CN100409580C - System and method for a feed forward amplifier employing a self-generated alignment list and an adaptive controller - Google Patents

Abstract

A method and apparatus for fast calibration of an adaptive feed forward power amplifier is disclosed. Successful alignment settings are correlated to operating conditions that affect the gain and phase of the amplifier. These operating conditions may include input power level (228), carrier frequency (230), temperature (236), DC supply voltage, and so forth. The successful alignment settings are stored with the corresponding operating conditions in a list (224) indexed with the multidimensional attribute vector. Elements of the list are automatically generated.

Description

Systems and methods for feedforward amplifiers employing self-generated calibration lists and adaptive controllers

Related application information

This application claims the benefit of Provisional Application Serial No. 60/442099, filed January 23, 2003, the disclosure of which is incorporated herein by reference in its entirety.

Background of the invention

1. Field of invention

The present invention relates to RF power amplifiers and amplification methods. More specifically, the present invention relates to feedforward amplifiers and methods for controlling feedforward amplifiers.

2. Description of prior art and relevant background information

RF amplifiers are devices that attempt to replicate the RF signal present at the input and produce an output signal with a much higher power level. The increase in power from input to output is called the "gain" of the amplifier. An amplifier is said to be "linear" when the gain is constant over the dynamic range of the input signal. Amplifiers have limited capacity in terms of delivered power because of gain and phase variations, especially saturation at high power, which makes all real amplifiers non-linear as input power levels vary. The ratio of the generated distortion power to the delivered signal power is a measure of the nonlinearity of the amplifier.

In RF communication systems, the maximum allowable nonlinearity of amplifiers is specified by government agencies such as the FCC or ITU. Because amplifiers are inherently nonlinear when operating near saturation, the linearity requirement often becomes a limitation on the rated power delivery capability. Generally speaking, when operating near saturation, the linearity of the amplifier deteriorates rapidly because the signal power increment delivered by the amplifier is proportionally smaller than the distortion power increment produced.

Traditionally, various compensation methods have been used to reduce the distortion of the system output, thereby increasing the rated power delivery capability. The preferred method is feedforward compensation. In a feed-forward RF power amplifier, an error amplifier is used to amplify the main amplifier distortion component, which is combined out-of-phase with the main amplifier output to cancel the main amplifier distortion component. In general, feed-forward compensation provides the power capability of the main amplifier and the linearity of the error amplifier.

Feedforward linearization of the amplifier is based on matching the gain and phase of the parallel RF paths to either cancel the carrier (input) signal (loop 1) or eliminate distortion (loop 2). Carrier cancellation is often referred to as "loop 1 error," which is an estimate of the distortion in the main amplifier path. Distortion cancellation takes place in loop 2, using the loop 1 error to remove distortion from the main amplifier. The matching of gain and phase in the respective loops is called "loop calibration control". When loop 2 is calibrated correctly, distortion on the output is minimized, making the entire feedforward system more linear than the main amplifier alone. When the calibration of loop 1 is correct, the power through the error amplifier (which amplifies the loop 1 error) is limited. In most cases, a Loop 1 calibration must be done before enabling the Loop 2 error amplifier. This ensures that the error amplifier is not being overdriven, a condition that would create unwanted distortion or damage the device.

Most end users of feedforward power amplifiers have a specification that limits the time it takes for the adaptive portion of the feedforward compensation to calibrate the loop. Consequently, it is important to have a good initial calibration when the adaptive controller starts searching for the best (or sufficient) calibration. Some of these techniques require times as low as 10 seconds.

Also, it is important to have a good calibration of loop 1 to limit the power going into the error amplifier. However, the loop 1 error power is roughly the product of the input power and the calibration quality (carrier cancellation amount). As a result, the calibration quality of loop 1 may be moderate if the input power is low. If the input power suddenly increases, the loop 1 error power will increase proportionally, potentially overdriving the error amplifier. Although loop 1 will automatically adjust its calibration settings in response to this increase, a fast change in the calibration of loop 1 is preferred because it reduces the transient effect of overdriving the error amplifier.

There are many prior methods for feed-forward linearization, first documented in the 1920s. In earlier methods, the calibration settings were static, using fixed settings for gain and phase, optimized for nominal operating conditions. Later methods introduce look-up tables for compensating for temperature and DC power variations. Still later, an adaptive approach was applied, where the offset of the loop was measured internally and used for subsequent calibration adjustments. Loop 1 error power is often used as a measure of the amount of loop 1 offset. To measure loop 2, a pilot signal is usually introduced in loop 1 as a "known distortion". The pilot power detected at the output of loop 2 is a measure of the amount of loop 2 offset. Unpiloted methods for measuring the offset of loop 2 have been implemented, which are based on distortion measurements. In such systems, the second loop convergence has a significant dependence on the input signal and the resulting distortion.

Look-up tables have been used in both static and adaptive versions of feedforward amplifiers. These look-up tables are typically used to compensate for temperature or DC power variations. However, even when LUTs are used in adaptive feed-forward systems, these LUTs are usually used to control parts of the system that are not adaptive, such as front-end voltage-controlled attenuators and phase shifters (which maintain the overall gain or phase). That is, loop adaptation and lookup tables are not linked.

As noted above, lookup tables have historically used a fixed structure. Inputs such as temperature are indices into the array. These indices are equally spaced over a range in ascending order, and the corresponding calibration settings are stored in the array. This structure is well suited to memory chips because the index corresponds to the address and the calibration settings correspond to the data. However, lookup tables are usually based on experimental data (calibration) and require a lot of time to populate the elements of the table. In addition, drift caused by component aging can render any look-up tables obsolete, necessitating recalibration.

Another difficulty associated with lookup tables is that if there are many operating conditions that affect the quality of the calibration, multidimensional arrays are required, which can be extremely difficult to manage. One can imagine the number of elements present in an equally spaced 4D array. For example, 10 samples per dimension yields 10000 elements.

One technique for managing multiple index dimensions is to assume that the effects are separable. Separable conditions allow separate arrays to be used for each operating condition, while the combined effect is the sum of these individual adjustments. (Not unlike the Taylor series expansion, where partial derivatives are specified). However, this approach is only effective for small (differential) calibration adjustments, since any cross-correlation between dimensions is ignored. The largest errors occur at the corners of multidimensional arrays. For example, the problematic corners in the temperature, DC power index space are high temperature and low voltage. It is these angular positions that are usually tested by experienced customers to determine if the amplifier is within specification.

A problem associated with array-based lookup tables is the choice of sampling interval (separation between adjacent indexes) within the index space. In general, the sensitivity of gain and phase settings varies over index space. The sampling density must be chosen according to the most sensitive area of the index space. The remaining regions will be oversampled. This oversampling problem is even more severe for multidimensional arrays.

A lookup table self-calibration or self-generation has been attempted. However, fixed array structures are difficult to manage. The key problem encountered is 'update fragmentation'. Consider the 4D array case mentioned earlier. When the lookup table is updated, only one element out of 10000 is changed. If the source of degradation is global (e.g. due to element drift), then all 10000 elements are affected. However, changes must be propagated as each index is accessed. There is a possibility that adjacent indexes have large differences just because one of the indexes is older.

Accordingly, a need currently exists for a system and method for fast loop calibration control in a feedforward amplifier system that avoids the aforementioned limitations of the prior art.

Summary of the invention

In a first aspect, the present invention provides a feedforward amplifier system comprising an input for receiving an RF input signal and a first control loop coupled to the input, the first control loop comprising a main amplifier, a main amplifier Sampling couplers, delay elements, and cancellation combiners. The feedforward amplifier system also includes a second control loop coupled to the first control loop and including a first signal path, a second signal path including an error amplifier, and an error coupler coupling the first and second signal paths. The output is coupled to the error coupler. The feedforward amplifier system also includes means for controlling at least one of the first and second control loops employing a calibration list having a plurality of list elements, each element having a calibration setting and an operating condition characterizing the feedforward amplifier system parameter set.

In a preferred embodiment of the feedforward amplifier system, the first control loop further includes a gain adjuster and a phase adjuster, and the respective calibration settings employed by the means for controlling may include the first control loop gain adjuster setting and the first control loop phase regulator setting. The second control loop may also include a gain adjuster and a phase adjuster, and the respective calibration settings employed by the means for controlling may include a second control loop gain adjuster setting and a second control loop phase adjuster setting Certainly. The set of parameters characterizing the operating conditions of the feedforward amplifier system may include one or more of temperature, DC power supply, input signal power, and input signal carrier frequency. Preferably, the set of parameters characterizing the operating conditions of the feedforward amplifier system is defined as property vectors, and a distance is defined between any two property vectors. The means for controlling obtains the current attribute vector and calculates the distances to the attribute vectors of the list elements, selecting the list element with the smallest distance for use as a calibration setting in the control function. The means for controlling may continuously measure an offset of the feedforward amplifier system, and retrieve a calibration setting from a calibration list when the measured offset exceeds a predetermined value. The means for controlling takes the selected elements as initial calibration settings and calculates new calibration settings from the initial settings using an iterative control algorithm. The means for controlling updates the calibration list with new calibration settings after completion of the iterative calculations. The distance between the closest list element attribute vectors can vary throughout the calibration list.

According to another aspect, the present invention provides an adaptive controller for controlling a loop of an amplifier system. The adaptive controller includes one or more inputs for receiving one or more property parameters corresponding to current operating conditions of the amplifier system. The adaptive controller also includes one or more processors coupled to the one or more inputs, the one or more processors having associated calibration lists and programmed with the calibration list algorithm and the controller algorithm to provide Loop adjustment settings to control the loop of the amplifier system. The calibration list algorithm generates a calibration list with the adjustment settings calculated by the controller algorithm and associates one or more property parameters with each adjustment setting.

In a preferred embodiment of the adaptive controller, the calibration list algorithm selects calibration settings from the calibration list for use by the controller algorithm at startup or when the loop becomes sufficiently detuned. The calibration list algorithm is preferably calculated by calculating the distance between one or more property parameters corresponding to the current operating conditions and the property parameter associated with each calibration setting in the list, and selecting the calibration corresponding to the property parameter with the smallest distance. Settings, select the calibration list to adjust the settings for use by the controller algorithm. The distance calculation may weight different attribute parameters with different weights. The attribute parameters may include one or more of temperature, DC supply voltage, input signal power, and input signal carrier frequency. The distance d _attr between two sets of attribute parameters "n" and "0" may be defined eg by a weighted _Linf -norm distance metric or a weighted L2 _- norm distance metric. The adaptive controller may also include one or more inputs for receiving calibration data. The one or more inputs for receiving calibration data may include, for example, a pilot signal input or an input for loop test data. Adjustment settings may include gain adjuster and phase adjuster settings.

According to another aspect, the invention provides a method for controlling an amplifier system having a control loop comprising a control loop input, a first signal path, a second signal path and a control loop output, the first At least one of the first and second signal paths includes an amplifier. The method includes providing a list of calibration settings, each calibration setting having an associated operating condition. The method also includes sensing a current operating condition of the amplifier system. The method also includes comparing the current operating conditions to those in the list of calibration settings and selecting the calibration settings associated with the most similar operating conditions in the list.

In a preferred embodiment of the method for controlling an amplifier system, the relevant operating conditions are configured as multidimensional property vectors. Comparing the current operating conditions to those in the list of calibration settings may include measuring the distance between the current property vector and each property vector in the list. Selecting the calibration setting associated with the most similar operating condition in the list may include determining the attribute vector with the smallest distance from the current operating condition attribute vector. The method may further include calculating a new calibration setting using an iterative loop controller algorithm, wherein the calibration setting associated with the most similar operating conditions is used as an initial calibration setting for the adaptive loop controller algorithm. The method may further include updating the calibration list with the new calibration settings calculated by the adaptive loop controller algorithm. The size of the calibration list can be dynamic. Also, the interval of stored adjustment settings defined by attribute vector distances preferably varies throughout the list. For example, a higher density of adjustment settings may be set in areas of the list that are most sensitive to the one or more operating conditions that include the property vector.

According to another aspect, the present invention provides a method of maintaining a list of calibration settings for a control loop of an amplifier system, the list comprising a plurality of elements each having a calibration setting and parameters corresponding to operating conditions of the amplifier system set. The method includes selecting an element of the calibration list and determining an element of the calibration list that has a corresponding operating condition most similar to the selected element. The method also includes determining whether two elements are similar enough to be considered redundant. The method also includes deleting the oldest of two elements of the calibration list if those elements are redundant.

In a preferred embodiment of the method of maintaining a list of calibration settings for a control loop of an amplifier system, selecting elements of the calibration list includes selecting an oldest element of the list that has not previously been subjected to a list maintenance process. Determining the element of the calibration list having a corresponding operating condition most similar to the selected element may include determining a distance measure to the operating condition parameter value of each remaining element of the calibration list and selecting the element having the smallest distance. A distance metric may include a weighted difference between parameter values corresponding to operating conditions. Parameters corresponding to operating conditions of the amplifier system may include one or more of temperature, DC power supply, input signal power, and input signal carrier frequency. Determining whether elements are similar enough to be considered redundant may include determining a distance metric between calibration settings and comparing the calibration distance to a redundancy distance threshold. Alternatively, determining whether elements are similar enough to be considered redundant may include comparing the distance between the operating condition parameters of two elements to an outdated distance threshold. The method may further include repeating the list maintenance process for each element in the calibration list.

According to another aspect, the present invention provides a method of generating a hierarchical list of calibration settings for a control loop of an amplifier system. The list includes a plurality of elements, each element has a calibration setting and a corresponding set of parameters corresponding to operating conditions of the amplifier system, and has a hierarchical structure including at least two levels. The method includes selecting an element in a first level of a calibration list. The method also includes determining the element in the first level of the calibration list that has a corresponding operating condition most similar to the selected element, and demoting the earliest of the two elements to a lower level of the hierarchical calibration list.

In a preferred embodiment of the method of generating a hierarchical list of calibration settings for a control loop of an amplifier system, determining an element in the calibration list having a corresponding operating condition most similar to the selected element includes determining to a first level of the calibration list The distance measure of the working condition of each remaining element of , and select the element with the smallest distance. The method may also include determining whether two elements are redundant, wherein earlier elements are demoted only if the elements are redundant. The method may also include repeating list processing for each level of the hierarchical list. The method may also include deleting earlier entries if the list maintenance process is at the lowest level of the hierarchy. Demoted elements are preferably associated as subset list entries of non-demoted redundant elements. Elements that are demoted and have subset lists are preferably merged with the subset lists of redundant elements that are not demoted.

According to another aspect, the invention provides a method for controlling an amplifier system having a control loop comprising a control loop input, a first signal path, a second signal path and a control loop output , at least one of the first and second signal paths includes an amplifier. The method includes providing a hierarchical list of calibration settings having at least two levels, each calibration setting having an associated operating condition and some or all of the calibration settings in the highest level having a subset of calibration settings in the lower levels. The method also includes detecting current operating conditions of the amplifier system and comparing the current operating conditions to those in the highest level of the hierarchical list of calibration settings. The method also includes selecting, at the highest level of the list, the calibration setting associated with the most similar operating conditions. The method also includes comparing the current operating conditions to those of the selected subset of the highest-level calibration settings, and selecting the calibration settings of the subset with the most similar operating conditions. The method also includes selecting a calibration setting having an operating condition most similar to the current operating condition in a higher or lower level.

In a preferred embodiment, the method for controlling an amplifier system comprises repeating the process for each level of the hierarchical list until the next lower subset is empty. The highest level preferably has a coarser calibration set interval than the lower levels. For example, any two calibration settings may have a certain calibration distance, with the highest level having a larger calibration distance between settings than the lower level. Calibration distances may include weighted differences between adjacent settings. For example, adjustment settings may include gain adjustment and phase adjustment settings, and weighting may include calibration sensitivity.

Other features and advantages of the invention are set forth in the following detailed description.

Brief introduction to the drawings

Fig. 1 is a block schematic diagram of a feedforward power amplifier according to the present invention.

Fig. 2 is a block schematic diagram of the control system of the feedforward amplifier system according to the present invention.

Figure 3 is a flow diagram of a processing control algorithm according to the present invention illustrating the pruning of the calibration list to remove outdated elements.

Figure 4 is a flow diagram of a processing control algorithm according to the present invention illustrating a second pruning of the calibration list to remove outdated elements.

5 is a flow diagram of a process control algorithm in accordance with the present invention illustrating loop calibration control processing including the interaction between calibration list processing and adaptive controller processing.

Figure 6 is a flow chart of a process control algorithm in accordance with the present invention illustrating the generation of a hierarchical calibration list structure.

Fig. 7 is a schematic illustration of a hierarchical alignment list structure prior to demoting an entry.

Figure 8 is a schematic illustration of the hierarchical calibration list structure after demoting an entry.

A detailed description

1 and 2 show block diagrams of a feedforward compensated power amplifier (PA) system according to a preferred embodiment of the present invention. Figure 1 illustrates the basic feed-forward amplifier, and Figure 2 illustrates the control system.

As shown in Figure 1, feed-forward amplifiers have a traditional architecture with two control loops. Loop 1 includes signal input 103, sampling coupler 106, gain adjuster 108, phase adjuster 109, pilot signal input coupler 112, main amplifier 115, main sampling coupler 118, input test coupler 130, delay 133 . Eliminate combiner 136 and loop 1 test coupler 139 . Loop 2 includes main sampling coupler 118, main path delay 121, error coupler 124, carrier cancellation combiner 136, loop 2 gain adjuster 141, loop 2 phase adjuster 142, error amplifier 145, loop 2 Test coupler 148 and output 127 . As shown in FIG. 2, the control system may include a processor 202 implementing the adaptive controller and calibration list algorithm detailed below. Calibration lists are stored in appropriate memory 224 and are organized and accessed as detailed below. Alternatively, separate processors may be provided for the adaptive controller and calibration list functions. Digital-to-analog converters 204, 206, 208, 210 convert the loop 1 and loop 2 digital gain adjustment and phase adjustment control signals into analog signals α ₁ , φ ₁ , α ₂ , φ ₂ , which are provided to gain and phase Regulators 108, 109, 141 and 142 (shown in Figure 1). The processor also receives input test data corresponding to the detected input power from input test coupler 130 via power detector 214 and A/D converter 212, and from loop 1 test coupler 139 via power detector 218 and A/D converter 218. D-converter 216 receives loop 1 test data. The (optional) pilot signal generator 220 generates a pilot signal, which is provided to the pilot signal input coupler 112 (shown in FIG. 1 ). Pilot signal receiver 222 detects any uncancelled pilot signal from loop 2 test coupler 148 (shown in FIG. 1 ) and provides the detected pilot signal to processor 202 in digital form. The processor also receives input corresponding to current operating conditions of the amplifier system. For example, inputs 226 , 228 , 230 may be provided for temperature, DC power, and the carrier frequency of the incoming RF signal, which are converted to digital form by analog-to-digital converters 236 , 238 , 240 . Other operating condition inputs 234 may be provided and converted to digital form by A/D converter 242 .

The general operating principle of the feed-forward amplifier system is first described. An adaptive feedforward amplifier control system provides fast convergence properties by memorizing and reusing successful calibration settings. The system is able to learn the effect that operating conditions such as temperature, DC power supply, input power level and carrier frequency have on the optimal calibration settings. As a result, an adaptive feedforward amplifier control system can respond to changes in these operating conditions more quickly than an adaptive controller acting alone.

More specifically, the processor's adaptive controller function measures the offset of the feed-forward loop and adjusts the calibration. The controller iteratively adjusts the calibration volume, searching for the minimum calibration error. During the brief period when the search is not completed, the loop misalignment degrades the performance of the feed-forward power amplifier and provides too much instantaneous energy, degrading the linearity of the system. A good initial calibration is desirable in order to minimize transient degradation. Also, a good initial calibration reduces the chance of instability caused by a diverging adaptive controller.

In order to obtain a good initial calibration, the feedforward power amplifier control system processor 202 maintains a list 224 of previous successful calibration settings. Before requesting use of the adaptive controller, processor 202 searches the calibration list for past calibration settings used under similar operating conditions. Operating conditions are represented as multidimensional attribute vectors. The attribute 'distance' defined below is calculated by comparing the current attribute with the stored value. In some cases, an initial calibration from the list will provide enough cancellation to avoid the use of an adaptive controller.

The property distance is also used to predict the state change of the feed-forward power amplifier system. The feed-forward power amplifier system adjusts its tasking when a sudden change in the current property vector is detected. (Note that the term 'current' is used throughout the document to describe the 'current time' and should not be interpreted as 'current'). Maintenance tasks are exited in favor of loop calibration, with loop 1 calibration receiving the highest priority.

Loop 1 calibration involves control of the adjustment settings of gain adjuster 108 and phase shifter 109 represented by α ₁ and φ ₁ , gain adjuster 108 may be a voltage variable attenuator (VVA) or a controlled gain device, which Used to minimize the power detected at 'loop 1 test ε(t)' in Figure 1. The adaptive control function of loop 1 may use conventional techniques, or may use the methods of US Patent Application No. 10/733087, filed December 11, 2003, the disclosure of which is incorporated herein by reference in its entirety. For loop 2, the calibration control minimizes the intermodulation (IMD) traces detected at 'loop 2 test, z(t)'. Optionally, a pilot tone 112 is injected before the main amplifier 115 as a known distortion, making loop 2 calibration control easier. For example, the use of a pilot tone for loop control of a feed-forward power amplifier (PA) is disclosed in US Patent No. 5,796,304 to Gentzler et al., the disclosure of which is incorporated herein by reference. This loop 2 adaptive control function may use conventional techniques or, alternatively, the method of US Patent Application No. 10/733498, filed December 11, 2003, the disclosure of which is incorporated herein by reference in its entirety . In this document, the gain adjuster and phaser values are referred to as 'calibration settings'. The offset of gain adjuster and phaser settings from their optimum values is called 'offset'. When the deviation from the optimal setting is small, the loop is said to be 'calibrated'.

There are many operating conditions that affect the optimum calibration settings. These include environmental conditions, application-specific conditions, and system-specific conditions. For example, temperature, input power level x(t), carrier frequency, and DC supply voltage are relevant operating conditions for most applications, and these are provided as inputs 226, 214, 230, and 228, as shown in FIG. Time can also be considered as a parameter affecting calibration, as components age. The loop 1 calibration settings can also affect the optimal loop 2 calibration settings. These parameters are measurable and can be monitored by processor 202 in a feedforward power amplifier system. In the control system of the present invention, the relevant measurable parameters are used to form property vectors. The property vectors and associated settings when the loop is calibrated are stored in a calibration list in memory 224 . The correlation of attribute vectors with calibration settings is achieved through calibration lists. The disclosed system combines calibration list processing with adaptive controller processing to allow fast loop calibration.

Next, a detailed embodiment of the present invention will be described with reference to FIGS. 1-8.

First, a preferred embodiment of the manner in which attribute vectors and calibration settings are represented and stored in a calibration list is described. A distance metric for measuring the similarity of attribute vectors is also described. A procedure for pruning the calibration list is also discussed to allow maintaining a manageable number of list entries while maintaining coverage of the attribute space. Next, the use of the calibration list and the adaptive controller to adjust the loop settings is described. Self-generation of elements within the calibration list is also described. Alternative calibration list structures based on hierarchical list structures are then described in conjunction with FIGS. 6-8.

As mentioned above, attribute parameter sets or attribute vectors are employed to allow the feed-forward power amplifier system to learn from past actions. By relating the attribute vectors to past calibration settings, loop convergence becomes faster and more robust. For a given application, the property parameters that affect the calibration of the feedforward power amplifier system are determined when defining the property vector. Temperature, average input power, and center frequency are often important parameters for cellular applications because they are time-varying and affect gain. Parameters such as modulation format and number of carriers also affect gain; however, if these parameters do not vary with time, they provide small values as part of the property vector and can be excluded from the property vector.

To determine the similarity of two attribute vectors, a distance metric is employed. The differences (squared or absolute) between the parameters are weighted according to their sensitivity to the gain of the feed-forward power amplifier system. That is, parameters that have a greater influence on the gain of the feed-forward power amplifier system are weighted more. These sensitivities can be estimated using part specifications or experimentation with specific implementation and application requirements.

Specifically, let the attribute vector with respect to calibration list element 'n' be defined as

(Formula 1) p(n)=[p ₀ (n)...p _m (n)]

where p _k is the value of an attribute parameter 'k' (eg temperature). In order to facilitate the implementation of the distance metric on the processor 202, a weighted _Linf norm can be used: that is, the distance between an element 'n' and '0' denoted by d _attr (n, 0) is defined for:

(Formula 2) d _attr (n, 0)＝max _k {w _k ·|p _k (n)-p _k (0)|}

where w _k is the weight of parameter 'k'. Alternative distance metrics, such as the _L2 norm, may also be employed. The weighted _L2- norm measure of the distance between element 'n' and '0' denoted by d _attr (n,0) is defined as:

(Formula 3) $d_{\mathrm{attr}}(\mathrm{no},0)={\left[\underset{k}{\mathrm{\Σ}}\right\{w_k\·{|p_k\left(\mathrm{no}\right)-p_k\left(0\right)|}^2\left\}\right]}^{0.5}$

It is also valuable to measure the similarity of the calibration settings. The L _inf norm can be used again:

(Formula 4) d _align (n, 0) = max _k {s _k |Δ _k (n, 0)}

where Δ ₀ (n, 0) = VVA(n)-VVA(0), Δ ₁ (n, 0) = PHS(n)-PHS(0), where VVA(n) is the nth gain calibration setting , PHS(n) is the nth phase alignment setting, and _sk is the corresponding sensitivity. The sensitivity term can be chosen such that the distance _dalign corresponds to the cancellation residual when [VVA(0), PHS(0)] is the best alignment. Alternative distance measures such as the _L2 norm can also be employed again. The _L2 norm measure of d _align is:

(Formula 5) $d_{\mathrm{align}}(\mathrm{no},0)={\left[\underset{k}{\mathrm{\Σ}}\right\{{\mathrm{the\; s}}_k\·{\left|{\mathrm{\Δ}}_k(\mathrm{no},0)\right|}^2\left\}\right]}^{0.5}$

The calibration list structure can be dynamic. Both list entries and number of entries can be changed dynamically. More specifically, along with the attribute vectors, the processor 202 keeps track of past successful calibration settings. Before implementing the adaptive controller function, the processor 202 tests the residual of the current calibration settings. If it is sufficient in terms of mass removal, no action is required. If it is not sufficient, the calibration set of the listed elements with the attribute vector closest to the current operating condition is retrieved. The newly set residuals are then tested. If the cancellation is still insufficient, the adaptive controller generates new calibration settings. After a given loop has converged, by means of the adaptive controller, new calibration settings are added to the calibration list.

It is important to limit the number of elements in the calibration list to limit processor computation time. The easiest way is to put an upper limit on the number of elements, and if a new calibration setting is received, the oldest elements are overwritten. An alternative is to use pruning. Pruning may be performed when processor 202 is not busy processing priority commands. Pruning removes redundancy by removing earlier elements when their calibration settings are nearly identical to their neighbors (that is, the neighbors of element 'k' have the lowest d _attr (n, k), and if d _align (n, k) is small, it is considered redundant). As a result, the number of elements required to represent a region of the attribute space is proportional to the variation in the calibration setting. This list-based approach yields the most compact representation of attribute calibration maps.

By way of example only, one implementation of the calibration list pruning process flow is represented in FIG. 3 . As shown, process flow begins at 302 when the processor 202 is not occupied by higher priority tasks. Process flow proceeds to 304 to select the earliest element from the calibration list. Next, processing utilizes the attribute vector distance metric d _attr as defined above to determine the closest element to the remaining elements of the list by first computing the distances to the remaining elements of the list at 306 and then selecting the element with the smallest distance at 308 . Next, at 310, the process flow calculates the alignment distance _dalign as defined above to select the closest element. If the calibration settings are the same or close enough, then at 312 the earlier elements are deleted. Getting close enough depends on the cancellation tolerance and the sensitivity of the cancellation to the corresponding calibration setting (gain adjuster or phase shifter). For example, a minimum distance d _redundant may be taken, and if d _align is less than or equal to d _redundant , the earlier element is deleted, while if d _align is greater than d _redundant , both elements are kept. At 314, the process flow checks to see if the entire list has been checked, and if not, repeats until the remaining elements are far enough apart in terms of gainer and phaser tolerances (or until the lower limit of the list size is reached).

The second pruning process is shown in Figure 4. This can be used to remove outdated elements from the calibration list. For example, component aging can change the relationship between optimal calibration settings and a given property vector. Small inter-element attribute distances potentially identify stale elements. For example, if the elements have incorrect calibration settings, the residual error will be too large, necessitating the use of the adaptive controller function. The adaptive controller will find the new calibration settings. As a result, two different calibrations will be listed for a given attribute vector (or two very similar vectors). Removing the earlier elements resolves any conflicts and keeps the list current.

Referring to the particular process flow shown in FIG. 4 for this second pruning process, the second pruning process begins at 402 when the processor is not executing higher priority tasks (including the first pruning process). At 404, the process flow selects the earliest element in the calibration list that has not been processed for the second pruning. At 406, the distance d _attr to each of the remaining elements in the list is calculated for this earliest element. Next at 408, processing determines the element having the smallest distance d _attr to the earliest element. If this minimum distance is less than or equal to the predetermined distance _doutdated , then at 410, processing deletes the earlier of the two elements from the calibration list. However, if the minimum distance is greater than d _outdated , the two elements are considered sufficiently different and both remain in the calibration list. At 412, process flow checks to see if there are more elements to check, and if so, process flow returns to 404 to check the next oldest element in the list. The second pruning process exits at 414 when all elements have been checked.

In case the calibration list is empty, the default calibration settings for each loop can be kept. Also, the factory default calibration setting can be kept separately so that it is not clipped.

Next, referring to FIG. 5 , loop calibration control processing will be described. Each calibration loop control process (ie, loop 1 control and loop 2 control) has a similar structure, which is depicted in FIG. 5 . The feedforward power amplifier system preferably maintains separate calibration lists for each loop (loop 1 and loop 2 described above with respect to Figure 1).

As generally represented in Figure 5, the loop calibration algorithm uses two parallel processes 500 and 501 and a process flow that controls the interaction between the two processes. The first process indicated at 501 generates new calibration settings for initial use in adaptive loop controller process 500 . This process 501 is used at startup and runs continuously thereafter to allow for controller processing when the amount of offset becomes too large. More specifically, the first process flow 501 continuously monitors the current working condition parameters to obtain the current attribute vector, as shown in 504 . For example, a processor may determine current temperature, DC power, carrier frequency, and input power to determine a current property vector for current operating conditions. Then, at 506, the process flow examines the calibration list for the list element with the smallest distance from the attribute of the current vector. This list element calibration setting (loop 1 or loop 2 calibration setting) is then retrieved at 510 . Whether this retrieved calibration setting is used to update the calibration of the adaptive controller process 500 can be controlled by the offset measurement process 514 and the changes in the retrieved elements. At startup, whenever the measured offset is too large, the calibration list settings are retrieved and used to update the calibration at 512 . The adaptive controller process 500 is then initiated, and the adaptive controller then calculates calibration correction values for either loop 1 or loop 2 at 516 using an iterative controller algorithm, as previously described.

For example, this process flow can be controlled by status flags. If the element retrieved at 510 changes, the status flag is cleared to indicate that the system is 'not iterating'. The process flow also continuously measures 514 the amount of offset. If the amount of offset is too large, check the status flags. If the flag is cleared, a significant change in the property vector is indicated and the calibration is updated with the new settings retrieved from the calibration list. After retrieving the new settings, the status flag is set to start the iterative adaptive controller process. If the flag has been set, then the adaptive controller process 500 is requested, which calculates the loop offset at 516 from the difference adjustment. The calibration is updated at 518 and the iterative process is repeated until the loop is calibrated. If the measured misalignment is small after the adaptive controller process is complete, then at 520 the calibration settings and current property vector are stored in a calibration list. Then complete the loop calibration routine. At this point, different loops can be calibrated or the various calibration lists can be pruned as described above. On system shutdown or after a timeout, the best calibration settings for the session can be selected at 522, which can be stored and used to initiate a fast boot.

Referring to Figures 6-8, an additional optional feature of calibration list processing is illustrated using a hierarchy of elements. In the above methods, there is a tradeoff when choosing the number of elements in the list. The benefit of allowing a large number of elements is dense coverage of the attribute space. However, the benefit of a small number of elements is that it takes less time to determine the element with the smallest distance from the attributes of the current vector. Utilizing a hierarchical calibration list structure allows for dense coverage and fast searching.

The use of pruning can be used to generate a hierarchical calibration list, as shown in Figures 6-8. In the aforementioned redundancy pruning process, when two elements are determined to be "similar", the earlier element becomes redundant and is removed, leaving only the other element surviving. In the hierarchical approach to list management, redundant elements are not removed; rather, they are "demoted" to a lower-level subset below the surviving elements. The creation of low-level subsets is recursive, which allows defining as many levels as desired. (Zero or one subset rank is sufficient in most cases).

The basic pruning process flow used to generate the hierarchical calibration list is shown in FIG. 6 . At 602, pruning begins when the processor is not occupied by higher priority tasks. At 604, redundant pairs of entries are identified. This process 604 may follow the same distance calculations (at 304, 306, 308, and 310) described in connection with FIG. 3 . At 606, the earlier entry is marked as redundant and the other is maintained at its current level in the hierarchical list structure. At 608, the pruning process flow checks whether the list is at the lowest level. If yes, then at 610 redundant entries are deleted. If the list is not at the lowest level, then at 612 the redundant entry is demoted to the next lower level. Its position in the lower rank is marked as a sublist entry of the surviving entry in the higher rank. Next, the pruning process flow goes to the next lower level at 614 and processing 602 begins at that level for the current entry sublist.

Redundant elements can have subsets. This type of hierarchy is shown in FIGS. 7 and 8 . Before a redundant element 710 is demoted, its lower level subset list 714 is merged (at the same level) with the surviving element's subset list 712 . Redundant elements 710 are then demoted to a lower level 704 . From the immediate point of view, merging subsets at the same level makes the subset too large; however, when the pruning process is initiated, the subset is too small to return to the desired value.

In the hierarchical list, the highest rank 702 preferably has the coarsest sampling (maximum value of the threshold _dalign ). Lower levels have finer resolution (smaller d _align ). By adjusting the threshold _dalign for each class, it is possible to adjust the number of entries in a given alignment list for a given class. By increasing d _align , the number of entries in the list decreases. It is desirable that all lists at various levels of the hierarchy have approximately equal numbers of entries.

The loop calibration process flow for the hierarchical list may generally follow process flow 501 of FIG. 5 as described above. However, the search at 506 for elements with the minimum attribute distance is restricted to one set; this set is called 'valid'. When the search 506 begins, the highest level set is available. A search for the calibration set closest to the current attribute vector first finds the closest entry in the top-ranked list, and then searches the list of subsets of that entry. Once the element within the highest rank with the smallest attribute distance is identified, its calibration is retrieved (as described above in connection with Figure 5). However, instead of requesting an adaptive controller as described above, the next lower-level subset of elements becomes available. The next lower-level subset is then searched for elements with the smallest attribute distance, and a new calibration set is retrieved. The process repeats recursively until the next lower subset is empty. At this point, the adaptive controller is requested and uses the retrieved calibration settings. This process is repeated recursively until the lowest level is reached. If any intermediate setting provides sufficient calibration quality, the process aborts before requesting the adaptive controller. Note that the subset search should include the parent entry (or multiple parents if there are two or more levels below the highest level), since it is likely to be the best match.

The complexity of the search time is proportional to the product of the number of entries per list (N) and the number of levels in the hierarchy (L). In contrast, exhaustive search scales with N to the L power, which is usually significantly higher. (This assumes that each list has N entries. Note that level L has N times as many subset lists as level L-1, which means that level L has N^L entries in total).

Note that elements removed as part of the second pruning process described in connection with Figure 4 are out of date and therefore should not be kept in lower level collections.

In summary, the disclosed system combines multidimensional calibration list processing with adaptive controller processing to calibrate the control loop of a feedforward amplifier system. These two processes are combined in a novel way to improve the dynamic response of the system. The multi-dimensional calibration list used in the disclosed method has a different structure than array-based look-up tables, thereby avoiding the above-mentioned problems of look-up tables. Instead of using the array structure to store elements, the elements are gathered together as a set. Each element has the following: (a) a set of parameters or attributes corresponding to the operating conditions affecting the amplifier; and (b) an optimal calibration setting to find under those operating conditions. Forms a metric that defines the 'distance' between two elements, based on the difference between the element's attributes. If a feed-forward system detects a misalignment in a given loop, a property associated with the current operating condition is measured. Next, the element in the calibration list with the smallest distance to the current attribute is identified and its corresponding calibration setting is retrieved from memory. If the new calibration settings are not appropriate, an adaptive controller is activated to further improve the loop calibration. Once the calibration is deemed to be close enough to the optimal value, the calibration settings are combined with the current attributes to form a new element within the set. Therefore, the calibration list is self-generated.

In order to limit the computational complexity of searching for minimum distance elements, the size of the element set needs to be limited. To identify redundant elements, the similarity of elements is measured in terms of attribute distance and calibration separation (performed during idle time). If the collection size exceeds a preset number, the earliest element in a pair of similar elements is deleted. By limiting the list size, the time to determine the element with the smallest distance to the current attribute is manageable. If additional elements are required for greater coverage, it is possible to form a list-level hierarchy. Instead of removing the earliest similar elements, they are 'relegated' to a lower-ranked subset below the surviving elements. If the downgraded element contains its own lower-ranked subsets, they are merged with the surviving element's subset. The creation of lower-level subsets is recursive, which allows defining as many levels as desired. (Zero or one subset rank is sufficient in most cases).

The hierarchical structure allows the search for minimum distance elements to be computationally efficient because each effective set has a finite size. Initially, the highest-ranked set is valid. If the element with the smallest distance in the highest-ranked set does not yield enough calibrations, then its subset (if any) becomes valid. The minimum distance matches from the lower rank subsets are tested for calibration quality. The subsets are searched recursively until the calibration quality is sufficient or the next lower subset is empty. For the latter case, an adaptive controller will be activated to improve calibration.

The combined operation of the adaptive feed-forward controller and the disclosed calibration list enables the disclosed amplifier system to learn from its experience and improve its performance in the presence of dynamic conditions, including, for example, varying input power levels. Level, changing (or jumping) carrier frequency, changing temperature or DC power or component aging. The system can accommodate any number of attributes (multidimensional index spaces) without significantly increasing complexity. Hierarchical collection management allows storing arbitrarily large numbers of elements without significantly increasing worst-case latency in finding the element with the smallest attribute distance.

Another advantage of the method of regulation for the first loop (carrier cancellation) is that the transient power through the error amplifier that occurs when the input power suddenly increases is minimized, thereby reducing the possibility of damage. This is especially important for amplifier designs that use a small error amplifier. For tuning of the second loop in unguided implementations, this approach provides a stable default value when false detections are not possible. This is another advantage for some amplifier designs.

An added benefit of the disclosed calibration list is that the attribute space can be non-uniformly sampled. In general, the sensitivity of gain and phase settings varies in the attribute space. The disclosed system of course intentionally produces higher elemental densities in regions of higher sensitivity.

The calibration list provided by the present invention is useful for dynamic waveforms. For example, one application of a feedforward power amplifier system is for frequency hopping beacons. In such applications, calibration settings for each carrier can be stored to allow fast frequency hopping (relative to typical PA setup times).

In view of the foregoing, it will be appreciated that the present invention provides a number of advantageous features. The use of the calibration list in conjunction with adaptive controller processing provides fast calibration of the loop within a feed-forward power amplifier system. The autogenous nature of the calibration list allows the system to learn from past experience, reducing the search time required for an adaptive controller. Transient energy surges are reduced, and the adaptive controller becomes more robust when good initial estimates of gain and phase settings are obtained. The hierarchical structure of the calibration list allows the search for minimum distance elements to be computationally efficient while providing broad coverage of the attribute space.

The present invention has been described in connection with presently preferred embodiments, however, those skilled in the art will appreciate that various modifications, too numerous to describe, may be made while remaining within the scope of the invention. Accordingly, the foregoing detailed description should be regarded as illustrative only, and not restrictive.

Claims (50)

1. A feed-forward amplifier system comprising: an input terminal for receiving an RF input signal; a first control loop coupled to the input and comprising a main amplifier, a main amplifier sampling coupler, a delay element, and an elimination combiner; a second control loop coupled to the first control loop and comprising a first signal path, a second signal path including an error amplifier, and an error coupler coupling the first and second signal paths; coupled to the output of the error coupler; and means for controlling at least one of said first and second control loops employing a calibration list having a plurality of list elements, each element having a calibration setting and a parameter characterizing an operating condition of said feedforward amplifier system set. 2. The feedforward amplifier system of claim 1 , wherein the first control loop further comprises a gain adjuster and a phase adjuster, and each of the calibration settings comprises a first control loop gain adjustment regulator setting and the first control loop loop phase regulator setting. 3. The feedforward amplifier system of claim 1 , wherein the second control loop further comprises a gain adjuster and a phase adjuster, and each of the calibration settings comprises a second control loop gain regulator setting and second control loop phase regulator setting. 4. The feedforward amplifier system of claim 1 , wherein the set of parameters characterizing the operating conditions of the feedforward amplifier system includes one or more of temperature, DC power supply, input signal power, and input signal carrier frequency. 5. The feedforward amplifier system of claim 4, wherein the set of parameters characterizing the operating conditions of the feedforward amplifier system is defined as a property vector, and a distance is defined between any two property vectors. 6. feed-forward amplifier system as claimed in claim 5, is characterized in that, described means for controlling obtains current attribute vector and calculates the distance to the attribute vector of list element, selects the list element with minimum distance, is used as Calibration settings in the control function. 7. A feedforward amplifier system as claimed in claim 6, wherein said means for controlling continuously measures the miscalibration of the feedforward amplifier system, when said measured miscalibration exceeds a predetermined value, from said Retrieve the calibration settings from the calibration list described above. 8. The feed-forward amplifier system of claim 7, wherein the means for controlling employs the selected elements as initial calibration settings and calculates new calibration settings from the initial settings using an iterative control algorithm . 9. The feedforward amplifier system of claim 8, wherein said means for controlling updates said calibration list with new calibration settings after completion of said iterative calculation. 10. The feed-forward amplifier system of claim 5, wherein the distance between the closest list element attribute vectors varies throughout the list. 11. An adaptive controller for controlling a loop of an amplifier system, comprising: one or more inputs for receiving one or more property parameters corresponding to current operating conditions of the amplifier system; and one or more processors coupled to the one or more inputs and having associated calibration lists and programmed with the calibration list algorithm and the controller algorithm to provide loop tuning settings to control the loop of the amplifier system , wherein the calibration list algorithm generates the list with the adjustment settings calculated by the controller algorithm and associates one or more property parameters with each adjustment setting. 12. The adaptive controller for controlling a loop of an amplifier system as recited in claim 11 , wherein said calibration list algorithm selects calibration settings from said calibration list for said controller algorithm to Used at startup or when the loop becomes sufficiently detuned. 13. The adaptive controller for controlling a loop of an amplifier system as recited in claim 12, wherein said calibration list algorithm calculates the sum of one or more property parameters corresponding to current operating conditions and the Each calibration sets the distance between associated property parameters, and selecting the calibration set corresponding to the property parameter with the smallest distance selects the calibration list adjustment set for use by the controller algorithm. 14. The adaptive controller for controlling a loop of an amplifier system as claimed in claim 12, wherein the distance calculation weights different property parameters with different weights. 15. The adaptive controller for controlling a loop of an amplifier system as claimed in claim 11 , wherein said property parameter comprises one of temperature, DC supply voltage, input signal power and input signal carrier frequency or more. 16. An adaptive controller for controlling a loop of an amplifier system as claimed in claim 14, characterized in that the distance d _attr between two attribute parameter sets "n" and "0" is given by the weighted L _inf Norm distance metric or weighted L2 _- norm distance metric to define. 17. The adaptive controller for controlling a loop of an amplifier system as recited in claim 11 , further comprising one or more inputs for receiving calibration data. 18. The adaptive controller for controlling a loop of an amplifier system according to claim 17, wherein said one or more inputs for receiving calibration data comprises a pilot signal input. 19. The adaptive controller for controlling a loop of an amplifier system according to claim 17, wherein said one or more inputs for receiving calibration data includes an input for loop test data end. 20. The adaptive controller for controlling a loop of an amplifier system as recited in claim 11 , wherein said adjustment settings include gain adjuster and phase adjuster settings. 21. A method for controlling an amplifier system having a control loop comprising a control loop input, a first signal path, a second signal path and a control loop output, the first and second At least one of the signal paths includes an amplifier, the method includes: Provides a list of calibration settings, each with associated operating conditions; Current operating conditions of the sense amplifier system; compare current operating conditions to those in the calibration set list; and Select the calibration setup that correlates to the most similar operating condition from the list. 22. The method for controlling an amplifier system according to claim 21, wherein the relevant operating conditions are configured as a multi-dimensional property vector. 23. A method for controlling an amplifier system as claimed in claim 22, wherein said comparing comprises measuring the distance between the current attribute vector and each attribute vector in the list. 24. A method for controlling an amplifier system as claimed in claim 23, wherein said selecting comprises determining the property vector having the smallest distance from the current operating condition property vector. 25. The method for controlling an amplifier system of claim 21 , further comprising calculating a new calibration setting using an iterative loop controller algorithm, wherein the calibration setting associated with the most similar operating condition is Used as an initial calibration setting for the adaptive loop controller algorithm. 26. The method for controlling an amplifier system of claim 25, further comprising updating the calibration list with new calibration settings calculated by the adaptive loop controller algorithm. 27. The method for controlling an amplifier system of claim 21 , wherein the size of the calibration list is dynamic. 28. A method for controlling an amplifier system as claimed in claim 23, wherein the intervals of the stored adjustment settings defined by the property vector distances vary throughout the list. 29. A method for controlling an amplifier system as claimed in claim 28, characterized by setting a higher density of adjustment settings in the region of the list where calibration is most sensitive to one or more operating conditions comprising property vectors. 30. A method of maintaining a list of calibration settings for a control loop of an amplifier system, said list comprising a plurality of elements each having a calibration setting and a set of parameters corresponding to operating conditions of the amplifier system, said method comprising : select elements of the calibration list; Identify elements of the calibration list that have corresponding operating conditions most similar to those of the selected element; determine whether two elements are similar enough to be considered redundant; and If two elements of the calibration list are redundant, the oldest of the two elements is deleted. 31. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 30, wherein said step of selecting elements of the calibration list includes selecting the oldest element in the list that has not previously been subjected to list maintenance processing. element. 32. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 30, wherein said step of determining an element in the calibration list having a corresponding operating condition most similar to the selected element comprises A distance measure is determined to the operating condition parameter value for each remaining element of the calibration list and the element with the smallest distance is selected. 33. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 32, wherein said distance metric comprises a weighted difference between parameter values corresponding to operating conditions. 34. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 33, wherein said parameters corresponding to operating conditions of the amplifier system include temperature, DC power, input signal power, and input One or more of the signal carrier frequencies. 35. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 30, wherein said step of determining whether elements are similar enough to be considered redundant comprises determining and compares the calibration distance to the redundancy distance threshold. 36. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 32, wherein said step of determining whether elements are similar enough to be considered redundant comprises combining the work of two elements The distance between conditional parameters is compared with the expired distance threshold. 37. The method of maintaining a list of calibration settings for a control loop of an amplifier system as recited in claim 30, further comprising repeating said list maintenance process for each element in the calibration list. 38. A method of generating a hierarchical list of calibration settings for a control loop of an amplifier system, said list comprising a plurality of elements, each element having a calibration setting and a corresponding set of parameters corresponding to operating conditions of the amplifier system, wherein Said list has a hierarchical structure comprising at least two levels, said method comprising: Select an element in the first level of the calibration list; identifying elements in the first level of the calibration list that have corresponding operating conditions most similar to the selected element; and Demotes the earliest of the two elements to the lower level of the hierarchical calibration list. 39. The method of generating a hierarchical list of calibration settings for a control loop of an amplifier system as recited in claim 38, wherein said determining the element in the calibration list having a corresponding operating condition most similar to the selected element The steps include determining a distance measure to the operating condition of each remaining element of the first level of the calibration list, and selecting the element with the smallest distance. 40. The method of generating a hierarchical list of calibration settings for a control loop of an amplifier system as recited in claim 38, further comprising determining whether two elements are redundant, wherein said earlier element is only These elements are only demoted if they are redundant. 41. The method of generating a hierarchical list of calibration settings for a control loop of an amplifier system as recited in claim 38, further comprising repeating said listing process for each level of the hierarchical list. 42. The method of generating a hierarchical list of calibration settings for a control loop of an amplifier system as claimed in claim 41 , wherein if the list maintenance process is at the lowest level of the hierarchy, said earlier entry is deleted. 43. The method of generating a hierarchical list of calibration settings for a control loop of an amplifier system as recited in claim 38, wherein said degraded elements are included as subset list entries of non-degraded redundant elements associated. 44. The method of generating a hierarchical list of calibration settings for a control loop of an amplifier system as claimed in claim 43, wherein the elements that are degraded and have a subset list are a subset of the redundant elements that are not degraded List merged. 45. A method for controlling an amplifier system having a control loop comprising a control loop input, a first signal path, a second signal path and a control loop output, the first and second At least one of the signal paths includes an amplifier, the method includes: providing a hierarchical list of calibration settings having at least two levels, each calibration setting having an associated operating condition and some or all of the calibration settings in the highest level having a subset of calibration settings in the lower levels; Current operating conditions of the sense amplifier system; Comparing the current operating conditions with those at the top of the hierarchical list of calibration settings; Select the calibration setting associated with the most similar operating conditions at the highest level of the list; Comparing current operating conditions to those operating conditions in a subset of the selected top-level calibration set; selecting a calibration set that has a subset of the most similar operating conditions; and Select the calibration setting that has the operating conditions most similar to the current operating conditions at a higher or lower level. 46. The method for controlling an amplifier system of claim 45, further comprising repeating the process for each level of the hierarchical list until the next lower level subset is empty. 47. A method for controlling an amplifier system as claimed in claim 45, wherein the highest level has a coarser calibration set interval than the lower level. 48. A method for controlling an amplifier system as claimed in claim 47, wherein any two calibration settings have a calibration distance and the highest level has a greater calibration distance between the settings than said lower level distance. 49. The method for controlling an amplifier system of claim 47, wherein the calibration distance is a weighted difference between adjacent settings. 50. A method for controlling an amplifier system as claimed in claim 49, wherein the adjustment settings are gain adjustment and phase adjustment settings and the weighting is calibration sensitivity.

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