
1
4
[
2
cos
(
ω
2
t
)
+
cos
(
2
ω
1
t
+
ω
2
t
)
+
cos
(
2
ω
1
t
-
ω
2
t
)
]
cos
(
ω
1
t
)
cos
2
(
ω
2
t
)
=
1
4
[
2
cos
(
ω
1
t
)
+
cos
(
2
ω
2
t
+
ω
1
t
)
+
cos
(
2
ω
2
t
-
ω
1
t
)
]
where the terms cos(2&ohgr;
1
t−&ohgr;
2
t)/4 and cos(2&ohgr;
2
t−&ohgr;
1
t)/4 are well known in the art to present particular difficulty in the design of communications equipment since they can produce undesired in-band distortion products at frequencies close to the desired linear signal frequencies. For example, at frequencies of f
1
=100 MHz and f
2
=100.1 MHz, with &ohgr;
1
=2&pgr;f
1
and &ohgr;
2
=2&pgr;f
2
, the undesired frequency component 2&ohgr;
1
−&ohgr;
2
is a frequency of 99.9 MHz and 2&ohgr;
2
−&ohgr;
1
is a frequency of 100.2 MHz. These two undesired frequencies at 99.9 and 100.2 MHz are created by the third-order nonlinearity of the polynomial (i.e., a
3
x
3
), and are so close to the desired linear signal frequencies of 100 and 100.1 MHz that they cannot easily be removed by filtering.
One prior art approach to the problem employs feedforward compensation wherein a distortion error signal is generated by taking the difference between a first amplified and distorted signal, and a second undistorted signal, and later subtracting the distortion error signal from the first amplified and distorted signal in order to remove the distortion components.
This prior art is illustrated in the schematic drawing of
FIG. 1
, in which an apparatus
10
incorporating feedforward compensation is shown by way of example. An input signal
12
is applied both to first amplifier
14
and a first delay device
16
. The time delay of the first delay device equals the time delay of the first amplifier. The output signal
18
of the first amplifier is attenuated by the attenuator
20
. The output signal
22
of the first delay device is subtracted from the output signal
24
of the attenuator in a first subtractor
26
resulting in error signal
28
. The output signal
18
of the first amplifier is also input to a second delay device
30
. The time delay of the second delay device equals the time delay of the second amplifier
32
that amplifies the error signal. The output signal
34
of the second amplifier is subtracted from the output signal
36
of the second delay in a second subtractor
38
to form the final output signal
40
.
For illustrative purposes, an example input frequency spectrum
42
is shown for input signal
12
comprised of two input spectral lines of equal amplitude at different frequencies. The spectrum at the output signal
18
of the first amplifier
14
is illustrated in second spectrum
44
where the two innermost spectral lines correspond to the original input frequencies illustrated in the input spectrum, but with larger amplitude, and the two outermost spectral lines representing third-order distortion components of the output signal
18
of the first amplifier. The spectrum at the error signal
28
is illustrated in third spectrum
46
where the two spectral lines correspond to an attenuated version of the third-order distortion components of the second spectrum
44
(the two outermost spectral lin
Alston & Bird LLP
Choe Henry
The University of North Carolina at Charlotte
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