Abstract:
A method of calibrating a polar amplification stage including a main signal path and a magnitude signal path, the method comprising: generating signals ( 108, 142 ) for the main signal path and the magnitude signal path for operating an amplifier ( 102 ) between a linear mode of operation and a saturated mode of operation; detecting ( 114 ) first and second peaks in a signal at the output of the amplifier ( 102 ) representing transitions between the linear and saturated modes of operation; and adjusting ( 124 ) the timing in one of the main signal path and the amplitude signal path in dependence on a relative difference between the size of the detected first and second peaks.

Description:
BACKGROUND TO THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polar amplification stage which is suitable for, but not limited to, a transmitter. The invention is particularly but not exclusively concerned with 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 
     Polar amplification stages, also known as polar modulators, are well-known in the art, and comprise an arrangement in which separate magnitude and main signal paths are utilised in order to amplify a signal for transmission. Polar amplification stages are typically used in transmitters, such as transmitters for RF (radio frequency) applications. A mobile communication system RF implementation may use a polar amplification stage. 
     Known polar amplifiers utilise, for example, envelope tracking (ET) techniques or envelope elimination and restoration (EER) techniques. In an envelope tracking amplification stage a modulated power supply is generated for an amplifier in dependence on the envelope (magnitude) of the input signal to be amplified by the amplifier, and the input signal to be amplified is provided as the input of the amplifier. 
     It is an objective in such amplification stages to minimise the distortion in the transmitted signal. It is known that one cause of distortion is a result of the total delay suffered by the main signal (in a main signal path) and the total delay suffered by the magnitude signal (in a magnitude signal path) not being equal to each other. In order to meet system specification requirements, it is necessary to align the signal in the main signal path and the magnitude signal path to ensure that any timing misalignment between those signals falls within a permitted range. Any misalignment of the timing signals in the paths results in distortion of the transmitted signal, and can reduce transmitter efficiency. 
     With reference to  FIG. 1  there is illustrated components of an exemplary 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. 
     A signal generation block  122  receives a baseband signal (not shown) to be amplified by the amplifier  102 . The signal generation block generates a signal on line  125   a  representing the envelope of the input signal to be amplified. The signal generation block  122  additionally generates I and Q components of the input signal to be amplified on lines  125   b  and  125   c.    
     The generation of the envelope signal and the I and Q components of the baseband input signal is known to one skilled in the art. Various techniques for the generation of such signals may be implemented. 
     As illustrated in  FIG. 1 , the envelope signal on line  125   a  representing the envelope of the 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 ET modulator  108  may be implemented using a variety of techniques. For example, the ET modulator  108  may incorporate a switched mode power supply for selecting between one of a plurality of supply voltages in dependence on the magnitude of the envelope signal, and a correction or adjustment stage for adjusting the selected supply voltage in dependence on an error determined between the selected supply voltage and a reference signal based on the envelope signal. An exemplary envelope tracking modulator is described in U.S. Pat. No. 7,482,869. 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 . 
     The baseband I and Q components of the input signal on lines  125   b  and  125   c  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 components of the input signal 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 inter-stage 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 . 
     As known in the art, the envelope 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 which vary on a unit-by-unit basis within a production tolerance. As also known in the art the input or RF path which the baseband signal follows 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. 
     In general, such delays 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). 
     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 delays may arise in the ET modulator  108  itself. 
     In the input or 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 . 
     It is an aim of the present invention to provide an improved technique for a polar modulator in which distortion in an amplified signal is reduced by controlling the relative delay between the main signal path and the magnitude signal path. 
     It is also an aim of the present invention to provide an improved technique for an envelope tracking power supply for an RF amplifier, in which distortion in an amplified signal is reduced by controlling the relative delay between in the RF signal path and the envelope signal path. 
     SUMMARY OF THE INVENTION 
     In one aspect the invention provides a method of calibrating a polar amplification stage including a main signal path and a magnitude signal path, the method comprising: generating signals for the main signal path and the magnitude signal path for operating the amplifier between a linear mode of operation and a saturated mode of operation; detecting first and second peaks in a signal at the output of the amplifier representing transitions between the linear and saturated modes of operation; and adjusting the timing in one of the main signal path and the amplitude signal path in dependence on a relative difference between the size of the detected first and second peaks. 
     The step of operating the amplifier between linear and saturated modes of operation may comprise switching the operating mode of the amplifier from linear or saturated mode to saturation or linear mode respectively, and back. 
     The step of generating signals for the main signal path and the magnitude signal path may comprise generating an input signal for the input path to the amplifier having rising and falling slopes; and generating an input signal for the envelope path to the amplifier having opposite slopes to the signal in the input path. The input signal in the input path may include a rising slope followed by a falling slope, the first peak in the output signal corresponding to a condition where the amplifier transitions from a linear mode of operation to a saturated mode of operation, and the second peak in the output signal corresponding to a condition where the amplifier transitions from a saturated mode of operation to a linear mode of operation. 
     If the first peak is determined to be larger than the second peak, the timing of the magnitude signal path may be delayed relative to the timing of the main signal path. If the first peak is determined to be smaller than the second peak, the timing of the main signal path may be delayed relative to the timing of the magnitude signal path. 
     The input signals may be periodic, and the steps may be repeated for each period. 
     The steps may be performed for a calibration phase of operation, and the step of adjusting the timing may be applied prior to a normal phase of operation. 
     In another aspect the invention provides a polar amplification stage including a main signal path to an amplifier input and a magnitude signal path to the amplifier supply, the apparatus further comprising: a signal generator for generating signals for the main signal path and the magnitude signal path arranged to operate the amplifier between a linear mode of operation and a saturated mode of operation; a detector for detecting first and second peaks in a signal at the output of the amplifier representing transitions between the linear and saturated modes of operation; and a delay adjustment block for adjusting the timing in one of the main signal path and the amplitude signal path in dependence on a relative difference between the size of the detected first and second peaks. 
     The signal generator may be arranged to generate the signals to operate the amplifier between linear and saturated modes of operation by switching the operating mode of the amplifier from linear or saturated mode to saturation or linear mode respectively, and back. 
     The signal generator may be arranged to generate an input signal for the input path to the amplifier having rising and falling slopes; and generate an input signal for the envelope path to the amplifier having opposite slopes to the signal in the input path. 
     The signal generator may be arranged to generate the signals such that the input signal in the input path includes a rising slope followed by a falling slope, the first peak in the output signal corresponding to a condition where the amplifier transitions from a linear mode of operation to a saturated mode of operation, and the second peak in the output signal corresponding to a condition where the amplifier transitions from a saturated mode of operation to a linear mode of operation. 
     The adjustment bock may be arranged such that if the first peak is determined to be larger than the second peak, the timing of the magnitude signal path is delayed relative to the timing of the main signal path. 
     The adjustment block may be arranged such that if the first peak is determined to be smaller than the second peak, the timing of the main signal path is delayed relative to the timing of the magnitude signal path. 
     The signal generator may be arranged to generate periodic input signals, and the steps are repeated for each period. 
     The timing adjustment block may be arranged to apply the timing adjustment prior to a normal phase of operation. 
     The polar amplification stage may be an envelope tracking modulator, the magnitude signal path comprising an envelope signal path and the main signal path comprising a path of a signal to be amplified. 
     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 exemplary RF amplification stage incorporating an envelope tracking power supply or modulator, in accordance with the prior art; 
         FIG. 2  illustrates a modified exemplary RF amplification stage incorporating an envelope tracking power supply or modulator in accordance with a preferred embodiment of the invention; 
         FIGS. 3(   a ) to  3 ( c ) illustrate waveforms in exemplary arrangements of embodiments of the invention; 
         FIG. 4  illustrates exemplary waveforms in an exemplary arrangement of an embodiment of the invention; 
         FIG. 5(   a ) illustrates the principle of operation of the amplifier with a decreasing power supply, and  FIG. 5(   b ) illustrates exemplary waveforms in an exemplary arrangement of an embodiment of the invention; and 
         FIG. 6  illustrates the steps in utilising the exemplary RF amplification stage of  FIG. 2  in an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is applicable in general to polar amplification stages. In the following description the invention is described with specific reference to the example of an amplification stage incorporating an envelope tracking (ET) polar modulation technique. However this is for the purposes of illustrating an exemplary implementation of the invention, and to aid in understanding the invention, and the invention is not limited to such a specific technique. The skilled person will appreciate the invention may also be implemented in other polar transmitter technologies including, for example, envelope elimination and restoration technologies. One skilled in the art will appreciate that the invention and its embodiments may be utilised in a broader range of polar transmitters than is set forth herein. 
     In the following description, where an element of one Figure corresponds to an element of another Figure, like reference numerals are used to denote a correspondence. The presentation of a combination of features in an embodiment does not represent a limitation that the combination of features is necessarily essential to an embodiment, nor exclude the possibility that elements of an embodiment may be used without other illustrated elements or with other non-illustrated elements. 
     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 , and a measurement block  120 , in accordance with an exemplary implementation of the present invention. 
     As illustrated in the embodiment of  FIG. 2 , the envelope signal, I component of the input signal, and Q component of the signal for the respective digital-to-analogue converters  126   a  to  126   c  are generated on lines  125   a ,  125   b  and  125   c  respectively by the signal generation block  122  via the programmable delay adjustment block  124 . 
     The signal generation block  122  optionally generates signals to the measurement and correlation block  120  on lines  156 , and the measurement block  120  generates signals to the programmable delay adjustment block  124  on lines  157 . 
     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  on line  121 . 
     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 for example be implemented using a receiver chain including an analogue to digital converter, with detection of the envelope being implemented in the digital domain. 
     The adaptation of an RF power amplification stage in accordance with the exemplary arrangement of  FIG. 2  provides for a calibration system that reduces the delay uncertainty in the envelope path and the RF path, and that can be implemented as a self-calibration. 
     In accordance with the principles of this invention the power amplifier is driven in and out of compression, such that it operates in both a linear mode of operation (without compression) and a saturated mode of operation (with compression). This is preferably achieved by providing a signal for the RF input path which has an increasing and decreasing slope, whilst at the same time the envelope path is driven with a signal with opposite slopes to that of the signal on the RF input path. 
     This is illustrated with respect to  FIG. 3 , which shows a triangular signal in  FIGS. 3(   a ) and  3 ( b ) which may be applied to the input envelope path, and an inverse triangular signal applied to the RF input path as shown in  FIG. 3(   b ). The signal to the input envelope path is denoted by reference numeral  308 , and the signal to the RF input path is denoted by reference numeral  310 . 
     As illustrated in  FIGS. 3(   a ) and  3 ( b ), the signals contain two distinct slopes, one increasing and one decreasing, and one signal is the inverse of the other. 
     As noted above, the levels of the signal applied to the RF input path and the envelope path must be set such that both linear and saturation modes of operation are obtained in the power amplifier. The effect of this is that when the signal in the RF input path is small enough, i.e. lower than the power supply voltage provided to the power amplifier as determined by the signal in the envelope path, the power amplifier operates in a linear mode of operation. In a linear mode of operation, the power amplifier output power is a strong function of the power amplifier input level. When the RF input path signal becomes large enough, i.e. higher than the power supply voltage provided to the power amplifier by the signal in the envelope path, the power amplifier operates in saturation mode and the power amplifier output level becomes a strong function of the signal in the envelope path to the power amplifier supply voltage. 
     This means that due to the opposite signal slopes on each path, there will be a peak of the power amplifier output voltage at the point when the power amplifier transitions from a linear mode of operation to a saturated (non-linear) mode of operation and a peak where the power amplifier transitions from a saturated (non-linear) mode of operation to a linear mode of operation. This will occur both during the up and down slopes of the input signals, which therefore generate two power amplifier output voltage peaks. 
     For the simple example of the input signals of  FIGS. 3(   a ) and  3 ( b ), the RF output signal is illustrated in  FIG. 3(   c ), and denoted by reference numeral  312 . As can be seen, the RF output signal has two peaks. The first peak represents the transition point from saturated operation to linear operation, and the second point represents the transition from linear operation to saturated operation. 
     Although in the example of  FIGS. 3(   a ) to  3 ( c ) the input signals are shown as triangular waves, the invention is not limited to the input signals being of any particular shape. The input signals can be, for example, sine waves or any other type of signal. The only characteristic required of the input signals is that they contain two distinct slopes, one increasing and one decreasing, and that the signal in one path is the inverse of the other. 
     With reference to  FIG. 4  there is illustrated a further example. In  FIG. 4  waveform  302  represents the input voltage on the RF input path, which as illustrated is a sinusoidal signal. Waveform  304  represents the supply voltage provided by the ET modulator. Waveform  306  represents the voltage at the output of the power amplifier. 
     As illustrated in  FIG. 4 , between times t 0  and t 1  the amplifier operates in non-linear or saturated mode, between times t 1  and t 2  the amplifier operates in linear mode, and between times t 2  and t 3  the amplifier operates in saturated or non-linear mode. At the time at which the amplifier changes from non-linear to linear mode, at time t 1 , a peak in the output signal is generated. Similarly at the time that the amplifier transitions from linear to non-linear mode, time t 2 , a peak in the output signal occurs. 
     If there is a timing misalignment in either the first or second peak at times t 1  and t 2  one peak would be larger than the other, due to the transition in power amplifier operating mode occurring at a slightly different operating condition. For example if the envelope path to the power amplifier supply voltage has a signal which is slightly earlier than the signal on the RF input path, then its rising flank will cause the transition into linear mode to occur earlier, and the first peak will be larger than the second peak. Similarly the transition out of linear mode also occurs slightly earlier and therefore non-linear mode will occur earlier and the second peak will be slightly less in amplitude. 
     Reference is made to  FIGS. 5(   a ) and  5 ( b ) to help further understand the occurrence of one peak being larger than the other when a timing misalignment is present. 
       FIG. 5(   a ) illustrates as a main plot a typical device transfer characteristic of a transistor of the amplifier stage, comprising numerous plots  550  of amplifier output voltage against amplifier input voltage. The numerous plots  550  reflect the sweeping of the power amplifier supply voltage, such that the higher the output voltage the higher the supply voltage. Such a transfer characteristic as represented by waveforms  550  is well-known in the art. 
     Also illustrated in  FIG. 5(   a ) is a plot  552  of supply voltage to the amplifier in a calibration mode of operation in accordance with an embodiment of the invention. In the illustration of  FIG. 5(   a ) there is represented a condition, in the calibration mode of operation, where there is no delay between the envelope path signal and the RF input path signal. As shown  552  illustrates a falling supply voltage as the input voltage increases. Further illustrated is a plot  554  of output voltage against the input voltage in association with the falling supply voltage  552 . 
     As illustrated in  FIG. 5(   a ), the output voltage increases in accordance with the normal behaviour of the transistor device characteristics, following the input voltage. However at some point denoted by time t a  the output voltage peaks and starts to fall, as the amplifier has reached saturation due to the decreasing supply voltage  552  in combination with the increasing input voltage. The output voltage the slopes off as the input voltage continues to rise and the supply voltage continues to decrease. 
     With reference to  FIG. 5(   b ), there is illustrated the effect on the amplitude of the peak of waveform  554  of  FIG. 5(   a ) as a result of relative delays between the envelope signal and the input signal, which results in the differences in peak amplitudes which are detected in the circuit of  FIG. 2 . 
     As illustrated by arrow  558 , for the device transfer characteristics waveforms  550  the output voltage of the amplifier increases as the input voltage increases, for increasing supply voltages. 
     As denoted by arrow  560 , during a calibration operation in accordance with the invention the slope of the supply voltage relative to the input voltage will vary in dependence on the relative delay in the input signal path and the envelope signal path. As denoted by arrow  560 , for a given input voltage the instantaneous supply voltage will vary in dependence on the relative delay. 
     The supply voltage waveform  552  of  FIG. 5(   a ) is thus replaced by supply voltage waveforms  552   a  and  552   b  in  FIG. 5(   b ).  FIG. 5(   b ) represents a timing misalignment with respect to  FIG. 5(   a ). 
     The output voltage waveform  554  of  FIG. 5(   a ) is also replaced by the output voltage waveforms  554   a  and  554   b . The output voltage waveform  554   a  is associated with the supply voltage  552   a , and the output voltage waveform  554   b  is associated with the supply voltage  552   b . These output voltage waveforms  554   a  and  554   b  show the effect of timing misalignment between the envelope and input signal paths on the size of the peaks in the output. 
     For the supply voltage waveform  552   a , the corresponding output voltage waveform is  554   a , which peaks at an output voltage level A. For the supply voltage waveform  552   b , the corresponding output voltage waveforms is  554   b , which peaks at output voltage level B. The voltage peak B is less than voltage peak A. The output voltage  554   b  is not able to reach as a high a level as the output voltage  554   a , because the decreasing supply voltage  552   b  results in saturation being reached at a lower input voltage than for supply voltage  552   a . For supply voltage  552   b  the amplifier enters saturation for a lower input voltage, and is thus not able to achieve as a high a peak as for supply waveform  554   a . In general, the later the supply voltage is in comparison to the input waveform, the higher the associated peak at a transition from linear mode to saturation will be. 
     In summary, the supply drops down to a certain level and then the amplifier output is dominated by the supply. The supply goes down as the input increases, due to the inverse nature of the signals. If the supply is early, then a low peak is obtained. If the supply is late, then a high peak is obtained. 
     The peaks also provide information about the direction of delay. If the first peak is larger than the second peak then the signal on the envelope path to the power amplifier supply voltage needs to be delayed, or alternatively the signal on the RF input path needs to be advanced, and if the second peak is larger than the first peak then the signal on the envelope path to the power amplifier voltage supply needs to be advanced (or the signal on the RF input path needs to be delayed). 
     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. 6 . 
     As denoted in step  502 , the signal generation block  122  is arranged to generate first and second signals for the envelope path and the RF input path. One signal is a signal with increasing and decreasing slopes, and the second signal is the inverse of the first signal, with opposite slopes. The first and second signals may be generated independently by the signal generation block  122 , or one signal may be generated for one path and then inverted for the other path. 
     In a step  504  the first and second signals are applied to the envelope path and the input path respectively. It should be noted that there is no requirement for the signals to be applied to a particular one of the paths, it is merely a requirement that the signal applied to the two paths have opposite slopes. In this exemplary arrangement, the first signal is processed by the envelope path and the second signal is processed by the input path. 
     The diode detector  114 , as denoted by step  506 , detects the power of 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  detects and measures a first peak, as denoted by step  508 . The measurement block then detects and measures a second peak as denoted by step  510 . As indicated in  FIG. 2 , the measurement block  120  may receive a signal from the signal generation block  122 , so that the measurement block  120  can associate detected peaks with a particular pair of input signals generated for the input path and envelope paths. For example, a signal on line  156  may provide a trigger to the measurement block  102  to associate two detected peaks with a single input sequence. 
     As denoted by step  512 , the measurement block then compares the first and second detected peaks. As discussed hereinabove, the measurement block makes a determination as to which of the input and envelope paths contains a signal which is more advanced than the other. In dependence upon determination of one signal being more advanced than the other, then an appropriate delay or adjustment is made as denoted by step  514 . In the event of the first peak being detected as greater than the second peak, the first signal is delayed (or the second signal advanced). In determination of the second peak being greater than the first peak, the second signal is delayed (or the first signal advanced). In detection of the first and second peaks being equal, no adjustment is made. 
     The adjustment is preferably made by the measurement block  120  providing an appropriate adjustment to the programmable delay adjustment block  124  on lines  157  in dependence on the measured difference of the peaks and if appropriate the direction of the measured difference. 
     The process may then be applied iteratively, until the measurement block  120  determines that the delay between the two paths is determined to fall within an acceptable tolerance. 
     The measurement timing resolution restrictions of the ADC  116  may be relaxed by post-processing the peak information to interpolate the peaks. 
     The technique as described for reducing delay between the signals in the RF input path and the envelope path has a number of advantages. 
     The main advantage of the technique described herein is that the delay is detected based on very large power amplifier output signals. On this basis there is no requirement for a particularly sensitive detection device. The technique is relatively insensitive to quantisation, noise or isolation effects. 
     A second advantage of the described technique is that the direction of required delay adjustment can be seen from the signal generated at the power amplifier output by comparing the two peaks generated when entering linear mode and exiting linear mode. This means that the detection of the correction direction of delay adjustment is not needed. Detection of the correction direction may take additional time and processing effort, which is not a problem incurred by the present techniques. 
     Thirdly, it is possible to calculate the amount of delay adjustment required due to the amplitude difference (or ratio, or some other characteristic of signal peak differences). If the delay can be calculated from the peak amplitudes then the requirement for a search algorithm is negated. However, tolerances, timing setup and other real world effects may make it impractical to directly calculate the delay requirement absolutely. Nevertheless the ability to provide some measurement of the delay is provided. 
     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.