Abstract:
An RF amplifier includes a phase modulator developing a phase modulated RF input signal to be transmitted. A power amplifier receives the RF input signal and amplifies the RF input signal to develop an RF output signal. An amplifier control is operatively associated with the phase modulator and the power amplifier. The amplifier control includes a memory for storing correction information correlating desired amplitude of the RF output signal relative to actual amplifier amplitude, and a control varies power amplifier supply voltage responsive to the correction information to linearize amplitude modulation in the power amplifier.

Description:
FIELD OF THE INVENTION 
     This invention relates to RF amplifiers and, more particularly, to a circuit and method having I/Q modulation and independent amplitude modulation in a power amplifier. 
     BACKGROUND OF THE INVENTION 
     Radio frequency (RF) transmitters, such as used in cellular telephones, develop an RF signal to be transmitted through the air. Information is carried on the signal via some form of modulation such as frequency modulation, phase modulation, amplitude modulation, or a combination of these. 
     It may be desirable to create a modulated signal with both amplitude and phase modulation. With the necessity of developing small and lightweight devices, particularly cellular telephones, it is important that such amplifier circuits use a minimum of components. One way to satisfy this desire is to directly modulate an oscillator phase lock loop (PLL) to impart the phase modulation component directly on the signal and then to amplitude modulate the power amplifier stage connected to a voltage controlled oscillator/phase lock loop (VCO/PLL) combination with the amplitude component. VCO/PLL circuits exist that have sufficient bandwidth relative to the information bandwidth of the signal to cause the phase modulation to occur directly on the output signal without any up-conversion. It remains, however, to put an amplitude signal onto this phase modulated signal. This is preferably done in the power amplifier stage, as it will permit this stage to run at high efficiency in a non-linear mode. A difficulty might arise in that the information bandwidth of the individual phase modulation component of the signal is larger than that of the composite signal. For complex quadrature amplitude modulation (QAM) signals, the PLL loop bandwidth may not be sufficient to impart all of the phase modulation components of the spectrum. 
     Previously, the above problem has been solved by direct I/Q modulation of the RF signal applying both amplitude and phase modulation to the RF signal. This requires a linear power amplifier to follow the I/Q modulator. A problem with the linear power amplifier is that it has an efficiency only in the range of thirty to forty percent and generates more heat than power. 
     The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a circuit and method independently controlling amplitude and phase modulation in an RF amplifier circuit. 
     Broadly, there is disclosed herein an RF amplifier including a phase modulator developing a phase modulated RF input signal to be transmitted. A power amplifier receives the RF input signal and amplifies the RF input signal to develop an RF output signal. An amplifier control is operatively associated with the the power amplifier. The amplifier control includes means for developing a control signal representing a desired amplitude of the RF output signal. Memory means store correction information correlating actual amplitude of the RF output signal relative to the control signal. Control means vary the power amplifier supply voltage using the control signal modified responsive to the correction information for the desired amplitude. 
     It is a feature of the invention that the phase modulator comprises an I/Q modulator. The phase modulator has no amplitude modulation. 
     It is another feature of the invention that the amplifier control comprises a processor. 
     It is still another feature of the invention that the memory means stores a transfer curve of the power amplifier RF output signal relative to the control signal. 
     It is still another feature of the invention to provide means coupled to the amplifier control for monitoring the RF output signal. The amplifier control periodically updates the correction information using the monitored RF output signal. 
     It is still another feature of the invention that the control means comprises a switching regulator developing the power amplifier supply voltage. Alternatively, the control means comprises a delta-sigma modulator, Class D amplifier stage and a low pass filter developing the power amplifier circuit supply voltage. 
     It is still a further feature of the invention that the control means comprises a programmed processor controlling the phase modulator. The phase modulator uses a high speed phase lock loop (PLL). The PLL includes a voltage-controlled oscillator (VCO) and a divider in a control loop for the VCO and the processor, in the form of a delta-sigma modulator, controls the divider integer to set channel frequency. 
     In accordance with another aspect of the invention an RF amplifier includes an I/Q modulator developing a phase modulated RF input signal to be transmitted. A power amplifier receives the RF input signal and amplifies the RF input signal to develop an RF output signal. An amplifier control is operatively associated with the I/Q modulator and the power amplifier, the amplifier control driving the I/Q modulator to impart phase modulation and varying the power amplifier supply voltage to linearize amplitude modulation in the power amplifier circuit. 
     In accordance with a further aspect of the invention there is disclosed the method of linearizing amplitude modulation in a power amplifier of an amplifier circuit comprising the steps of developing a phase modulated RF input signal to be transmitted, a power amplifier receiving the RF input signal and amplifying the RF input signal to develop an RF output signal, storing correction information correlating actual amplitude of the RF output signal relative to an amplifier control signal, and varying the power amplifier supply voltage using the amplitude control signal modified responsive to the correction information for the desired amplitude. 
     Further features and advantages of the invention will be readily apparent from the specification and from the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram illustrating an amplifier circuit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a transmitter  10  which uses an RF amplifier circuit  12  according to the invention is illustrated. The transmitter  10  may be a device that transmits an RF signal through the air such as in a mobile cellular telephone or the like. More generally, the transmitter  10  may be utilized in any device which creates a modulated signal with both amplitude and phase modulation. The invention is particularly directed to a circuit and method to use in-phase and quadrature channel (I/Q) phase modulation in a transmitter with independent, high efficiency amplitude modulation. The amplifier circuit  12  learns a transfer curve of a power amplifier RF signal amplitude output versus supply voltage control signal to linearize amplitude modulation, as described more particularly below. 
     In the illustrated embodiment of the invention, the transmitter  10  utilizes a digital signal processor (DSP) and related circuitry for developing the output signal to be transmitted. As will be apparent, the circuit functions could be implemented in an ASIC, a programmed DSP, or a programmed microprocessor, or other similar type device. 
     The transmitter  10  includes a waveform generator  14 . The waveform generator  14  creates the total modulation, including amplitude and phase, appropriate to the digital data being transmitted and the modulation characteristics. The RF amplifier circuit  12  receives the waveform and delivers it to a phase modulation path  16  and an amplitude modulation path  18 . 
     The phase modulation path  16  includes an I/Q function  20 . This is a mathematical function that converts the mathematical modulation waveform into mathematical sine and cosine of the phase versus time of the modulation waveform. The combination of the sine and cosine of the phase has a constant amplitude and varies only in phase. The function block  20  also converts the format from digital to analog. The sine and cosine signals are then applied to two modulation ports of an I/Q modulator  22 . The I/Q modulator  22  varies only the phase of a continuous wave RF signal, as described further below. 
     The output of the I/Q modulator  22  is amplified by an RF driver stage  24 . The RF driver stage  24  supplies sufficient signal level to a power amplifier  26  so that it is overdriven. The output of the power amplifier  26  is an output of the transmitter represented at a block  28 . 
     The amplitude modulation path  18  includes a magnitude function  30  to create an amplitude modulation control signal A(t) representing a desired amplitude of the RF output signal. Various implementations and methods could be used to create this signal. This signal must be time synchronized with the I/Q function  20  such that the net effect through the amplifier circuit  12  is to create the desired composite signal at the output  28 . 
     In the amplitude modulation path  18 , the control signal from the magnitude function  30  is applied to a correction table  34 . The correction table  34  is stored in a suitable memory of the amplifier circuit associated with the DSP. The memory stores a transfer curve of the power amplifier RF signal amplitude output versus the control signal. Particularly, the correction table  34  modifies the value of the desired amplitude to an amplitude that, when applied to the system, results in the correct amplitude of the RF signal out. The modified control signal is applied to a modulator  36  that creates a sequence of one bit digital signals whose average mimics the input waveform. Any pulse density modulator could be used. However, a delta-sigma modulator has the advantage that its noise versus frequency is low at low frequencies and high at high frequencies. A Class D amplifier stage  38  boosts the current capacity of the modulated signal as its output is either the full battery voltage or zero, depending on the binary state of the signal input to the modulator  36 . The amplified signal is applied through a low pass filter  40  with the smooth voltage being connected to the drain or collector of the power amplifier  26 . Thus, when empowered by the power amplifier  26  on the RF signal from the I/Q modulator  22 , the final output signal is the original waveform created by the waveform generator  14  but now on an RF carrier signal at frequency ƒ 0 . 
     To maintain a faithful reproduction of the signal from the waveform generator  14  to the output  28 , it is necessary to continually correct for non-linearities in the amplitude modulation process. The power level at the output of the power amplifier  26  is measured with a power detector circuit  42 . The power signal is sampled at an analog-to-digital converter  44 . A conversion block  46  converts the power level to amplitude by taking the square root and scaling it to the appropriate level with a constant so that it can be compared with the desired amplitude at a block  48 . Particularly, the block  48  compares the desired amplitude from the block  30  and the measured amplitude from the block  46  and a new correction value is calculated for the particular level of the desired amplitude. The new value is inserted in the correction table at the block  34  if it is sufficiently different from the prior stored value for that specific value of the control signal A(t). 
     The correction table at the block  34  is maintained over varying conditions of temperature, power amplifier loading, battery voltage, etc. The maintenance of the correction table in the block  34  is easily done at a very low sampling rate. 
     Returning to the phase modulation path  16 , the continuous wave signal supplied to the I/Q modulator  22  is developed by a combination voltage-controlled oscillator (VCO)/phase locked loop (PLL) function  50 . A delta-sigma modulator or channel generator  52  develops a set of binary control signals Z i(t)  to control the divisor number of a frequency divider  54 . The divider  54  receives an output of a VCO  56 . The divider  54  covers a large range under the control of the delta-sigma modulator  52 . This creates a highly fractional N division at a very low divider number. Thus, the reference frequency of the VCO/PLL  50  is kept high and the loop frequency is also kept high. The divisor ratio sets the character frequency of the VCO  56 . A system reference oscillator  58 , phase detector  60 , and PLL filter  62 , along with the oscillator  56  and divider  54  make up a phase lock loop source that creates the continuous wave signal modulated by the I/Q modulator  22 . 
     The advantage of using this VCO/PLL configuration is that the loop bandwidth is significantly higher and encompasses that necessary to pass the phase modulation frequency components. 
     As will be appreciated by one of ordinary skill in the art, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining hardware and software aspects. The present invention has been described with respect to the block diagram illustrations of FIG.  1 . It will be understood that many of the blocks can be implemented by computer program instructions. These program instructions, which represent steps, may be provided to a processor to produce a machine. 
     Accordingly, blocks of the illustration support combinations of means for performing the specified functions in combinations of steps for performing the specified functions. It will be understood that each block of the illustrations, and combinations of blocks in the illustrations, can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
     Thus, in accordance with the invention, there is disclosed a transmitter amplifier circuit independently controlling an I/Q modulator and a power amplifier to provide independent, high efficiency amplitude modulation.