The present invention relates to electronic communication systems, and more particularly to systems in which multiple signals are simultaneously transmitted at varying power levels. 
In electronic communication systems, it is often necessary that groups of information signals be amplified and transmitted simultaneously.  For example, a cellular radio base station transmitter typically transmits signals to many active receiving mobile stations within a single geographic cell.  The signals typically appear at multiple predetermined frequencies in such multi-carrier signals.  Similarly, a satellite communications transponder amplifies and transmits large number of information signals destined for various participating remote stations.  Because such systems customarily employ a frequency division multiple access (FDMA) scheme, in which information signals are modulated on signal carriers occupying several frequency channels within an allocated frequency band, care must be taken to avoid inter-channel interference which may corrupt signal transmissions. 
One possible source of such cross-channel interference is known as intermodulation distortion (IMD), which may result when two or more signals of different frequencies are mixed.  For example, if two carriers of different frequencies are amplified using a non-linear amplifier, spurious outputs occur at the sum and difference of integer multiples of the original carrier frequencies. 
As described in detail below, third order intermodulation products resulting from two relatively strong signals may disrupt transmission of a third relatively weak signal being transmitted on a carrier having a frequency equal to the frequency of the intermodulation product. 
Various solutions have been proposed for improving linearity and reducing inter-channel effects in multi-carrier amplifiers.  One such solution is the feed-forward amplifier circuit.  In the feed-forward amplifiers two or more feed-forward loops are typically used to cancel distortion.  Alternative feed-forward amplifier configurations may utilize more loops to further reduce distortion.  In a first loop, a portion of the signals at the input to the amplifier are fed forward and, following suitable amplitude and phase adjustment, are subtracted from the amplifier output to generate an error signal.  The error signal is proportional to distortion components of the output.  The first loop that generates the error signal is known as the signal-cancellation loop.  The error signal is then amplified, phase-adjusted and subtracted from the amplifier output to give a corrected signal output with reduced distortion levels.  This portion of the circuit is known as the error-cancellation loop. 
In one design, a “pilot tone” is introduced to the first loop of the feed-forward amplifier (i.e. signal cancellation loop).  Then, the amplitude and phase adjustments in the error cancellation loops are performed by varying the amplitude and phase until a desired output is obtained.  The adjustments made in this manner, however, are inherently narrowband, typically giving optimal amplifier performance across a narrow frequency band. 
Another technique for reducing distortion involves the addition of more cancellation loops to further improve IMD cancellation in a feed forward amplifier.  The additional loops typically have additional phase and gain controls that introduce complexity to overall amplifier adjustment, as well as additional cost.  Typically any small improvement in performance obtained, is typically not deemed worth the extra complexity and cost of utilizing multiple feedforward loops. 
Accordingly, it would be desirable to provide other techniques which reduce intermodulation distortion at the output of feedforward power amplifiers in multi-carrier environments. 
Those having skill in the art would understand the desirability of having a multiple loop feedforward power amplifier that tends to reduce distortion, with a minimum of adjustment, and additional circuitry.  This type of feed-forward linear amplifiers would tend to more completely cancel in-band distortion, thus allowing the linearity of the amplifier to be improved while allowing control of the multiple loop feedforward amplifier loops to be simplified. 
Several attempts have been made to minimize such intermodulation distortion, notably U.S. Pat.  No.  5,796,304, issued to Gentzler, U.S. Pat.  No.  5,444,418 issued to Mitzlaff, and U.S. Pat.  No.  5,528,196 issued to Baskin, all of which are incorporated by reference herein.  These patents, along with others, have focused on feedforward amplifier topologies which aim to minimize intermodulation distortion. 
Intermodulation distortion tends to cause problems in transmitters that amplify and send out multicarrier signals.  The intermodulation distortion often falls in the spectrum between two of the carrier signals, and is difficult to filter out, because such a filter would likely also filter out the carrier signal, removing that signal and the associated data from the signal that is ultimately amplified by the power amplifier.  Thus, it is desirable to use an amplifier, such as a feed forward amplifier, that tends to suppress intermodulation distortion without filtering. 
FIG.  1 is a block diagram of a conventional feed-forward linear amplifier.  An input signal is applied to coupler 100a which couples portions of the input signal to delay line 140 and to main amplifier 110 via first complex gain and phase modulator 120.  Main amplifier 110 produces an amplified output having intermodulation products generated due to non-linearities in main amplifier 110.  A portion of the amplified output signal is coupled to summer 150 by coupler 100b.  Delay line 140 delays the input signal with respect to the output of the amplifier 110 producing a delayed signal such that the two signals reach summer 150 at the same time, but reversed in phase where they are better served. 
The output of summer (or coupler) 150 is an error signal which is coupled to auxiliary (or error) amplifier 160 via second complex gain and phase modulator 121.  Auxiliary amplifier 160 increases the amplitude of the error signal producing an error correction signal.  The error correction signal should be matched in amplitude to the intermodulation products (i.e. spurious component) generated by main amplifier 110 and delay line 141, but reversed in phase.  The resultant vector cancellation of the intermodulation products is performed in coupler 100c where the error correction signal is subtracted from the amplified input signal.  The vector cancellation must be performed with a high degree of accuracy.  If the error correction signal is matched in amplitude and phase to the intermodulation products, the error correction signal can completely cancel the intermodulation products of the main amplifier.  However, even with the high-precision components used in the amplifier, the error signal in reality cannot completely cancel the spurious components generated by main amplifier 110.  In general, complete cancellation requires that the error correction signal be maintained with greater than 0.5 degrees phase accuracy and 0.1 dB amplitude accuracy which is difficult to achieve in production. 
The feed-forward technique can be used in a multi-carrier power amplifier to effectively suppress intermodulation products, but at the cost of lower power efficiency and a high demand on complexity and component cost.  In particular, high power multi-carrier power amplifiers are difficult to master in production.  In addition some distortion typically remains at the output of the feedforward power amplifier. 
It may be desirable to further reduce the distortion in the output of a feed forward power amplifier by adding one or more additional feed forward compensation loops. 
Vector cancellation can be degraded by small deviations in delay, phase or amplitude from an initial set of optimized values of control signals to produce a desired delay, phase and amplitude response.  The basic structure described above is relatively sensitive to both long and short term component values that change due to aging, operating conditions or temperature. 
In order to address these problems, some feedforward amplifiers have incorporated automatic control processes in an attempt to correct or adjust one or more vector modulators.  Correction is attempted in a direction which restores and maintains the cancellation, by utilizing one or more pilot signals.  Pilot signals are desirable because they are always present and have known characteristics.  The pilot signals are injected into the main power amplifier output path.  The automatic control process minimizes, to the extent possible, the pilot signal amplitude monitored at the feedforward amplifier output.  Due to the finite bandwidth in a practical system, vector modulator and delay values corresponding to minimization of the pilot signal typically do not coincide precisely with the modulator and delay values required to achieve the lowest IM distortion signal output. 
U.S. Pat.  No.  5,528,196 to Baskin describes a feedforward amplifier in which a pilot tone is utilized in this manner.  All IM distortion cancellation occurs within a single vector summer. 
In addition to environmental and aging factors, the observed intermodulation distortion (IMD) performance of a power amplifier is sensitive to changes in output power, the number of carriers and their frequency separation.  For some radio services, including cellular communications, the transmitted output power is commonly required to be varied rapidly over a wide range.  Under these conditions, the automatic control system provides an incorrect, or delayed, estimate of the phase and amplitude adjustments used in the feedforward amplifier, and as a result, the IMD performance is degraded. 
When the performance of a feedforward system is limited by the accuracy in estimating the phase and amplitude adjustment control signals, IMD performance may be improved by adding additional stages of vector cancellation. 
FIG.  2 illustrates a prior art dual feedforward system according to U.S. Pat.  No.  5,444,418 issued to Mitzlaff.  In FIG.  2, a vector modulator is adjusted according to information based on the degree of cancellation of a pilot signal.  In this system, alternate embodiments place a first vector modulator 310 in the main amplifier path, shown as being implemented through power splitter 312, or in the delay path through the delay element 352. 
In this system, first loop controller 350 adjusts the controls of vector modulator 332 so as to minimize the pilot signal power measured at sampling point 340 by pilot receiver 344 via switched path 342.  Similarly, second loop controller 370 adjusts the controls of vector modulator 360 so as to minimize the pilot signal power measured at sampling point 368 by pilot receiver 344 via switched path 342. 
The topology of the feedforward system of FIG.  2 incorporates multiple closed loop structures that are dependent on each other, and must therefore be adjusted according to the control sequence explicitly described and/or used by the system.  Should a different input signal be used, the characteristics of the input signal vary over time, or the components age or degrade over time, the amplifier will not maintain adequate performance.  That is adjusting any one of the four loops will cause the performance of the other loops to change.  Typically an explicit sequence of adjustments are utilized to minimize the disruptions to the control of the remaining loops where one of the loops is adjusted. 
In addition, when a control loop is selected to be adjusted according to the specified sequence, the control points (vector modulator settings) of the remaining loops are temporarily frozen at their previous values while the control point of the selected loop is adjusted.  As such, switch 342 must be used to switch between the first loop coupler 340 and the second loop coupler 368 to properly correct the IM distortion correction of this type of amplifier.  This switching introduces additional errors into the feedforward amplifier that could prevent such a topology from being applicable to certain applications.