Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2000-08-24
2003-04-08
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S622000, C330S149000, C455S295000, C455S232100
Reexamination Certificate
active
06545487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to a signal amplification system producing an amplified signal with reduced distortion.
2. Description of Related Art
Amplifiers often add undesired distortion to a signal, creating an output signal comprising distortion or nonlinear components and the signal component. The distortion includes any undesired signals added to or affecting adversely the input signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modern amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of input RF carrier pattern as well as the resultant distortion location, a known frequency component, i.e. a pilot signal, is injected in the main signal path with the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot signal along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot signal, the distortion can also be removed.
The pilot signal is an electrical signal comprising at least one frequency component spectrally located near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in
FIG. 1
which shows the frequency response of a radio frequency (RF) amplifier including the location of the pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot
1
) and/or located near the upper edge of the band of operation (e.g., pilot
2
). The pilot is positioned a spectral distance of &Dgr;f from an edge of the band of operation whose center frequency is f
0
. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known.
The feed forward distortion reduction circuitry reduces distortion produced by the RF amplifier by applying the pilot signal to the RF amplifier and making adjustments based on information obtained from the applied pilot signal.
FIG. 2
discloses feed-forward correction circuitry
10
and its use of information obtained from the pilot signal to reduce distortion produced by RF amplifier
12
. An input signal, for example including at least one carrier signal, is applied to a splitter
14
. The splitter
14
replicates the input signal on a main signal path
16
and a feed forward path
18
. The splitter
14
is part of a carrier cancellation loop referred to as loop #
1
, which in addition to the splitter
14
, comprises gain & phase circuit
20
, coupler
22
, the RF amplifier
12
, delay circuit
24
and couplers
26
and
28
. The signal on the main path
16
is applied to gain & phase circuit
20
. The output of gain & phase circuit
20
and the pilot signal are applied to the coupler
22
. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of the amplifier
12
. The output of the coupler
22
is applied to the amplifier
12
whose output comprises the amplified input signal, the amplified pilot signal and distortion signals produced by the amplifier
12
.
A portion of the output of the amplifier
12
is obtained from the coupler
26
and is combined at the coupler
28
via coupling path
30
with a delayed version of the input signal on the feed forward path
18
to isolate the pilot signal with distortion on the feed forward path
18
. The input signal on the feed forward path
18
is sufficiently delayed by delay circuit
24
so that such signal experiences the same delay as the signal appearing at the coupler
28
via the path
30
. The resulting error signal contains the distortion produced by the amplifier
12
along with any portion of the carrier signal remaining at the output of the coupler
28
and the pilot signal. The amount of carrier cancellation in the carrier cancellation loop depends on the proper gain and out-of-phase relationship between the two paths from the splitter
14
to the coupler
28
.
The gain & phase circuit
20
adjusts the phase and gain of the input signal according to control signals on control paths
32
and
33
such that the signal appearing at the coupler
28
via the path
30
is substantially the inverse (equal in amplitude but 180° out of phase) of the delayed input signal at the coupler
28
. The gain and phase control signals appearing on the control paths
32
and
33
of the gain & phase circuit
20
are derived from the signal at the output of the coupler
28
in a well known manner by sampling the output of the coupler
28
with a coupler
34
and using signal detection and control circuitry
35
. In general, the signal detection and control circuitry
35
detects an error signal for the carrier cancellation loop. The error signal represents the amplitude of the carrier signal(s) at point A. The signal detection and control circuitry
35
attempts to reduce the amplitude of the carrier signal(s) at point A by providing gain and/or phase control signals.
In this embodiment, the signal detection and control circuitry
35
includes a detector
36
, such as a log detector, which produces a signal representing the amplitude of the carrier signal(s) at point A. A filter
38
filters the output of the log detector to produce a DC-type amplitude signal representing the amplitude of the carrier signal(s). The amplitude signal is provided to a nulling circuit
40
. In response to the amplitude signal, the nulling circuit
40
provides the control signals on the control paths
32
and
33
to adjust the relative gain and/or phase between the combining signals at the coupler
28
and reduce the carrier signal(s). When the carrier signal(s) is minimized, the carrier signals combined at the coupler
28
substantially cancel each other leaving at the output of the coupler
28
the pilot signal with distortion produced by the amplifier
12
. Loop #
1
is thus a carrier cancellation loop which serves to isolate on the feed forward path
18
the pilot signal with distortion produced by the amplifier
12
.
A distortion reduction loop or loop #
2
attempts to reduce the pilot signal on the main signal path
16
, thereby reducing the distortion produced by the amplifier
12
, using the output of the coupler
28
. The pilot signal with distortion on the feed forward path
18
is fed to a gain & phase circuit
42
. The output of the gain and phase circuit
42
is fed to amplifier
44
whose output is applied to coupler
46
. The coupler
46
combines the amplified pilot signal and distortion on the feed forward path
18
with the signals from the amplifier
12
on the main signal path
16
(carrier signal(s), pilot signal with distortion). A delay circuit
40
on the main signal path
16
delays the signals from the output of the amplifier
12
on the main signal path
16
to experience substantially the same delay as the corresponding signals from the output of the amplifier
12
which pass over the coupling path
30
through the coupler
28
to the coupler
46
.
A coupler
48
provides an error signal representative of the signal at the output of the coupler
46
onto a pilot detection path
50
. Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detection and control circuitry
52
can detect the amplitude of the remaining portion of the pilot signal from the error signal on the pilot detection path
50
. The pilot detection and control circuitry
52
determines the amplitude of the pilot signal, and in response to the amplitude of the remaining pilot signal, the pilot detection and control circuitry
52
provides contro
Deb Anjan K.
Le N.
Lucent Technologies - Inc.
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