JP3765402B2 - Distortion compensation amplifier and control method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a distortion compensation amplifier that detects distortion generated in a main amplifier by a distortion detection loop and outputs an amplified signal with very little distortion by removing the detected distortion from the output signal of the main amplifier by a distortion removal loop. About.
[0002]
[Prior art]
This type of distortion compensation amplifier can be used to amplify a signal having a plurality of carriers in a predetermined band because it can amplify an input signal over a wide band without generating very little distortion. ing.
[0003]
FIG. 17 shows the configuration of a conventional distortion compensation amplifier.
[0004]
Referring to FIG. 17, the distortion compensation amplifier according to the conventional example includes a distortion detection loop 901, a distribution synthesizer 911, a distortion removal loop 921, a distribution synthesizer 931, a distributor 941, a first pilot signal level detection unit 951, a first A control unit 954, a second pilot signal level detection unit 961 and a second control unit 964. The distortion detection loop 901 includes a first pilot signal generator 902, an adder 903, a distributor 904, a first vector adjuster 905, a main amplifier 906, a second pilot signal generator 907, an adder 908, and a first. Delay signal path 909. The distribution synthesizer 912 includes a distributor 912 and a synthesizer 913. The distortion removal loop 921 includes a second delay signal path 922, a distributor 923, a second vector adjuster 924, and an error amplifier 925. The first pilot signal level detector 951 includes a first band filter 952 and a first detector 953. The second pilot signal level detector 961 includes a second band filter 962 and a second detector 963.
[0005]
The input signal is distributed by a distributor 904 into a main input signal and a sub input signal. Although the main input signal is amplified by the main amplifier 906, due to the nonlinearity of the main amplifier 906, the output signal of the main amplifier 906 includes a distortion component in addition to the signal component. The distributor 912 transmits the output signal of the main amplifier 906 to the second delay signal path 922 and transmits the attenuated output signal to the combiner 913. The combiner 913 subtracts the sub input signal from the attenuated output signal of the main amplifier 906. The gain, phase and delay time from the input terminal of the signal distributor 904 via the addition input terminal of the combiner 913 to the output terminal of the combiner 913 are the signal distributor 904 via the subtraction input terminal of the combiner 913. Is adjusted to be equal to the gain, phase and delay time from the input terminal to the output terminal of the synthesizer 913, the synthesizer 913 generates from the main amplifier 906, and the divider 912 and the synthesizer 913 Only attenuated distortion components are obtained.
[0006]
The gain of the error amplifier 925 of the distortion elimination loop 921 is such that the gain from the input terminal of the signal distributor 912 via the addition input terminal of the combiner 932 to the output terminal of the combiner 932 passes through the subtraction input terminal of the combiner 932. The gain from the input terminal of the signal distributor 912 to the output terminal of the combiner is adjusted to be equal. Then, the synthesizer 932 of the distribution synthesizer 931 subtracts the output signal of the error amplifier 925 from the output signal of the main amplifier 906, so that an amplified signal from which the distortion component is removed can be obtained.
[0007]
Here, the input signal that has passed through the main amplifier 906 and the input signal that has not passed through the main amplifier 906 must be equalized by the synthesizer 913, but the change in the characteristics of the main amplifier 906 over time, temperature The two cannot always be made equal because of dependency. Therefore, the first pilot signal generated by the first pilot signal generator 902 is added to the input signal by the adder 903 before the input signal is distributed by the distributor 904, and the distortion component extracted by the distributor 923 is obtained. And a signal including the pilot signal are supplied to the first band filter 952, only the first pilot signal is extracted by the first band filter 952, and the level of the first pilot signal is detected by the first detector 953. The first controller 954 controls the characteristics of the first vector adjuster 905 so that the level of the detected first pilot signal is lowered. The first vector adjuster 905 adjusts the amplitude and / or phase of the input signal to be amplified by the main amplifier 906 according to the control by the first controller 954. Note that the first delay signal path 909 delays the input signal, but this delay is provided to compensate for the delay of the input signal in the first vector adjuster 905 and the main amplifier 906. .
[0008]
Similarly, the distortion component that has passed through the error amplifier 925 and the distortion component that has not passed through the error amplifier 925 must be equalized by the synthesizer 932 of the distribution synthesizer 931. The two cannot always be made equal because of the change with time, temperature dependency, and the like. Therefore, before the output signal of the main amplifier 906 is distributed by the distributor 912, the second pilot signal generated by the second pilot signal generator 907 is added to the output signal by the adder 908, and the distributor 941 The extracted main signal and the signal including the second pilot signal are supplied to the second band filter 962, the second band filter 962 extracts only the second pilot signal, and the second detector 963 outputs the second signal. The pilot signal level is detected, and the second controller 964 controls the characteristics of the second vector adjuster 924 so that the detected second pilot signal level is lowered. The second vector adjuster 924 adjusts the amplitude and / or phase of the distortion component to be amplified by the error amplifier 925 in accordance with control by the second control unit 964. The second delay signal path 922 delays the output signal of the main amplifier 906. This delay is used to compensate for the distortion component delay in the second vector adjuster 924 and the error amplifier 925. Is provided.
[0009]
[Problems to be solved by the invention]
However, in general, a distortion compensating amplifier that is controlled using a pilot signal has a problem in that the detection accuracy is deteriorated due to a decrease in the pilot signal level after loop pull-in, and stable control cannot be performed. There is.
[0010]
To solve this problem, for example, as disclosed in Japanese Patent Laid-Open No. 10-233628, a variable attenuator is provided at the input of the pilot signal detector to improve control accuracy, speed up response, and stabilize operation. A method has been proposed. The technique disclosed in this prior art document is shown in a simplified manner in FIG. In order to optimize the S / N by providing an attenuator at the detector input, and further to optimize the control speed, a double operation of varying the perturbation width given by the control system to find the equilibrium state is performed. Yes. For this reason, there exists a problem that control is complicated.
[0011]
The main object of the present invention is to provide a distortion compensation amplifier that does not reduce the response speed of the characteristic adjustment of the vector adjuster at the time of pull-in, and that can stably obtain a large distortion compensation after pull-in.
[0012]
[Means for Solving the Problems]
According to a first aspect of the present invention, a distortion detection loop for detecting distortion generated in a main amplifier, a distortion removal loop for removing the distortion detected in the distortion detection loop from an output signal of the main amplifier, A distortion compensation amplifier comprising a pilot signal and adjusting means for adjusting the characteristics of the distortion removal loop, wherein the pilot signal is passed through the filter, and the level of the pilot signal detected from the final output signal is high There is provided a distortion compensation amplifier characterized in that the adjustment means includes a signal having a wide pass bandwidth and a narrow pass bandwidth when the level of the pilot signal detected from the final output signal is low.
[0013]
According to a second aspect of the present invention, a distortion detection loop for detecting distortion generated in a main amplifier, a distortion removal loop for removing the distortion detected in the distortion detection loop from an output signal of the main amplifier, An adjustment means for adjusting the characteristics of the distortion detection loop using a pilot signal, a filter that passes the pilot signal, and the level of the pilot signal detected by the distortion removal loop is A distortion compensation amplifier is provided in which the adjustment means has a wide pass bandwidth when it is high and a narrow pass bandwidth when the level of the pilot signal detected by the distortion elimination loop is low.
[0014]
According to a third aspect of the present invention, a main amplifier that amplifies an input signal, an adder that adds a pilot signal to an output signal of the main amplifier, and a distortion that detects a distortion component of the output signal of the main amplifier Detection means; a first delay signal path through which the output signal of the main amplifier to which the pilot signal is added; a first adjuster that adjusts an amplitude and / or phase of the distortion component; An error amplifier that amplifies the distortion component, the amplitude and / or phase of which is adjusted by one adjuster, and the pilot signal added to the output signal of the main amplifier that has propagated through the first delay signal path. A first synthesizer that subtracts the output signal of the error amplifier and outputs a subtraction result as a final output signal; a pilot detection unit that detects the level of the pilot signal from the final output signal; A distortion compensation amplifier comprising: a first control unit that controls the first adjuster so that a level of the output pilot signal is reduced; and the pilot detection unit detects the level of the detected pilot signal. There is provided a distortion compensation amplifier comprising a filter having a wide pass bandwidth when the signal is high and a narrow pass bandwidth when the level of the detected pilot signal is low.
[0015]
In the distortion compensating amplifier according to the third aspect of the present invention, the filter may be a band pass filter, and the center frequency of the filter may be the same as the frequency of the pilot signal.
[0016]
In the distortion compensating amplifier according to the third aspect of the present invention, the pilot detection unit may further include a circuit that converts a frequency of the pilot signal to a predetermined frequency, and the filter may be a bandpass filter. The center frequency of the filter may be the same as the predetermined frequency.
[0017]
In the distortion compensating amplifier according to the third aspect of the present invention, the pilot detection unit may further include a circuit that converts the frequency of the pilot signal into a direct current or a frequency in the vicinity thereof, and the filter is a low-pass filter. It may be.
[0018]
In the distortion compensating amplifier according to the third aspect of the present invention, the distortion detecting means propagates the first distributor for branching a part of the input signal and the input signal branched by the first distributor. A second delay signal path; a second distributor for branching a part of the output signal of the main amplifier; and a part of the output signal of the main amplifier branched by the second distributor. A second combiner that subtracts the input signal propagated through the second delay signal path and outputs the subtraction result as the distortion component.
[0019]
A distortion compensation amplifier according to a third aspect of the present invention includes a second adjuster that adjusts the amplitude and / or phase of the input signal before the input signal is amplified by the main amplifier, and the distortion component. And a second controller for controlling the second adjuster.
[0020]
According to a fourth aspect of the present invention, an adder for adding a pilot signal to an input signal, a first adjuster for adjusting the amplitude and / or phase of the input signal to which the pilot signal is added, A main amplifier for amplifying the input signal to which the pilot signal is added and the amplitude and / or the phase are adjusted; a first distributor for branching a part of the input signal to which the pilot signal is added; A first delay signal path through which the pilot signal is added and the input signal branched by the first distributor propagates; a second distributor for branching a part of the output signal of the main amplifier; The pilot signal is added from a part of the output signal of the main amplifier branched by the second distributor, branched by the first distributor, and propagated through the delay signal path A first synthesizer that outputs a subtraction result as a first distortion component, a pilot detection unit that detects the level of the pilot signal from the first distortion component, and the detected pilot signal A distortion compensation amplifier comprising: a first control unit that controls the first adjuster so as to reduce a level, wherein the pilot detection unit has a pass bandwidth when the level of the detected pilot signal is high There is provided a distortion compensating amplifier comprising a filter having a wide passband when the pilot signal detected is low and the level of the detected pilot signal is low.
[0021]
In the distortion compensating amplifier according to the fourth aspect of the present invention, the filter may be a band pass filter, and the center frequency of the filter may be the same as the frequency of the pilot signal.
[0022]
In the distortion compensation amplifier according to the fourth aspect of the present invention, the pilot detection unit may further include a circuit that converts the frequency of the pilot signal to a predetermined frequency, and the filter may be a bandpass filter. The center frequency of the filter may be the same as the predetermined frequency.
[0023]
In the distortion compensating amplifier according to the fourth aspect of the present invention, the pilot detection unit may further include a circuit for converting the frequency of the pilot signal into a direct current or a frequency in the vicinity thereof, and the filter is a low-pass filter. It may be.
[0024]
A distortion compensation amplifier according to a fourth aspect of the present invention includes a second delay signal path through which an output signal of the main amplifier propagates, and a second adjuster that adjusts the amplitude and / or phase of the first distortion component. An error amplifier that amplifies the first distortion component whose amplitude and / or phase is adjusted by the second adjuster, and the error from the output signal of the main amplifier that has propagated through the second delay signal path A second synthesizer that subtracts the output signal of the amplifier and outputs a subtraction result as a final output signal, a distortion component detection circuit that detects a second distortion component from the final output signal, and the second distortion component A second control unit that controls the second adjuster so as to be smaller.
[0025]
According to a fifth aspect of the present invention, there is provided a distortion detection loop for detecting distortion generated in a main amplifier, and a distortion removal loop for removing the distortion detected in the distortion detection loop from an output signal of the main amplifier. In a control method for adjusting a characteristic of the distortion removal loop of a distortion compensation amplifier comprising a pilot signal, a step of extracting a branch signal from a final output signal, a step of passing the branch signal through a filter, and the step of passing through the filter A step of detecting the level of the pilot signal from the branched signal; a step of adjusting the distortion elimination loop so that the level of the pilot signal is low; and a high passband of the filter when the level of the pilot signal is high. When the pilot signal level is low, the pass band of the filter is narrow. Control method characterized by comprising a setting step of setting a serial filter is provided.
[0026]
In the control method according to the fifth aspect of the present invention, in the setting step, when the level of the pilot signal is higher than a threshold value, a filter having a wide pass band is selected, and the level of the pilot signal is set to the threshold value. If lower, a filter with a narrow passband may be selected.
In the control method according to the fifth aspect of the present invention, in the setting step, when the level of the pilot signal is higher than a threshold value, a parameter is selected so that a pass band of the filter is widened. When the level is lower than the threshold value, a parameter that narrows the pass band of the filter may be selected.
[0027]
In the control method according to the fifth aspect of the present invention, in the setting step, when the level of the pilot signal is higher than a threshold, the pass band of the filter is widened, and the level of the pilot signal is lower than the threshold. When it is low, the pass band of the filter may be narrowed, and the threshold value may be updated with the level of the pilot signal.
[0028]
According to a sixth aspect of the present invention, there is provided a distortion detection loop for detecting distortion generated in a main amplifier, and a distortion removal loop for removing the distortion detected in the distortion detection loop from an output signal of the main amplifier. In a control method for adjusting characteristics of the distortion detection loop of a distortion compensation amplifier comprising a pilot signal, a step of extracting a branch signal from the distortion removal loop, a step of passing the branch signal through a filter, and a step of passing through the filter Detecting the level of the pilot signal from the branch signal, adjusting the distortion detection loop so that the level of the pilot signal is low, and when the level of the pilot signal is high, the passband of the filter is When the pilot signal level is wide, the pass band of the filter is narrow Control method characterized by comprising a setting step of setting a sea urchin said filter, is provided.
[0029]
In the control method according to the sixth aspect of the present invention, in the setting step, when the level of the pilot signal is higher than a threshold value, a filter having a wide pass band is selected, and the level of the pilot signal is set to the threshold value. If lower, a filter with a narrow passband may be selected.
[0030]
In the control method according to the sixth aspect of the present invention, in the setting step, when the level of the pilot signal is higher than a threshold value, a parameter is selected so that the pass band of the filter is widened, and When the level is lower than the threshold value, a parameter that narrows the pass band of the filter may be selected.
[0031]
In the control method according to the sixth aspect of the present invention, in the setting step, when the level of the pilot signal is higher than a threshold, the pass band of the filter is widened, and the level of the pilot signal is lower than the threshold. When it is low, the pass band of the filter may be narrowed, and the threshold value may be updated with the level of the pilot signal.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0033]
[Embodiment 1]
FIG. 1 shows a configuration of a distortion compensation amplifier according to Embodiment 1 of the present invention.
[0034]
Referring to FIG. 1, the distortion compensation amplifier according to the first exemplary embodiment of the present invention includes a distortion detection loop 101, a distribution synthesizer 111, a distortion removal loop 121, a distribution synthesizer 131, a distributor 141, a pilot signal level detection unit 161, 1 control part 171 and 2nd control part 164 are provided. The distortion detection loop 101 includes a distributor 104, a first vector adjuster 105, a main amplifier 106, a pilot signal generator 107, an adder 108, and a first delay signal path 109. The distribution synthesizer 111 includes a distributor 112 and a synthesizer 113. The distortion removal loop 121 includes a second delay signal path 122, a distributor 123, a second vector adjuster 124, and an error amplifier 125. The distribution synthesizer 131 includes a synthesizer 132. The pilot signal level detection unit 161 includes a variable band filter 162 and a detector 163.
[0035]
The distributor 104 distributes the input signal to the first vector adjuster 105 and the delayed signal path 109. The first vector adjuster 105 adjusts the amplitude and / or phase of the input signal distributed by the distributor 104 according to the control signal from the first controller 171. The main amplifier 106 amplifies the input signal whose amplitude and / or phase has been adjusted by the first vector adjuster 105. The pilot signal generator 107 generates a pilot signal. The pilot signal is, for example, a sine wave having a predetermined frequency. However, the pilot signal may be frequency-variable or phase-modulated as long as its main spectrum is within the band of the variable band filter. Adder 108 adds the pilot signal to the output signal of main amplifier 106. The output signal of the adder 108 includes the amplified main signal, the distortion component generated by the main amplifier 106, and the pilot signal. The first delay signal path 109 delays the input signal distributed by the distributor 104 for a predetermined time.
[0036]
The distributor 112 distributes the output signal of the adder 108 to the second delay signal path 122 and the combiner 113. The combiner 113 subtracts the input signal delayed by the first delay signal path 109 from the first output signal of the distributor 112, and outputs a signal including the distortion component generated by the main amplifier 106 and the pilot signal. To do.
[0037]
The second delay signal path 122 delays the second output signal of the distributor 112 for a predetermined period. The distributor 123 distributes the output signal of the combiner 113 to the second vector adjuster 124 and the first controller 171. The second vector adjuster 124 adjusts the amplitude and / or phase of the distortion component and pilot signal distributed by the distributor 123. The error amplifier 125 amplifies the distortion component and the pilot signal whose amplitude and / or phase are adjusted by the second vector adjuster 124.
[0038]
The combiner 132 subtracts the distortion component and pilot signal output from the error amplifier 125 from the main signal, distortion component, and pilot signal output from the second delay signal path 122, and outputs a signal mainly including the main signal. It should be noted that the output signal of the distribution synthesizer 131 ideally does not include a distortion component and a pilot signal, but generally includes a small amount thereof.
[0039]
The distributor 141 distributes the output signal of the synthesizer 132 to the output terminal and the variable band filter 162.
[0040]
The variable band filter 162 passes only the pilot signal among the main signal, the distortion component and the pilot signal supplied from the distributor 141. Note that the variable band filter 162 is a band pass filter whose center frequency is the frequency of the pilot signal. The detector 163 detects the level of the pilot signal that has passed through the variable band filter 162.
[0041]
The second control unit 164 controls the amount of adjustment of the distortion component and the amplitude and / or phase of the pilot signal in the second vector adjuster 124 so that the level of the pilot signal detected by the detector 163 is lowered. To do.
[0042]
In addition, the second control unit 164 increases the pass band width of the variable band filter 162 when the level of the pilot signal detected by the detector 163 is high, and when the level of the pilot signal detected by the detector 163 is low. The pass band width of the variable band filter 162 is controlled by the variable band filter control signal so that the pass band width of the variable band filter 162 is narrowed.
[0043]
The first control unit 171 detects the distortion component from the distortion component and the pilot signal distributed from the distributor 123, and the amplitude of the input signal by the first vector adjuster 105 so that the detected distortion component becomes small. And / or control the amount of phase adjustment.
[0044]
The first vector adjuster 105 may be of a polar type that divides a vector into gain and phase and adjusts the gain and phase, respectively, or divides the vector into an in-phase component and a quadrature component, A Cartesian type of adjusting the in-phase component and the quadrature component may be used. Ideally, the gain of the vector adjuster 105 is constant regardless of the frequency, and the amount of phase shift is a linear phase.
[0045]
The difference between the distortion compensation amplifier according to the first embodiment and the distortion compensation amplifier according to the conventional example is that a variable band filter 162 is provided instead of the second filter 962 of the distortion compensation amplifier according to the conventional example, and the pass bandwidth of the variable band filter 162 is increased. Is controlled by the second control unit 164.
[0046]
The variable band filter 162 has a pass bandwidth set continuously or stepwise according to the detected pilot signal level. Specifically, when the pilot signal level is low, the bandwidth is set narrow, and when it is high, the bandwidth is set wide. FIG. 2 shows how the pass band width of the variable band filter 162 changes.
[0047]
Therefore, during the pull-in operation of the feedforward control, the pass bandwidth is set wide, but the pilot signal level is high and the signal-to-noise (S / N) ratio remains high. The pilot signal can be detected without reducing the speed.
[0048]
In addition, when the pilot signal is canceled due to the balance of the feedforward control loop and the detection level of the pilot signal becomes low, the passband width of the variable band filter is set narrow. The noise-to-noise ratio (S / N ratio) is improved, and errors due to detection level noise can be reduced.
[0049]
Therefore, as a result, there is an effect that large distortion compensation can be obtained stably.
[0050]
Next, the operation of the distortion compensation amplifier according to the first embodiment will be described.
[0051]
First, the operation of the distortion removal loop during the initial operation will be described.
[0052]
Until the distortion elimination loop 121 reaches equilibrium, the level of the pilot signal input to the pilot signal detector 161 is considerably higher than that in the balanced state, and the pilot signal input to the pilot signal level detector 161 in the balanced state. The level of the pilot signal that is input to the pilot signal level detector 161 in the unbalanced state when the level is set as a reference is, for example, about several tens of dB.
[0053]
As control of the distortion removal loop 121 is started and the distortion removal loop 121 approaches the equilibrium state, the level of the pilot signal input to the pilot signal level detection unit 161 decreases. That is, as the equilibrium state of the distortion elimination loop 121 is achieved, the level of the pilot signal gradually decreases. Accordingly, the pilot signal level and the signal-to-noise ratio (S / N ratio) of noise in the pilot signal level detection unit 161 deteriorate.
[0054]
Here, the noise indicates the thermal noise level at the output of the distortion compensation amplifier, the main signal itself, or the pilot signal frequency band of intermodulation distortion generated from the main signal system or the leakage power to the vicinity. ing. In addition, pilot detector 161C (FIG. 11) using heterodyne detection described later includes noise generated due to the noise figure characteristic of pilot detector 161B itself. Since these noise levels are components that do not decrease in the same manner as the pilot signal level, the S / N ratio deteriorates.
[0055]
The relationship between the signal levels is schematically shown in FIG. Referring to FIG. 3, although the noise level is constant, the pilot signal level is high in the initial state, that is, before the distortion elimination loop is in an equilibrium state, and when the distortion elimination loop is in an equilibrium state, The level of is low. Accordingly, the S / N is large in the initial state, whereas the S / N is small in the equilibrium state.
[0056]
Next, the operation in which the second control unit 164 controls the pass bandwidth of the variable band filter 162 provided in the pilot signal level detection unit 161 according to the pilot signal level detected by the pilot signal level detection unit 161. explain. Here, for the sake of simplicity, a case will be described in which the bandwidth of the variable band filter is set in two stages, but it can be easily extended to other cases.
[0057]
First, the pass band width of the variable band filter 162 is set by the second control unit 164 continuously or stepwise according to the level of the detected pilot signal. Specifically, when the pilot signal level is low, the pass bandwidth is set narrow, and when it is high, the pass bandwidth is set wide. These relationships are schematically shown in FIG. 4 together with the case where the filter band is continuously varied.
[0058]
A method of controlling the band of the variable band filter 162 by the control unit 164 will be described in more detail with reference to FIGS.
[0059]
The operation of the control unit 164 when the control unit 164 controls the band of the variable band filter 162 stepwise will be described with reference to FIG.
[0060]
First, the pilot signal level is detected (step S201). Next, it is determined whether the level of the pilot signal detected in step S201 is higher than a threshold value (step S202). If it is determined in step S202 that the pilot signal level is higher than the threshold value, a filter parameter corresponding to a wide band is selected (step S203). If it is determined in step S202 that the pilot signal level is equal to or lower than the threshold value, the filter parameter 162 corresponding to the narrow band is selected (step S204). After step 203 or step S204, the parameter of the second vector adjuster 124 is adjusted so that the level of the pilot signal detected in step S201 is lowered (step S205). Then, the process returns from step S205 to step S201, and steps S201 to S205 are repeated.
[0061]
The operation of the control unit 164 when the control unit 164 continuously controls the band of the variable band filter 162 will be described with reference to FIG.
[0062]
First, the pilot signal level is detected (step S221). Next, it is determined whether or not the level of the pilot signal detected in step S221 is higher than a threshold value (step S222). If it is determined in step S222 that the level of the pilot signal is higher than the threshold value, the filter parameters are changed so that the band of the filter 162 becomes wider (step S223). If it is determined in step S202 that the pilot signal level is equal to or lower than the threshold value, the filter parameter 162 is changed so that the filter band becomes narrower (step S224). After step 223 or step S224, the level of the pilot signal detected in step S221 is substituted for the threshold value (step S225). Next, the parameter of the second vector adjuster 124 is adjusted so that the level of the pilot signal detected in step S221 is lowered (step S226). And it returns to step S221 from step S226, and repeats step S221-S226.
[0063]
When the pass band width of the filter is constant, the S / N ratio of pilot signal to noise deteriorates as the pilot signal level decreases as described above. However, in the present invention, the pilot signal level decreases. of The pass band width of the variable band filter 162 is controlled so that the pass band width becomes narrower in response to. When the pass band width of the variable band filter 162 is narrowed, the pilot signal level does not change, and the noise level decreases in proportion to the bandwidth, so that the S / N ratio of the pilot signal is improved.
[0064]
For example, as in the example shown in FIG. 4, when the pilot signal level is lower than the threshold value, the passband width is set to be 1/10 narrower than when the pilot signal level is higher than the threshold value. In this case, since the noise level decreases by 10 dB in proportion to the bandwidth, the S / N is improved by 10 dB.
[0065]
FIG. 7 shows the relationship between these signals together with the case where the pass bandwidth is not variable.
[0066]
In this way, by controlling the pass bandwidth according to the pilot signal level, it is possible to suppress the pilot detection level error due to noise and the erroneous control caused thereby, so that the distortion removal loop 121 is balanced with higher accuracy. It is possible to bring the vicinity of the state, and it is possible to prevent the distortion compensation amount from being deteriorated due to erroneous control. That is, stable large distortion compensation can be obtained.
[0067]
Next, the reason why the pass band width of the variable band filter 162 is set wide during the feed-forward control pull-in operation will be described.
[0068]
At this time, since the pilot signal level is high and the signal-to-noise (S / N) ratio remains high as described above, it is not necessary to narrow the band of the variable band filter 162. On the other hand, since the level of the pilot signal changes rapidly during the pull-in operation, the band of the variable band filter 162 is widened in order to increase the control response time of the second vector adjuster 124. There is a need.
[0069]
This relationship is shown in FIG. FIG. 8A shows the relationship between the pass bandwidth of the variable band filter 162 and the response time of the variable filter 162 when the pass bandwidth of the variable band filter 162 changes in two stages. The relationship between the pass bandwidth of the variable band filter 162 and the response time of the variable filter 162 when the pass band width of the variable band filter 162 continuously changes is shown.
[0070]
Therefore, in a state where the response speed is important as in the initial startup, it is desirable to make the pass band width of the variable band filter 162 wide enough to obtain a desired response speed.
[0071]
Further, in a state where the loop is balanced, the control system only needs to follow the temperature change and aging of the circuit elements such as the error amplifier 125, and these time changes are very slow compared to the operation time of the control circuit. Therefore, even if the pass band width of the variable band filter 162 is narrowed, this hardly causes a problem that the response speed of the control system becomes slow.
[0072]
[Embodiment 2]
The second embodiment of the present invention differs from the first embodiment only in that the pilot signal level detection unit 161 is changed to a pilot signal level detection unit 161B shown in FIG.
[0073]
As shown in FIG. 9, the same effect as in the first embodiment can be obtained even when a plurality of filters having different bandwidths are used by switching with a variable band filter control signal from the second control unit 164. In FIG. 9, when the detection level of the pilot signal is high, the filter 162A having a wide pass bandwidth is selected, and when the detection level of the pilot signal is low, the filter 162B having a narrow band is selected. The characteristics of each filter are shown in FIG. The method of operation and control is the same as that of the embodiment shown in FIG.
[0074]
Of course, this can also be easily expanded when the number of filters to be switched is N (N> = 3).
[0075]
Furthermore, a plurality of variable band filters can be switched and used, or a fixed band filter and a variable band filter can be used in combination.
[0076]
Examples of combinations of the fixed band filter and the variable band filter are as follows.
[0077]
(1) A configuration in which a fixed wide band filter and a variable narrow band filter are connected in series
(2) A fixed narrow band filter and a variable wide band filter are prepared. If the pilot signal level is equal to or higher than the threshold value, the variable wide band filter can be used while changing the band, and the pilot signal level must be smaller than the threshold value. For example, using a fixed narrowband filter
[0078]
[Embodiment 3]
The distortion compensation amplifier according to the third embodiment is different from the distortion compensation amplifier according to the first embodiment only in that the pilot signal level detection unit 161 is changed to a pilot signal level detection unit 161C illustrated in FIG.
[0079]
In the third embodiment, the signal including the pilot signal supplied from the distributor 141 is generated by the local oscillator 160. , The multiplier 165 multiplies an oscillation signal having a frequency different from that of the pilot signal to frequency-convert the signal including the pilot signal. Then, the frequency-converted signal is passed through a variable band filter 166 whose center frequency is the frequency of the pilot signal after frequency conversion, so that only the frequency-converted pilot signal is extracted, and then the level of the pilot signal is detected by the detector 163. Is detected.
[0080]
Similar to the variable band filter 162, the variable band filter 166 has a wide pass band when the level of the pilot signal detected by the second controller 164 is high, and a pass band when the level of the detected pilot signal is low. The width is controlled to be narrow.
[0081]
[Embodiment 4]
The configuration of the distortion compensation amplifier according to the fourth embodiment is shown in FIG. As is clear from comparison between FIG. 1 and FIG. 12, the distortion compensation amplifier according to the fourth embodiment is different from the distortion compensation amplifier according to the first embodiment in that the pilot signal level detection unit 161 is changed to a pilot signal level detection unit 161D. Only the difference.
[0082]
In the fourth embodiment, the signal including the pilot signal supplied from the distributor 141 is multiplied by the pilot signal in the multiplier 165 to frequency-convert the signal including the pilot signal. After frequency conversion, the center frequency of the pilot signal is DC. Then, the frequency-converted signal is passed through the variable band filter 167 to extract only the frequency-converted pilot signal, and then the level of the pilot signal is detected by the detector 163. The variable band filter 167 is a low-pass filter.
[0083]
Similarly to the variable band filter 162, the variable band filter 167 has a wide pass band when the level of the pilot signal detected by the second controller 164 is high, and a pass band when the level of the detected pilot signal is low. The width is controlled to be narrow.
[0084]
[Embodiment 5]
The distortion compensation amplifier according to the fifth embodiment is different from the distortion compensation amplifier according to the first embodiment only in that the pilot signal level detection unit 161 is changed to a pilot signal level detection unit 161E illustrated in FIG.
[0085]
In the fifth embodiment, the signal including the pilot signal supplied from the distributor 141 is passed through the variable band filter 162, and then this signal is generated by the local oscillator 160. , The multiplier 165 multiplies an oscillation signal having a frequency different from that of the pilot signal, thereby frequency-converting the output signal of the variable band filter 162. Then, the frequency-converted signal is passed through a variable band filter 166 whose center frequency is the frequency of the pilot signal after frequency conversion, so that only the frequency-converted pilot signal is extracted, and then the level of the pilot signal is detected by the detector 163. Is detected.
[0086]
The variable band filter 162 of the pilot signal level detector 161E is the same as the variable band filter 162 of the pilot signal level detector 161, and the pass bandwidth is controlled by the second controller 164 as in the first embodiment. . Further, the variable band filter 166 of the pilot signal level detection unit 161E is the same as the variable band filter 166 of the pilot signal level detection unit 161C, and the pass bandwidth by the second control unit 164 is controlled as in the third embodiment. Is done. The control of the pass bandwidth of the variable band filter 162 and the control of the pass bandwidth of the variable band filter 166 may be performed simultaneously or sequentially.
[0087]
[Embodiment 6]
The distortion compensation amplifier according to the sixth embodiment has the same basic configuration as the distortion compensation amplifier according to the fifth embodiment. However, the control parameters of the variable band filter 162 and the variable band filter 166 are different.
[0088]
In the sixth embodiment, by shifting the center frequency of the variable band filter 162 and / or the center frequency of the variable band filter 166, the pass band width when both are combined is adjusted. That is, when the level of the detected pilot signal is high, the center frequency of the variable band filter 162 is matched with the frequency of the pilot signal, and the center frequency of the variable band filter 166 is matched with the frequency of the pilot signal subjected to frequency conversion. Increase the passband width. On the other hand, when the level of the detected pilot signal is low, the center frequency of the variable band filter 162 is made higher than the frequency of the pilot signal, and the center frequency of the variable band filter 166 is made lower than the frequency of the frequency-converted pilot signal. This narrows the passband width. Alternatively, when the level of the detected pilot signal is low, the center frequency of the variable band filter 162 is made lower than the frequency of the pilot signal, and the center frequency of the variable band filter 166 is made higher than the frequency of the pilot signal subjected to frequency conversion. This narrows the passband width.
[0089]
[Embodiment 7]
The seventh embodiment of the present invention is different from the first embodiment only in that the pilot signal level detection unit 161 is changed to a pilot signal level detection unit 161F shown in FIG.
[0090]
In the seventh embodiment, a signal including a pilot signal supplied from the distributor 141 is A / D converted by the A / D converter 168, and then the digital signal after the A / D conversion is passed through the digital variable band filter 162B. Thus, the pilot signal is extracted, and the extracted pilot signal is passed through the digital detector 163B to obtain the detection level of the digital pilot signal.
[0091]
The digital variable band filter 162B is controlled by the second control unit 164 in the same manner as the variable band filter 162.
[0092]
Note that a D / A converter for D / A converting the output of the digital variable band filter 162B may be provided, and the detector 163 may be used as the detector without using the digital detector 163B.
[0093]
In addition, in the pilot signal level detection unit 161C shown in FIG. 11, the pilot signal level detection unit 161D shown in FIG. 12, and the pilot signal level detection unit 161E shown in FIG. 13, components subsequent to the multiplier 165 may be digitized. .
[0094]
[Embodiment 8]
The configuration of the distortion compensation amplifier according to the eighth embodiment is shown in FIG.
[0095]
As is apparent from a comparison between FIG. 1 and FIG. 15, in the first embodiment, the pilot signal level detection unit 161 and the second control unit 164 for controlling the second vector adjuster 124 in the distortion removal loop 121. In Although having features, the eighth embodiment has features in the pilot signal level detection unit 151 and the first control unit 154 for controlling the first vector adjuster 105 in the distortion detection loop 101.
[0096]
Pilot signal level detection section 151 has the same configuration as pilot signal detection level detection section 161 and operates in the same manner. Therefore, the variable band filter 152 is the same as the variable band filter 162, and the detector 153 is the same as the detector 163. The first control unit 154 has the same configuration as the second control unit 164 and performs the same operation.
[0097]
The first control unit 154 controls the first vector adjuster 105 so that the level of the pilot signal included in the signal distributed by the distributor 123 is lowered. As in the first embodiment, the first control unit 154 controls the pilot signal. The pass band width of the variable band filter 152 is controlled so that the pass band width of the variable band filter 152 is wide when the level of the variable band filter 152 is high, and the pass band width of the variable band filter 152 is narrow when the level of the pilot signal is low.
[0098]
The distortion component detection circuit 181 detects a distortion component from the signal distributed by the distributor 141 from the final output signal. For example, a non-linear distortion component having a predetermined frequency is detected. The second control unit 182 controls the second vector adjuster 124 so that the distortion component detected by the distortion component detection circuit 181 becomes small.
[0099]
Needless to say, the same embodiments as those of Embodiments 2 to 7 can be derived from Embodiment 8 in the same manner as Embodiments 2 to 7 of Embodiment 1 are derived.
[0100]
[Embodiment 9]
FIG. 16 shows the configuration of a distortion compensation amplifier according to the ninth embodiment. The ninth embodiment is a combination of the first embodiment and the eighth embodiment.
[0101]
The first vector adjuster 105 in the distortion detection loop 101 is controlled by the first controller 154 so that the level of the pilot signal detected by the pilot signal level detector 151 is lowered, and The second vector adjuster 124 is controlled by the second controller 164 so that the level of the pilot signal detected by the pilot signal level detector 161 is lowered.
[0102]
Needless to say, the same embodiments as those of Embodiments 2 to 7 can be derived from Embodiment 9 in the same manner as Embodiments 2 to 7 of Embodiment 1 are derived.
[0103]
[Other Embodiments]
In the above-described embodiment, the band pass filter has been described as the variable band filter. However, the variable pass filter may be configured by other types of filters such as a low pass filter and a high pass filter.
[0104]
Moreover, although the case where the bandwidth of the variable band filter is controlled in two stages has been described in the above embodiment, it goes without saying that the control may be performed by further dividing into a plurality of stages. Alternatively, it is of course possible to control the bandwidth continuously, or control by combining both.
[0105]
In addition, regarding the variable band-pass filter described in the above embodiment, it is preferable that the bandwidth can be electrically controlled in consideration of performing the bandwidth control from the control circuit, but an element whose capacitance changes with voltage, for example, a varactor This can be realized by configuring a variable band filter with a diode or an element whose dielectric constant varies with voltage, such as a voltage variable dielectric.
[0106]
【The invention's effect】
As described above, since the pass band width of the filter is controlled so that the pass band width becomes narrower in response to a decrease in the pilot signal level, the noise level is lower than the same pilot signal level. Since it decreases in proportion to the width, the signal-to-noise ratio can be improved.
[0107]
Therefore, the distortion detection loop and the distortion elimination loop can be further controlled near the equilibrium state, and the distortion compensation amount can be prevented from being deteriorated due to erroneous control. As a result, a large distortion compensation operation that is stable as a distortion compensation amplifier is achieved. Can be obtained.
[0108]
Further, when the pilot signal level is high, the pass band of the filter is widened, so that the control loop can be settled in an equilibrium state at an early stage.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a distortion compensation amplifier according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a manner in which a pass band width of a variable band filter according to an embodiment of the present invention changes.
FIG. 3 is a diagram comparing a pilot signal level and S / N in an initial state with a pilot signal level and S / N in a loop equilibrium state.
FIG. 4 is a diagram illustrating a state in which a pass band of a variable band filter is changed according to a detection level of a pilot signal according to an embodiment of the present invention.
FIG. 5 is a flowchart for explaining the operation of the control unit when the control unit according to the embodiment of the present invention controls the band of the variable band filter step by step.
FIG. 6 is a flowchart for explaining the operation of the control unit when the control unit according to the embodiment of the present invention continuously controls the band of the variable band filter.
FIG. 7 is a diagram showing that the S / N is improved by changing the pass band of the variable band filter according to the detection level of the pilot signal in the embodiment of the present invention.
FIG. 8 is a diagram illustrating how the response time is changed by changing the pass bandwidth of the variable band filter in the embodiment of the present invention. (A) is a case where the pass band width of the variable band filter is changed stepwise, and (b) is a case where the pass band width of the variable band filter is continuously changed.
FIG. 9 is a block diagram showing a configuration of a pilot signal level detection unit according to Embodiment 2 of the present invention.
FIG. 10 is a diagram illustrating characteristics of a filter used in a pilot signal level detection unit according to Embodiment 2 of the present invention.
FIG. 11 is a block diagram showing a configuration of a pilot signal level detection unit according to Embodiment 3 of the present invention.
FIG. 12 is a block diagram showing a configuration of a distortion compensation amplifier according to Embodiment 4 of the present invention.
FIG. 13 is a block diagram showing a configuration of a pilot signal level detection unit according to Embodiment 5 of the present invention.
FIG. 14 is a block diagram showing a configuration of a pilot signal level detection unit according to Embodiment 7 of the present invention.
FIG. 15 is a block diagram showing a configuration of a distortion compensation amplifier according to an eighth embodiment of the present invention.
FIG. 16 is a block diagram showing a configuration of a distortion compensation amplifier according to a ninth embodiment of the present invention.
FIG. 17 is a block diagram showing a configuration of a distortion compensation amplifier according to a conventional example.
FIG. 18 is a block diagram showing a configuration of a distortion compensation amplifier according to a conventional example.
[Explanation of symbols]
101 Distortion detection loop
102 Pilot signal generator
103 adder
104 Distributor
105 First vector adjuster
106 Main amplifier
107 Pilot signal generator
108 Adder
109 First delay signal path
111 Distributor / Synthesizer
112 distributor
113 Synthesizer
121 Distortion removal loop
122 Second delay signal path
123 distributor
124 second vector adjuster
125 error amplifier
131 Distributor / Synthesizer
132 Synthesizer
141 Distributor
151 Pilot signal level detector
152 Variable Band Filter
153 detector
154 First control unit
161 Pilot signal level detector
162 Variable band filter
163 detector
164 Second control unit

Claims (21)

A distortion detection loop for detecting distortion generated in the main amplifier;
A distortion removal loop for removing the distortion detected by the distortion detection loop from the output signal of the main amplifier;
Adjusting means for adjusting the characteristics of the distortion elimination loop using a pilot signal;
In a distortion compensation amplifier comprising:
A filter that passes the pilot signal, and has a wide pass bandwidth when the level of the pilot signal detected from the final output signal is high, and the pass when the level of the pilot signal detected from the final output signal is low A distortion compensation amplifier characterized in that the adjustment means has a narrow bandwidth. A distortion detection loop for detecting distortion generated in the main amplifier;
A distortion removal loop for removing the distortion detected by the distortion detection loop from the output signal of the main amplifier;
Adjusting means for adjusting the characteristics of the distortion detection loop using a pilot signal;
In a distortion compensation amplifier comprising:
A filter that passes the pilot signal, and has a wide pass bandwidth when the level of the pilot signal detected by the distortion removal loop is high, and when the level of the pilot signal detected by the distortion removal loop is low, A distortion compensation amplifier characterized in that the adjusting means includes a narrow passband. A main amplifier for amplifying the input signal;
An adder for adding a pilot signal to the output signal of the main amplifier;
Distortion detecting means for detecting a distortion component of the output signal of the main amplifier;
A first delayed signal path through which the output signal of the main amplifier to which the pilot signal has been added propagates;
A first adjuster for adjusting the amplitude and / or phase of the distortion component;
An error amplifier that amplifies the distortion component whose amplitude and / or phase is adjusted by the first adjuster;
A first combiner for adding the pilot signal and subtracting the output signal of the error amplifier from the output signal of the main amplifier propagated through the first delay signal path and outputting a subtraction result as a final output signal; ,
A pilot detector for detecting a level of the pilot signal from the final output signal;
A first control unit that controls the first adjuster so that a level of the detected pilot signal is reduced;
In a distortion compensation amplifier comprising:
The pilot detection unit includes a filter having a wide passband when the level of the detected pilot signal is high and a narrow passband when the level of the detected pilot signal is low. Distortion compensation amplifier. The distortion compensation amplifier according to claim 3.
The distortion compensation amplifier, wherein the filter is a band pass filter, and a center frequency of the filter is the same as a frequency of the pilot signal. The distortion compensation amplifier according to claim 3.
The pilot detection unit further includes a circuit that converts the frequency of the pilot signal to a predetermined frequency,
The filter is a bandpass filter;
The distortion compensating amplifier according to claim 1, wherein a center frequency of the filter is the same as the predetermined frequency. The distortion compensation amplifier according to claim 3.
The pilot detection unit further includes a circuit that converts the frequency of the pilot signal into a direct current or a frequency in the vicinity thereof,
The distortion compensation amplifier, wherein the filter is a low-pass filter. The feedforward amplifier according to any one of claims 3 to 6,
The strain detecting means includes
A first distributor for branching a part of the input signal;
A second delayed signal path through which the input signal branched by the first distributor propagates;
A second distributor for branching a part of the output signal of the main amplifier;
A second synthesis that subtracts the input signal propagated through the second delay signal path from a part of the output signal of the main amplifier branched by the second distributor and outputs a subtraction result as the distortion component And
A distortion compensation amplifier comprising: The feedforward amplifier according to any one of claims 3 to 7,
A second regulator for adjusting the amplitude and / or phase of the input signal before the input signal is amplified by the main amplifier;
A second control unit for controlling the second adjuster based on the distortion component;
A distortion compensation amplifier, further comprising: An adder for adding a pilot signal to the input signal;
A first adjuster for adjusting the amplitude and / or phase of the input signal to which the pilot signal has been added;
A main amplifier that amplifies the input signal to which the pilot signals are added and the amplitude and / or phase is adjusted;
A first distributor for branching a part of the input signal to which the pilot signal is added;
A first delayed signal path through which the pilot signals are added and the input signal branched by the first distributor propagates;
A second distributor for branching a part of the output signal of the main amplifier;
The pilot signal is added from a part of the output signal of the main amplifier branched by the second distributor, and the input signal branched by the first distributor and propagated through the delay signal path is subtracted. A first synthesizer that outputs a subtraction result as a first distortion component;
A pilot detector for detecting a level of the pilot signal from the first distortion component;
A first control unit that controls the first adjuster so that a level of the detected pilot signal is reduced;
In a distortion compensation amplifier comprising:
The pilot detection unit includes a filter having a wide passband when the level of the detected pilot signal is high and a narrow passband when the level of the detected pilot signal is low. Distortion compensation amplifier. The distortion compensation amplifier according to claim 9, wherein
The distortion compensation amplifier, wherein the filter is a band pass filter, and a center frequency of the filter is the same as a frequency of the pilot signal. The distortion compensation amplifier according to claim 9, wherein
The pilot detection unit further includes a circuit that converts the frequency of the pilot signal to a predetermined frequency,
The filter is a bandpass filter;
The distortion compensating amplifier according to claim 1, wherein a center frequency of the filter is the same as the predetermined frequency. The distortion compensation amplifier according to claim 9, wherein
The pilot detection unit further includes a circuit that converts the frequency of the pilot signal into a direct current or a frequency in the vicinity thereof,
The distortion compensation amplifier, wherein the filter is a low-pass filter. The feedforward amplifier according to any one of claims 9 to 12,
A second delayed signal path through which the output signal of the main amplifier propagates;
A second adjuster for adjusting the amplitude and / or phase of the first distortion component;
An error amplifier that amplifies the first distortion component whose amplitude and / or phase is adjusted by the second adjuster;
A second synthesizer that subtracts the output signal of the error amplifier from the output signal of the main amplifier propagated through the second delay signal path and outputs the subtraction result as a final output signal;
A distortion component detection circuit for detecting a second distortion component from the final output signal;
A second controller that controls the second adjuster so that the second distortion component is reduced;
A distortion compensation amplifier, further comprising: A distortion detection loop for detecting distortion generated in the main amplifier;
In a control method for adjusting a characteristic of the distortion elimination loop of a distortion compensation amplifier comprising a distortion elimination loop that removes the distortion detected by the distortion detection loop from an output signal of the main amplifier using a pilot signal,
Extracting a branch signal from the final output signal;
Passing the branch signal through a filter;
Detecting the level of the pilot signal from the branched signal that has passed through the filter;
Adjusting the distortion removal loop so that the level of the pilot signal is low;
A setting step for setting the filter so that a pass band of the filter is wide when the level of the pilot signal is high and a pass band of the filter is narrow when the level of the pilot signal is low;
A control method comprising: The control method according to claim 14, wherein
In the setting step, when the pilot signal level is higher than the threshold value, a filter having a wide pass band is selected, and when the pilot signal level is lower than the threshold value, a filter having a narrow pass band is selected. A control method characterized by: The control method according to claim 14, wherein
In the setting step, when the level of the pilot signal is higher than a threshold value, a parameter is selected so that a pass band of the filter is widened. When the level of the pilot signal is lower than the threshold value, A control method characterized by selecting a parameter that narrows the passband of the filter. The control method according to claim 14, wherein
In the setting step, when the level of the pilot signal is higher than a threshold value, the pass band of the filter is widened, and when the level of the pilot signal is lower than the threshold value, the pass band of the filter is narrowed, A control method, wherein a threshold value is updated with a level of the pilot signal. A distortion detection loop for detecting distortion generated in the main amplifier;
In a control method for adjusting a characteristic of the distortion detection loop of a distortion compensation amplifier including a distortion removal loop that removes the distortion detected in the distortion detection loop from an output signal of the main amplifier using a pilot signal,
Extracting a branch signal from the distortion elimination loop;
Passing the branch signal through a filter;
Detecting the level of the pilot signal from the branched signal that has passed through the filter;
Adjusting the distortion detection loop so that the level of the pilot signal is low;
A setting step for setting the filter so that a pass band of the filter is wide when the level of the pilot signal is high and a pass band of the filter is narrow when the level of the pilot signal is low;
A control method comprising: The control method according to claim 18, wherein
In the setting step, when the pilot signal level is higher than the threshold value, a filter having a wide pass band is selected, and when the pilot signal level is lower than the threshold value, a filter having a narrow pass band is selected. A control method characterized by: The control method according to claim 18, wherein
In the setting step, when the level of the pilot signal is higher than a threshold value, a parameter is selected so that a pass band of the filter is widened. When the level of the pilot signal is lower than the threshold value, A control method characterized by selecting a parameter that narrows the passband of the filter. The control method according to claim 18, wherein
In the setting step, when the level of the pilot signal is higher than a threshold value, the pass band of the filter is widened, and when the level of the pilot signal is lower than the threshold value, the pass band of the filter is narrowed, A control method, wherein a threshold value is updated with a level of the pilot signal.

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