JP2010021644A - Input level correction circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To output an amplified high frequency signal stably while preventing the internal circuit of a power amplification means from being damaged by correcting the input level of a plurality of high frequency signals having different input levels from a plurality of signal sources at the prestage of the power amplification means. <P>SOLUTION: The input level of high frequency signals having different input levels from a plurality of signal sources 40a-40c is corrected appropriately by a matching circuit (PLD) 118 as a means for changing the amount of attenuation which changes the amount of attenuation of a variable attenuator 112 as an attenuation means in accordance with the amount of attenuation which is obtained by linear approximation of an appropriate input level at a frequency between the lower limit frequency and the upper limit frequency based on the amount of attenuation which is changed in correspondence with the lower limit frequency and the upper limit frequency of the high frequency signal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an input level correction circuit that performs input level correction of high-frequency signals having different input levels.

  Conventionally, as shown in FIG. 5, for example, a wireless system has a configuration in which a signal source 10 and a power amplifying unit (PA) 20 are housed in one casing. In such a wireless system, when a high frequency signal generated by oscillation of the crystal resonator 11 of the signal source 10 is converted to an arbitrary frequency by the frequency converter 12, the variable attenuator 21, amplifiers 22 and 23, the circulator 24, and the band pass filter are used. The signal is output from the output connector 26 to the antenna 30 via the (BPF) 25 and the directional coupler 26.

  A signal entering from the antenna 30 side is guided to the arrow c side by the circulator 24 and absorbed by the resistor R.

  In such a wireless system, the attenuation amount of the variable attenuator 21 is varied in accordance with the increase / decrease of the APC (automatic power control) voltage from the direct conversion circuit 26 obtained via the amplifier 28, thereby providing an output connector. The amplification output of the high-frequency signal output from the antenna 27 to the antenna 30 is held substantially constant.

  Further, in such a wireless system, the signal source 10 and the power amplification unit (PA) 20 are housed in one casing, and the input level of the high frequency signal from the signal source 10 to the power amplification unit (PA) 20. Therefore, stable amplification output can be performed from the power amplifying unit (PA) 20 without correcting the input level of the high frequency signal from the signal source 10 to the power amplifying unit (PA) 20. It has become.

  By the way, under the present circumstances, such a radio system is significantly changed, and a plurality of signal sources and a plurality of power amplifiers are provided separately, and any one of the signal sources and any one of the power amplifiers is freely combined by a switch. A wireless system is also being developed. In this case, if the power amplifier (PA) 20 of FIG. 5 can be used as it is for each power amplifier, development of a new power amplifier becomes unnecessary, which is advantageous in terms of development cost.

  However, if any one of the signal sources and any one of the power amplifiers are freely combined by the switch, not only the input level of the high frequency signal from each signal source is different, but also any one of the signal sources and any one of them. The length of the path with the power amplifier increases, and the loss (mainly cable loss) between the paths increases in proportion to the frequency, which may affect the frequency characteristics. The amplified output may be hindered.

  Moreover, if the frequency characteristics are affected by loss (mainly cable loss) and the input level becomes undefined, the transient amplification operation during transmission will be affected, and the internal circuit of the power amplifier will be damaged. There is also a risk.

  In order to eliminate such a problem at the time of transmission, in Patent Document 1, in a transmission output control circuit that obtains a constant transmission output by controlling the amplification degree of a transmission amplifier circuit by a transmission output control voltage, At the rising edge, the transmission output control voltage at the time of the previous transmission is read from the memory circuit and the transmission output control is performed. A transmission output control circuit provided with means for switching is proposed.

  In Patent Document 2, a high-frequency transmission signal subjected to frequency hopping within a predetermined frequency band is generated, and the high-frequency transmission signal is output by power amplification. A variable attenuator that electronically changes the amount of attenuation, a detection circuit that detects the level of a part of the power-amplified high-frequency signal, detects the detected high-frequency signal and outputs it as a detection voltage value, and a detection voltage value An automatic power control circuit having a writable and readable memory and configured to output a detection voltage value read from the memory in response to frequency hopping for controlling the attenuation of the variable attenuator. And a hopping synthesizer that outputs a local oscillation frequency signal that is frequency hopped and frequency swept when the power is turned on. Has proposed a transmitter was.

JP 07-212255 A JP 2007-295153 A

  In Patent Document 1 described above, when the transmission output is stabilized, the transmission output control is performed using the detection voltage at which the transmission output is detected as the transmission output control voltage, so that the waveform at the time of the transmission rising becomes an appropriate output. Therefore, the internal circuit is prevented from being damaged due to the transient amplification operation at the time of transmission.

  Further, in Patent Document 2 described above, the attenuation amount of the variable attenuator is controlled by the detected voltage value read from the memory in response to frequency hopping, and therefore occurs at the rise of the frequency after switching when the transmission frequency is switched. Damage to the internal circuit due to a transient amplification operation due to overshoot or the like is prevented.

  However, in each of these Patent Documents 1 and 2, the signal source and the power amplifying unit (PA) are one-to-one, and as described above, a plurality of signal sources and a plurality of power amplifiers are used. If it is intended to be applied to a wireless system that is provided separately and any signal source and any power amplifier are freely combined by a switch, as described above, any signal source and any power The length of the path to the amplifier becomes longer, and the loss (mainly cable loss) between the paths increases in proportion to the frequency, which may affect the frequency characteristics. Stable amplified output from each power amplifier There has been a problem that may be disturbed.

  In addition, as described above, the frequency characteristics are affected by loss (mainly cable loss), and if the input level becomes indefinite, the transient amplification operation at the time of transmission is affected, and the internal power amplifier There was also a problem that the circuit might be damaged.

  The present invention has been made in view of such a situation, and an object thereof is to provide an input level correction circuit capable of solving the above-described problems.

An input level correction circuit according to the present invention is an input level correction circuit for correcting an input level of a plurality of high-frequency signals having different input levels from a plurality of signal sources before the power amplification means. Attenuating means for attenuating according to the frequency, and attenuation amount changing means for detecting an appropriate input level of each of the lower limit frequency and the upper limit frequency of the high-frequency signal and changing the attenuation amount by the attenuation means, and the attenuation amount The changing means is based on the attenuation obtained by linearly approximating an appropriate input level at a frequency between the lower limit frequency and the upper limit frequency based on the attenuation changed corresponding to the lower limit frequency and the upper limit frequency. The attenuation amount by the attenuation means is changed.
In the input level correction circuit of the present invention, the attenuation amount changing means linearly approximates an appropriate input level at a frequency between the lower limit frequency and the upper limit frequency based on the attenuation amount changed corresponding to the lower limit frequency and the upper limit frequency. By changing the attenuation amount by the attenuation means in accordance with the obtained attenuation amount, input level correction of high-frequency signals having different input levels from a plurality of signal sources is appropriately performed.

  According to the input level correction circuit of the present invention, an appropriate input level at a frequency between the lower limit frequency and the upper limit frequency is linearly approximated based on the attenuation amount changed corresponding to the lower limit frequency and the upper limit frequency by the attenuation amount changing means. The input level correction of high-frequency signals with different input levels from a plurality of signal sources is appropriately performed by changing the attenuation amount by the attenuation means according to the attenuation amount obtained in this way. Even if the path length with the signal source becomes longer and the loss (mainly cable loss) between the paths increases in proportion to the frequency, which may affect the frequency characteristics, the appropriate input level should be linear. Since the attenuation amount by the attenuation unit is changed according to the attenuation amount obtained by approximation, stable amplification output by the power amplification unit can be performed.

  Even if the frequency characteristics are affected by loss (mainly cable loss) and the input level becomes indefinite, the attenuation means according to the attenuation obtained by linearly approximating the appropriate input level as described above. Since the amount of attenuation due to is changed, the transient amplification operation at the time of transmission is not affected, and damage to the internal circuit of the power amplification means can be prevented.

  Details of the embodiment of the present invention will be described below. FIG. 1 is a diagram for explaining an outline of a wireless system to which an input level correction circuit according to an embodiment of the present invention is applied.

  The wireless system shown in the figure includes a plurality of signal sources 40a to 40c, a switch 50, and a plurality of power amplifiers 100a to 100c. Output connectors 41 a to 41 c of the signal sources 40 a to 40 c are connected to input connectors 51 a to 51 c of the switch 50. The output connectors 52a to 52c of the switch 50 are connected to the input connectors 101a to 101c of the power amplifiers 100a to 100c. Antennas 130a to 130c are connected to the output connectors 102a to 102c of the power amplifiers 100a to 100c.

  In such a wireless system, the signal sources 40a to 40c and the power amplifiers 100a to 100c are freely combined on a one-to-one basis by switching by the selector switch 53 of the switch 50, and are amplified by the respective power amplifiers 100a to 100c. Signals are output from the antennas 130a to 130c connected to the output connectors 102a to 102c.

  Here, when a path based on a combination of the signal sources 40a to 40c and the power amplifiers 100a to 100c is set by switching by the switch 50, the power amplifiers 100a to 100c are set to one of the signal sources 40a corresponding to the path. A transmission command for transmitting a lower limit frequency and an upper limit frequency, which are high-frequency signals for matching, is issued to ˜40c.

  When the signal sources 40a to 40c have information related to the transmission at the lower limit frequency and the transmission at the upper limit frequency of the high frequency signal for matching, the signal sources 40a to 40c are transmitted from the power amplifiers 100a to 100c. The transmission command to the 40c side does not require a command for instructing the lower limit and the upper limit frequency, and only needs to be a transmission instruction.

  FIG. 2 is a circuit block diagram for explaining the internal configuration of the power amplifiers 100a to 100c. In the following drawings, the internal configurations of the power amplifiers 100a to 100c are substantially the same, and therefore, the power amplifier 100a will be described as a representative.

  That is, the power amplifier 100 a includes an input level correction circuit 110 and a power amplification unit (PA) 120.

  The input level correction circuit 110 is provided in front of the power amplification unit (PA) 120, and includes an attenuator 111, a variable attenuator 112, an attenuator 113, a changeover switch 114, an amplifier 115, a detection diode 116, a comparator 117, and a matching circuit. (PLD) 118 and D / A converter 119 are provided.

  The attenuator 111 attenuates the level of the high-frequency signal input from the input connector 101a to a predetermined amount. Here, the high-frequency signal input from the input connector 101a is a signal from one of the signal sources 40a to 40c switched by the switch 50 described above, but when input level correction is performed, a matching signal is used. It is said.

  The variable attenuator 112 attenuates the level of the high-frequency signal from the attenuator 111 according to the attenuation control voltage from the D / A converter 119 controlled by the matching circuit (PLD) 118. The attenuator 113 attenuates the level of the high-frequency signal attenuated by the variable attenuator 112 to a predetermined amount.

  The changeover switch 114 is switched to the a side when the input level is corrected, and is switched to the b side during normal transmission after the input level correction. That is, when the switch 50 described above sets a path by a combination of the signal sources 40a to 40c and the power amplifiers 100a to 100c, the changeover switch 114 is switched from the b side to the a side, and the input level correction processing is performed. The details will be described later.

  The amplifier 115 amplifies the high frequency signal attenuated by the attenuator 113 to a predetermined amount. The detection diode 116 detects the high frequency signal amplified by the amplifier 115 and outputs the detected voltage (DC voltage).

  The comparator 117 compares the reference voltage and the detection voltage (DC voltage), and outputs when the detection voltage (DC voltage) exceeds the reference voltage.

  The matching circuit (PLD) 118 controls the attenuation amount of the variable attenuator 112 at the time of input level correction and normal transmission after input level correction. That is, at the time of input level correction, when the changeover switch 114 is switched from the b side to the a side, the variable attenuator 112 is controlled to the maximum attenuation amount via the D / A converter 119, and then again the D / A The variable attenuator 112 is controlled via the A converter 119 so that the amount of attenuation gradually decreases.

  Such control of the attenuation of the variable attenuator 112 is performed while a transmission command is issued from the power amplifier (PA) 100a to any one of the signal sources 40a to 40c switched by the switch 50. Further, the control of the attenuation amount of the variable attenuator 112 is the same for the lower limit frequency and the upper limit frequency that are high-frequency signals for matching from any one of the signal sources 40a to 40c switched by the switch 50. To be done.

  Here, after controlling the variable attenuator 112 to the maximum attenuation with respect to the lower limit frequency for matching from any of the signal sources 40a to 40c, the variable attenuator 112 is controlled via the D / A converter 119. When the attenuation is controlled so as to gradually decrease, the detection voltage (DC voltage) by the detection diode 116 gradually increases, and when the detection voltage (DC voltage) exceeds the reference voltage in the comparator 117, the variable attenuator. The amount of attenuation 112 is fixed, and the D / A value for the lower limit frequency corresponding to the amount of attenuation when the amount is fixed is stored by the matching circuit (PLD) 118. When the attenuation amount of the variable attenuator 112 is fixed in this manner, the lower limit frequency becomes an appropriate input level.

  Similarly, after controlling the variable attenuator 112 to the maximum attenuation with respect to the matching upper limit frequency from one of the signal sources 40a to 40c, the variable attenuator 112 is controlled via the D / A converter 119. When the attenuation is controlled so as to gradually decrease, the detection voltage (DC voltage) by the detection diode 116 gradually increases, and when the detection voltage (DC voltage) exceeds the reference voltage in the comparator 117, the variable attenuator. The amount of attenuation 112 is fixed, and the D / A value for the upper limit frequency corresponding to the amount of attenuation when the amount is fixed is stored by the matching circuit (PLD) 118. When the attenuation amount of the variable attenuator 112 is fixed in this way, the upper limit frequency becomes an appropriate input level.

  Then, an appropriate D / A value between the lower limit frequency and the upper limit frequency is obtained by linear approximation from the D / A value corresponding to the attenuation amount of the appropriate variable attenuator 112 for each of the lower limit frequency and the upper limit frequency, and input. At the time of normal transmission after level correction, the attenuation amount of the variable attenuator 112 is controlled according to the D / A value obtained by the linear approximation based on the frequency data from one of the signal sources 40a to 40c. The details will be described later.

  The D / A converter 119 outputs a signal for changing the attenuation amount of the variable attenuator 112 according to the control from the matching circuit (PLD) 118.

  The power amplifier (PA) 120 performs power amplification of the high-frequency signal level-corrected by the input level correction circuit 110, and includes a variable attenuator 121, amplifiers 122 and 123, a circulator 124, a bandpass filter (BPF) 125, A directional coupler 126 and an amplifier 127 are provided.

  The variable attenuator 121 attenuates the level of the high-frequency signal whose input level has been corrected by the input level correction circuit 110 in accordance with the increase or decrease of the APC (automatic power control) voltage from the direct conversion circuit 126 obtained via the amplifier 127. . The amplifiers 122 and 123 amplify the high frequency signal attenuated by the variable attenuator 121.

  The circulator 124 has a three-port configuration, for example, and outputs the high-frequency signal amplified by the amplifier 123 to the band-pass filter (BPF) 125 side. The circulator 124 guides a signal entering from the antenna 130a side to the arrow c side. At this time, the signal is absorbed by the resistor R.

  The band-pass filter (BPF) 125 passes only a predetermined band with respect to the high-frequency signal that has passed through the circulator 124. The direct conversion circuit 126 detects a traveling wave of the high-frequency signal that has passed through the band-pass filter (BPF) 125 and outputs it as an APC (automatic power control) voltage. The amplifier 127 amplifies the APC (automatic power control) voltage from the direct conversion circuit 126 to a predetermined value, and outputs the amplified value to the variable attenuator 121.

  Next, the input level correction by the input level correction circuit 110 will be described with reference to FIGS. 3 is a flowchart for explaining the input level correction by the input level correction circuit 110, and FIG. 4 is a diagram for explaining the input level correction by the matching circuit (PLD) 118. As shown in FIG. In the following description, it is assumed that the switch 50 sets a path by a combination of the signal source 40c and the power amplifier 100b, for example.

  First, when a path based on a combination of the signal source 40c and the power amplifier 100b is set by the switch 50 (step S1), a command for transmitting a high-frequency signal for matching is sent from the power amplifier 100b to the signal source 40c. (Step S2). In this case, a transmission command is issued from the power amplifier 100b to the signal source 40c so as to transmit at the lower limit frequency. This transmission command is continuously output until the correction of the input level with respect to the lower limit frequency and the upper limit frequency described later is completed by the input level correction circuit 110.

  In addition, when transmitting a high-frequency signal for matching to the signal source 40c side, if there is information to transmit at the lower limit frequency, as described above, the power amplifier 100b to the signal source 40c An instruction to transmit at the lower limit frequency is not necessary, and only a transmission instruction may be used. Also in this case, the transmission instruction is continuously output until the input level correction for the lower limit frequency and the upper limit frequency described later is completed by the input level correction circuit 110.

  Next, on the power amplifier 100b side, when the changeover switch 114 of the input level correction circuit 110 is switched from the b side during normal transmission after the input level correction to the a side during input level correction (step S3), the matching circuit (PLD) ) 118 so that the attenuation of the variable attenuator 112 is maximized via the D / A converter 119, and then the attenuation of the variable attenuator 112 is gradually reduced via the D / A converter 119. It is controlled (step S4).

  At this time, a matching high-frequency signal from the signal source 40 c input from the input connector 101 b passes through the attenuator 111, the variable attenuator 112, the attenuator 113, and the changeover switch 114 (switched to the “a” side). Amplified to a predetermined value, the detection diode 116 outputs a detection voltage (DC voltage) of the amplified high frequency signal.

  As described above, when the attenuation amount of the variable attenuator 112 is gradually reduced, the detection voltage (DC voltage) by the detection diode 116 gradually increases, and the detection voltage (DC voltage) becomes the reference voltage in the comparator 117. When exceeding (step S5), at this time, the matching circuit (PLD) 118 fixes the attenuation of the variable attenuator 112, and the D / A value for the lower limit frequency corresponding to the attenuation at the time of fixing is Stored by the matching circuit (PLD) 118 (step S6). When the attenuation amount of the variable attenuator 112 is fixed in this manner, the lower limit frequency becomes an appropriate input level.

  Next, a transmission command is issued from the power amplifier 100b to the signal source 40c to transmit at the upper limit frequency (step S7). In this case, as described above, if the signal source 40c has information to transmit at the upper limit frequency, a command to transmit at the upper limit frequency from the power amplifier 100b to the signal source 40c is not necessary. Yes, only a transmission instruction is required.

  Then, the matching circuit (PLD) 118 controls the variable attenuator 112 through the D / A converter 119 so that the attenuation amount of the variable attenuator 112 becomes maximum, and then the variable attenuator 112 through the D / A converter 119, as described above. Is controlled so that the amount of attenuation gradually decreases (step S8).

  At this time, as described above, as the attenuation of the variable attenuator 112 gradually decreases, the detection voltage (DC voltage) by the detection diode 116 gradually increases, and the detection voltage (DC voltage) is supplied to the comparator 117. When the reference voltage is exceeded (step S9), at this time, the matching circuit (PLD) 118 fixes the attenuation amount of the variable attenuator 112, and the D / A value for the upper limit frequency according to the attenuation amount at the time of fixing. Is stored by the matching circuit (PLD) 118 (step S10). When the attenuation amount of the variable attenuator 112 is fixed in this way, the upper limit frequency becomes an appropriate input level.

  At this time, the matching circuit (PLD) 118, as shown in FIG. 4, based on the D / A value at which the lower limit frequency becomes an appropriate input level and the D / A value at which the upper limit frequency becomes an appropriate input level. The slope (D / A value) between the lower limit frequency (for example, 200 MHz) and the upper limit frequency (for example, 400 MHz) is obtained by linear approximation, and between the lower limit frequency (for example, 200 MHz) and the upper limit frequency (for example, 400 MHz). Frequency division with a predetermined width is performed (step S11).

  The slope (D / A value) obtained by such linear approximation is used as an appropriate amount of attenuation in the variable attenuator 112 with respect to the frequency of the high-frequency signal from the signal source 40c during normal transmission after input level correction.

  That is, based on the D / A value at which the lower limit frequency becomes an appropriate input level by the matching circuit (PLD) 118 and the D / A value at which the upper limit frequency becomes an appropriate input level, the lower limit frequency (for example, 200 MHz) and an upper limit frequency (for example, 400 MHz) so as to obtain the slope (D / A value), the above-mentioned according to the frequency data from the signal source 40c at the time of normal transmission after the input level correction. The attenuation amount of the variable attenuator 112 is appropriately controlled by the optimum D / A value corresponding to the frequency division.

  FIG. 4 shows a case where the lower limit frequency is 200 MHz, for example, and the upper limit frequency is 400 MHz, for example, and the slope (D / A value) between the lower limit frequency and the upper limit frequency is obtained. When the frequency data from the signal source 40c is other than 200 MHz to 400 MHz, the optimum D / A value is obtained along the slope (D / A value) between the lower limit frequency and the upper limit frequency, and the attenuation of the variable attenuator 112 is obtained. The amount is properly controlled.

  As described above, in the present embodiment, the matching circuit (PLD) 118 serving as the attenuation amount changing unit is set between the lower limit frequency and the upper limit frequency based on the attenuation amount changed corresponding to the lower limit frequency and the upper limit frequency of the high frequency signal. By changing the attenuation amount by the variable attenuator 112 as the attenuation means in accordance with the attenuation amount obtained by linearly approximating an appropriate input level at the frequency, high frequencies having different input levels from the plurality of signal sources 40a to 40c. The signal input level was corrected appropriately.

  As a result, the path length with any one of the signal sources 40a to 40c is increased, and the loss (mainly cable loss) between the paths increases in proportion to the frequency, which may affect the frequency characteristics. However, since the attenuation amount by the variable attenuator 112 is changed according to the attenuation amount obtained by linearly approximating an appropriate input level, stable amplification output is performed by the power amplifying unit (PA) 120 as the power amplifying means. be able to.

  In addition, even if the frequency characteristics are affected by loss (mainly cable loss) and the input level becomes indefinite, the variable attenuator can be used according to the attenuation obtained by linearly approximating the appropriate input level as described above. Since the attenuation amount by 112 is changed, the transient amplification operation at the time of transmission is not affected, and damage to the internal circuit of the power amplification unit (PA) 120 can be prevented.

  In this embodiment, the matching circuit (PLD) 118 obtains an optimum D / A value along the slope (D / A value) between the lower limit frequency and the upper limit frequency, and the attenuation amount of the variable attenuator 112. Is appropriately controlled, digital control operations can be performed, and the number of circuit adjustment points can be reduced as compared with an analog control circuit.

  In this embodiment, the input level correction circuit 110 includes a relatively inexpensive attenuator 111, variable attenuator 112, attenuator 113, changeover switch 114, amplifier 115, detector diode 116, comparator 117, and matching circuit (PLD) 118. Since it can be configured with the D / A converter 119, the circuit configuration can be made inexpensive.

  In the present embodiment, if the attenuation amount of the variable attenuator 112 is set in the matching operation, it is fixed until the next matching operation. Therefore, the modulation in which the amplitude varies and the radio frequency (RF) at the time of transmission rising are unnecessarily followed. Without limitation, the conventional power amplification unit (PA) 120 having the configuration of transmission output detection feedback control can be used as it is.

  In the present embodiment, the attenuation amount of the variable attenuator 112 can be changed at the timing when the frequency data from any one of the signal sources 40a to 40c changes. In addition, since it is possible to set a D / A value that provides an appropriate amount of attenuation for high-speed frequency switching such as frequency hopping, it is possible to perform appropriate transmission output control even during a transient operation.

  The present invention can be applied to a wireless system such as mobile communication or fixed communication.

It is a figure for demonstrating the outline | summary of the radio | wireless system to which the input level correction circuit which concerns on one Embodiment of this invention is applied. FIG. 2 is a circuit block diagram for explaining an internal configuration of the power amplifier of FIG. 1. 3 is a flowchart for explaining input level correction by the input level correction circuit of FIG. 2. It is a figure for demonstrating the input level correction | amendment by the circuit for a matching (PLD) of FIG. It is a circuit block diagram for demonstrating an example of the conventional radio system.

Explanation of symbols

100a to 100c Power amplifier 101a to 101c Input connector 102a to 102c Output connector 110 Input level correction circuit 111 Attenuator 112 Variable attenuator 113 Attenuator 114 Changeover switch 115 Amplifier 116 Detection diode 117 Comparator 118 Matching circuit (PLD)
119 D / A converter 121 Variable attenuator 122, 123 Amplifier 124 Circulator 125 Band pass filter (BPF)
126 Directional Coupler
127 Amplifier 130a-130c Antenna

Claims (1)

An input level correction circuit that performs input level correction of a plurality of high-frequency signals having different input levels from a plurality of signal sources at a stage preceding the power amplification means,
Attenuating means for attenuating according to the input level of the high-frequency signal;
Attenuation amount changing means for detecting an appropriate input level of each of the lower limit frequency and the upper limit frequency of the high-frequency signal and changing the attenuation amount by the attenuation means,
The attenuation amount changing means is responsive to an attenuation amount obtained by linearly approximating an appropriate input level at a frequency between the lower limit frequency and the upper limit frequency based on the attenuation amount changed corresponding to the lower limit frequency and the upper limit frequency. An input level correction circuit, wherein the attenuation amount by the attenuation means is changed.

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