Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1998-09-29
2002-06-25
Ghayour, Mohammad H. (Department: 2634)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S254000, C375S278000, C375S296000
Reexamination Certificate
active
06411644
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a control system comprising an electrical circuit that uses a pilot signal to substantially cancel distortion produced by the electrical circuit and more particularly to a technique of frequency hopping about the frequency band of operation of the electrical circuit.
2. Description of the Related Art
Electrical signals when applied to electrical circuits are often distorted as a result of being processed by the circuits. Also electrical circuits generate signals for various useful purposes. The distortion comprises any undesired signals produced by the electrical circuits which are added to or are somehow combined with the applied or generated signals. A well known technique for substantially canceling distortion produced by an electrical circuit uses a control system coupled to the electrical circuit to which a pilot signal is applied. The applied pilot signal is detected by the control system. The applied pilot signal can have a single spectral component (i.e., one frequency) of a certain amplitude or the applied pilot signal can comprise a plurality of spectral components of various amplitudes. Typically, the applied pilot signal has an amplitude that is at least 60 dB lower than signals applied to or generated by the electrical circuit. The control system obtains information (about the distortion) from the detected pilot signal and uses that information to substantially cancel the distortion produced by the electrical circuit.
A control system comprises at least one circuit which uses external signals or signals generated by the at least one circuit to process signals applied to the at least one circuit. A particular implementation of the technique discussed above is shown in FIG.
1
.
FIG. 1
depicts a control system, comprising two feed forward loops (loop
1
and loop
2
) and detection circuit
132
. A pilot signal is applied to electrical circuit
108
via coupler
105
. Electrical circuit
108
can be any electrical and/or electronic (e.g., Radio Frequency (RF) linear amplifier, power amplifier) circuit. Loop
1
comprises coupler
105
, Gain & Phase circuit
104
, splitter
102
and delay circuit
126
. Coupler
105
is typically a device which combines two or more input signals and allows access to all or a portion of the combined signal. A coupler is also used to obtain a portion of a signal appearing at its input and output. Gain & Phase circuit
104
is typically a circuit which modifies the amplitude and phase of signals applied to its input based on the values of control signals applied to its control inputs (not shown). Splitter
102
is a circuit with one input and at least two outputs where a signal applied to the input is substantially replicated at the outputs. Delay circuit
126
is typically a circuit which applies a certain amount of delay to a signal applied to its input.
When a signal is applied to the input of the control system (i.e., to splitter
102
), the distortion experienced by the applied signal due to electrical circuit
108
is isolated at point A (i.e., path
123
). In particular, an input signal is applied to splitter
102
. Splitter
102
substantially replicates the input signal on paths
103
and
127
. In path
103
the input signal is applied to Gain & Phase circuit
104
, coupler
105
and electrical circuit
108
. In path
127
, the input signal is delayed by delay circuit
126
and then fed to cancellation circuit
124
via path
125
. Although not shown, it will be readily understood to those skilled in the art that the amplitude and phase of the input signal on path
125
can be detected (using well known detection circuitry) and converted to control signals that are applied to the control inputs (not shown) of Gain and Phase circuit
104
. Using coupler
112
, a portion of the input signal (plus any distortion produced by electrical circuit
108
) appearing at the output of electrical circuit
108
is fed to cancellation circuit
124
via path
113
. Cancellation circuit
124
can be implemented as a combiner circuit which has at least two inputs and one output. A combiner circuit combines signals applied to its inputs and transfers the combined signal to its output. Gain and Phase circuit
104
is adjusted such that the amplitude and phase of the input signal on path
113
are modified resulting in that signal being substantially 180° out of phase (+/−1°) and relatively the same amplitude (i.e., substantially the inverse) with the input signal on path
125
such that when the two signals are combined by cancellation circuit
124
they substantially cancel each other leaving the distortion (produced by electrical circuit
108
) at point A (path
123
). Thus Loop
1
is designed to isolate the distortion produced by electrical circuit
108
.
Loop
2
, which comprises delay circuit
114
, coupler
116
, Gain & Phase circuit
122
, and amplifier
120
, is designed to use information obtained by Detection circuit
132
from a pilot signal applied to electrical circuit
108
to substantially cancel the distortion produced by electrical circuit
108
. In particular, a pilot signal is applied to electrical circuit
108
via coupler
105
. The pilot signal (processed by electrical circuit
108
) appears on path
115
and at the output of coupler
116
, i.e., on path
117
. The pilot signal also appears at point A on path
123
after having propagated through path
113
via coupler
112
. A portion of the pilot signal processed by electrical circuit
108
is fed to detection circuit
132
via coupler
130
and path
128
. Detection circuit
132
comprises well known circuits (e.g., Log detector/amplifier, Sample & Hold circuit, Null circuit) to detect signal characteristics (e.g., amplitude, spectral content, phase response) of the pilot signal. Some or all of the characteristics may have been altered due to the distortion effects of electrical circuit
108
. Detection circuit
132
detects the characteristics of the input and uses this information to generate control signals on path
131
to cause Gain & Phase circuit
122
to modify the pilot signal. The pilot signal at point A is modified such that the pilot signal appearing on path
118
is substantially the inverse (relatively same amplitude, 180° out of phase, +/−1°) of the pilot signal appearing on path
115
. Amplifier
120
provides additional gain to the output of Gain & Phase circuit
122
. The additional gain is calculated such that the signal appearing on path
118
has an amplitude substantially equal to the amplitude of the signal on path
115
. Delay circuit
114
is designed such that the two pilot signals arrive at coupler
116
at substantially the same moment; that is, the two pilot signals are substantially synchronized (aligned in time) to each other. When the two pilot signals are combined by coupler
116
they cancel each other.
Detection circuit
132
now has the information that allows Gain & Phase circuit
122
to modify distortion appearing at point A and thus cancel distortion appearing at the output of electrical circuit
108
. When an input signal is applied to the control system, any distortion produced by electrical circuit
108
is isolated at point A (on path
123
) as discussed above. The signal on path
115
is the input signal (processed by electrical circuit
108
) plus any distortion produced by electrical circuit
108
. The distortion at point A is modified by Gain and Phase circuit
122
based on the information (i.e., signal characteristics) obtained from the previously applied pilot signal so that the distortion on path
129
is substantially the inverse of the distortion on path
115
. The distortions on path
115
and path
118
are combined at coupler
116
causing the distortions to substantially cancel each other resulting in an output signal that is substantially free of distortion.
Electrical circuit
108
has a bandwidth that defines a frequency band of operation. It is desirable that the pilot signal be spectrally locate
Ghayour Mohammad H.
The Law Office of John Ligon
LandOfFree
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