Abstract:
The invention relates to a method of calibrating an envelope path and an input path of an amplification stage including an envelope tracking power supply, the method comprising: generating input signals having a known relationship for each of the input and envelope paths; and varying an amplitude and a delay of the signal in one of the envelope and input paths in order to reduce the variation in the power detected in a signal at the output of the amplification stage.

Description:
BACKGROUND TO THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an amplification stage in which an envelope tracking (ET) modulator is utilised to provide a power supply to an RF amplifier. 
     2. Description of the Related Art 
     With reference to  FIG. 1  there is illustrated components of a known RF amplification architecture in which an envelope tracking (ET) modulator is used to provide a power supply to a radio frequency (RF) power amplifier. 
     As illustrated in  FIG. 1 , an RF power amplifier  102  receives an RF input signal to be amplified on an input line  136 , and receives a modulated power supply voltage V supply  on line  138 . The RF power amplifier  102  generates an RF output signal on line  140 . An example implementation of such an RF power amplifier is in mobile communication systems, with the RF output on line  140  connected to the front end of radio transmission equipment. 
     As illustrated in  FIG. 1 , an envelope signal representing the envelope of the RF input signal to be amplified is converted by a digital-to-analogue converter  126   a  into an analogue signal, filtered by an optional envelope filter  128   a , and then provided as an input to an ET modulator  108 . The output of the ET modulator  108  forms an input to an output filter  106 , and a modulated supply voltage is then provided through a supply feed  104  to provide the supply voltage on line  138 . 
     Baseband I and Q signals are converted into analogue signals via respective digital-to-analogue converters  126   b  and  126   c , and optionally filtered through respective I and Q filters  128   b  and  128   c . The filtered I and Q signals are provided as inputs to a vector modulator, illustrated as respective multipliers  130   a  and  130   b  and a combiner  132 . The combined output of the combiner  132  forms an input to a variable gain amplifier  134 , the output of which forms an input to an optional interstage surface acoustic wave (SAW) filter  112 . The output of the filter  112  provides the RF input signal to be amplified on input line  136  to the RF power amplifier  102 . 
     The generation of the envelope signal and the I and Q baseband signals is known to one skilled in the art. Various techniques for the generation of such signals may be implemented. In  FIG. 1  signal generation block  122  generally denotes the generation of these signals. 
     As known in the art, the path which the envelope signal follows from the digital-to-analogue converter  126   a  to generation of the supply voltage on line  138  to the power amplifier  102  suffers from delays and attenuation which vary on a unit by unit basis within a production tolerance. As also known in the art the path which the baseband signals follow from the digital-to-analogue converters  126   b  and  126   c  to generation of the RF input signal to be amplified on line  136  suffers from delays and attenuation. 
     In general, such delays and attenuation need to be controlled so as to ensure that they fall within certain tolerances, usually smaller than the production tolerances, to ensure maximum operating efficiency of the power amplifier and to ensure certain spectral emissions requirements are met (such as a minimum distortion of the amplified output signal). To achieve this, the signal processing in the envelope path prior to magnitude calculation must match that through the input (RF) path accurately and precisely. Further, the relationship between the amplitude of the signal in the envelope path and the amplitude of the signal in the input path must be correctly aligned. 
     In the envelope path delays may be introduced by several stages, such as the filter  128   a , the output filter  106 , and the supply feed  104 . In addition, as denoted by an internal delay block  110  of the ET modulator  108 , delays may arise in the ET modulator  108  itself. It should be noted that block  110  is illustrative of delays suffered in the ET modulator  108 , and is not representative of specific circuitry or functionality of the ET modulator  108 , which is not shown as it is not relevant to the present invention. 
     In the RF path delays may also be introduced by several stages, such as the respective I and Q filters  128   b  and  128   c , and in the inter-stage SAW filter  112 . 
     Amplitude errors may be introduced in an amplifier stage  150  of the envelope tracking modulator, the amplitude stage  134  of the input path, and in the supply feed network  104  to the amplifier. The filters  126   b  and  126   c  in the input path, and the filter  126   a  in the envelope path, are also sources of amplitude errors. 
     It is an aim of the present invention to provide an improved technique for controlling the relative delay and amplitude between the RF and envelope paths. 
     SUMMARY OF THE INVENTION 
     The invention provides a technique for the relative signal delay and relative amplitude attenuation between the two paths to be controlled. 
     In one aspect the invention provides a method of calibrating an envelope path and an input path of an amplification stage including an envelope tracking power supply, the method comprising: generating input signals having a known relationship for each of the input and envelope paths; and varying an amplitude and a delay of the signal in one of the envelope and input paths in order to reduce the variation in the power detected in a signal at the output of the amplification stage. 
     The input path may be defined as a path along which a signal is delivered to a signal input of an amplifier of the amplification stage. The envelope path may be defined as a path along which a signal is delivered to a power supply input of the amplifier. The envelope path may include an envelope detector for generating a signal representing the envelope of a signal to be amplified. The envelope path may include a modulator for generating a voltage supply for the amplifier. 
     The method may further comprise varying the delay in the one path to determine the delay minimising the variation in detected power and varying the amplitude in the one path to determine the amplitude minimising the variation in detected power. The method may further comprise varying the amplitude with the delay set at that determined to minimise variation in detected power or varying the delay with the amplitude set at that determined to minimise variation in detected power. 
     The method may further comprise repeating each determination. 
     The method may further comprise varying one of the relative delay or relative amplitude between the paths over a plurality of values; detecting the power at the output of the amplification stage for each value; determining the value of the relative delay or relative amplitude which generates the minimum detected output power variation; applying the determined value; varying the other of the relative delay or relative amplitude between the paths over a plurality of values; detecting the power at the output of the amplification stage for each value; and determining the value of the other of the relative delay or relative amplitude which generates the minimum detected output power variation. 
     The steps of varying each of the relative delay and the relative amplitude may comprise varying the delay and amplitude of the signal in one path whilst applying no variation to the signal in the other path. 
     The generated input signals for the input and envelope paths may be correlated. 
     The method may further comprise applying a sinusoidal signal to each of the envelope and input paths, wherein the applied signals in each path are 180° out of phase with each other. 
     The method may further comprise the steps of determining the delay and amplitude variations in the one path minimising the variation in the detected output power; and setting the delay and amplitude variations in the one path corresponding to such. 
     In another aspect the invention provides an amplification stage including an amplifier and an envelope tracking power supply, and having an input path and an envelope path, the amplification stage comprising a signal generator arranged to generate a signal on each of the input and envelope paths having a known relationship, and a detector for detecting the power in a signal at the output of the amplifier, the signal generator being adapted to vary the delay and amplitude of the signal in one of the envelope and input paths in order to reduce the variation in the power detected in the signal at the output. 
     The signal generator may be adapted to vary the delay and the amplitude in separate stages of operation. 
     The amplification stage may further include a measurement block for measuring the power variation of the output signal. 
     The signal generator may be adapted to generate the signal on one of the input or envelope paths for a plurality of delay values, and the measurement block is adapted to measure the output power variation for each delay value, the measurement block being further adapted to determine the delay value associated with the minimum detected power variation. 
     The measurement block may be further adapted to apply the determined delay value. 
     The signal generator may be adapted to generate the signal on one of the input or envelope paths for a plurality of amplitude values, and the measurement block is adapted to measure the output variation for each amplitude value, the measurement block being further adapted to determine the amplitude value associated with the minimum detected power variation. 
     The measurement block may be further adapted to apply the determined amplitude value. 
     The signal generator may be arranged to generate the signal on each of the input and envelope paths with a correlation therebetween. 
     The signal generator may be arranged to generate a sinusoidal signal on each of the input and envelope paths which are 180° out of phase. 
     The amplification stage may be further adapted to set the delay and amplitude variations in the one path determined to minimise the variation in the power detected in the output. The amplification stage may be an RF amplification stage. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will now be described by way of example with reference to the accompanying Figures in which: 
         FIG. 1  illustrates an RF amplification stage adapted in accordance with an embodiment of the present invention; 
         FIG. 2  illustrates a modified RF amplification stage in accordance with a preferred embodiment of the invention; and 
         FIG. 3  illustrates the steps in utilising the exemplary RF amplification stage of  FIG. 1  in an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described with further reference to the exemplary RF amplification architecture of  FIG. 2 , which modifies the arrangement of  FIG. 1  in accordance with exemplary embodiments of the invention. The invention, and its embodiments, is not however limited in its applicability to the exemplary architecture and implementation as illustrated in  FIG. 2 . 
     With reference to  FIG. 2 , the RF amplification architecture is adapted to include a calibration control stage  142  including the signal generation block  122 , a programmable delay adjustment block  124 , a measurement block  120 , and a power control block in accordance with an exemplary implementation of the present invention. 
     As illustrated in the embodiment of  FIG. 2 , the envelope signal, I data signal, and Q data signal for the respective digital-to-analogue converters  126   a  to  126   c  are generated by the signal generation block  122  via the programmable delay adjustment block  124 . The signal generation block  122  generates signals to the measurement and correlation block  120 , and the measurement block  120  generates signals to the programmable delay adjustment block  124 . 
     A diode  114  is connected to the output of the power amplifier  102  on line  140  in order to provide the functionality of a power detector. The diode  114  is further connected to a filter  118 , which in turn is connected to an analogue-to-digital converter  116  to provide a digital and filtered representation of the signal detected by the diode  114  to the measurement block  120 . 
     The implementation shown is exemplary, and the invention is not limited to the use of a diode as a power detector to provide feedback to the measurement block  120 . In general, the diode  114  represents a functional block for providing a signal representing the amplitude or power of the signal at the output of the RF power amplifier  102  on line  140 . In an alternative implementation, the detection could be implemented using a receiver chain including an analogue to digital converter, with detection of the envelope being implemented in the digital domain. 
     The power control block  152  is connected to receive an input from the measurement block  120 , and generate outputs to the signal generation block  122 , and the variable gain amplifier  134  of the input path. 
     The adaptation of an RF power amplification stage in accordance with the exemplary arrangement of  FIG. 2  provides for a calibration system that removes the delay and attenuation uncertainty in the envelope path and the RF path to arbitrary precision that can be implemented as a self-calibration. The variation in the delay and amplitude in the input and envelope paths causes a power variation in the output of the amplifier. The point at which the variations between the signals in the envelope and input paths are controlled is denoted by reference numerals  158  and  160  respectively in  FIG. 1 . 
     The principles of the present invention as exemplified by the arrangement of  FIG. 2  are now further described with reference to an exemplary procedure as set out in the flow diagram of  FIG. 3 . 
     As denoted in step  200 , the signal generation block  122  is arranged to generate sinusoidal signals for the input path and envelope path. The sinusoidal signals for the input and envelope paths are generated independently. The signals are generated to be 180° out of phase with each other. The two sinusoidal signals are preferably also arranged such that in an ideal system their amplitudes would cancel, i.e. to have the same magnitudes. It is assumed that the envelope path is pre-calibrated, in accordance with techniques known in the art, to sufficient accuracy and precision that it does not affect the determination of the amplitude of the envelope signal to ensure that the amplitudes cancel. 
     In a first phase of a calibration operation, as denoted by step  202  the signal generation block  122  is arranged to apply the generated sinusoidal signal to the envelope path, via the programmable delay adjustment block  124 , but with no delay applied. For the entire first phase of operation, the signal applied to the envelope path has no delay applied to it. 
     The signal generation block  122  is further arranged, as denoted by step  202 , to apply the constant amplitude signal to the RF input path via the programmable delay adjustment block  124 , with a controlled applied delay. The programmable delay adjustment block  124  is controlled to vary the delay in the sinusoidal signal applied to the input path. Preferably the delay is varied through successive values in successive time periods. 
     In accordance with the standard operation of the power amplification stage, the constant amplitude signal is processed by the RF input path and amplified by the power amplifier. 
     The diode detector  114 , as denoted by step  204 , detects the power at the output of the RF amplifier, which is delivered to the measurement block  122  through the feedback path formed by the diode  114 , the filter  118 , and the analogue-to-digital converter  116 . 
     The measurement block  120  receives the generated sinusoidal signal applied to the input path and the detected output signal. The detected output signal represents the RF output power. If the signal in the envelope and input paths are correctly aligned in time and amplitude, the output power is a constant level. If they are not correctly aligned, a ripple is present on the output signal. The measurement block measures the peak-to-peak value of the detected output power to determine the size of the ripple. This peak-to-peak determination is made for each successive applied delay, such that a plurality of peak-to-peak values are determined corresponding to the plurality of applied delays. The measurement block  120  then assesses the peak-to-peak values, using standard techniques, to determine the smallest peak-to-peak, and hence the delay value associated with best alignment of the signals, as denoted by step  206 . 
     As denoted by step  208 , the delay associated with the smallest peak-to-peak value is then applied in the input path using the programmable delay adjustment block  124 . 
     In a second phase of the calibration process, the signal generation block  122  is adapted to apply the sinusoidal signal to the input path as I and Q signals, via the programmable delay adjustment block  124 , with the delay being set to that determined as optimum in the first phase of operation. The signal generation block  122  is further adapted as denoted by step  210  to apply a variable amplitude to the signal in the input path under the control of the power control block  152 . The power control block  152  provides amplitude adjustment information on line  156  to the signal generation block  122  and on line  154  to the variable gain amplifier  134  of the input path. Preferably the amplitude is varied through successive values in successive time periods. 
     In accordance with the standard operation of the power amplification stage, the constant amplitude signal is processed by the RF input path and amplified by the power amplifier. As in the first phase of operation the diode  114  detects the power of the output of the RF amplifier as denoted by step  212 , and the detected power is provided to the measurement block  120 . 
     The measurement block  120  receives the generated sinusoidal signal applied to the input path and the detected output signal. A peak-to-peak determination is made for each successive applied amplitude, such that a plurality of peak-to-peak values are determined corresponding to the plurality of applied amplitudes. The measurement block  120  then assesses the peak-to-peak values, using standard techniques, to determine the smallest peak-to-peak, and hence the amplitude value associated with best alignment of the amplitude of the signals in the envelope and input paths, as denoted by step  214 . 
     As denoted by step  216 , the amplitude associated with the smallest peak-to-peak value is then applied in the input path using the signal generator or the variable gain amplifier. 
     After completion of the first and second phases of the correlation process, the measurement block  120  has calculated a delay and amplitude attenuation to be applied in the input path. These values may be stored, and applied in the input path during normal operation, as denoted by step  218 . 
     In a preferred embodiment, where the amplitude variation is being controlled/applied in the input path, the appropriate control/variation is applied in the VGA  134 , although this could be applied in the signal generator  122 . In a preferred embodiment, where the amplitude variation is being controlled/applied in the envelope path, the appropriate control/variation is applied in the signal generator  122 . 
     It should be noted that the first and second phases of the calibration process may be carried out in any order, such that the second phase may take place before the first phase, i.e. the calibration based on amplitude may be carried out before the calibration based on delay. 
     The first and second phases of the calibration process may additionally be cycled through a certain number of times to improve the alignment and avoid a local minima. 
     It should also be noted that in the first and second phases of the calibration process, the signal in the input path may be kept unchanged whilst the delay and attenuation is varied in the envelope path. 
     The described technique may be implemented as internal self-calibration, avoiding the need for expensive and time-consuming factory calibration. 
     Since the delay and attenuation information is determined using a relative measurement technique, the uncertainty of the bandwidth in the power detector is removed. 
     The bandwidth of the signal applied to the RF path in either the first or second phases of the calibration process must lie within the bandwidth of the envelope tracking system. 
     The invention seeks to minimise the variation in the detected output power for all harmonics or for the fundamental frequency. 
     As denoted by step  220 , the process may be repeated for different frequencies of operation, in order to determine delay and attenuation values to be applied for other frequencies. 
     Similarly, as denoted by step  222 , the process may be repeated for different power control levels at each frequency of operation, in order to determine delay and attenuation values to be applied for other power control levels. 
     The invention is described herein with reference to particular examples and embodiments, which are useful for understanding the invention and understanding a preferred implementation of the invention. The invention is not, however, limited to the specifics of any given embodiment, nor are the details of any embodiment mutually exclusive. The scope of the invention is defined by the appended claims.