Patent Publication Number: US-6340915-B1

Title: Feed forward amplifier

Description:
FIELD OF THE INVENTION 
     The present invention relates to amplifier circuits. More specifically, the present invention relates to a feed forward amplifier. 
     BACKGROUND OF THE INVENTION 
     Feed forward amplifiers (“FFA”) are well known. Within the art, it is also known to improve the linear behavior of FFAs using a pilot tone. Put in simple terms, a pilot tone is generated by a pilot tone generator and injected prior to the main amplifier. The injected pilot tone is then detected by a pilot tone receiver at the output of the circuit. The detected pilot tone is used to adjust the gain and phase of the output of the correctional amplifier, such that the pilot tone and error or distortion introduced by the main amplifier are substantially eliminated. The overall result is a substantially linear feed forward amplifier. More detailed explanations of pilot tone usage in FFAs can be found in a variety of sources, such as in U.S. Pat. No. 5,874,850 to Van Horn. 
     While the use of a pilot tone can improve the overall linearity of FFAs, current pilot tone generators and receivers tend to add complexity and overall cost to the FFA circuit. For example, current FFA circuits that utilize a pilot tone typically require multiple oscillators—i.e., a first oscillator for the pilot tone generator, and a second oscillator for the pilot tone receiver. Furthermore, these two oscillators must be programmed with complementary frequencies: namely, a first frequency for the first oscillator, and a second frequency for the second oscillator, whereby the second frequency differs from the first frequency by the intermediate frequency (“IF”) of the pilot tone receiver. Another problem with using two oscillators is that phase noise, (jitter in the time domain) can exist between the oscillators, thus requiring additional care, cost and/or complexity when designing both the generator and receiver to compensate for any phase noise. 
     Yet another problem with the prior art is that certain elements in the pilot tone receiver, such as the filter, can be difficult to appropriately size so that they properly complement the correctional amplifier and delay element used in the error signal path of the FFA circuit, thus increasing the overall complexity and cost of designing the FFA. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel feed forward amplifier that obviates or mitigates at least one of the above-identified disadvantages of the prior art. 
     In an aspect of the invention, there is provided a pilot tone generator-receiver for a feed-forward amplifier (FFA). The feed forward amplifier includes a main amplifier and an error correction pathway. The generator-receiver comprises an oscillator for producing a signal and a vector modulator connected to the oscillator. The vector modulator is operable to either add or subtract a modulating frequency to or from, respectively, the signal in order to generate a pilot tone that is suitable for injection prior to the main amplifier. The generator-receiver also includes a receiver circuit connected to the oscillator, which is for detecting a pilot tone from an output of the feed-forward amplifier. The receiver circuit is also operable to use the detected pilot tone to modify the controls of the error correction pathway, such that the error pathway is then operable introduce a signal at the output of the feed-forward amplifier that substantially eliminates the pilot tone and any error introduced by the main amplifier. 
     In another aspect of the invention, there is provided a method for generating and receiving a pilot tone for a feed forward amplifier having a main amplifier, an error correction pathway, and an output pathway, the method comprising the steps of: 
     producing an oscillating signal; 
     applying a vector modulation to the oscillating signal to produce a pilot tone; 
     injecting the pilot tone prior to the main amplifier; 
     mixing the oscillating signal with a measured output of the output pathway to produced a mixed signal; 
     filtering an intermediate frequency from the mixed signal to produce a filtered signal; and, 
     detecting the pilot tone from the filtered signal. 
     The present invention can reduce expense and complexity over certain prior art FFAs. For example, in various aspects of the present invention, a feed forward amplifier is provided that only requires a single oscillator to generate a pilot tone and provide an oscillating signal for pilot tone receiver circuitry. In contrast, certain prior art FFAs can require two oscillators; one for the pilot tone generator and one for the pilot tone receiver. In another embodiment of the invention, a method is provided of generating and detecting a pilot tone from a single oscillating signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
     FIG. 1 is a block diagram of a feed forward amplifier according to the prior art; 
     FIG. 2 is a block diagram of a feed forward amplifier according to an embodiment of the invention; and, 
     FIG. 3 is a flow chart showing a method of generating and receiving a pilot tone in accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a prior art feed-forward amplifier (“FFA”) is indicated generally at  20 . FFA  20  comprises an amplifier portion  24 , a pilot tone generator  28 , and a pilot tone receiver  32 . Amplifier portion  24  includes a coupler  36  that is connected to an input signal path  40 . Coupler  36  is operable to split an incoming signal from path  40  into a first signal path  44  and a second signal path  48 . First signal path  44 , which carries the main signal from coupler  36 , is characterized by a gain and phase adjuster  52 , a main amplifier  56  and a delay element  60  that outputs to an output signal path  64 . Second signal path  48 , which carries a sample of the input signal generated by coupler  36 , is characterized by a delay element  68 , a gain and phase adjuster  72  and a correctional amplifier  74 , the output of which connects to output signal path  64  via a coupler  76 . Those of skill in the art will recognize that adjuster  72  and correctional amplifier  74  form an error correction pathway  73  within amplifier portion  24 . Accordingly, when the output of correctional amplifier  74  is coupled to the output of delay  60 , error and pilot tones are substantially eliminated from the output signal path  64 , such that the output signal path  64  represents a substantially linear amplification of input signal path  40 . 
     Amplifier portion  24  is further characterized by a coupled path  78  that interconnects a first coupler  80  (which is connected to the output of main amplifier  56 ) and a second coupler  82  (which is connected to the output of delay element  68 ), in order to provide a sample of the error introduced by main amplifier  56  to the error correction pathway  73  commencing at adjuster  72 . Additionally, a detector-controller  84  connects to the input of adjuster  72 , via a coupler  86 , in order to measure the total power just prior to adjuster  72 . Detector-controller  84  uses the measurement to modify the controls of adjuster  52  in order to minimize the level of the total power. The remaining power coupled through path  78 , which consists of the pilot tone and any intermodulation products created in the main amplifier  56 , is fed to error pathway  73 . 
     Pilot tone generator  28  includes a reference signal generator  88  that provides a reference signal to a phase locked loop (PLL) chip  92 . (While not expressly shown in FIG. 1, those of skill in the art will recognize that PLL chip  92  includes both PLL circuitry and loop filter circuitry). PLL chip  92  is operable to receive a programming input  96  to allow a user to set a suitable pilot tone for use in FFA  20 . PLL chip  92  connects to an oscillator  100 , which feeds its output back to PLL chip  92 , while also injecting its output into amplifier portion  24  via a coupler  102 , just prior to the input of main amplifier  56 . Thus, overall, pilot tone generator  28  is operable to inject a pilot tone into main amplifier  56 . 
     Pilot tone receiver  32  includes a reference signal generator  104  that provides a reference signal to a PLL chip  108 . PLL chip  108  is also operable to receive the same programming input  96  which is introduced to PLL chip  92  of pilot tone generator  28  to ensure that the pilot tone received at receiver  32  will correspond with the pilot tone generated by pilot tone generator  28 . PLL chip  108  is tunable to a frequency that is different from PLL chip  92  by an amount equal to the intermediate frequency of filter  124 . The difference can either be less or more than the frequency of PLL  92  depending on desired system parameters. PLL chip  108  connects to an oscillator  112 , which feeds its output back to PLL chip  108  in order to correct the rate of oscillation in the usual manner. Accordingly, oscillator  112  operates at the frequency controlled by PLL chip  92 . Those of skill in the art will now recognize that this is one of the disadvantages of the prior art, in that two oscillators are required for FFA  20 , thereby increasing the overall cost and complexity of FFA  20 . 
     The output of oscillator  112  is also directed to a mixer  116 , which also receives input from a coupler  120  joined to output signal path  64 . The output of mixer  116  is presented to a filter  124 , which is selected to match the intermediate frequency, as determined from the difference in frequency between oscillator  100  and  112 . The filtered signal outputted from filter  124  is inputted to a detector-controller  128 , which detects the pilot tone received at coupler  120  and modifies the controls of adjuster  72  in order to minimize the level of the pilot tone. Thus, overall, pilot tone receiver  32  is operable to detect the pilot tone at output signal path  64  and use the level of the pilot tone to modify gain and phase adjuster  72  so as to minimize the level of the pilot tone at coupler  120 . 
     The operation of prior art FFA  20  will now be discussed with reference to the foregoing and FIG. 1. A signal is introduced to input signal path  40  where it is split into first signal path  44  and second signal path  48 . A pilot tone is injected into first signal path  44  just prior to main amplifier  56  by pilot tone generator  28 . The output along signal output path  64  is monitored by pilot tone receiver  32 , which detects the magnitude of the pilot tone and directs it to adjuster  72 . Adjuster  72  also receives, via coupled path  78 , output from main amplifier  56 . Correcting amplifier  74  then uses the output from gain and phase adjuster  72  to generate an output that is injected into output signal path  64  via coupler  76 , such that the pilot tone and any error introduced by main amplifier  56  are substantially eliminated. 
     A feed forward amplifier in accordance with an embodiment of the present invention will now be discussed with reference to FIG. 2, and is indicated generally at  20   a . FFA  20   a  includes an amplifier portion  24  which, in a present embodiment, is identical to amplifier portion  24  of the prior art FFA  20  discussed above. Accordingly, the individual components of amplifier  24  in FFA  20   a  are identical to amplifier portion  24  of FFA  20 , and accordingly, are indicated with the same reference numbers as amplifier  24  in FIG.  1 . 
     FFA  20   a  also includes a pilot tone generator-receiver  132 , which includes a reference signal generator  136  that provides a reference signal to a PLL chip  140 . PLL chip  140  is operable to receive a programming input  96   a  (which can be the same as the programming input  96  discussed in the prior art shown in FIG. 1) which is introduced to PLL chip  140  of pilot tone generator-receiver  132  in order to allow a user to set a desired pilot tone for FFA  20   a . PLL chip  140  connects to an oscillator  144 , that is operable to generate a signal at the desired pilot tone frequency, plus or minus the intermediate frequency. Oscillator  144  feeds its output back to phase locked loop chip  140  in order to correct the desired rate of oscillation the usual manner. 
     Oscillator  144  also outputs its signal to a vector modulator  160  that generates a pilot tone for injection into amplifier portion  24  at a coupler  102   a , just prior to main amplifier  56 . Vector modulator  160  produces the pilot tone from the signal produced by oscillator  144  by adding or subtracting a desired modulating frequency (Fm) to the oscillator signal. The addition or subtraction of the modulating frequency is selectable by the phase reference given to the SIN and COS inputs of modulator  160 . These SIN and COS inputs can be operated with a digital signal processor (DSP) or other type of master controller for FFA  20   a , as will occur to those of skill in the art. 
     Any suitable modulation frequency can be chosen. One suitable range of frequencies from which the modulating frequency (Fm) can be the chosen is any frequency that is less than about one megahertz (1 MHz). However, other ranges of frequencies can be from near direct current to about fifty megahertz. Yet another range of frequencies can be from about ten kilohertz to about ten megahertz. In the present embodiment however, a modulating frequency of four-hundred-and-fifty kilohertz (450 kHz) is presently preferred, due to the ready-supply of 450 kHz IF filters. The output of vector modulator  160  thus presents an accurate pilot tone, which can be injected into amplifier portion  24  of FFA  20   a  at coupler  102   a , in the substantially same manner the pilot tone was injected at coupler  102  in the prior art FFA  20  discussed above. 
     The output of oscillator  144  is also directed to pilot tone receiver circuitry, which in a present embodiment includes a mixer  148  that also receives input from a coupler  120   a  joined to output signal path  64 . The output of mixer  148  is then presented to a filter  150 , which has a center of its intermediate frequency that is sized to correspond with the modulating frequency (Fm). The filtered signal outputted from filter  150  is then inputted to a detector-controller  154 , which detects the pilot tone received at coupler  120   a  and makes adjustments to adjuster  72  in order to minimize the level of the pilot tone. 
     Table I shows a table of exemplary part numbers for use in pilot tone generator-receiver  132 . 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Reference 
                   
                   
               
               
                 Item 
                 Number in 
               
               
                 Name 
                 FIG. 2 
                 Part Supplier 
                 Part Number 
               
               
                   
               
             
            
               
                 Vector 
                 160 
                 RF Micro Devices, Inc. 
                 RF2422 
               
               
                 Modulator 
                   
                 5550 Scotts Valley Drive, 
               
               
                   
                   
                 Suite 400 
               
               
                   
                   
                 Scotts Valley, CA 95066 
               
               
                 Oscillator 
                 144 
                 Z˜Communications Inc. 
                 V622ME10 
               
               
                   
                   
                 9939 Via Pasar 
               
               
                   
                   
                 San Diego, CA 92126 
               
               
                 PLL chip 
                 140 
                 National Semiconductor 
                 LMX2326 
               
               
                   
                   
                 2900 Semiconductor Drive 
               
               
                   
                   
                 P.O. Box 58090 
               
               
                   
                   
                 Santa Clara, CA 
               
               
                   
                   
                 95052-8090 
               
               
                 Filter 
                 150 
                 Murata Electronics 
                 CFUX450C100H 
               
               
                   
                   
                 2200 Lake Park Drive 
               
               
                   
                   
                 Smyra, GA 30080 7604, 
               
               
                 Detector- 
                 154 
                 Analog Devices 
                 AD8310 (detector) 
               
               
                 controller 
                   
                   
                 ADSP-2186 
               
               
                   
                   
                   
                 (DSP controller) 
               
               
                 Mixer 
                 148 
                 Mini-Circuits 
                 SYM-22H 
               
               
                   
               
            
           
         
       
     
     A method of generating and receiving a pilot tone, in accordance with another embodiment of the invention, will now be discussed with reference to the flow-chart shown in FIG.  3 . In order to assist in the explanation of the method, reference will be made to FFA 20   a  shown in FIG. 2, and the above discussion. At step  300 , an oscillating signal is produced. In a present embodiment, oscillator  144  produces the oscillating signal, however, other ways of producing an oscillating signal will occur to those of skill in the art. While not critical, it is presently preferred that the oscillating signal is any signal that does not include frequencies which will be amplified by FFA  20   a  Further functionality is provided in FFA  20   a  by including input  96   a  to allow a user to set the frequency of the oscillating signal. PLL chip  140  and reference signal  136  provide further input and control over oscillator  144  to ensure that a reliable oscillating frequency is produced. In general, it is to be understood that any means for producing an oscillating signal that is usable for pilot tone generation and detection can be used, and thus other functional equivalents will occur to those of skill in the art. 
     At step  305 , a vector modulation is applied to the oscillating signal produced at step  100  in order to generate a pilot tone. In a present embodiment, the vector modulation is performed by vector modulator  160 , which, depending on the phase relationship of a SIN(Fm) input and a COS(Fm) input being received, vector modulator  160  will add or subtract a desired modulating frequency thus generating a pilot tone at the output of vector modulator  160 . In other embodiments, it is to be understood that any means for applying a desired vector modulation to the oscillating signal can be used, or any means for adding (or subtracting) a modulating frequency to (or from, respectively) the oscillating signal can be used, and thus other types of equipment for performing step  305  will occur to those of skill in the art. 
     At step  310 , the pilot tone is injected prior to the main amplifier. In a present embodiment, the pilot tone is injected through coupler  102   a  which is located just prior to main amplifier  56 , however, other means for injecting the pilot tone into amplifier portion  24  will occur to those of skill in the art. 
     Having injected the pilot tone, amplifier portion  24  behaves in the usual manner, namely the main signal and the injected pilot tone are amplified by main amplifier  56 . After experiencing a delay at delay element  60 , the amplified signal is output from delay element  60  along output signal path  64 . Additionally, a sample of the amplified signal generated by amplifier  56  is directed into the error pathway. 
     At step  315 , the oscillating signal produced at step  100  is mixed with a measured output of the amplifier portion. In a present embodiment, the measured output is obtained via coupler  120   a , which is joined to output signal path  64 . The measured output is mixed using mixer  148 , which outputs the mixed signals to filter  150 . 
     At step  320 , the intermediate frequency is filtered from the mixed signal. In a present embodiment, this step is accomplished by filter  150 , which presents the filtered signal to detector-controller  154 , however, other filter means operable to filter the intermediate frequency from the mixed signal are within the scope of the invention. 
     At step  325 , the pilot tone is detected from the filtered signal. In a present embodiment, this is accomplished by detector-controller  154 . 
     At step  330 , the controls of error correction pathway  73  are modified to minimize the level of the detected pilot signal. In a present embodiment, the adjustment is performed by detector-controller  154 , which utilizes the detected pilot tone from step  325 . The modification is performed on adjuster  72  within error correction pathway  73 . 
     It will now be understood by those of skill in the art that the steps  300 - 330  continuously cycle, and need not occur in the exact order as shown in FIG.  3 . 
     While the embodiments discussed herein are directed to specific implementations of the invention, it will be understood that combinations, sub-sets and variations of the embodiments are within the scope of the invention. For example, while presently less preferred, in other embodiments of the invention it is contemplated that vector modulator  160  can be eliminated, and substituted with a phase shifter or phase adjustment means for adaptively adjusting the phase of the oscillating signal generated by oscillator  144 . The phase adjustment means can be placed in any suitable location, such as either prior to coupler  102   a  where the pilot tone is injected, or it can be located between mixer  148  and oscillator  144 . The signal generated by oscillator  144  will be the pilot tone as well as the local oscillator for the pilot tone receiver. In this example, means are provided to maintain a desired phase relationship between the pilot tone at coupler  120   a  and the signal coming from oscillator  144 , at the inputs to mixer  148 . Additional means are provided to adaptively control the phase adjustment means to compensate for temperature and component variation. According to this variation, regardless of where the phase adjustment means is located, the output of mixer  148  is a DC voltage that corresponds to the level of the pilot tone received at coupler  120   a . Those of skill in the art will thus recognize that this variation is a “zero-IF” system. 
     The present invention provides a novel feed forward amplifier. The feed forward amplifier includes any circuitry commonly found in the amplifier portion of feed forward amplifiers, and also includes a pilot tone generator-receiver that, when incorporated into a complete feed forward amplifier, only requires a single oscillator in order to allow the feed forward amplifier to produce a substantially error-free, linear amplification. Because only a single oscillator is required, the present invention is generally easier to implement, and less costly, than the two-oscillator systems of in the prior art. Furthermore, the pilot tone generator-receiver of the present invention can be implemented using readily-available vector modulators, in order to obtain a pilot tone generator combined with a pilot tone receiver circuit. By using a common oscillator, tedious and complex matching of filters and detector-controllers in the pilot tone receiver with the elements of the pilot tone generator can be obviated. Additionally, because one oscillator is used for both pilot tone generation and reception, it is largely possible to cancel the phase noise produced by the oscillator yielding a lower noise IF signal and a generally more sensitive pilot tone receiver. 
     The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art.