Telecommunications – Transmitter – With feedback of modulated output signal
Reexamination Certificate
1998-08-20
2003-04-22
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S127500, C455S114100, C330S151000, C330S052000
Reexamination Certificate
active
06553211
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to radio frequency (RF) amplifiers and, more particularly, to a pilot and input signal synchronization scheme for feed-forward RF amplifiers.
2. Description of Related Art
RF amplifiers often add undesired distortion to an input signal, creating an output RF signal on a main path comprising an amplified input signal and distortion, a significant component of which is intermodulation distortion (IMD). The distortion includes any undesired signals added to or affecting adversely the amplified input signal. IMDs of a signal arises from intermodulation of the frequency components in the signal by each other in a nonlinear system, producing waves having frequencies, among others, equal to the sums and differences of the frequency components of the original signal. Feed-forward correction is routinely deployed in modern RF amplifiers to reduce the distortion produced from the RF amplifier on the main signal path. The essence of the feed-forward correction is to isolate the distortion produced from the amplifier on a correction path so that at the final summing point, the distortion on the correction path cancels out the distortion of the RF signal on the main path.
Due to the unpredictability of input RF carrier pattern as well as the resultant location of the distortion, a known frequency component, i.e. the pilot, is injected in the main loop to mimic the distortion produced by the amplification process. In feed-forward amplifiers, the correction circuitry isolates the amplified pilot signal along with the distortion onto the correction path and manipulates the pilot signal and the distortion on the correction path to combine with and reduce the pilot and the distortion on the main signal path. The correction circuitry detects the pilot signal and attempts to cancel the pilot signal from the main signal path. In cancelling the pilot signal from the main signal path, the correction circuitry cancels the distortion.
There are two general types of pilot signals: continuous wave (CW) and spread spectrum (SS) pilot signals. The CW pilot is easy to detect and measure, but runs the risk of being overlaid by one of the input carriers. Therefore, it is advantageous to move the CW pilot outside of the band of operation. Otherwise, the pilot frequency should be constantly updated to find a quiet location in-band where there is no input carriers. For example,
FIG. 1
shows the frequency response of an RF amplifier including the location of a CW pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot
1
) 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
. As mentioned above, the pilot signal can also be located somewhere within the band of operation of the RF amplifier. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as a single spectral component of a certain amplitude, the pilot signal can comprise a plurality of spectral components having various amplitudes. Furthermore, an SS pilot can be spread across the entire operating band for the RF amplifier. The SS pilot is harder to detect and measure, but it is immune to the input carrier placement when placed in-band.
FIG. 2
discloses typical feed-forward correction circuitry
10
, which uses information obtained from the pilot signal to reduce distortion produced by RF amplifier
12
. An input signal is applied to a splitter
14
. The splitter
14
replicates the input signal on a main signal path
16
and a second path
18
. The splitter
14
is part of a feed forward 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 input signal on the main signal 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 create additional significant IMD components from the amplifier
12
due to the pilot signal. The output of 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 with a delayed version of the input signal (signal on path
18
) at the coupler
28
via coupling path
30
. The input signal on the path
18
has experienced sufficient delay provided by delay circuit
24
, the delay of which is designed so that such signal experiences the same delay as the signal appearing at the coupler
28
via the path
30
.
The gain & phase circuit
20
is controlled via control path
32
with two control signals to adjust the gain and phase of the input signal such that the input 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 control signal appearing on the control path
32
of the gain & phase circuit
20
is derived from the signal at point A in a well known manner through the use of detection circuits. The detection circuits detect well known electrical signal characteristics such as amplitude, phase, and frequency of the signal. Therefore, the input signals applied to the coupler
28
cancel each other leaving at point A essentially the pilot signal and the distortion produced by the amplifier
12
. Loop #
1
is thus a feed forward loop which serves to isolate at point A the pilot signal and distortion produced by the amplifier
12
.
The signals appearing at point A (pilot signal and distortion signals) are fed to gain & phase circuit
34
whose output is fed to amplifier
36
whose output is applied to coupler
38
. A portion of the output signals (input signal, pilot signal and distortion signals) of the amplifier
12
is fed to delay circuit
40
whose output is fed to the coupler
38
. The delay circuit
40
is designed such that signals from the output of the amplifier
12
applied to the coupler
38
experience the same delay as the signals from the output of the amplifier
36
applied to the coupler
38
.
Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detect circuit
42
can use circuits such as a mixer connected to a log detector (or other well known detection circuits) to detect the pilot signal or a portion of the pilot signal via coupler
44
. The pilot signal is used to obtain information about the distortion left at the final output. The information is obtained by detecting well known electrical signal characteristics of the pilot signal. In particular, the characteristics (e.g., amplitude, spectral content, phase response) of the pilot signal are known and thus when the pilot detect circuit
42
detects alterations to the pilot signal, detection circuit
42
will use such information to generate control signals onto path
46
. The control signals on the path
46
cause the gain & phase circuit
34
to modify the pilot signal and distortion at point A such that the pilot signal and the distortion on the main path
16
at the coupler
38
is the inverse (equal in amplitude but 180° out of phase) of the pilot signal and the distortion on the second path
18
at the coupler
38
. The corresponding pilot signals and the distortion signals at the coupler
38
cancel each other at the coupler
38
essentially leaving the amplified version of the input signal at the output of the system. Therefore, loop #
2
, which comprises the coupler
26
, the coupler
28
, the gain & phase circuit
34
, t
Chin Vivian
Garceran Julio A.
Lee John J
Lucent Technologies - Inc.
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