Abstract:
A single continuous closed-loop power control feedback system provides seamless power control for a power amplifier and also enables an AM signal to be injected into the power amplifier through the power amplifiers&#39; control port. The AM signal is developed by an I/Q modulator and supplied to a comparator located in the power control loop. By using leakage from the power amplifier as feedback to a phase locked loop during initial power amplifier power ramp-up, the single continuous closed-loop power control system provides continuous feedback to the phase locked loop during the entire power amplification ramp-up period and eliminates the need for multiple feedback loops.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to the versatile transmission of radio frequency power in a wireless communication device transmitter, and more particularly, to a continuous closed-loop power control system including modulation injection into a wireless transceiver&#39;s power amplifier.  
           [0003]    2. Related Art  
           [0004]    With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.  
           [0005]    As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, portable communications systems complying with the IS-136 standard specify the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8 QPSK), offset π/4 differential quadrature phase shift keying (π-DQPSK) or variations and the access format is time division multiple access (TDMA). Other standards may require the use of, for example, code division multiple access (CDMA).  
           [0006]    Similarly, in Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrowband TDMA access environment.  
           [0007]    Furthermore, in a typical GSM mobile communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a very clean phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used, thus allowing efficient transmission of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, such as that employed in IS-136, however, use a modulation scheme in which both a PM signal and an amplitude modulated (AM) signal are transmitted. Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.  
           [0008]    This condition is further complicated because transmitters typically employed in GSM communication systems transmit in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. In GSM this power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the control port of the power amplifier.  
           [0009]    When attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will attenuate the amplitude variations present in the signal in an attempt to maintain a constant output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal.  
           [0010]    In such systems in which transmit signals contain both PM and AM components, the output power can be controlled by applying a pre-determined control voltage to the power amplifier. Unfortunately, this requires the use of a highly linear, and therefore very inefficient, power amplifier. In non-burst transmission systems the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method to control the output power is to “pre-distort” the modulated signal in such a way that the power control loop will cancel the effect of the pre-distortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, these methods are costly and inefficient.  
           [0011]    Known multi-mode transmitter architectures require multiple variable elements, which are chosen depending upon the desired transmit mode. These architectures are complex, unreliable, require periodic calibration, and cannot support multiple transmission standards without significant adjustments to the supporting analog and digital circuitry.  
           [0012]    Further, in those transmission standards in which both a PM signal and an AM signal are sent to a power amplifier, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.  
           [0013]    With the increasing desirability of developing one worldwide portable communication standard, it would be desirable to have a multi-band and multi-mode portable transceiver that can transmit a signal containing both a PM component and an AM component, while max minimizing the efficiency of the power amplifier. Furthermore, it would be desirable to have such a multi-band and multi-mode portable transceiver that can use conventional in-phase (I) and quadrature (Q) transmit signal components without requiring separate baseband signals for phase modulation and amplitude modulation. Further still, as the GSM standard evolves further, such as with the development of enhanced data rates for GSM evolution (EDGE), it is desirable to have one portable transceiver that may operate in all systems.  
         SUMMARY  
         [0014]    The invention provides a continuous closed-loop power control system, which includes modulation injection into a wireless transceiver&#39;s power amplifier that allows the use of non-linear, power efficient amplifiers. The invention uses a single continuous closed-loop power control system that allows an AM signal to be injected into the power amplifier through the power amplifier control port. The AM signal is derived from the output of an I/Q modulator and supplied to a comparator located within the power control feedback loop. By using the leakage from the power amplifier as feedback to a translation loop during the initial power amplifier ramp-up, continuous phase feedback to the translation loop is achieved during the entire power amplification ramp-up period, thus eliminating the need for multiple feedback loops.  
           [0015]    Related methods of operation and computer readable media are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0016]    The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.  
         [0017]    [0017]FIG. 1 is a block diagram illustrating a simplified portable transceiver.  
         [0018]    [0018]FIG. 2 is a block diagram illustrating the upconverter and power control element of FIG. 1 including the continuous closed-loop power control system of the invention.  
         [0019]    [0019]FIG. 3 is a graphical representation of the average power output of the power amplifier of FIG. 2.  
         [0020]    [0020]FIG. 4 is a schematic view illustrating an alternative embodiment of the power amplifier circuit of FIG. 2.  
         [0021]    [0021]FIG. 5 is a schematic view illustrating another alternative embodiment of the power amplifier circuit of FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Although described with particular reference to a portable transceiver, the continuous closed-loop power control system including modulation injection can be implemented in any system where it is desirable to transmit a combined signal including a PM component and an AM component. Furthermore, the continuous closed-loop power control system can be implemented independently from the modulation injection, where both systems are applicable to any system where it is desirable to implement a closed power control feedback loop and where a PM signal and an AM signal are amplified by a power amplifier.  
         [0023]    Further still, the continuous closed-loop power control system including modulation injection can be implemented in software, hardware, or a combination of hardware and software. In a preferred embodiment(s), selected portions of the continuous closed-loop power control system including modulation injection are implemented in hardware and software. The hardware portion of the invention can be implemented using specialized hardware logic. The software portion can be stored in a memory and be executed by a suitable instruction execution system (microprocessor). The hardware implementation of the continuous closed-loop power control system including modulation injection can include any or a combination of the following technologies, which are all well known in the art: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit having appropriate logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc.  
         [0024]    Furthermore, the continuous closed-loop power control system including modulation injection software, which comprises an ordered listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions.  
         [0025]    In the context of this document, a “computer-readable medium” can be any means that can contain store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a nonexhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory) (magnetic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory.  
         [0026]    [0026]FIG. 1 is a block diagram illustrating a simplified portable transceiver  100 . Portable transceiver  100  includes speaker  102 , display  104 , keyboard  106 , and microphone  108 , all connected to baseband subsystem  110 . In a particular embodiment, portable transceiver  100  can be, for example but not limited to, a portable telecommunication handset such as a mobile cellular-type telephone. Speaker  102  and display  104  receive signals from baseband subsystem  110  via connections  112  and  114 , respectively, as known to those skilled in the art. Similarly, keyboard  106  and microphone  108  supply signals to baseband subsystem  110  via connections  116  and  118 , respectively. Baseband subsystem  110  includes microprocessor (μP)  120 , memory  122 , analog circuitry  124 , and digital signal processor (DSP)  126  in communication via bus  128 . Bus  128 , though shown as a single bus, may be implemented using a number of busses connected as necessary among the subsystems within baseband subsystem  110 . Microprocessor  120  and memory  122  provide the signal timing, processing and storage functions for portable transceiver  100 . Analog circuitry  124  provides the analog processing functions for the signals within baseband subsystem  110 . Baseband subsystem  110  provides control signals to radio frequency (RF) subsystem  130  via connection  132 . Although shown as a single connection  132 , the control signals may originate from DSP  126  or from microprocessor  120 , and are supplied to a variety of points within RF subsystem  130 . It should be noted that, for simplicity, only the basic components of portable transceiver  100  are illustrated.  
         [0027]    Baseband subsystem  110  also includes analog-to-digital converter (ADC)  134  and digital-to-analog converters (DACs)  136  and  142 . ADC  134  and DACs  136  and  142  also communicate with microprocessor  120 , memory  122 , analog circuitry  124  and DSP  126  via bus  128 . DAC  136  converts the digital communication information within baseband subsystem  110  into an analog signal for transmission to RF subsystem  130  via connection  140 . Connection  140 , while shown as two directed arrows, includes the information that is to be transmitted by RF subsystem  130  after conversion from the digital domain to the analog domain. In accordance with an embodiment of the invention, DAC  136  supplies baseband in-phase (I) and quadrature (Q) components of the information signal to be transmitted via connection  140  to modulator  146 . In such an embodiment, modulator  146  is an I/Q modulator. DAC  142  supplies control signals to various components with RF subsystem  130  via connection  132 .  
         [0028]    Modulator  146 , after receiving a frequency reference signal, also called a “local oscillator,” signal, or “LO,” from synthesizer  148  via connection  150 , modulates the I and Q information signals received from the DAC  136  onto the LO signal and provides a modulated signal via connection  152  to upconverter  154 . Modulator  146  also supplies an intermediate frequency (IF) signal containing only the desired amplitude modulated (AM) signal component on connection  138  for input to the power control element  300  via connection  138 . The power control element  300  also supplies to the modulator  146  via connection  144  a constant level IF signal containing both the phase modulated (PM) and AM components of the transmit signal. The operation of the power control element  300  will be described below with reference to FIG. 2.  
         [0029]    Upconverter  154  also receives a frequency reference signal from synthesizer  148  via connection  156 . Synthesizer  148  determines the appropriate frequency to which upconverter  154  will upconvert the modulated signal on connection  152 .  
         [0030]    Upconverter  154  supplies the filly modulated signal at the appropriate transmit frequency via connection  158  to power amplifier  160 . Power amplifier  160  amplifies the modulated signal on connection  158  to the appropriate power level for transmission via connection  162  to antenna  164 . Illustratively, switch  166  controls whether the amplified signal on connection  162  is transferred to antenna  164  or whether a received signal from antenna  164  is supplied to filter  168 . The operation of switch  166  is controlled by a control signal from baseband subsystem  110  via connection  132 .  
         [0031]    A portion of the amplified transmit signal power on connection  162  is supplied via connection  170  to power control element  300 . Power control element  300  forms a continuous closed power control feedback loop and supplies an information signal on connection  172  instructing the power amplifier  160  as to the power to which the signal on connection  158  should be amplified. The power control element  300  also receives the LO signal from synthesizer  148  via connection  198 . The operation of power control element  300  will be described in further detail with respect to FIG. 2.  
         [0032]    A signal received by antenna  164  may, at the appropriate time determined by baseband subsystem  110 , be directed via switch  166  to receive filter  168 . Receive filter  168  will filter the received signal and supply the filtered signal on connection  174  to low noise amplifier (LNA)  176 . Receive filter  168  may be a bandpass filter that passes all channels of the particular cellular system where the portable transceiver  100  is operating. As an example, for a 900 MHz GSM system, receive filter  168  would pass all frequencies from 935.1 MHz to 959.9 MHz, covering all 124 contiguous channels of 200 kHz each. The purpose of this filter is to reject all frequencies outside the desired region. LNA  176  amplifies the very weak signal on connection  174  to a level at which downconverter  178  can translate, the signal from the transmitted frequency back to a baseband frequency. Alternatively, the functionality of LNA  176  and downconverter  178  can be accomplished using other elements, such as, for example but not limited to, a low noise block downconverter (LNB).  
         [0033]    Downconverter  178  receives an LO signal from synthesizer  148 , via connection  180 . The LO signal determines the frequency to which to downconvert the signal received from LNA  176  via connection  182 . The downconverted frequency is called the intermediate frequency (IF). Downconverter  178  sends the downconverted signal via connection  184  to channel filter  186 , also called the “IF filter.” Channel filter  186  filters the downconverted signal and supplies it via connection  188  to amplifier  190 . The channel filter  186  selects the one desired channel and rejects all others. Using the GSM system as an example, only one of the 124 contiguous channels is actually to be received. After all channels are passed by receive filter  168  and downconverted in frequency by downconverter  178 , only the one desired channel will appear precisely at the center frequency of channel filter  186 . The synthesizer  148 , by controlling the local oscillator frequency supplied on connection  180  to downconverter  178 , determines the selected channel. Amplifier  190  amplifies the received signal and supplies the amplified signal via connection  192  to demodulator  194 . Demodulator  194  recovers the transmitted analog information and supplies a signal representing this information via connection  196  to ADC  134 . ADC  134  converts these analog signals to a digital signal at baseband frequency and transfers it via bus  128  to DSP  126  for further processing.  
         [0034]    [0034]FIG. 2 is a block diagram illustrating the upconverter  154  and power control element  300  of FIG. 1 including the continuous closed-loop power control system and modulation injection of the invention. Beginning with a description of the power control loop  300 , a portion of the output power present at the output of power amplifier  160  on connection  162  is diverted by coupler  222  via connection  170  and input to mixer  226  in the power control element  300 . Mixer  226  also receives the local oscillator (LO) signal from synthesizer  148  via connection  198 .  
         [0035]    The mixer  226  down converts the RF signal on connection  170  to an intermediate frequency (IF) signal on connection  228 . For example, mixer  226  takes a signal having a frequency of approximately 2 gigahertz (GHz) on connection  170  and down converts it to a frequency of approximately 100 megahertz (MHz) on connection  228  for input to variable gain element  232 . Variable gain element  232  can be, for example but not limited to, a variable gain amplifier or an attenuator. In such an arrangement, variable gain element  232  might have a dynamic range of approximately 70 decibels (dB) i.e., +35 dB/−35 dB. Variable gain element  232  receives a control signal input from the non-inverting output of amplifier  236  via connection  234 . The input to amplifier  236  is supplied via connection  132  from the DAC  142  of FIG. 1. The signal on connection  132  is a reference voltage signal for the transmit power level and provides the power profile. This signal on connection  132  is supplied to a reconstruction filter, which includes resistor  240  and capacitor  242 . In this manner, a reference voltage for the transmit power level and power profile is supplied via connection  234  to the control input of the variable gain element  232 .  
         [0036]    The output of variable gain element  232  on connection  246  is at an IF and includes modulation having both an AM component and a PM component and is called the “power measurement signal.” This power measurement signal is related to the absolute output power of power amplifier  160 , and includes a very small error related to the AM and PM components present in the signal. The output of variable gain element  232  on connection  246  is supplied to the input of power detector  262  and is also supplied to a limiter  248  in the phase locked loop  220 . The IF signal on connection  246  includes both an AM component and a PM component. The signal on connection  246  is supplied to power detector  262 , which provides, on connection  264 , a baseband signal representing the instantaneous level of  1 F power present on connection  246 . The output of power detector  262  on connection  264  is supplied to the inverting input of amplifier  268 .  
         [0037]    Amplifier  268 , capacitor  266  and capacitor  270  form a comparator  284 , which provides the error signal used to control the power amplifier  160  via connection  272 . The non-inverting input to the amplifier  268  is supplied via connection  138  from the output of the modulator  146  through the power detector  276 . The signal on connection  138  supplied to the non-inverting input of amplifier  268  contains the AM modulation developed by the modulator  146  in the phase locked loop  220  for input to the control port  172  of power amplifier  160 .  
         [0038]    The gain of the power control loop  300  amplifies the signal on connection  272  such that the difference between the signals on connections  264  and  138  input to amplifier  268  provide an error on connection  272  that is used to control the output of the power amplifier  160 . The error on connection  272  is supplied to variable gain element  274 , which can be similar in structure to variable gain element  232 . However, the variable gain element  274  has a function that is inverse to that of variable gain element  232 . The control input to variable gain element  274  is supplied from the inverting output of amplifier  236 . In this manner, the power amplifier control signal on connection  172  supplied to the control port of power amplifier  160  drives the power amplifier  160  to provide the proper output on connection  162 .  
         [0039]    The level of the signal on connection  264  and the level of the signal on connection  138  should be equal. For example, if the output level of the variable gain element  232  is increased by a factor of 10, then the level of the output of power amplifier  160  should be decreased accordingly, to maintain equilibrium at the input of the amplifier  268 . The output of the power amplifier  160  changes to cancel the gain change of variable gain element  232 . In this manner, the amplitude of the signal on connection  264  remains equal to the amplitude of the signal on connection  138 . However, this implies that the signal on connection  228  lags the signal on connection  234  with the result that the two signals will not completely cancel. In this manner, an error signal with an AM and a PM portion is present on connection  246 . The signal on connection  246  is converted by power detector  262  from an IF signal to a baseband signal on connection  264 . The signal on connection  264  is amplified by amplifier  268  and amplifier  274  to drive the power amplifier control port on connection  172  so that the desired signal is achieved at the output of the power amplifier  160  on connection  162 . Power control loop  300  has sufficient gain so that the error signal on connection  264  can be kept small. In such a case, the gain changes of variable gain element  232  and the power amplifier  160  will substantially be the inverse of each other.  
         [0040]    In addition to amplifying the error signal on connection  264 , the amplifier  268  also compares the power measurement signal on connection  264  with a reference voltage signal including an AM portion on connection  138 , supplied by the modulator  146  within the phase locked loop  220 . The DC voltage level on connection  138  affects the desired static output power for the power amplifier  268 , irrespective of AM modulation. Amplifier  268  compares the signal level on connection  264  with the signal level on connection  138  and then amplifies the difference, thus providing a power control signal on connection  272 . The comparator  284  functions as an integrator, which is also a low pass filter.  
         [0041]    The power control signal on connection  272  drives the variable gain amplifier  274 , which corrects for the effect that variable gain element  232  has on the transfer function of power control loop  300 . The variable gains of variable gain element  232  and variable gain element  274  are complimentary. Because the power measurement signal is present on connection  264  and the AM error signal is present on connection  138 , the amplifier  268  provides a dual function; (1) it amplifies the AM error signal on connection  138  so as to modulate the power output of power amplifier  160  via connection  172  to have the correct amount of AM; and (2) it performs the average power comparison and amplifies the result, thus providing a control signal on connection  272  that drives the power amplifier  160  to the correct average power output. Therefore, at connection  172  both the AM error signal and the power control error signal are amplified to a level sufficient to drive the power amplifier  160  to the desired average power with the desired AM signal. In this manner, the desired AM portion of the signal is supplied to the control input  172  of power amplifier  160  and made present on the power amplifier output on connection  162 . Mixer  226 , variable gain element  232 , power detector  262 , amplifier  268  and variable gain element  274  provide a continuous closed-loop power control feedback system to control the power output of power amplifier  160 , while allowing for the introduction of the AM portion of the transmit signal via connection  138 .  
         [0042]    At all times, the continuous power-control feedback loop allows the correction of any phase shift caused by power amplifier  160 . In this manner, the PLL  220  now includes a feedback loop for looping back the output of power amplifier  160  to the input of phase/frequency detector  208 . Any unwanted phase shift generated by the power amplifier  160  will be corrected by the PLL  220 . The output of variable gain element  232  passes any phase distortion present via connection  246  to limiter  248  for correction by the PLL  220 . As such, the phase of the output of power amplifier  160  is forced to follow the phase of the LO signal on connection  156 .  
         [0043]    In order to remove the AM from the output of variable gain element  232 , the variable gain element  232  is connected via connection  246  and connection  144  to the input of limiter  248 . Limiter  248  develops a local oscillator signal containing only a PM component on connection  250 . This LO signal is supplied via connection  250  to the modulator  146 . In addition, the baseband I and Q information signals are supplied via connections  278  and  282 , respectively, to the modulator  146 . The I and Q baseband information signal interface is understood by those having ordinary skill in the art. As a result of the operation of the modulator  146 , the output on connection  252  is an intermediate frequency signal including an AM component in the form of an AM reference signal and a small PM error signal. The output of modulator  146  is supplied via connection  252  to power detector  276 . In this manner, the output of power detector  276  also includes the AM portion of the desired transmit signal. In this manner, the signal provided on connection  138  is a reference signal for input to the power control loop  300 . Because the power control loop  300  has limited bandwidth, the rate at which the amplitude modulation occurs on connection  138  is preferably within that power control loop bandwidth.  
         [0044]    The output of limiter  248  is supplied via connection  250  as a local oscillator signal having a PM component, but substantially no AM component to the modulator  146 . The modulator  146  removes virtually the entire PM component and applies an AM modulated component to the signal and supplies this signal via connection  252 . In order to remove the PM component present on connection  250 , the I and Q signals are reversed on connections  278  and  282 , respectively. In this manner, the output of modulator  146  on connection  252  contains a very small PM portion and a significant AM portion. With respect to the PM component of the signal on connection  252 , the modulator  146  acts as a comparator, comparing the I and Q signals on connections  278  and  282 , respectively, with the LO signal supplied from the output of the variable gain element  232 , through limiter  248  and on connection  250 . The components within the phase locked loop  220  provide gain for the comparison of the PM on connection  250  and the modulator connections  278  and  282 , thus providing a phase error output of the modulator  146  on connection  252 . This phase error signal is then supplied to limiter  256 , which outputs a signal on connection  258  containing the small PM phase error component.  
         [0045]    In this manner, a feedback signal taken from the output of variable gain element  232  on connection  246  is supplied as continuous feedback to the phase locked loop  220 . The error signal output of modulator  146  on connection  252  containing the phase error, will get smaller and smaller as the gain of the phase locked loop  220  increases. However, there will always be some error signal present, thus enabling the phase locked loop  220  to achieve phase lock. It should be noted that even when the power amplifier  160  is not operating, there will always be some small leakage through the power amplifier  160  onto connection  162 . This small leakage is sufficient to provide a feedback signal through the variable gain element  232  and into the phase locked loop  220  such that the phase locked loop  220  can be locked using just the leakage output of power amplifier  160 . In this manner, a single feedback loop can be used to continuously control the output power of power amplifier  160  from the time that the amplifier is off through the time when the amplifier  160  is providing full output power. Phase/frequency detector  208  receives an unmodulated input signal from synthesizer  148  via connection  156 . The unmodulated input signal is frequency divided by a number “x” in order to provide a signal having an appropriate frequency on connection  204 . The number “x” is chosen so as to minimize the design complexity of the synthesizer  148  and can be, for example, but not limited to, chosen to convert the output of the synthesizer  148  to a frequency of 100 MHz. Phase/frequency detector  208  also receives the output of divider  260  via connection  206 . The number “y” is chosen in similar manner to that of the number “x.” Phase/frequency detector  208  detects any phase difference between the signal on connection  204  and the signal on connection  206  and places a signal on connection  210  that has an amplitude proportional to the difference. When the phase difference reaches 360°, the output of phase/frequency detector  208  on connection  210  will become proportional to the frequency difference between the signals on connections  204  and  206 .  
         [0046]    The output of phase/frequency detector  208  on connection  210  is a digital signal having a value of either a 0 or a 1 with a very small transition time between the two output states. This signal on connection  210  is supplied to low-pass filter  212 , which integrates the signal on connection  210  and places a DC signal on connection  214  that controls the frequency of the transmit voltage control oscillator (TX VCO)  216 . The output of TX VCO  216  is supplied via connection  158  directly to the power amplifier  160 . In this manner, the synthesizer  148 , limiter  248 , modulator  146 , limiter  256 , divider  260 , divider  202 , phase/frequency detector  208 , low-pass filter  212  and TX VCO  216  form a phase locked loop (PLL)  200 , which is used to determine the transmit frequency on connection  158 . When the PLL  220  is settled, or “locked,” then the two signals entering the phase/frequency detector  208  on connections  204  and  206  have precisely the same phase and frequency, and the output of the phase/frequency detector  208  on connection  210  goes to zero. The output of the integrating low-pass filter  212  on connection  214  stabilizes, resulting in a fixed frequency out of TX VCO  216 . For example, the synthesizer  148  and the mixer  226  ensure that the frequency of the signal output from the TX VCO  216  on connection  158  tracks the sum of the frequencies of the local oscillator signal supplied by synthesizer  148  and the IF frequency on connection  206 .  
         [0047]    When the phase locked loop  220  is locked, the phase of the signal on connection  204  and the phase of the signal on connection  206  will be equal. Because the amount of PM on connection  206  should be very small, the gain in the phase locked loop  220  has to be sufficiently high to amplify the error signal on connection  206  to a level at which the phase/frequency detector  208  can make a comparison. By using the modulator  146  to impose the I and Q information signals on the signal on connection  250  in a direction opposite from which it is desirable for the phase of the TX VCO to move, and because it is desirable for the phase locked loop  220  to remain locked, the phase of the signal output from the TX VCO  216  on connection  158  will move opposite that of the phase imposed by the modulator  146 . In this manner, the PM error signal present on connection  206  is minimized by the very high sensitivity, of the order of many MHz per volt, of the TX VCO  216 .  
         [0048]    Because the power control loop  300  is a closed loop for AM signals at connection  138 , it is possible to use a non-linear, and therefore highly efficient, power amplifier  160 . Furthermore, the undesirable and detrimental AM-to-PM conversion, which occurs due to the amplitude dependence of an amplifier&#39;s phase shift, is rectified by the power amplifier  160  being included within the phase locked loop  220 . By separating the AM and the PM modulation and by providing closed loop control for both the AM and PM modulation, a non-linear, and therefore highly efficient power amplifier can be used.  
         [0049]    In some applications it is desirable to allow the power amplifier  160  to output a signal containing both an AM component and a PM component, while maintaining the power amplifier  160  as a non-linear (and therefore highly efficient) power amplifier. In such a case, the output of modulator  146  will include both a very small AM and PM component, with limiter  256  used to cancel the AM component present on connection  252 , thereby preventing any AM-to-PM conversion in the phase/frequency detector  208 .  
         [0050]    [0050]FIG. 3 is a graphical representation of the average power output of power amplifier  160  of FIG. 2. The vertical axis of graph  350  represents average power output of the power amplifier  160  and the horizontal axis of graph  350  represents time. Point “a” represents the point in time at which a transmission burst is initiated. At this time leakage from the power amplifier  160  is used to provide feedback from the variable gain element  232  (FIG. 2) to the phase locked loop  220  (FIG. 2) with the variable gain element  232  set to maximum gain. During the following ramp-up time the PLL  220  of FIG. 2 tracks the output of the power amplifier  160  with the gain of variable gain element  232  (and therefore the amplitude fed back to phase locked loop  220 ) reducing as the ramp progresses in time, thus allowing the PLL  220  to correct any phase distortion present at the output of power amplifier  160 . The point “c” in FIG. 3 represents the point at which the power amplifier  160  has developed sufficient power so that transmission of data may begin. In this manner, a single power control loop provides continuous power detection and feedback to the PLL  220 .  
         [0051]    [0051]FIG. 4 is a schematic view illustrating an alternative embodiment  400  of the power amplifier circuit of FIG. 2. In some applications it may be desirable to have the ability to transmit an AM signal having a very wide bandwidth. Therefore, and in a departure from that discussed above with respect to FIG. 2, the power amplifier circuit  400  of FIG. 4 includes a mixer  492  added to the phase locked loop  420 . The mixer  492  receives the output of the TX VCO  416  on connection  494  and also receives as input the output of a low-pass filter  490  via connection  496 .  
         [0052]    To develop the signal for input to the mixer  492 , the output of modulator  446  on connection  452  is supplied to mixer  480 . Mixer  480  combines the PM component of the signal on connection  458  with the AM component of the signal on connection  452 . The mixer  480  combines the signal on connection  452  containing the AM and very small PM component and the signal on connection  458  containing the very small PM component, and combines them, thus extracting the AM signal and placing it on connection  484 . The AM signal on connection  484  is at a baseband frequency and is supplied to amplifier  486 . Amplifier  486  scales the signal on connection  484  and supplies the scaled signal, via connection  488 , to low-pass filter  490 . The AM slope information is supplied to the control input to the amplifier  486  from the DAC  142  via connection  132  (FIG. 1). Low-pass filter  490  removes any high frequency components from the signal on connection  488  and supplies the AM signal via connection  496  to the mixer  492 .  
         [0053]    The mixer  492  combines the AM signal on connection  496  with the PM signal supplied from the TX VCO  416  on connection  494  and supplies a combined modulated signal containing both AM and PM on connection  458 . This combined signal is then supplied to the power amplifier  160 .  
         [0054]    With respect to the power control loop  400 , as described above, a reference voltage signal containing the AM signal component is supplied from the output of the modulator  446  via connection  438  to the non-inverting input of amplifier  468  in the comparator  484 . The signal supplied from power detector  462  via connection  464  contains an AM component. Because the AM signal component on connection  464  is in phase with respect to the AM signal component on connection  438 , the two AM components will substantially cancel in the comparator  484 , thus eliminating the AM portion of the signal from the output of amplifier  468  on connection  472 . The output of amplifier  468  on connection  472  is the error signal used to adjust the output power of power amplifier  160  as described above.  
         [0055]    [0055]FIG. 5 is a schematic view illustrating another alternative embodiment  500  of the power amplifier circuit of FIG. 2. The power amplifier circuit  500  includes power control loop  500 , where modulator  546  is placed at the output of variable gain element  532 . The input signal to modulator  546  on connection  538  is a constant level signal supplied by variable gain element  532 . The output of variable gain element  532  includes both an AM and PM component. The baseband I and Q information signals are supplied to the modulator  546  via connections,  578  and  582 , respectively.  
         [0056]    With respect to the PM signal on connection  538 , when a PM signal is supplied to modulator  546 , the I and Q components will remove, or greatly reduce the level of the PM signal on connection  538  within the loop bandwidth of phase locked loop  520 . With respect to the AM portion of the signal on connection  538 , the I and Q portions will also reduce the AM component by a function equal to the gain of the power control loop  500 . Therefore, the value of the AM and PM components at the output of modulator  546  on connection  550  are very small error signals as mentioned above. In accordance with this aspect of the invention, the inverse of the I and Q information signals are supplied to the modulator  546  on connections  578  and  582 , respectively, thus providing the error signal on connection  550 . The error signal on connection  550  includes both PM and AM components.  
         [0057]    This small error signal is supplied on connection  550  to the phase/frequency detector  508 , which, because there is virtually no AM present on the signal on connection  550 , will measure the phase difference between the signal on connection  550  and the signal on connection  504 . The phase/frequency detector  508  provides a signal on connection  510  as described above with respect to FIG. 2.  
         [0058]    The error signal on connection  550  is also supplied to power detector  562 , which converts the IF signal on connection  550  to a DC plus small AM error signal on connection  564 , the DC component representing the average power output of power amplifier  160 . The signal on connection  564  is supplied to the inverting input of amplifier  568 . The non-inverting input to amplifier  568  is coupled from a common mode voltage signal V REF . Amplifier  568  functions as a phase inverter, thus inverting the phase of the signal on connection  564  and supplying this inverted phase signal as a power amplifier control signal on connection  572 . The control signal on connection  572  is supplied to variable gain element  574 , which functions similar to the variable gain element  274  of FIG. 2. The variable gain element  574  supplies a control output to the power amplifier  160  via connection  172 .  
         [0059]    Advantageously, the embodiment illustrated in FIG. 5 eliminates one of the power detectors (power detector  276 ) shown in FIG. 2. In this manner, it is unnecessary to match the operational characteristics of the power detector  276  and the power detector  262  of FIG. 2. Furthermore, the limiters  248  and  256  of FIG. 2 are also eliminated.  
         [0060]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.