Abstract:
Methods for generating a look-up table relating a plurality of complex reflection coefficients to a plurality of matched states for a tunable matching network. Typical steps include measuring a plurality of complex reflection coefficients resulting from a plurality of impedance loads while the tunable matching network is in a predetermined state, determining a plurality of matched states for the plurality of impedance loads, with a matched state determined for each of the plurality of impedance loads and providing the determined matched states as a look-up table. A further step is interpolating the measured complex reflection coefficients and the determined matching states into a set of complex reflection coefficients with predetermined step sizes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/236,662 filed Sep. 24, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to methods for tuning an impedance matching network, and more specifically to methods for tuning an impedance matching network with a look-up table. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electronic communication products which transmit and receive radio frequency (RF) signals need to match the impedances between the internal circuitry, such as an RF power amplifier, and the antenna for optimum performance of the product. The optimum mode may also depend upon the modulation scheme, the frequency of transmission and other considerations. 
         [0004]    Electronically tunable filters are frequently used to compensate for any impedance mismatch between the circuitry and the antenna. Such tunable filters utilize various tuning elements, such as tunable capacitors, varactors, micro-electronic mechanical systems (MEMS), and doped semiconductor materials. 
         [0005]    Adaptive impedance matching modules (AIMMs) sense continually sense impedance mismatch and retune the impedance for any changed conditions. AIMMs may take up to 20 iterative steps to converge on the best impedance match, such as by using gradient search methods. 
         [0006]    There is a need for an adaptive impedance matching network which can rapidly tune to the best or optimum matching impedance in fewer steps and in less time. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to methods for generating and using a look-up table relating a plurality of complex reflection coefficients to a plurality of matched states for a tunable matching network. In one embodiment, typical steps of the methods include measuring a plurality of complex reflection coefficients resulting from a plurality of impedance loads while the tunable matching network is in a predetermined state, determining a plurality of matched states for the plurality of impedance loads, with a matched state determined for each of the plurality of impedance loads, and providing the determined matched states in a look-up table. These steps may be reiterated for different frequency bands, different frequencies or for different use cases. 
         [0008]    In other embodiments, the methods may include additional steps such as interpolating the measured complex reflection coefficients and the determined matching states into a set of complex reflection coefficients with predetermined step sizes, selecting the predetermined state used in measuring the complex reflection coefficients to minimize tolerance variations based upon temperature coefficient, component tolerance or tolerance over time, using the determined matched states in the look-up table to tune the tunable matching network, tuning the tunable matching network to adaptively match the impedance of an antenna, controlling the tunable impedance elements with digital to analog converters. 
         [0009]    The tunable matching network plurality of tunable impedance elements, may include a which may be, for example, ferroelectric capacitors, voltage variable capacitors, MEMS, tunable inductors or networks thereof. 
         [0010]    Yet another embodiment includes methods for generating a look-up table relating a set of measured parameters to a set of matched states for a tunable matching network, including the steps of measuring a set of parameters associated with a plurality of impedance loads while the tunable matching network is in a predetermined state, determining a matched state for each of the plurality of impedance loads, and providing the determined matched states as a look-up table. The set of measured parameters may be selected from the group consisting of complex reflection coefficients, current drain, incident power, reflected power, control setting of the tunable matching network, temperature, input power level, reliability considerations and linearity considerations. Preferably, a sufficient number of matched states are provided such that a transition from one matched state to an adjacent matched state provides a smooth transition. 
         [0011]    A further embodiment may include methods of adaptively tuning a tunable impedance matching network with a lookup table, the lookup table containing a plurality of complex reflection coefficients corresponding to a plurality of impedance mismatches between an input terminal and an output terminal of the tunable impedance matching network, including the steps of determining the impedance mismatch between the input and output terminals of the tunable impedance matching network, determining the closest impedance mismatch in the lookup table, and using the complex reflection coefficients in the lookup table which correspond to the determined closest impedance mismatch to tune the tunable impedance matching network. Further steps may include controlling the ferroelectric capacitors with digital to analog converters, and tuning the tunable impedance matching network to adaptively match the impedance of an antenna. 
         [0012]    Another embodiment may include methods of adaptively tuning a tunable impedance matching network with a lookup table, the lookup table containing sets of parameters corresponding to matched states for the tunable impedance matching network, including the steps of selecting a parameter from the group consisting of complex reflection coefficients, current drain, incident power, reflected power, control setting of the tunable matching network, temperature, input power level, reliability considerations and linearity considerations, determining the closest match to the selected parameter in the lookup table, and using the closest match to the selected parameter in the lookup table to tune the tunable impedance matching network. The parameter may also be selected to minimize tolerance variations based upon temperature coefficient, component tolerance or tolerance over time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention, together with its objects and the advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the figures, and in which: 
           [0014]      FIG. 1  is a schematic diagram of an impedance matching circuit for including variable capacitors for impedance optimization; 
           [0015]      FIG. 2  is a table of exemplary definitions of independent variables and scaling factors, related to the methods of the present invention; 
           [0016]      FIG. 3  is a block diagram of a typical characterization set-up for the methods of the present invention; 
           [0017]      FIG. 4  is a sample look-up table which correlates complex impedance loads with measured complex reflection coefficients and determined settings of a plurality of digital to analog converters; 
           [0018]      FIG. 5  is a table illustrating how an address pointer corresponds to the independent variables and the adaptive impedance matching network settings; and 
           [0019]      FIG. 6  is another sample look-up table with interpolated settings for a plurality of digital to analog converters resulting from interpolation of measured complex reflection coefficients and determined settings which are found in the table shown in  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    It will be understood that the present invention may be embodied in other specific forms without departing from the spirit thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details presented herein. 
         [0021]      FIG. 1  illustrates an impedance matching circuit, generally designated  100 , for impedance optimization between a source of radiofrequency (RF) power, such as an RF amplifier  102 , and an RF load, such as an antenna  104 . An adaptive impedance matching network  106  is coupled between RF amplifier  102  and antenna  104 . In the example of  FIG. 1 , the adaptive impedance matching network  106  is an adaptive impedance matching module (AIMM). AIMM  106  dynamically adjusts its internal impedance matching circuit to minimize reflected power to achieve a near-optimal impedance match. For example, a common application for AIMM  106  is to correct antenna-RF amplifier impedance mismatch that often occurs with handheld radios and body-worn antennas. Similarly, handheld communications devices, such as cellular telephones encounter impedance mismatch when the communication device is held close to the user&#39;s head or torso. Yet another application for the AIMM  106  is in sensor networks where the antenna of the sensor can be detuned by proximity effects of the ground, foliage or debris. In general, AIMM  106  may be used in any application where forward power needs to be maximized and reverse power needs to be minimized 
         [0022]    AIMM  106  may be a multi-chip module comprising a tunable impedance network, which contains one or more tunable ferroelectric capacitors  108 - 109 . Preferably, the tunable ferroelectric capacitors  108 - 109  are Paratek&#39;s ParaTune™ family of passive tunable integrated circuits (PTICs), which are commercially available from Paratek Microwave, Inc. of Columbia, Md. These PTICs  108 - 109  utilize a Parascan® tunable dielectric material which is further described in U.S. Pat. Nos. 7,107,033 and 6,514,895, which are assigned to the same assignee as the present invention and which are incorporated herein by reference in their entirety. These PTICs  108 - 109  overcome the power limitations common to other tunable technologies such as varactor diodes and MEMS and can handle in excess of 2 watts of RF power with extremely low inter-modulation distortion. 
         [0023]    The adaptive impedance matching module or AIMM  106  in  FIG. 1  has an input terminal  110  for receiving an RF signal from RF amplifier  102 . A forward and reverse power detector  112  provides a signal on line  114  to a rectifier  116  which is representative of the forward power from RF amplifier  102 . Rectifier  116  provides a rectified value of the forward RF power to an analog to digital converter (ADC)  118 , which in turn provides a digital representation of the forward power to a microprocessor  120 . In a similar fashion, the power detector  112  provides a signal on line  115  to a rectifier  117  which is representative of the reverse or reflected power from antenna  104 . Rectifier  117  provides a rectified value of the reverse RF power to an ADC  119 , which in turn provides a digital representation of the reverse power to a microprocessor  120 . 
         [0024]    Based upon the values of the determined forward and reverse power levels, and in accordance with one aspect of the present invention, microprocessor  120  uses a lookup table, such as table  400  in  FIG. 4  or table  600  in  FIG. 6 , which may be resident in memory  124  to determine bias adjustments to be made to the PTICs  108 - 109  for a first step of retuning the adaptive impedance of the AIMM  106 . To this end, microprocessor  124  supplies digital signals to digital to analog converters (DACs)  122 - 123  which control the analog bias signals to PTICs  108 - 109 . Microprocessor  120  may continue with additional steps of retuning the PTICs, as needed, to provide a near-optimum impedance matching between RF amplifier  102  and antenna  104 . 
         [0025]      FIG. 2  illustrates a table, generally designated  200 , which is an example of how independent variables  202  may be defined and organized. For example, the independent variables  202  may include frequency information  204 , reflection coefficient magnitude  205  and reflection coefficient phase  206 . Furthermore, each of these independent variables may have a symbol  208 , a normalized independent variable  210 , a normalized symbol  212  and a number of points  214 . 
         [0026]    As shown in the example of  FIG. 2 , the frequency information  204  may be associated with a symbol Fq and a normalized symbol Fq, and may consist of 12 values including one value for each transmit frequency band and one value for each receive frequency band. The frequency variable for the frequency information  204  may also be accorded 12 points. Similarly, the reflection coefficient magnitude  205  has a symbol mag_S11, a normalized symbol N_mag_S11, and a normalized independent variable determined as mag_S11 times 8 and then rounded to the nearest integer. The reflection coefficient magnitude  205  is accorded 6 points. Lastly, the reflection coefficient phase  206  has a symbol ph_S11, a normalized symbol Nyh_S11, and a normalized independent variable determined as divided by 45 and then rounded to the nearest integer. The reflection coefficient phase  106  is accorded 8 points. 
         [0027]    The purpose of a look-up table, such as look-up table  400  in  FIG. 4  or look-up table  600  in  FIG. 6  is to make it possible for an adaptive impedance matching network, such as adaptive impedance matching module  106  in FIG. I, to take a large first step in tuning. A large first tuning step reduces the time to convergence of final settings, reduces power consumption and decreases traffic on the communication bus. 
         [0028]    In accordance with one embodiment of the present invention, the look-up table  400  may contain pairs (or sets) of digital-to-analog (DAC) settings  402  that are to be put into the high voltage application specific integrated circuit (HV-ASIC) for controlling tunable impedance module  106 . elements in the adaptive impedance matching The pairs (or sets) of DAC settings  402  can be identified by an index which is correlated to independent variables  202 , for example, frequency  204 , magnitude S11  205  and phase S11  206 . The index would be related to the position in the table  400  of the desired information. Hence, the index information would not need to be stored in the table. The index could be the sum of three independent variables, such as independent variables  202 . 
         [0029]    In this example, the address pointer, such as address pointer  602  in  FIG. 6 , may be calculated as pointer=6*8*Fq+8*N_ma9 —811 +N-ph_s11. The table then has 432 rows (=12*6*8). Each has 2 or 3 bytes of information, one for each tunable element setting. The total memory usage for a two tunable element adaptive impedance network is 1152 bytes. The total memory usage for an adaptive impedance matching network with 3 tunable elements is 1728 bytes in this example. 
         [0030]    A worst case analysis may typically be required to determine how many different phases and magnitudes are sufficient or insufficient. A key consideration is how accurate the first step needs to be and how much the tolerances may degrade the accuracy. Also, the magnitude and phase of 811 do not have to be scaled linearly. They could be scaled non-linearly to give better accuracy to areas of the Smith chart that are common. 
         [0031]    The operation of the adaptive impedance matching module  106  with the look-up table  400  will now be considered. When the adaptive impedance matching module  106  is initially turned-on, it may be programmed to an initial or default state. From the initial state, adaptive impedance matching module  106  will measure the magnitude and phase of the reflection coefficient  205  and  206 , lookup the DAC setting  402  in table  400  that corresponds to that reflection coefficient, and take a large first tuning step. The first tuning step can be improved by interpolating between table entries. The first step may be broken into several steps if the modulation accuracy or time mask specifications do not allow for a full step. The default setting may have the following characteristics: a) all tunable elements may be set to the same voltages, and b) the voltage that the tunable elements are set to would correspond to the voltage at which the tolerances have minimal impact, for example: a temperature coefficient that is 0 ppm/C. Many tolerances may degrade the accuracy of the first step. Using the 0 ppm/C temperature coefficient will reduce the contribution of temperature to the tolerance stack-up. The default setting could also be chosen to correspond to the expected load impedance, such as the load-pull system  306 , if known. 
         [0032]    The magnitude, phase and band information will then be used to calculate the index of the look-up table that represents the best first guess for the first tuning step. Thereafter, adaptive impedance matching network  304  may switch to its standard adaptive search algorithm. Once optimal tuning is achieved, adaptive impedance matching network  304  can switch to a tracking mode, in which step sizes are smaller and/or less often. 
         [0033]    Consideration will now be given to generating the look-up table, such as tables  400  and  600 . These tables may be generated during the design phase after the topology and the tuner values are fixed. A block diagram of the characterization test set-up  300  is shown in  FIG. 3 . It consists of a signal generator  302  coupled via a conductor  303  to the adaptive impedance matching network  304 , which in turn, is coupled via a conductor  305  to a load-pull system  306 . This test set-up  300  simulates an end use application of the adaptive impedance matching network  304 , in which signal generator  302  may be an RF power amplifier, for example, in a portable communication device such as a cellular telephone, personal digital assistant, or the like. In such an end use application, the load-pull system  306  may be the RF antenna for the portable communication device. 
         [0034]    The procedure to generate the table  400  begins with the adaptive impedance matching network  304  in a pause mode, and the tunable element in the adaptive impedance matching network  304  set to the default setting. This may be the bias voltage at which the capacitive temperature coefficient is 0 ppm/C. Note that different settings may be preferable if it results in improved tolerance performance. For example, if there is knowledge of the expected load, the default setting could also be set based on the expected load, such as presented by the load pull system  306 . 
         [0035]    The signal generator is preferably set to the middle of the frequency range being characterized. As the design is better understood, the designer may choose to use a different frequency that better represents the center of performance. If frequency information is available, a look-up table could be generated for each frequency, as the availability of memory allows or permits. 
         [0036]    The load-pull system  306  is run through a fine mesh of the magnitudes and phases. The magnitude and phase of 811 is recorded as measured by adaptive impedance matching network  304  for each load. Alternatively, other parameters could be recorded for use in the table. These parameters may include (but are not limited to) complex reflection coefficient, current drain, incident power, reflected power, reliability metrics, linearity metrics, and the like. 
         [0037]    The adaptive impedance matching module  106  is set in the run mode. That is the adaptive impedance matching module  106  is permitted to optimize the match as it would in an end use application. The optimization may be part of the module  106  or apart from it. If the optimization is apart from the module  106 , it may be referred to as a tunable impedance matching network. If the optimization is part of the network r it may be referred to as an adaptive impedance matching network. An adaptive impedance matching module  106  always includes a tunable impedance matching network. 
         [0038]    As the load-pull system  306  runs through a fine mesh of the magnitudes  404  and phases  405 , the DAC settings  402  that adaptive impedance matching network  304  settles on for each load  306  are recorded, as by storage in available memory. At this point, all of the information is available for a look-up table, such as look-up table  400  in  FIG. 4 . 
         [0039]    Using the two data sets  304  and  305 , generate contours correlating the 811 measurements to the preferred DAC settings  302 . The contours are then used to interpolate to find the preferred or interpolated DAC settings  404  in table  400  ( FIG. 4 ) for each 811 that is to be represented in the look-up table  400 . 
         [0040]    At this point, the look-up table would be like that of table  600  in  FIG. 6 . This is an extremely memory efficient table implementation because only the output interpolated DAC settings  604  need to be stored. An address pointer  602  will retrieve the applicable interpolated DAC settings  604 . The interpolated DAC settings  604  are preferably organized such that no searching is required, thereby saving processing time. 
         [0041]    It will be appreciated by those skilled in the art that the above steps of generating the look-up table, will be performed by a microprocessor or the like. For example, the above steps may be performed by a microprocessor in the product for which the antenna matching is occurring, such as in a cellular telephone, PDA, or the like. Alternately, the microprocessor may be provided in the adaptive impedance matching module  106 . 
         [0042]    While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention in its broader aspects.