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 traversing or sweeping the pilot signal across the 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 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.degree.  out of phase (+/-1.degree.)  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) of the pilot signal.  Detection circuit 132 detects the amplitude 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.degree.  out of phase, +/-1.degree. )  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 located substantially in the middle of the frequency band of operation of electrical circuit 108.  However, placing the pilot signal anywhere in the band of operation of electrical circuit 108 causes interference to occur between the input signal and the pilot signal adding more distortion to the input signal.  The interference is any interaction between the pilot signal and an applied or generated signal that adversely affects one or more characteristic (e.g., amplitude, frequency, phase) of the applied or generated signal and/or the pilot signal.  Thus, interference not only distorts any signal that is applied or generated by the electrical circuit, but also affects the pilot signal.  As discussed above, the pilot signal is typically 60 dB lower than the applied or generated signals and thus would be interfered with by such signals.  A distorted pilot signal provides inaccurate information about the distortion and thus the very purpose of such a pilot signal is defeated.  Also, even when the pilot signal is located in the middle of the frequency band of operation, it does not experience the distortions located in other parts (e.g., lower band or upper band) of the frequency band of operation.  What is therefore needed is to use a pilot signal that obtains information about the entire frequency band of operation of the electrical circuit and does so without interfering with any signals applied to or generated by the electrical circuit.