Abstract:
A feed forward amplifier and method of amplification are disclosed. The amplifier output is used to generate a pilot signal via feedback using uncancelled noise in the amplifier output. An automatic level control circuit maintains the pilot signal at a substantially constant level when the detected uncancelled noise in the amplifier output is above a threshold level. The generated pilot signal strength is allowed to vary when the detected uncancelled noise in the amplifier output is below the threshold and disappears automatically when the amplifier is aligned.

Description:
RELATED APPLICATION INFORMATION 
     The present application is a continuation-in-part of Ser. No. 10/838,985 filed May 5, 2004, now U.S. Pat. No. 7,123,086 which claims priority under 35 U.S.C. 119(e) to provisional application Ser. No. 60/468,444 filed May 7, 2003 and the present application also claims priority under 35 U.S.C. 119(e) to provisional application Ser. No. 60/659,744 filed Mar. 8, 2005, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to RF power amplifiers and amplification methods. More particularly, the present invention relates to feed forward power amplifiers and methods of using a pilot to align the loops of a feed forward amplifier. 
     2. Description of the Prior Art and Related Information 
     A primary goal of RF power amplifier design is linearity over the range of power operation. Linearity is simply the ability to amplify without distortion. This requirement is critical for modern wireless communication systems but it is increasingly difficult to achieve. This is due primarily to the bandwidth requirements of modern wireless communication systems which are placing increasing demands on amplifier linearity. Feed forward compensation is a well known approach applied to amplifiers to improve linearity by estimating and canceling distortion. In feed forward RF power amplifiers an error amplifier is employed to amplify only distortion components which are then combined with the main amplifier output to cancel the main amplifier distortion component.  FIG. 1  illustrates a conventional feed forward amplifier design having a main amplifier  1  and an error amplifier  2 . The basic elements also include delays  3 ,  4  in the main and error path, respectively, and main to error path couplers  5 ,  6 ,  7  and  8 . Additional elements not shown are also typically present in a conventional feed forward architecture as is well known to those skilled in the art. The delays, couplers and error amplifier are designed to extract distortion components from the main path and inject out of phase distortion components from the error path into the main amplifier output at coupler  8  to substantially eliminate the distortion component in the main amplifier path. 
     The performance of a feed forward amplifier may typically be analyzed based on two cancellation loops. Loop 1 , called the carrier cancellation loop, ideally provides a signal at the output of coupler  7  with the input RF carrier component cancelled and only a distortion component remaining. Loop  2  is referred to as the error cancellation loop or auxiliary path loop. In loop  2  the distortion component provided from coupler  7  is amplified by the error amplifier  2  and injected at coupler  8  to cancel the distortion component in the main path and ideally provide a distortion free signal at the output. 
     The quality of the distortion estimate (carrier cancellation) is determined by the alignment of the first loop in terms of gain and phase. The distortion cancellation in turn is determined by the alignment of the second loop in terms of gain and phase. In prior art systems, a pilot  9  is injected into the main amplifier path of the first loop, acting like a known distortion signal. The pilot signal is detected at the feed forward amplifier output by a pilot detector  10  and used to aid the alignment process for the second loop. When the second loop is aligned, the pilot is cancelled. If the second loop is misaligned, residual pilot power will be detected at the output of the feed forward amplifier. The degree of the misalignment is estimated from the measured power of the residual pilot. The alignment of the second loop is adjusted in an iterative manner with the goal of reducing the residual pilot power. The estimate of the pilot power must be reliable in order to determine if a given change in the gain and/or phase alignment represents an improvement. 
     Prior art pilot generation and detection systems must contend with various problems. First, there is a phase offset between the circuitry modulating and demodulating the pilot. As a result, it is necessary to compute the quadrature terms of the detected pilot in order to obtain a reliable estimate of the pilot power. Second, the pilot is ‘always on’ in order to measure the second loop alignment, even when the second loop is almost aligned fully. As a result, the residual pilot can appear at the output of the feed forward amplifier as a spectral spur. Third, the pilot power consumes part of the rated power handling capability of the main and error amplifiers. As a result, larger transistors are required to meet customer specifications, which in turn increases the cost. 
     In the prior art, the quadrature terms are obtained using two general approaches. The first approach generates a pilot tone without modulation and uses quadrature detection. The second approach modulates the pilot tone with quadrature components and uses scalar detection. In this approach the quadrature components are time-multiplexed to produce two independent measurements at the detector. The quadrature terms are then squared and added to obtain the pilot power. In general, the quadrature requirement adds expense and complexity to the pilot generation or detection circuitry, and adds complexity to the post-detector digital processing. 
     The residual pilot is considered to be an unwanted spectral emission from the feed forward amplifier. It must be limited when the amplifier is in an operational mode, after the second loop alignment has converged sufficiently to meet customer specifications. For prior art approaches, the amount of pilot power injected into the main amplifier path is therefore limited to prevent excessive residual spurs. This makes the detection circuitry more susceptible to noise, making the alignment process for the second loop less robust. 
     In the prior art, the pilot power consumes part of the power rating of the main and error amplifiers. In general, the power rating of the amplifier is determined primarily by linearity requirements rather than device failure. That is, the presence of the pilot power affects the amount of distortion produced rather than damaging the transistor. As a result, it would be desirable to reduce or turn off the pilot signal when the second loop is aligned fully or at least sufficiently to meet the spectral mask requirements. In addition to improving the power handling capability, turning off the pilot reduces the residual pilot spur appearing at the output. The problem with turning off the pilot is that subsequent misalignments in the second loop cannot be detected. This would make the amplifier very susceptible to thermally induced drift in the second loop gain or phase. 
     Accordingly, a need exists for a pilot generation and detection system which solves the above-mentioned problems in a simple, inexpensive, and effective manner. 
     SUMMARY OF THE INVENTION 
     In a first aspect the present invention provides a feed forward amplifier comprising an RF input for receiving an RF signal, a carrier cancellation loop, an error cancellation loop, and an RF output. The carrier cancellation loop comprises a main amplifier receiving and amplifying the RF signal, a main amplifier output sampling coupler, a first delay coupled to the RF input and providing a delayed RF signal, and a carrier cancellation combiner coupling the delayed RF signal to the sampled output from the main amplifier. The error cancellation loop comprises an error amplifier receiving and amplifying the output of the carrier cancellation combiner, a second delay coupled to the output of the main amplifier, and an error injection coupler combining the output from the error amplifier and the delayed main amplifier output from the second delay so as to cancel distortion introduced by the main amplifier. The RF output is coupled to the error injection coupler output and provides an amplified RF signal. The feed forward amplifier further comprises an output sampling coupler for providing a sampled output of the amplified RF signal and a positive feedback pilot generator circuit for generating a pilot signal from the sampled output of the amplified RF signal and providing the pilot signal to the input of the main amplifier, the pilot signal having a substantially constant strength when the uncancelled distortion from the error cancellation loop is above a threshold level. The positive feedback generator circuit also provides a detected pilot power signal from the sampled output of the amplified RF signal which varies with the strength of the uncancelled distortion from the error cancellation loop. 
     In a preferred embodiment the feed forward amplifier further comprises a controller and the threshold level is provided to the positive feedback pilot generator circuit from the controller and the positive feedback pilot generator circuit provides the detected pilot power signal to the controller. The feed forward amplifier preferably further comprises a phase adjuster and a gain adjuster coupled between the carrier cancellation combiner and the error amplifier, wherein the controller controls the gain adjuster and/or phase adjuster based on the detected pilot power signal. The feed forward amplifier also preferably comprises a phase adjuster and a gain adjuster coupled between the RF input and the main amplifier, wherein the controller controls the gain adjuster and/or phase adjuster based on the pilot power signal. 
     According to another aspect the present invention provides a pilot detection and generation system adapted for use with an RF amplifier having an RF input and an RF output. The pilot detection and generation system comprises a detector circuit for detecting a signal component representative of a band limited portion of the amplifier RF output and providing the signal component as an output, an automatic level control circuit coupled to the detector circuit and providing a level stabilized signal therefrom and a pilot generator circuit coupled to the automatic level control circuit for generating a pilot signal from the level stabilized signal output of the automatic level control circuit. 
     In a preferred embodiment of the pilot detection and generation system, the automatic level control circuit further comprises one or more outputs providing a variable signal representative of the signal power of the output of the detector circuit. The automatic level control circuit preferably also comprises a power detector and a feedback circuit for stabilizing the maximum level of the output of the detector circuit and providing it as the level stabilized signal, and a variable voltage attenuator configured in the feedback circuit for reducing the level of the signal from the detector circuit. The pilot detection and generation system preferably further comprises a local oscillator and the detector circuit comprises a first multiplier coupled to the local oscillator and mixing the local oscillator signal with the amplifier RF output and a bandpass filter coupled to the output of the first multiplier and providing as an output the signal component representative of a band limited portion of the amplifier RF output. The pilot generator circuit preferably further comprises a second multiplier coupled to the local oscillator for mixing the local oscillator signal with the level stabilized signal and providing as an output the pilot signal. 
     According to another aspect the present invention provides a method for generating a pilot signal from the output of an RF amplifier. The method comprises detecting a signal representative of a band limited sample of the amplifier output, performing an automatic level control operation on the signal to provide a level controlled signal and generating a pilot signal derived from the level controlled signal. 
     In a preferred embodiment of the method for generating pilot signal, detecting a signal representative of a band limited sample of the amplifier output comprises sampling an RF output signal from the amplifier, down converting the output signal to an intermediate frequency, and bandpass filtering the intermediate frequency signal. The down converting preferably comprises mixing the sampled RF output signal with a local oscillator signal. The method for generating a pilot signal also preferably comprises detecting the power of the band limited signal. Generating a pilot signal derived from the level controlled signal preferably further comprises mixing the level controlled signal with a local oscillator signal. 
     According to another aspect the present invention provides a method of controlling an amplifier. The method comprises sampling an output of the amplifier to provide a sampled signal, bandpass filtering the sampled signal to provide a band limited signal, deriving a level controlled signal from the band limited signal, generating a pilot signal derived from the level controlled signal, and injecting the pilot signal into a signal path of the amplifier. The method further comprises detecting any residual pilot signal in the band limited signal and controlling at least one of the gain and phase of a signal path in the amplifier in response to the detected pilot signal. 
     In a preferred embodiment of the method of controlling an amplifier, deriving a level controlled signal comprises performing a feedback level control on the band limited signal to limit the maximum power level of the signal. 
     According to another aspect the present invention provides a method for amplifying an RF input signal having an RF carrier with a carrier bandwidth. The method comprises receiving the RF input signal, amplifying the RF input signal employing a main amplifier, sampling the main amplifier output, delaying the RF input signal and providing a delayed RF input signal, and coupling the delayed RF input signal to the sampled output from the main amplifier so as to provide a distortion component of the sampled output from the main amplifier. The method further comprises amplifying the distortion component employing an error amplifier, delaying the output of the main amplifier, and combining the amplified distortion component and the delayed output of the main amplifier so as to cancel distortion introduced by the main amplifier and providing an amplified RF output. The method further comprises sampling the amplified RF output to provide a sampled output, band limiting the sampled output to a frequency band outside the RF carrier band, deriving a level controlled signal from the band limited signal, and generating a pilot signal from the band limited signal and injecting the pilot signal as an input to the main amplifier. The method further comprises detecting any residual pilot signal in the sampled output and controlling at least one of the gain or phase of the signal input to the error amplifier in response to the detected pilot signal. 
     In a preferred embodiment of the method for amplifying an RF input signal, deriving a level controlled signal comprises performing a feedback level control on the band limited signal to limit the maximum power level of the signal. Controlling the gain or phase of the signal input to the error amplifier preferably comprises adjusting at least one of the gain or phase so as to reduce the level of the generated pilot signal. 
     Further features and aspects of the invention are set out in the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block schematic drawing of a prior art feed forward amplifier. 
         FIG. 2  is a block schematic drawing of a feed forward amplifier employing a positive feedback pilot generation system in accordance with the present invention. 
         FIG. 3  is a block schematic drawing of a positive feedback pilot generation system employed in the feed forward amplifier of  FIG. 2 . 
         FIG. 4  is a block schematic drawing of a preferred embodiment of the ALC circuit of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A feed forward amplifier in accordance with a preferred embodiment of the present invention is shown in  FIG. 2  in a block schematic drawing. The feed forward amplifier employs a positive feedback pilot generation system, a preferred embodiment of which is shown in  FIG. 3 . The feed forward amplifier of the present invention may incorporate known features other than the novel aspects described in detail herein and such known features will not be described in detail. For example, additional features of a feed forward amplifier architecture and control system are described in U.S. patent application Ser. No. 10/365,111 filed Feb. 12, 2003, the disclosure of which is incorporated herein by reference in its entirety. 
     Referring to  FIG. 2 , the feed forward amplifier includes an input  12  which receives an input RF signal to be amplified and an output  14  which outputs the amplified RF signal. The RF signal may be a high bandwidth signal such as a CDMA (Code Division Multiple Access) spread spectrum communication signal or WCDMA (Wide Code Division Multiple Access) signal or other RF signal. The input RF signal is split into a main amplifier signal path and an error amplifier signal path at input coupler  30  in accordance with well known feed forward amplifier design. The main amplifier signal path includes main amplifier  16 . The main amplifier signal path further includes input and pre-distortion circuitry  20 . The input circuitry may include conventional preamplifier and group delay circuitry (not shown), and gain and phase control circuitry  50 ,  52 , respectively, implemented in accordance with conventional feed forward design. The pre-distortion circuitry  48  in turn pre-distorts the input signal to reduce IMDs introduced by main amplifier  16  and may be optional in some implementations. Input and predistortion circuitry  20  is controlled by loop  1  control signals  44  provided from controller  24 . In particular, these control signals include predistortion control signals  49 , gain adjuster settings  51  and phase adjuster settings  53 . 
     A positive feedback pilot generation circuit  22  (described in detail in relation to  FIG. 3  below) provides a pilot signal  58  which is injected into the main amplifier input at pilot injection coupler  23  as illustrated and is used to control loop  2  alignment (as described below). The pilot signal is extracted at the amplifier output by pilot sampling coupler  25  and detected by circuit  22  and the detected pilot signal  60  is used by controller  24  to provide the loop control to minimize the pilot signal in the output signal and thereby minimize distortion in the output signal (as described in more detail below). Controller  24  may also provide a set point signal to circuit  22  (as described below in relation to  FIG. 4 ). The main amplifier signal path further includes a main amplifier output sampling coupler  26  and delay  28 , generally in accordance with conventional feed forward design. 
     Still referring to  FIG. 2 , the error amplifier signal path includes input signal coupler  30  which samples the RF input signal and provides it to the error amplifier  34  via delay  32 , carrier cancellation combiner  36  and pre-error input circuitry  38 . More specifically, delay  32  and carrier cancellation combiner  36  operate as in a conventional feed forward amplifier such that the sampled output of the main amplifier  16  is attenuated by attenuator  40  and combined with the delayed input signal at carrier cancellation combiner  36  to substantially cancel all but the distortion component of the sampled signal from the main signal path. This carrier cancellation completes loop  1  of the feed forward amplifier. The output of carrier cancellation combiner  36  is sampled by coupler  37  and the sampled signal is provided to carrier cancellation detector  39 . The detected carrier cancelled signal  41  is provided to controller  24  which uses the detected signal to control the loop  1  gain and phase adjuster settings  51 ,  53  to minimize the detected carrier. In some applications and implementations it may be advantageous to control the loop  1  cancellation at combiner  36  to retain some RF carrier component in the resulting signal and the resulting signal is not purely the distortion component of the main amplifier. Nonetheless, for the purposes of the present application the resulting signal will be referred to as the distortion component and it should be understood some carrier component may be included. This distortion component of the signal is provided to pre-error input circuitry  38 . Pre-error input circuitry  38  may include conventional preamplifier and group delay circuitry (not shown), and gain and phase control circuitry  54 ,  56 . Controller  24  provides loop  2  control signals  46 , comprising gain adjuster settings  55  and phase adjuster settings  57 , to minimize the detected pilot from pilot detector  22 . Unlike the main path a predistortion circuit is typically not required in the error path due to the more linear nature of the error amplifier operation. The output of circuitry  38  is provided to error amplifier  34  which restores the magnitude of the sampled distortion components (IMDs) to that in the main signal path. The amplified distortion component output from error amplifier  34  is combined out of phase with the delayed main amplifier output at error injection coupler  42  to cancel the distortion component in the main signal path. This error cancellation completes loop  2  of the amplifier. A substantially distortion free amplified signal is then provided to the output  14 . 
     A sample of the output signal  18  is provided by coupler  25  to pilot detector and generator circuit  22 . Any residual pilot signal in the output is detected by the pilot detector circuitry  22  and provided as a pilot power signal  60 . The pilot power  60  is used by the controller  24 , along with the carrier cancelled signal  41 , to provide control signals  44  and  46 . The two controls  44 ,  46  may be essentially independent and may be viewed as separate control of the two loops; loop 1  comprising circuitry  20 , main amplifier  16 , main amplifier output sampling coupler  26 , attenuator  40 , input signal coupler  30 , group delay  32  and carrier cancellation combiner  36 ; and loop  2  comprising main amplifier sampling coupler  26 , attenuator  40 , carrier cancellation combiner  36 , pre-error circuit  38 , error amplifier  34 , delay  28  and error injection coupler  42 . Loop  1  control by controller  24  employs signal  41  to adjust gain and phase adjusters  50 ,  52  to minimize the detected carrier  41  at the output of Loop  1 . Loop  2  control by controller  24  employs the detected pilot power  60  to adjust the gain and phase adjusters  54 ,  56  to minimize the detected pilot power  60 . Suitable loop control algorithms are known to those skilled in the art and may be implemented by controller  24  which may be a suitable programmed microprocessor. Additional feed forward Loop  1  and Loop  2  control algorithms are also described in U.S. patent application Ser. No. 10/733,087 filed Dec. 11, 2003 and Ser. No. 10/733,498 filed Dec. 11, 2003, the disclosures of which are incorporated herein by reference. 
     Referring to  FIG. 3 , a preferred embodiment of the positive feedback pilot generator  22  is illustrated in a block schematic drawing. As shown the circuit comprises a detection signal path  62  and a pilot generation signal path  64 . The sampled RF output  18  of the feed forward amplifier is the input to the detection path  62 . (An alternative approach is to measure the output of a dynamic range extender (DRE), which provides the feed forward amplifier output with some carrier cancellation. Such a dynamic range extender is described in U.S. Pat. No. 6,147,555 issued Nov. 14, 2000, e.g., in  FIGS. 14 and 15  thereof, the disclosure of which is incorporated herein by reference.) The detection portion  62  of the system preferably comprises a bandpass power detector circuit, which detects uncancelled power in a relatively narrow bandwidth portion of the sampled amplifier output  18  at a frequency outside of the RF carrier bandwidth. The bandpass power detector circuit preferably comprises a mixer  66 , bandpass filter  72 , and an automatic level control (ALC)  90 . IF gain stages  70 ,  74  may also be employed, depending on the signal strength of the sampled output  18 . The RF input  18  to the detection path is down-converted to an IF frequency by Local Oscillator (LO)  68  and mixer  66 . The IF signal is then bandpass filtered by filter  72  to provide a relatively narrow bandwidth signal including the pilot signal frequency. The power of this bandpass limited signal  92  is then detected by combining an attenuation signal (ATTN)  95  and a detector signal (DET)  96  from ALC  90  using a combining network  97  to form detected power output  60 . The output  60  corresponds to the residual pilot power after the second loop cancellation. This pilot power output  60  is provided to the feed forward loop controller  24  ( FIG. 2 ). 
     In an alternative embodiment the attenuation signal (ATTN)  95  may be used directly as a measure of the detected pilot signal strength and provided to controller  24 . In such an embodiment combining network  97  may be dispensed with. In another alternate embodiment the power of the band pass limited signal  92  may be detected by a separate power detector and provided to the controller  24 . In such an embodiment output lines  95  and  96  from ALC  90  may be dispensed with as well as combining network  97 . 
     Still referring to  FIG. 3 , the pilot generation circuitry  64  is preferably the reverse line-up of the bandpass power detector circuit. The pilot generation circuit  64  preferably comprises bandpass filter  84 , mixer  88 , and IF gain stage  86 . Additional or fewer IF gain stages may be employed, depending on signal strength. The pilot generation circuit  64  uses the bandpass filtered IF signal  94  from the ALC  90  as an input. The signal  94  is bandpass filtered by filter  84  (if spurious rejection from the ALC circuit is necessary) then up-converted to RF by mixer  88  and LO  68 , after a second IF gain stage  86  (if necessary). 
     The above-mentioned ALC  90  controls the amplitude level of the generated pilot. One possible implementation of an ALC is shown in  FIG. 4 . The ALC  90  regulates the amplitude at its output  94  by adjusting the attenuation in the signal path from input  92  to output  94 . The signal path comprises a variable attenuator (WA)  100  and a gain stage  102 . A power detector  104  is connected to the output  94 . A control feedback loop adjusts the WA  100  in an attempt to keep the detected power DET  96  constant. The feedback path comprises a summer  106 , loop gain k  108 , and an integrator  110 . The input to the feedback loop is the detected power DET  96  and the output is ATTN  95 . The integrator  110  is typically clamped to prevent the ATTN  95  from exceeding the control voltage range of the WA  100 . The integrator can be replaced with a low pass filter. 
     It is important to distinguish between the detected residual pilot and the generated pilot. The signal DET  96  measures the latter. The detected residual pilot  60 , measuring the uncancelled pilot at the output  14  of the feed forward system, is a combination of ATTN  95  and DET  96 . When both ATTN  95  and DET  96  are logarithmic, the combining network  97  performs an addition to form signal  60 . Even if ATTN  95  and DET  96  are not logarithmic, a summing operation is sufficient because the signal  60  will be monotonic with the detected residual pilot. 
     In a typical operation, DET  96  will be constant, at a level determined by the set point  98  which may be provided from loop controller  24  ( FIG. 2 ). The set point  98  is made negative using  112  then added to DET  96 . When DET  96  is lower (higher) than the set point  98  the attenuation  100  is decreased (increased). If the attenuation  100  reaches its minimum value, the signal DET  96  will decrease as the second loop alignment improves further. This corresponds to the onset of the pilot turn off. 
     The same LO  68  frequency is preferably used for both the pilot detection down-conversion at mixer  66  and the pilot generation up-conversion at mixer  88 . The frequency of LO  68  is chosen to place the pilot signal outside of the bandwidth of the RF carrier of the input signal to the feed forward amplifier and to facilitate detection of the signal in circuit  62 . Also, a suitable choice of LO frequency may allow a relatively inexpensive IF filter  72  to be employed. For example, a LO frequency of about 85 MHz frequency shift from the carrier band will allow an inexpensive SAW filter to be used, e.g. with a 5 MHz pass band. Various other choices of LO frequency and filter passband are also possible, however. 
     In operation, the pilot detection and generation circuit  22  creates a narrow bandwidth, positive feedback loop through the main amplifier  16  and the second loop of the feed forward amplifier ( FIG. 2 ). When combined with the ALC  90 , a limit-cycle oscillation will develop using noise present in the feed forward amplifier and the pilot system, assuming that the loop has sufficient gain. The cancellation of the second loop affects the gain and phase of the positive feedback loop. As a result, good alignment of the second loop will suppress the limit-cycle oscillation. The degree of alignment required to suppress the limit cycle is selectable based on the amount of IF gain provided by the IF gain stages preceding the ALC  90  or by adjusting the set point  98  of ALC  90 . 
     A number of modifications of the illustrated implementation of the positive feed back pilot generation circuit  22  are possible. For example, an implementation of the bandpass power detector circuit  62  may employ an RF filter which is placed before the mixer  66  to reject image frequencies. In such an approach, a similar RF filter is preferably included within the pilot generation path  64  after the mixer  88 . Also, it is possible to eliminate the bandpass filter  84  within the pilot generation path  64  if ALC spurious emissions are low. Also, as noted above, the number of IF gain stages, the set point  98  of the ALC  90 , the LO frequency and the filter passband bandwidth may all be varied in accordance with the particular implementation and the particular RF carrier being amplified. 
     From the foregoing it will be appreciated that the present invention provides a number of advantages over prior approaches. As discussed above, prior approaches to pilot generation and detection must contend with various problems. First, there is a phase offset between the circuitry modulating and demodulating the pilot. As a result, it is necessary to compute the quadrature terms of the detected pilot in order to obtain a reliable estimate of the pilot power. Second, the pilot is ‘always on’ in order to measure the second loop alignment, even when the second loop is almost aligned fully. As a result, the residual pilot can appear at the output of the feed forward amplifier as a spectral spur. Third, the pilot power consumes part of the rated power handling capability of the main and error amplifiers. As a result, larger transistors are required to meet customer specifications, which in turn increases the cost. 
     In accordance with the present invention a pilot generation and detection system is disclosed which is based on a bandpass power detector circuit and positive feedback. The operation of the positive feedback pilot generation has many advantages, solving the above-mentioned problems in a simple, inexpensive, and novel manner. 
     More specifically, with respect to the quadrature detection problem experienced by the prior art, the LO phase alignment is achieved automatically within the present system by adjusting the frequency of the pilot, exploiting the delay around the feedback loop. The frequency adjustment is a by-product of the positive feedback. That is, the maximum loop gain producing the desired phase alignment will be sought as a natural mode of the system. As a result, the pilot power is measured directly as a scalar value, not requiring additional digital signal processing to transform quadrature components into power. The bandpass filters in the detection and generation paths limit the range of frequencies that the pilot system will generate. 
     The pilot system of the present invention has an additional benefit: the pilot turns off when the alignment is complete and turns back on if the alignment degrades subsequently. The ‘on and off’ feature of the pilot system is a by-product of the positive feedback and the ALC within the generation path. When the misalignment of the second loop is large, the pilot will have nearly constant amplitude. This is due to the ALC placed in the generation path. The variations in the pilot amplitude will be due primarily to the bandpass filter placed after the ALC. As the alignment of the second loop improves, the detected pilot decreases. Once the detected value drops sufficiently low so that the ALC is at its minimum attenuation, the pilot power falls rapidly. As a result, the pilot will turn off because the second loop cancellation has reduced the loop gain so low that it cannot sustain the limit-cycle oscillation. However, if the second loop becomes misaligned, the pilot signal will return automatically. As a result of this “on and off” feature, a higher power pilot signal may be generated without negatively affecting amplifier performance. For example, a 10 dB stronger pilot signal than is conventionally used may be generated. This provides better pilot signal detection and more robust loop control. 
     In summary, the novel pilot signal generation system of the present invention is simple, inexpensive, robust, and better performance is obtained at lower costs. 
     A preferred embodiment of the present invention of an RF power amplifier design which provides an improved pilot tone generation technique has been described in relation to the various figures. Nonetheless, it will be appreciated by those skilled in the art that a variety of modifications and additional embodiments are possible within the teachings of the present invention. For example, a variety of specific pilot generation circuit implementations may be provided employing the teachings of the present invention and limitations of space prevent an exhaustive list of all the possible circuit implementations or an enumeration of all possible control implementations. A variety of other possible modifications and additional embodiments are also clearly possible and fall within the scope of the present invention. Accordingly, the described specific embodiments and implementations should not be viewed as in any sense limiting in nature and are merely illustrative of the present invention.