Patent Publication Number: US-9841486-B2

Title: Detection calibration circuit and transmission apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a detection calibration circuit that calibrates the power of a transmission signal and a transmission apparatus. 
     2. Description of the Related Art 
     In recent years, for wireless communication with high-speed transmission, a high-frequency band (for example, a frequency band of some hundreds MHz such as a millimeter wave band) is used in one channel to secure the band of a modulation signal and perform higher-speed transmission. Furthermore, for wireless communication with high-speed transmission over a long distance, the signal level needs to be maintained constant between a transmission apparatus and a reception apparatus to stably maintain the quality of the high-speed communication. 
     In the transmission apparatus, a power control circuit that detects power by using a detection circuit and adjusts a gain of an amplification circuit is used so that the power of a transmission signal is maintained constant regardless of an external perturbations (for example, a temperature fluctuation and a power supply fluctuation). For example, because the gain characteristic of a transistor used in a detection circuit is insufficient in a high frequency region such as a millimeter wave band, fluctuations in the gain characteristic due to the temperature, the power source, and aging are increased. Therefore, in the detection circuit, variations in the input-output characteristic are increased, whereby an error is generated in the output voltage of the detection circuit (hereinafter, referred to as “detected output voltage”). 
     Furthermore, because the input control range of the detection circuit is narrow in a high-frequency band, when variations in the input-output characteristic of the detection circuit are great, the input voltage level of a high frequency signal to be input deviates from the input signal range and the detected output voltage becomes inaccurate. 
     As a related art for reducing variations in an input-output characteristic of a detection circuit, a transmission power control circuit described in Japanese Patent No. 4304296 is proposed, for example. Furthermore, as a related art for suppressing variations in an input-output characteristic of a detection circuit due to aging, a transmission power detection circuit described in Japanese Unexamined Patent Application Publication No. 2006-13753 is proposed, for example. The details of each of the documents mentioned above will be described later with reference to  FIG. 9(A)  and  FIG. 9(B) . 
     SUMMARY 
     With the structures disclosed in Japanese Patent No. 4304296 and Japanese Unexamined Patent Application Publication No. 2006-13753 mentioned above, when a high frequency signal (for example, a microwave and a millimeter wave) is used, a sufficient gain characteristic cannot be obtained for a transistor used in the detection circuit. There have been cases where the detection accuracy of the detection circuit is insufficient due to temperature fluctuations, power supply fluctuations, and individuality variations, for example. 
     One non-limiting and exemplary embodiment provides a detection calibration circuit and a transmission apparatus that suppress fluctuations in the detected output voltage of a high frequency signal even when a temperature fluctuation, a power supply fluctuation, or aging is generated. 
     In one general aspect, the techniques disclosed here feature a detection calibration circuit that includes a first distributor, a first amplifier, a second distributor, a reference signal generator, a switcher, a detector, a sensitivity switcher, and a calibration control circuit. The first distributor distributes a first high frequency input signal into a first high frequency signal and a second high frequency signal. The first amplifier amplifies the first high frequency signal. The second distributor distributes the amplified first high frequency signal further into a third high frequency signal and a fourth high frequency signal. The reference signal generator uses the second high frequency signal to generate a reference signal in accordance with a predetermined switchable reference voltage. The switcher selects the third high frequency signal or the reference signal of the reference signal generator. The detector outputs a detection signal obtained by detecting the selected signal. The sensitivity switcher adjusts an input-output sensitivity for the detection signal. The calibration control circuit adjusts a detection gain of the detector and an input-output sensitivity for the detection signal. 
     According to the present disclosure, even when a temperature fluctuation, a power supply fluctuation, or aging is generated, fluctuations in the detected output of a high frequency signal can be reduced and deterioration of the detection characteristic can be suppressed. 
     It should be noted that general or specific embodiments may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating the internal structure of a detection calibration circuit according to Embodiment 1; 
         FIG. 2  includes a diagram illustrating a first example of the circuit structure of a saturation amplifier, a diagram illustrating an example of a waveform of an input signal to transistors, and a diagram illustrating an example of a waveform of an output signal of the saturation amplifier; 
         FIG. 3  includes a diagram illustrating a second example of the circuit structure of the saturation amplifier, a diagram illustrating an example of a waveform of an input signal to one transistor, a diagram illustrating an example of a waveform of an input signal to the other transistor, and a diagram illustrating an example of a waveform of an output signal of the saturation amplifier; 
         FIG. 4  includes a diagram illustrating an example of the circuit structure of a detection circuit, a diagram illustrating an example of a waveform of a signal input to the detection circuit, a diagram illustrating an example of a waveform of an output signal before smoothing, and a diagram illustrating an example of a waveform of an output signal after smoothing; 
         FIG. 5  includes a diagram illustrating an example of a detection gain in accordance with the relation between a detected input voltage and a detected output voltage, a diagram illustrating an example of the sensitivity for a detected output voltage corresponding to a detected input voltage, a diagram illustrating an example of a sensitivity switching circuit, and a diagram illustrating another example of the sensitivity switching circuit; 
         FIG. 6  is a circuit diagram illustrating the internal structure of a detection calibration circuit according to Embodiment 2; 
         FIG. 7  is a circuit diagram illustrating the internal structure of a detection calibration circuit according to Embodiment 3; 
         FIG. 8  includes a diagram illustrating an example of a matching switching circuit, an explanatory diagram illustrating an example of a Smith chart in which a high frequency signal is input to a detection circuit in the matching switching circuit, and an explanatory diagram illustrating an example of a Smith chart in which a reference (voltage) signal is input to the detection circuit in the matching switching circuit; 
         FIG. 9  includes a circuit diagram illustrating the internal structure of a transmission power control circuit in a first conventional example and a circuit diagram illustrating the internal structure of a transmission power detection circuit in a second conventional example; and 
         FIG. 10  is a circuit diagram illustrating the internal structure of a detection calibration circuit according to Embodiment 4. 
     
    
    
     DETAILED DESCRIPTION 
     (Background Leading to the Contents of the Respective Embodiments) 
     Before the contents of the respective embodiments of a detection calibration circuit and a transmission apparatus according to the present disclosure are described, the background leading to the contents of the respective embodiments will be described below with reference to  FIG. 9(A)  and  FIG. 9(B) .  FIG. 9(A)  is a circuit diagram illustrating the internal structure of a transmission power control circuit  100  in a first conventional example.  FIG. 9(B)  is a circuit diagram illustrating the internal structure of a transmission power detection circuit  200  in a second conventional example. 
     The transmission power control circuit  100  disclosed in Japanese Patent No. 4304296 illustrated in  FIG. 9(A)  includes an amplification circuit  101 , a coupler  102 , a detection circuit  103 , a storage unit  104 , and a control circuit  105 . In the transmission power control circuit  100 , an input signal of which the power has been amplified in the amplification circuit  101  is distributed into two types of signals in the coupler  102 . One type of signals are output to a post stage (not illustrated) and the other type of signals are input to the detection circuit  103 . 
     The detection circuit  103  detects an input signal and outputs a signal of a detected output voltage as a detection result to the control circuit  105 . The control circuit  105  adds an offset correction voltage value previously retained in the storage unit  104  to the detected output voltage and outputs the value obtained by the addition. At this point, the offset correction voltage value represents the offset amount of a voltage value in accordance with fluctuations in the temperature or the power supply voltage. With this, the transmission power control circuit  100  illustrated in  FIG. 9(A)  suppresses variations in the input-output characteristic (gain characteristic) of the detection circuit  103  even when the temperature or the power supply voltage fluctuates. 
     However, in the transmission power control circuit  100  illustrated in  FIG. 9(A) , the offset amount once retained in the storage unit  104  does not fluctuate. Therefore, when the input-output characteristic of the detection circuit  103  is deteriorated and changed due to aging, for example, errors of the detected output voltage are increased and require measures. 
     Furthermore, the gain characteristic and the input-output sensitivity characteristic of the detection circuit  103  are not adjusted. Therefore, in a state in which the gain characteristic of a transistor constructing the detection circuit  103  cannot be obtained sufficiently, the input signal range of a high frequency signal (for example, a millimeter wave signal) may be narrowed in the detection circuit  103 . 
     This is because in the detection circuit  103 , when a fluctuation is generated in the temperature or the power supply, the input signal range that can be detected by the detection circuit  103  is displaced due to a gain fluctuation and the input-output sensitivity of the detection circuit  103  is increased, whereby the input signal range is narrowed. 
     With this structure, in the detection circuit  103 , there are some cases where the input voltage of a high frequency signal deviates from the input signal range of the detection circuit  103  and the detection voltage is not output. 
     The transmission power detection circuit  200  illustrated in  FIG. 9(B)  includes an amplification circuit  201 , a coupler  202 , a detection circuit  203 A, a dummy amplifier  203 B, and a synthetic circuit  204 . In the transmission power detection circuit  200 , an input signal of which the power has been amplified in the amplification circuit  201  is distributed into two types of signals in the coupler  202 . One type of signals are output to a post stage (not illustrated) and the other type of signals are input to the detection circuit  203 A. 
     The detection circuit  203 A detects an input signal and outputs a signal of a detected output voltage as a detection result to the synthetic circuit  204 . The dummy amplifier  203 B uses a transistor of the same type as that of the transistor constructing the detection circuit  203 A to output an offset voltage to the synthetic circuit  204 . The synthetic circuit  204  outputs a detected output voltage being a synthetic wave obtained by subtracting the offset voltage of the dummy amplifier  203 B from the detected output voltage of the detection circuit  203 A. 
     With this structure, in the transmission power detection circuit  200 , when the input-output characteristic (gain characteristic) of the detection circuit  203 A fluctuates due to fluctuations in the temperature or the power supply including aging, the output (offset voltage) of the dummy amplifier  203 B similarly fluctuates. Each of the fluctuation amounts of the detected output voltage of the detection circuit  203 A and the offset voltage of the dummy amplifier  203 B is thus cancelled in the synthetic circuit  204 , whereby a stable detected output voltage can be obtained. 
     However, in the transmission power detection circuit  200  illustrated in  FIG. 9(B) , the gain characteristic and the input-output sensitivity characteristic of the detection circuit  203 A are not adjusted. Therefore, similarly to the case of the transmission power control circuit  100  illustrated in  FIG. 9(A) , when a fluctuation is generated in the temperature or the power supply, the input signal range that can be detected by the detection circuit  203 A is displaced due to a gain fluctuation and the input-output sensitivity of the detection circuit  203 A is increased, whereby the input signal range is narrowed. Therefore, there have been some cases where the input voltage of a high frequency signal deviates from the input signal range of the detection circuit  203 A and the detection voltage is not output. 
     Furthermore, in the transistor used in the detection circuit  203 A and the dummy amplifier  203 B, a relative error of the gain characteristic is superimposed on the detected output voltage. Therefore, in the transmission power detection circuit  200 , when a high frequency signal (for example, a millimeter wave) is used, the output of the detected output voltage becomes inaccurate due to a variation caused by the relative error. 
     Based on the background described above, each embodiment below describes an example of a detection calibration circuit and a transmission apparatus that suppress fluctuations in the detected output voltage of a high frequency signal even when a temperature fluctuation, a power supply fluctuation, or aging is generated, with reference to the drawings. 
     Embodiment 1 
       FIG. 1  is a circuit diagram illustrating the internal structure of a detection calibration circuit  10  according to Embodiment 1. A transmission apparatus  1  illustrated in  FIG. 1  includes a central processing unit (CPU)  5 , a detection calibration circuit  10 , and a transmission antenna Ant. Furthermore, the detection calibration circuit  10  illustrated in  FIG. 1  includes a coupler  11 , an amplification circuit  12 , a coupler  13 , a reference signal circuit  14 , a selector switch  15 , a detection circuit  16 , a sensitivity switching circuit  17 , and a calibration control circuit  18 . 
     To an input terminal of the detection calibration circuit  10  illustrated in  FIG. 1 , a high frequency signal (for example, a millimeter wave) RF 0  generated in a quadrature modulator (not illustrated) of the transmission apparatus  1  is input, for example. 
     The CPU  5  controls an operation performed by the transmission apparatus  1 . For example, the CPU  5  generates a control signal for adjusting the gain of the amplification circuit  12  in accordance with a detected output voltage Vdet of the detection calibration circuit  10  and outputs the generated control signal to the amplification circuit  12 . Furthermore, the CPU  5  switches the detection calibration circuit  10  to a calibration mode or a detection mode, and further generates a control signal for switching to either mode and outputs the generated control signal to the calibration control circuit  18 . The control signal from the CPU  5  is input to a mode determiner  18   a  of the calibration control circuit  18 . 
     The coupler  11  as an example of the first distributor is constructed with a directional coupler, for example. The coupler  11  distributes the high frequency signal RF 0  input to the input terminal of the detection calibration circuit  10  into two types of high frequency signals RF 01  and RF 02 , for example, and outputs the high frequency signals RF 01  and RF 02 . The high frequency signal RF 02  generated by distribution performed by the coupler  11  is input to the reference signal circuit  14 . 
     To the amplification circuit  12  as an example of an amplifier, the high frequency signal RF 01  generated by distribution performed by the coupler  11  is input, and the gain of the amplification circuit  12  is adjusted in accordance with the control signal from the CPU  5 . The amplification circuit  12  amplifies the high frequency signal RF 01  in accordance with the initial value of the gain or the value of the gain adjusted in accordance with the control signal from the CPU  5 , and outputs the amplified high frequency signal RF 01  to the coupler  13 . 
     The coupler  13  as an example of the second distributor is constructed with a directional coupler, for example. The coupler  13  distributes the high frequency signal RF 01  amplified by the amplification circuit  12  into two types of high frequency signals RF 011  and RF 012 , for example, and outputs the high frequency signals RF 011  and RF 012 . The high frequency signal RF 011  generated by distribution performed by the coupler  13  is transmitted from the transmission antenna Ant. 
     The reference signal circuit  14  as an example of the reference signal generator includes a saturation amplifier  14   a - 1  or a saturation amplifier  14   a , and a reference voltage switcher  14   b . The saturation amplifier  14   a - 1  or the saturation amplifier  14   a  receives the high frequency signal RF 02  and outputs a reference (voltage) signal RF 02   a  in accordance with a reference power supply voltage Vref supplied from the reference voltage switcher  14   b  to the selector switch  15 . 
     The saturation amplifier  14   a - 1  will now be described with reference to  FIG. 2(A) ,  FIG. 2(B) , and  FIG. 2(C) .  FIG. 2(A)  is a diagram illustrating a first example of the circuit structure of the saturation amplifier.  FIG. 2(B)  is a diagram illustrating an example of a waveform of an input signal to transistors.  FIG. 2(C)  is a diagram illustrating an example of a waveform of an output signal of the saturation amplifier  14   a - 1  illustrated in  FIG. 2(A) . 
     The saturation amplifier  14   a - 1  illustrated in  FIG. 2(A)  performs a push-pull operation in a low frequency band. The high frequency signal RF 02  that has been input to the saturation amplifier  14   a - 1  is input to the gate terminals of a p-channel metal-oxide-semiconductor (pMOS) transistor tr 1  and an n-channel metal-oxide-semiconductor (nMOS) transistor tr 2 , which have a bias voltage of Vg, via the DC cut capacitor C 1 . The source terminal of the pMOS transistor tr 1  and the source terminal of the nMOS transistor tr 2  are connected to the output side of the saturation amplifier  14   a - 1 . To the drain terminal of the pMOS transistor tr 1 , the reference power supply voltage Vref is supplied. The drain terminal of the nMOS transistor tr 2  is connected to a ground potential (GND potential, 0 V). 
     To simplify the description below, it is defined that the pMOS transistor tr 1  and the nMOS transistors tr 2 , tr 3 , and tr 4  that are used in the saturation amplifiers  14   a - 1  and  14   a  are operated with the same bias voltage Vg. 
     In  FIG. 2(B) , with respect to a voltage Vin of the high frequency signal RF 02  input to the gate terminals of the pMOS transistor tr 1  and the nMOS transistor tr 2 , when the voltage Vin has an amplitude in a region P 1  and a region P 2  centered on the bias voltage Vg, the nMOS transistor tr 2  is turned ON and the pMOS transistor tr 1  is turned OFF in the region P 1 . The nMOS transistor tr 2  is thus operated with the pMOS transistor tr 1  serving as a high impedance load. Furthermore, the output signal (reference (voltage) signal) of the saturation amplifier  14   a - 1  is set to the GND potential (0 V). 
     By contrast, in the region P 2 , the pMOS transistor tr 1  is turned ON, and the nMOS transistor tr 2  is turned OFF. The pMOS transistor tr 1  is thus operated with the nMOS transistor tr 2  serving as a high impedance load. Furthermore, the output signal (reference (voltage) signal) of the saturation amplifier  14   a - 1  is set to the reference power supply voltage Vref. 
     The output signal (reference (voltage) signal: voltage Vout) of the saturation amplifier  14   a - 1  is thus a reference (voltage) signal RF 02   a  of a rectangular waveform having an amplitude of the reference power supply voltage Vref from the reference voltage switcher  14   b , regardless of external factors (for example, a temperature fluctuation, a power supply fluctuation, and aging) (See  FIG. 2(C) ). 
     Furthermore, the reference voltage switcher  14   b  supplies a plurality of predetermined and different types of reference power supply voltages Vref to the saturation amplifier  14   a - 1  in accordance with a voltage control signal from a reference voltage controller  18   b  described later. With this, the saturation amplifier  14   a - 1  can output the reference (voltage) signal RF 02   a  of a rectangular waveform having the amplitude of the different types of reference power supply voltage Vref in accordance with the reference power supply voltage Vref supplied from the reference voltage switcher  14   b.    
     As the frequency band of the frequency of the reference (voltage) signal RF 02   a  becomes closer to that of the high frequency signal RF 012  detected, the detected output voltage error between the level of the reference (voltage) signal and the level of the high frequency signal RF 012  due to the frequency band difference is decreased, whereby the detection accuracy is improved. It should be noted that in the structure of the saturation amplifier  14   a - 1  illustrated in  FIG. 2(A) , the pMOS transistor tr 1  is used, and the operation frequency thereof is thus from 5 GHz to 10 GHz approximately. 
     By contrast, by using the saturation amplifier  14   a  illustrated in  FIG. 3(A) , for example, the operation frequency of the reference signal circuit  14  can be improved up to a millimeter wave band. Next, the saturation amplifier  14   a  will be described with reference to  FIG. 3(A) ,  FIG. 3(B) ,  FIG. 3(C) , and  FIG. 3(D) .  FIG. 3(A)  is a diagram illustrating a second example of the circuit structure of the saturation amplifier.  FIG. 3(B)  is a diagram illustrating an example of a waveform of an input signal to one transistor.  FIG. 3(C)  is a diagram illustrating an example of a waveform of an input signal to the other transistor.  FIG. 3(D)  is a diagram illustrating an example of a waveform of an output signal of the saturation amplifier  14   a  illustrated in  FIG. 3(A) . 
     The saturation amplifier  14   a  illustrated in  FIG. 3(A)  performs a push-pull operation in a high frequency band. The high frequency signal RF 02  that has been input to the saturation amplifier  14   a  is converted from a one-phase signal into a differential signal (positive-phase high frequency signal and negative-phase high frequency signal) in a transformer trns via a DC cut capacitor C 1 . 
     An output signal (positive-phase high frequency signal and negative-phase high frequency signal) of the transformer trns is input to gate terminals of two nMOS transistors tr 2   a  and tr 2   b , which have a bias voltage of Vg. Each of the source terminals of the nMOS transistors tr 2   a  and tr 2   b  is connected to the output side of the saturation amplifier  14   a . To the drain terminal of the nMOS transistor tr 2   a , the reference power supply voltage Vref is supplied. The drain terminal of the nMOS transistor tr 2   b  is connected to a ground potential (GND potential, 0 V). 
     In  FIG. 3(B)  and  FIG. 3(C) , when the voltages Vin 1  of the output (positive-phase high frequency signal) of the transformer trns is a positive voltage in a region P 1  centered on the bias voltage Vg, and the voltages Vin 2  of the output (negative-phase high frequency signal) of the transformer trns is a positive voltage in a region P 2  centered on the bias voltage Vg, the nMOS transistors tr 2   a  and tr 2   b  are never turned ON at the same time because the region P 1  and the region P 2  alternately appear. 
     In the region P 1  in  FIG. 3(B) , the nMOS transistors tr 2   b  is turned ON and the nMOS transistor tr 2   a  is turned OFF. The nMOS transistors tr 2   b  is thus operated with the nMOS transistor tr 2   a  serving as a high impedance load. Furthermore, the output signal (reference (voltage) signal) of the saturation amplifier  14   a  is set to the GND potential (0 V) in the voltage region P 1  in  FIG. 3(D) . 
     By contrast, in the region P 2  in  FIG. 3(B) , the nMOS transistors tr 2   a  is turned ON and the nMOS transistor tr 2   b  is turned OFF. The nMOS transistors tr 2   a  is thus operated with the nMOS transistor tr 2   b  serving as a high impedance load. Furthermore, the output signal (reference (voltage) signal) of the saturation amplifier  14   a  is set to the reference power supply voltage Vref in the voltage region P 2  in  FIG. 3(D) . 
     The output signal (reference (voltage) signal: Vout) of the saturation amplifier  14   a  is thus a reference (voltage) signal RF 02   a  of a rectangular waveform having an amplitude of the reference power supply voltage Vref from the reference voltage switcher  14   b , regardless of external factors (for example, a temperature fluctuation, a power supply fluctuation, and aging) even when a high frequency signal (for example, a millimeter wave of 60 GHz or higher) is used (See  FIG. 3(D) ). 
     In the case of a conventional amplifier, because the gain thereof is insufficient at a millimeter frequency, the rectangular wave in  FIG. 3(D)  for an output signal thereof does not extend from the GND to the reference power supply voltage Vref. More specifically, because the conventional amplifier is not in a saturated operation but in a linear operation, the amplitude of the output signal in  FIG. 3(D)  fluctuates in accordance with fluctuations in the gain due to external factors. 
     By contrast, in the case of the saturation amplifier in  FIG. 3 , because the gain thereof is sufficient at a millimeter frequency, the amplitude of the output signal in  FIG. 3(D)  extends from the GND to the reference power supply voltage Vref. The amplitude of the output signal in  FIG. 3(D)  thus does not fluctuate even if the gain fluctuates due to external factors. 
     Similarly, the reference voltage switcher  14   b  supplies a plurality of predetermined and different types of reference power supply voltages Vref to the saturation amplifier  14   a  in accordance with a voltage control signal from the reference voltage controller  18   b  described later. With this, the saturation amplifier  14   a  can output the reference (voltage) signal RF 02   a  of a rectangular waveform having the amplitude of the different types of reference power supply voltage Vref in accordance with the reference power supply voltage Vref supplied from the reference voltage switcher  14   b.    
     The selector switch  15  as an example of the switcher switches between the high frequency signal RF 012  from the coupler  13  and the reference (voltage) signal RF 02   a  from the reference signal circuit  14  in accordance with a switching control signal from the mode determiner  18   a  described later, that is, selects either the high frequency signal RF 012  or the reference (voltage) signal RF 02   a , and outputs the selected signal to the detection circuit  16 . 
     Specifically, when the transmission apparatus  1  is in the calibration mode, the selector switch  15  outputs the reference (voltage) signal RF 02   a  from the reference signal circuit  14  to the detection circuit  16  in accordance with the switching control signal from the mode determiner  18   a  described later. When the transmission apparatus  1  is in the detection mode, the selector switch  15  outputs the high frequency signal RF 012  from the coupler  13  to the detection circuit  16 . 
     The calibration mode is a mode for adjusting (calibrating) a detection gain and an input-output sensitivity of the detection circuit  16  in the detection calibration circuit  10 . The detection mode is a mode for detecting the high frequency signal RF 012  from the coupler  13  in the detection calibration circuit  10 . The mode of the transmission apparatus  1  is determined by the CPU  5 , and a mode control signal output by the CPU  5  is input to the mode determiner  18   a . In accordance with the mode control signal output by the CPU  5 , the mode determiner  18   a  generates the switching control signal indicating either the calibration mode or the detection mode and outputs the generated switching control signal to the selector switch  15 . 
     The detection circuit  16  as an example of a detector detects the reference (voltage) signal RF 02   a  from the reference signal circuit  14  or the high frequency signal RF 012  from the coupler  13  and outputs a signal (detection signal) of the detected output voltage as a detection result to the sensitivity switching circuit  17 . It should be noted that the detection circuit  16  converts the high frequency signal RF 012  or the reference (voltage) signal RF 02   a  into a direct current (DC) signal. However, the detection circuit  16  may be a detection circuit using a diode, for example, in a band of a high frequency signal (for example, a millimeter wave of 60 GHz or higher), and may be a detection circuit using an nMOS transistor that can adjust the detection gain. 
     The detection circuit  16  will now be described with reference to  FIG. 4(A) ,  FIG. 4(B) ,  FIG. 4(C) , and  FIG. 4(D) .  FIG. 4(A)  is a diagram illustrating an example of the circuit structure of the detection circuit.  FIG. 4(B)  is a diagram illustrating an example of a waveform of a signal input to the detection circuit  16 .  FIG. 4(C)  is a diagram illustrating an example of a waveform of an output signal before smoothing.  FIG. 4(D)  is a diagram illustrating an example of a waveform of an output signal after smoothing. 
     In the detection circuit  16  illustrated in  FIG. 4(A) , the reference (voltage) signal RF 02   a  or the high frequency signal RF 012  to be input is input to the gate terminal of the nMOS transistor tr 3  of the bias voltage Vg via a DC cut capacitor C 2 . In  FIG. 4(B) , the voltage Vin 3  of the reference (voltage) signal RF 02   a  or the high frequency signal RF 012  that is input to the detection circuit  16  periodically varies centered on the bias voltage Vg. 
     The source terminal of the nMOS transistor tr 3  is connected to the ground potential (GND potential, 0 V). To the drain terminal of the nMOS transistor tr 3 , a predetermined power supply voltage VDD is supplied. In  FIG. 4(C) , a voltage Vout 2   a  of an output signal of the nMOS transistor tr 3  is a signal (half-wave signal) higher than the voltage Vin 3  of the reference (voltage) signal RF 02   a  or the high frequency signal RF 012  illustrated in  FIG. 4(B) . In  FIG. 4(D) , a voltage Vout 2   b  of a signal obtained by smoothing the output signal of the nMOS transistor tr 3  with a smoothing capacitor C 3  is a detected output voltage being a DC voltage value. 
     The detection circuit  16  thus can adjust the amplitude of the half-wave signal in accordance with a bias control signal from a bias controller  18   c  described later and adjust a detection gain in the detection circuit  16  (that is, an input/output signal voltage ratio in the detection circuit  16 ). Accordingly, the detection circuit  16  can adjust the detection gain by adjusting the bias voltage Vg in accordance with a bias control signal from a bias controller  18   c  regardless of external factors (for example, a temperature fluctuation, a power supply fluctuation, and aging), whereby a desired input-output characteristic DSR 1  can be obtained (see  FIG. 5(A) ). 
       FIG. 5(A)  is a diagram illustrating an example of a detection gain in accordance with the relation between a detected input voltage and a detected output voltage. The input-output characteristic DSR 1  is a characteristic of a detected output voltage with respect to a detected input voltage for obtaining a desired detection gain in the detection circuit  16  even when an external factor (for example, a temperature fluctuation, a power supply fluctuation, and aging) is generated. 
     The sensitivity switching circuit  17  as an example of the sensitivity switcher adjusts the input-output sensitivity for an output signal (more specifically, a signal of the detected output voltage) of the detection circuit  16  in accordance with the sensitivity control signal from a sensitivity controller  18   d  described later (see  FIG. 5(B) ).  FIG. 5(B)  is a diagram illustrating an example of the sensitivity for a detected output voltage corresponding to a detected input voltage. An output of the sensitivity switching circuit  17  is detected in the CPU  5  as a detected output voltage Vdet in the detection calibration circuit  10 . 
     At this point, the sensitivity switching circuit  17  can adjust the input-output sensitivity by switching the voltage dividing resistance ratio using a variable resistances R 1  and R 2  illustrated in  FIG. 5(C) , for example. Furthermore, similarly to the adjustment of the detection gain in the detection circuit  16 , even when an external factor (for example, a temperature fluctuation, a power supply fluctuation, and aging) is generated, the sensitivity switching circuit  17  can obtain a desired input-output characteristic DSR 2  with respect to the input-output sensitivity by switching the input-output sensitivity of the detection circuit  16  when the detection gain fluctuates.  FIG. 5(C)  is a diagram illustrating an example of the sensitivity switching circuit. 
     It should be noted that the output signal of the saturation amplifier  14   a  or  14   a - 1  serves as a reference input signal for checking the deviation amount from input and output signals of the input-output characteristics DSR 1  and DSR 2  in  FIG. 5(A)  and  FIG. 5(B) . Outputs (detected input voltages) from the saturation amplifier  14   a  or  14   a - 1  at two or more points are thus used to adjust the detection circuit  16  and the sensitivity switching circuit  17  such that the detected output voltage corresponds to the input-output characteristics DSR 1  and DSR 2 . 
     With this, the detection calibration circuit  10  can adjust the detection gain and the input-output sensitivity in the detection circuit  16  even when the detected output voltage Vdet fluctuates due to occurrence of an external factor (for example, a temperature fluctuation, a power supply fluctuation, or aging), compared with the transmission power control circuit  100  and the transmission power detection circuit  200  described above, which are illustrated in  FIG. 9(A)  and  FIG. 9(B)  as conventional examples. Accordingly, in the detection calibration circuit  10 , even when a high frequency signal (for example, a millimeter wave) is used, for which the input signal range is narrow and the input-output sensitivity is high in the detection circuit  16 , the signal input to the detection circuit  16  does not deviate from the input signal range, whereby a detected output voltage with less variations can be obtained. 
     Furthermore, as another example of the sensitivity switching circuit, the circuit structure illustrated in  FIG. 5(D)  may be used that can switch the voltage dividing resistance ratio using an operational amplifier OP 1  and variable resistances R 3  and R 4 .  FIG. 5(D)  is a diagram illustrating another example of the sensitivity switching circuit. In a sensitivity switching circuit  17   a  illustrated in  FIG. 5(D) , because the operational amplifier has a function as an output buffer, a stable detected output voltage can be supplied to a load (not illustrated) connected to a post stage of the sensitivity switching circuit  17   a.    
     The calibration control circuit  18  as an example of the calibration control circuit includes the mode determiner  18   a , the reference voltage controller  18   b , the bias controller  18   c , the sensitivity controller  18   d , and a detected voltage comparator  18   e.    
     In accordance with the mode control signal output by the CPU  5 , the mode determiner  18   a  generates the switching control signal indicating either the calibration mode or the detection mode of the transmission apparatus  1  and outputs the generated switching control signal to the selector switch  15 . 
     The reference voltage controller  18   b  generates a voltage control signal for switching to a plurality of different reference power supply voltages Vref based on the output of the detected voltage comparator  18   e  described later and outputs the generated voltage control signal to the reference voltage switcher  14   b . The ranges and the differentials (resolutions) of the reference power supply voltages Vref are determined in advance such that the reference power supply voltages Vref are in the input signal range of the detection circuit  16  as two or more input signals even when the detected output voltage of the detection circuit  16  fluctuates due to occurrence of an external factor (for example, a temperature fluctuation, a power supply fluctuation, or aging). The reference voltage switcher  14   b  supplies the reference power supply voltage Vref in accordance with the voltage control signal to the saturation amplifiers  14   a  and  14   a - 1 . 
     The bias controller  18   c  generates a bias control signal for switching the bias voltage Vg of the detection circuit  16  based on the output of the detected voltage comparator  18   e  described later and outputs the generated bias control signal to the detection circuit  16 . The detection circuit  16  is operated in accordance with the bias voltage Vg corresponding to the bias control signal. 
     The sensitivity controller  18   d  generates a sensitivity control signal for switching the input-output sensitivity of the detection circuit  16  based on the output of the detected voltage comparator  18   e  described later and outputs the generated sensitivity control signal to the sensitivity switching circuit  17 . The voltage dividing resistance ratio is adjusted in accordance with the sensitivity control signal, whereby the input-output sensitivity of the detection circuit  16  is adjusted. 
     The detected voltage comparator  18   e  uses the detected output voltage Vdet for each of two or more reference power supply voltages Vref switched by the reference voltage controller  18   b  to calculate the detection gain and the input-output sensitivity of the detection circuit  16 . 
     The detected voltage comparator  18   e  retains a setting value of the detection gain and the input-output sensitivity for obtaining a desired input-output characteristic in the detection circuit  16 . The detected voltage comparator  18   e  compares each of the calculated values of the detection gain and the input-output sensitivity of the detection circuit  16  and each of the setting values of the detection gain and the input-output sensitivity of the detection circuit  16 . The detected voltage comparator  18   e  controls the reference power supply voltage Vref and the voltage dividing resistance ratio in sensitivity switching circuit  17  such that errors (differentials) of the calculated values and the setting values, which are comparison results, are minimized. 
     With the structure described above, the detection calibration circuit  10  can adjust the detection gain and the input-output sensitivity of the detection circuit  16  in accordance with the detected output voltage for each of a plurality of different reference power supply voltages. 
     The detection calibration circuit  10  thus can obtain desired input-output characteristics DSR 1  and DSR 2  of the detection circuit  16  regardless of external factors (for example, a temperature fluctuation, a power supply fluctuation, and aging), whereby dispersion errors of the detection circuit  16  itself can be reduced and deterioration of the detection accuracy can be suppressed. 
     Furthermore, even when an external factor (for example, a temperature fluctuation, a power supply fluctuation, and aging) is generated, the detection calibration circuit  10  can obtain a desired detected output voltage Vdet when the high frequency signal RF 012  is detected in the detection mode by using the detection gain and the input-output sensitivity of the detection circuit  16  which have been obtained in the calibration mode, whereby variations of the power of the high frequency signal RF 011  transmitted from the transmission antenna Ant can be reduced. The power of the high frequency signal RF 011  thus can be maintained constant. 
     Embodiment 2 
     In Embodiment 2, a detection calibration circuit  10 A provided on a transmission apparatus  1 A outputting high frequency signals at the same time from a plurality of transmission branches as in beam forming, for example, will be described. In the transmission apparatus  1 A corresponding to beam forming, the larger the number of the transmission branches, the directivity of the radiation pattern of the high frequency signals radiated from the transmission antenna becomes narrower. The transmission apparatus  1 A is thus highly effective as a beam forming device. 
     However, when the power of the high frequency signals vary among the transmission branches, the radiation pattern of the high frequency signals are disturbed, and due to fluctuations in the detected output voltage when an external factor (for example, a temperature fluctuation, a power supply fluctuation, and aging) is generated, the variation in the power of the high frequency signals among the transmission branches exceeds a desired value. Therefore, there is a need to maintain the power constant. 
       FIG. 6  is a circuit diagram illustrating the internal structure of the detection calibration circuit  10 A according to Embodiment 2. The transmission apparatus  1 A illustrated in  FIG. 6  includes a CPU  5 A, the detection calibration circuit  10 A, a transmission antenna Ant 1  connected to a transmission branch Tx-Br 1 , a transmission antenna Ant 2  connected to a transmission branch Tx-Br 2 , and a transmission antenna Ant 3  connected to a transmission branch Tx-Br 3 . 
     Furthermore, the detection calibration circuit  10 A illustrated in  FIG. 6  includes the transmission branches Tx-Br 1 , Tx-Br 2 , and Tx-Br 3 , a reference signal circuit  14 , detection circuit groups DET 1  and DET 2 , and calibration control circuits  18   m  and  18   n . It should be noted in the description of various sections illustrated in  FIG. 6 , components having the same structure and performing the same operation as in the sections in  FIG. 1  are denoted with the same reference characters and the explanations thereof are simplified or omitted, except for those having any different features. Furthermore, in  FIG. 6 , the detection calibration circuit  10 A having three types of transmission branches, for example, will be described, for ease of explanation. To each of the transmission branches Tx-Br 1 , Tx-Br 2 , and Tx-Br 3 , high frequency signals RF 1 , RF 2 , and RF 3  are input. 
     The transmission branch Tx-Br 1  includes a coupler  11 , an amplification circuit  12 , and a coupler  13 . The high frequency signal RF 11  from the coupler  11  is input to the amplification circuit  12  to be amplified. The high frequency signal RF 12  from the coupler  11  is input to a saturation amplifier  14   a  in the reference signal circuit  14 . The high frequency signal RF 111  from the coupler  13  is radiated from the transmission antenna Ant 1 . The high frequency signal RF 112  from the coupler  13  is input to a selector switch  15   a . A reference (voltage) signal RF 12   a  from the saturation amplifier  14   a  is input to the selector switch  15   a.    
     The transmission branch Tx-Br 2  includes an amplification circuit  12   a  and a coupler  13   a . The high frequency signal RF 2  is amplified in the amplification circuit  12   a  and distributed into two types of high frequency signals RF 21  and RF 22  in the coupler  13   a . The high frequency signal RF 21  from the coupler  13   a  is radiated from the transmission antenna Ant 2 . The high frequency signal RF 22  from the coupler  13   a  is input to the selector switches  15   a  and  15   b.    
     The transmission branch Tx-Br 3  includes an amplification circuit  12   b  and a coupler  13   b . The high frequency signal RF 3  is amplified in the amplification circuit  12   b  and distributed into two types of high frequency signals RF 31  and RF 32  in the coupler  13   b . The high frequency signal RF 31  from the coupler  13   b  is radiated from the transmission antenna Ant 3 . The high frequency signal RF 32  from the coupler  13   b  is input to the selector switch  15   b.    
     The detection circuit group DET 1  includes the selector switch  15   a , a detection circuit  16   a , and a sensitivity switching circuit  17   a . When the transmission apparatus  1 A is in the calibration mode, the selector switch  15   a  outputs the reference (voltage) signal RF 12   a  from the reference signal circuit  14  to the detection circuit  16   a . When the transmission apparatus  1 A is in the detection mode, the selector switch  15   a  outputs the high frequency signal RF 112  of the transmission branch Tx-Br 1  to the detection circuit  16   a  and outputs the high frequency signal RF 22  of the transmission branch Tx-Br 2  to the detection circuit  16   a.    
     At this point, the calibration control circuit  18   m  generates a switching control signal indicating either the calibration mode or the detection mode in accordance with a mode control signal output from the CPU  5 A and outputs the generated switching control signal to each of the selector switches  15   a  and  15   b , similarly to the calibration control circuit  18  in Embodiment 1. 
     Furthermore, the calibration control circuit  18   n  starts an operation after completion of adjustment of the detection gain and the input-output sensitivity of the detection circuit  16   a  for each reference (voltage) signal RF 12   a  in the calibration control circuit  18   m . The calibration control circuit  18   n  adjusts the gains of the amplification circuits  12 ,  12   a , and  12   b  respectively in the transmission branches Tx-Br 1 , Tx-Br 2 , and Tx-Br 3  such that the detected output voltages become the same that correspond to the high frequency signals RF 111 , RF 21 , and RF 31  respectively from the transmission branches Tx-Br 1 , Tx-Br 2 , and Tx-Br 3 . 
     The operations performed by the transmission apparatus  1 A in  FIG. 6  will be described below. 
     First Operation: Adjusting the Detection Circuit Group DET 1   
     Firstly, when the transmission apparatus  1 A is in the calibration mode, the calibration control circuit  18   m  adjusts the detection gain and the input-output sensitivity of the detection circuit  16   a  for each reference (voltage) signal RF 12   a  in the reference signal circuit  14 , similarly to the calibration control circuit  18  in Embodiment 1. 
     It should be noted that the description of the adjustment of the detection gain and the input-output sensitivity in the calibration control circuit  18   m  is the same as in Embodiment 1 and thus omitted. 
     Second Operation: Setting Adjustment Results of the Detection Circuit Group DET 1  to the Detection Circuit Group DET 2   
     As the values of the detection gain and input-output sensitivity in the detection circuit  16   b  in the detection circuit group DET 2  being another detection circuit group, the calibration control circuit  18   m  uses the same values as those of the detection gain and input-output sensitivity adjusted in the detection circuit group DET 1 . 
     In other words, the calibration control circuit  18   m  controls the bias voltage of the detection circuit  16   b  and the voltage dividing resistance ratio of the sensitivity switching circuit  17   b  such that the values of the detection gain and the input-output sensitivity in the detection circuit  16   b  in the detection circuit group DET 2  become the same values as those of the detection gain and the input-output sensitivity adjusted in the detection circuit group DET 1 . 
     With this, the detection circuit group DET 2  can obtain a detected output voltage range of the detected output voltage Vdet 2  that is the same as the detected output voltage range of the detected output voltage Vdet 1  of the detection circuit group DET 1  even when an external factor (for example, a temperature fluctuation, a power supply fluctuation, and aging) is generated. 
     Third Operation: Detecting the High Frequency Signal RF 112  in the Detection Circuit Group DET 1   
     After the values of the detection gain and the input-output sensitivity of the detection circuit  16   a  are adjusted by the calibration control circuit  18   m , the calibration control circuit  18   n  stores therein the value of output of the detection circuit group DET 1  for the high frequency signal RF 112  from the coupler  13  of the transmission branch Tx-Br 1  (that is, the detected output voltage Vdet 1 ). 
     Fourth Operation: Adjusting the Gain of the Amplification Circuit  12   a  Using the Detection Circuit Group DET 1   
     The calibration control circuit  18   n  inputs the high frequency signal RF 22  from the coupler  13   a  of the transmission branch Tx-Br 2  to the selector switch  15   a  and adjusts the gain of the amplification circuit  12   a  of the transmission branch Tx-Br 2  such that the value of the corresponding detected output voltage becomes the value of the output of the detection circuit group DET 1  of the transmission branch Tx-Br 1  (that is, the detected output voltage Vdet 1 ). 
     With this, the power of the high frequency signal RF 111  from the transmission branch Tx-Br 1  and the power of the high frequency signal RF 21  from the transmission branch Tx-Br 2  are adjusted to be the same value. 
     Fifth Operation: Detecting the High Frequency Signal RF 22  in the Detection Circuit Group DET 2   
     After the gain of the amplification circuit  12   a  of the transmission branch Tx-Br 2  is adjusted, the calibration control circuit  18   n  stores therein the value of the output of the detection circuit group DET 2  for the high frequency signal RF 22  from the coupler  13   a  of the transmission branch Tx-Br 2  (that is, the detected output voltage Vdet 2 ). 
     Sixth Operation: Adjusting the Gain of the Amplification Circuit  12   b  Using the Detection Circuit Group DET 2   
     Firstly, the calibration control circuit  18   n  causes the selector switch  15   b  to output the high frequency signal RF 32  from the coupler  13   b  of the transmission branch Tx-Br 3  to the detection circuit  16   b.    
     Next, the calibration control circuit  18   n  uses the detection gain and the input-output sensitivity of the detection circuit  16   b  that have been adjusted by the calibration control circuit  18   m  to adjust the gain of the amplification circuit  12   b  of the transmission branch Tx-Br 3  such that the value of the detected output voltage corresponding to the high frequency signal RF 32  from the coupler  13   b  of the transmission branch Tx-Br 3  becomes the same as the value of the detected output voltage Vdet 2 . 
     With this, the power of the high frequency signal RF 21  from the transmission branch Tx-Br 2  and the power of the high frequency signal RF 31  from the transmission branch Tx-Br 3  are adjusted to be the same value. 
     If the number of the transmission branches becomes larger, the transmission apparatus  1 A can similarly adjust the power of the output among each of the transmission branches (high frequency signal transmitted) to be the same value. 
     With the structure described above, in the detection calibration circuit  10 A according to the present embodiment, the reference signal circuit  14  generating the reference (voltage) signal RF 12   a  can be one signal, whereby dispersion errors of the reference (voltage) signal can be reduced compared with the case where a plurality of reference signal circuits  14  are used, the circuit structure can be simplified, and the circuit area can be reduced. Furthermore, because the detected output voltages corresponding to the high frequency signals input from two types of transmission branches are compared for each of the detection circuit groups DET 1  and DET 2 , the influence of dispersion errors can be suppressed and the powers of the high frequency signals from the transmission branches Tx-Br 1 , Tx-Br 2 , and Tx-Br 3  can be adjusted to be the same value even when the detected output values of the detection circuit groups DET 1  and DET 2  vary. 
     It should be noted that a separate reference signal circuit  14  may be provided in the detection circuit group DET 2 . By providing a reference signal circuit  14  in each detection circuit group DET, the adjustment period can be shortened. 
     Embodiment 3 
     In Embodiment 3, a detection calibration circuit  10 B corresponding to a millimeter wave using a saturation amplifier  14   c  that is operated also in a low frequency band, unlike in Embodiment 1, will be described. 
       FIG. 7  is a circuit diagram illustrating the internal structure of the detection calibration circuit  10 B according to Embodiment 3. A transmission apparatus  1 B illustrated in  FIG. 7  includes a CPU  5 , the detection calibration circuit  10 B, and a transmission antenna Ant. Furthermore, the detection calibration circuit  10 B illustrated in  FIG. 7  includes an amplification circuit  12 , a coupler  13 , a reference signal circuit  14 Bg, a matching switching circuit  15 Bs, a detection circuit  16 , a sensitivity switching circuit  17 , and a calibration control circuit  18 Bc. It should be noted in the description of various sections illustrated in  FIG. 7 , components having the same structure and performing the same operation as in the sections in  FIG. 1  are denoted with the same reference characters and the explanations thereof are simplified or omitted, except for those having any different features. 
     In the detection calibration circuit  10 B illustrated in  FIG. 7 , a high frequency signal RF 4  is amplified in the amplification circuit  12  and distributed into two types in the coupler  13 . A high frequency signal RF 41  from the coupler  13  is radiated from the transmission antenna Ant. A high frequency signal RF 42  from the coupler  13  is input to the matching switching circuit  15 Bs. 
     The reference signal circuit  14 Bg as an example of the reference signal generator includes a reference signal source  14   d , the saturation amplifier  14   c , and a reference voltage switcher  14   b . It should be noted that the reference signal source  14   d  may not be provided inside the reference signal circuit  14 Bg and may be a reference signal source provided inside the transmission apparatus  1 B. The reference signal source  14   d  generates a local signal RF 5  of which the frequency is in a frequency band (for example, 5 GHz to 10 GHz approximately) in which the saturation amplifier  14   c  can be operated and outputs the generated local signal RF 5  to the saturation amplifier  14   c.    
     The saturation amplifier  14   c  has the same structure as the saturation amplifier  14   a - 1  illustrated in  FIG. 2(A)  and the detailed explanation thereof is thus omitted. The saturation amplifier  14   c  receives the local signal RF 5  from the reference signal source  14   d  and outputs a reference (voltage) signal RF 5   a  to the matching switching circuit  15 Bs. It should be noted that because the saturation amplifier  14   c  uses the pMOS transistor tr 1  and the nMOS transistor tr 2  illustrated in  FIG. 2(A) , in the detection calibration circuit  10 B according to the present embodiment, the circuit structure of the reference signal circuit  14 Bg can be simplified and the circuit area thereof can be reduced. 
     Furthermore, the upper limit frequency at which the saturation amplifier  14   c  can be operated is approximately 10 GHz at which the pMOS transistor tr 1  illustrated in  FIG. 2(A)  can be operated. The lower limit frequency at which the saturation amplifier  14   c  can be operated is a frequency at which a smoothing capacitor C 3  provided in the output stage of the detection circuit  16  can be operated and a high frequency signal that is input can be converted into a DC detected output voltage. 
     At this point, the capacity value of the smoothing capacitor C 3  needs to be set in reverse proportion to the frequency of the high frequency signal that is input. Even if the capacity value of the smoothing capacitor C 3  is dynamically switched in the calibration mode, for example, the capacity value is approximately 20 times higher in an actual layout. Accordingly, the lower limit frequency is approximately one-twentieth of the high frequency signal that is input. 
     When a high frequency signal (for example, a millimeter wave of 60 GHz or higher) is used, in the reference signal circuit  14 Bg, the reference signal source  14   d  that generates a local signal having a frequency in a frequency band of 3 GHz to 10 GHz approximately is used, for example. 
     According to the present embodiment, the high frequency signal RF 42  is input to the detection circuit  16  in the detection mode, and the reference (voltage) signal RF 5   a  is input to the detection circuit  16  in the calibration mode. At this point, in the detection circuit  16 , the operating frequency of the high frequency signal RF 42  is approximately 10 times higher than the operating frequency of the reference (voltage) signal RF 5   a.    
     The matching switching circuit  15 Bs adjusts impedance matching in accordance with the frequency of the high frequency signal RF 42  as the detection mode or the frequency of the reference (voltage) single RF 5   a  as the calibration mode, in accordance with the switching control signal from an input switching controller  18   f  described later, and thereby outputs the high frequency signal RF 42  or the reference (voltage) signal RF 5   a  to the detection circuit  16 . 
     With this, the detection calibration circuit  10 B uses the matching switching circuit  15 Bs and thereby adjusts matching conditions in accordance with the high frequency signal RF 42  or the reference (voltage) signal RF 5   a , whereby the structure of the selector switch  15  can be omitted, unlike in Embodiment 1. It should be noted that inside the matching switching circuit  15 Bs, the selector switch  15  may be provided similarly to Embodiment 1. 
       FIG. 8(A)  is a diagram illustrating an example of the matching switching circuit  15 Bs.  FIG. 8(B)  is an explanatory diagram illustrating an example of a Smith chart in which the high frequency signal RF 42  is input to the detection circuit  16  in the matching switching circuit  15 Bs.  FIG. 8(C)  is an explanatory diagram illustrating an example of a Smith chart in which the reference (voltage) signal RF 5   a  is input to the detection circuit  16  in the matching switching circuit  15 Bs. 
     In  FIG. 8(A) , in accordance with the switching control signal from the input switching controller  18   f , the impedance matching of the high frequency signal RF 42  input from a high frequency signal input terminal or the reference (voltage) signal RF 5   a  input from a reference signal terminal is performed in a matching circuit  21   a  or a matching circuit  21   b , and the high frequency signal RF 42  or the reference (voltage) signal RF 5   a  is input to the detection circuit  16 . 
     The matching conditions of the matching circuit  21   a  and the matching circuit  21   b  will now be described with reference to the Smith charts illustrated in  FIG. 8(B)  and  FIG. 8(C) . 
     As the matching conditions for outputting the high frequency signal RF 42  to the detection circuit  16 , the matching switching circuit  15 Bs adjusts the impedance matching at the frequency of the high frequency signal RF 42  illustrated in  FIG. 8(B)  and implements a high impedance in the frequency band of the reference (voltage) signal RF 5   a  (see the right side of the largest circle in  FIG. 8(B) : open). A high frequency signal is thus input to the detection circuit  16 . 
     By contrast, as the matching conditions for outputting the reference (voltage) signal RF 5   a  to the detection circuit  16 , the matching switching circuit  15 Bs adjusts the impedance matching at the frequency of the reference (voltage) signal RF 5   a  illustrated in  FIG. 8(C)  and implements a high impedance in the frequency band of the high frequency signal RF 42  (open). A reference signal is thus input to the detection circuit  16 . 
     With these operations, the matching switching circuit  15 Bs combines the output signals from the matching circuit  21   a  and the matching circuit  21   b  and outputs the combined signal to the detection circuit  16 , whereby an isolation characteristic can be secured such that the high frequency signal RF 42  and the reference (voltage) signal RF 5   a  do not affect each other. 
     With this structure, in the detection calibration circuit  10 B, the matching switching circuit  15 Bs is used, whereby the selector switch  15  in Embodiment 1 can be omitted. Furthermore, the circuit of the detection calibration circuit  10 B can be simplified and the circuit area of the detection calibration circuit  10 B can be reduced. 
     The input switching controller  18   f  generates a switching control signal in accordance with the determination result of the mode determiner  18   a  (that is, the determination result indicating whether the transmission apparatus  1 B is in the calibration mode or in the detection mode) and outputs the generated switching control signal to the matching switching circuit  15 Bs. 
     The input switching controller  18   f  generates a switching control signal satisfying the matching conditions for outputting the high frequency signal RF 42  to the detection circuit  16  in the detection mode and generates a switching control signal satisfying the matching conditions for outputting the reference (voltage) signal RF 5   a  to the detection circuit  16  in the calibration mode, and outputs the generated switching control signal to the matching switching circuit  15 Bs. 
     Furthermore, because the detection gain depends on the frequency band of the signal input to the detection circuit  16 , the value of the detected output voltage Vdet output from the detection circuit  16  or the sensitivity switching circuit  17  depends on whether in the detection mode or in the calibration mode. 
     Therefore, in the calibration control circuit  18 Bc, an offset voltage value corresponding to the difference between the detected output voltage in the detection mode and the detected output voltage in the calibration mode may be retained in advance in an offset voltage retainer  18   g.    
     When the transmission apparatus  1 B is in the calibration mode, an adder  18   h  adds the offset voltage value retained in the offset voltage retainer  18   g  to the detected output voltage Vdet from the detection circuit  16  and the sensitivity switching circuit  17  and outputs the value obtained by the addition to the detected voltage comparator  18   e . With this, in the calibration control circuit  18 Bc, the detection gain generated due to the difference in the frequencies of signals input to the detection circuit  16  can be appropriately corrected and the detection gain and the input-output sensitivity can be adjusted to be constant, similarly to in the case of the detection mode. 
     With the structure described above, in the calibration mode of the transmission apparatus  1 B, the detection calibration circuit  10 B according to the present embodiment can calibrate the detected output voltage Vdet in the calibration mode, similarly to the detection calibration circuit  10  in Embodiment 1, even when the reference signal circuit  14 Bg is used that includes the saturation amplifier  14   c  (saturation amplifier  14   a - 1 ) of which the operating frequency is lower than that of the saturation amplifier  14   a  in the reference signal circuit  14  in Embodiment 1. 
     With this structure, in the detection calibration circuit  10 B, effects similar to those of the detection calibration circuit  10  in Embodiment 1 can be obtained, and furthermore, the circuit can be simplified and the circuit area can be reduced compared with the detection calibration circuit  10 . 
     Embodiment 4 
     In Embodiment 4, an input amplification circuit  19  will be described. Unlike in Embodiment 1, the input amplification circuit  19  (a second amplifier) provided for an input to a detection circuit  16  corresponds to switching between a saturation operation and a linear operation in accordance with the mode switching between the calibration mode and the detection mode. 
       FIG. 10  is a diagram illustrating the structure of the input amplification circuit according to Embodiment 4. A transmission apparatus  1 C illustrated in  FIG. 10  includes a CPU  5 , a detection calibration circuit  10 C, and a transmission antenna Ant. Furthermore, the detection calibration circuit  10 C illustrated in  FIG. 10  includes an amplification circuit  12 , a coupler  13 , the input amplification circuit  19 , the detection circuit  16 , a sensitivity switching circuit  17 , and a calibration control circuit  18 Bc. It should be noted in the description of various sections illustrated in  FIG. 10 , components having the same structure and performing the same operation as in the sections in  FIG. 1  are denoted with the same reference characters and the explanations thereof are simplified or omitted, except for those having any different features. 
     In the detection calibration circuit  10 C illustrated in  FIG. 10 , a high frequency signal RF 4  (first amplifier) is distributed into two types in the coupler  13  after being amplified in the amplification circuit  12 . A high frequency signal RF 41  from the coupler  13  is radiated from the transmission antenna Ant. A high frequency signal RF 42  from the coupler  13  is input to the input amplification circuit  19 . 
     The input amplification circuit  19  includes an input amplifier  19   b  (high frequency amplification circuit) that amplifies the high frequency signal RF 42  and a power supply voltage switcher  19   a  that switches the power supply voltage to be supplied to the input amplifier  19   b . The input amplifier  19   b  is an amplifier that amplifies a high frequency signal similarly to the amplification circuit  12  and can adjust an output single voltage range from a linear operation to a saturation operation in accordance with the power supply voltage to be supplied. 
     The reference voltage controller  18   b  thus sets the power supply voltage of the power supply voltage switcher  19   a  high because the input amplifier  19   b  is in a linear operation in the detection mode, and sets the power supply voltage of the power supply voltage switcher  19   a  low because the input amplifier  19   b  is in a saturation operation in the calibration mode. Furthermore, the reference voltage controller  18   b  can generate output signals of a plurality of saturation operation voltages by using a plurality of power supply voltage values in the calibration mode. 
     With this, in the detection calibration circuit  10 C, a reference signal circuit  14  and a selector switch  15  can be omitted in the calibration mode of the transmission apparatus  1 C. Furthermore, in the detection calibration circuit  10 C, the power supply voltage switcher  19   a  sets a plurality of power supply voltages, whereby reference (voltage) signals of rectangular waveforms being a plurality of saturation operation voltages can be output as the output signals of the input amplification circuit  19 . In other words, similarly to the detection calibration circuit  10  in Embodiment 1, the detection calibration circuit  10 C can calibrate the detected output voltage Vdet in the calibration mode. 
     With this structure, in the detection calibration circuit  10 C, the same effects as in the detection calibration circuit  10  in Embodiment 1 can be obtained, and furthermore, the circuit can be simplified and the circuit area can be reduced compared with the detection calibration circuit  10 . 
     Various aspects of embodiments according to the present disclosure include those described below. 
     A detection calibration circuit according to a first aspect includes a first distributor, a first amplifier, a second distributor, a reference signal generator, a switcher, a detector, a sensitivity switcher, and a calibration control circuit. The first distributor distributes a first high frequency input signal into a first high frequency signal and a second high frequency signal. The first amplifier amplifies the first high frequency signal. The second distributor distributes the amplified first high frequency signal further into a third high frequency signal and a fourth high frequency signal. The reference signal generator uses the second high frequency signal to generate a reference signal in accordance with a switchable reference voltage. The switcher selects the third high frequency signal or the reference signal of the reference signal generator. The detector outputs a detection signal obtained by detecting the selected signal. The sensitivity switcher adjusts an input-output sensitivity for the detection signal. The calibration control circuit adjusts a detection gain of the detector and an input-output sensitivity for the detection signal. 
     A detection calibration circuit according to a second aspect is the detection calibration circuit according to the first aspect described above. The calibration control circuit further includes a bias controller that switches a bias voltage of the detector to one of a plurality of different bias voltages and a sensitivity controller that switches an input-output sensitivity for the detection signal in the sensitivity switcher to one of a plurality of input and output sensitivities. 
     A detection calibration circuit according to a third aspect is the detection calibration circuit according to the first aspect described above. The calibration control circuit further includes a reference voltage controller that switches a reference voltage of the reference signal generator to one of a plurality of reference voltages. 
     A detection calibration circuit according to a fourth aspect is the detection calibration circuit according to the third aspect described above. The calibration control circuit further includes a detected voltage comparator that calculates the detection gain and the input-output sensitivity corresponding to the reference signal for each of the reference voltages, that compares between each of setting values of the detection gain and the input-output sensitivity corresponding to a desired input-output characteristic of the detector and each of calculated values of the detection gain and the input-output sensitivity, and that adjusts the detection gain and the input-output sensitivity in accordance with comparison results. 
     A detection calibration circuit according to a fifth aspect is the detection calibration circuit according to the first aspect described above. The calibration control circuit further includes a mode determiner that determines either a detection mode for detecting the third high frequency signal or a calibration mode for adjusting the detection gain and the input-output sensitivity. The switcher selects the third high frequency signal in the calibration mode and selects the reference signal in the detection mode. 
     A detection calibration circuit according to a sixth aspect is the detection calibration circuit according to the first aspect described above. The reference signal generator includes a transformer that converts the second high frequency signal from a one-phase signal into a differential signal, a first transistor element that receives a first positive-phase high frequency output signal of the transformer with a gate terminal thereof and outputs the reference signal in accordance with the reference voltage supplied to a drain terminal thereof, and a second transistor element that receives a first negative-phase high frequency output signal of the transformer with a gate terminal thereof and has a source terminal connected to a source terminal of the first transistor element and a drain terminal grounded. 
     A detection calibration circuit according to a seventh aspect is the detection calibration circuit according to the first aspect described above. The reference signal generator includes a reference signal source that outputs a local signal having a frequency lower than the high frequency input signal, a third transistor element that receives a local signal from the reference signal source with a gate terminal thereof and outputs a low frequency reference signal in accordance with the reference voltage supplied to a drain terminal thereof, and a fourth transistor element that receives a local signal from the reference signal source with a gate terminal thereof and has a source terminal connected to a source terminal of the third transistor element and a drain terminal grounded. The calibration control circuit further includes an adder that adds a predetermined offset voltage to the detection signal. 
     A detection calibration circuit according to an eighth aspect is the detection calibration circuit according to the seventh aspect described above. The switcher includes a first matching circuit that matches impedance corresponding to the third high frequency signal and a second matching circuit that matches impedance corresponding to the reference signal and selects either the third high frequency signal or the reference signal in accordance with impedance matching in the first matching circuit or the second matching circuit. 
     A detection calibration circuit according to a ninth aspect is the detection calibration circuit according to the first aspect described above. The detection calibration circuit further includes a third distributor that distributes the second high frequency input signal into a fifth high frequency signal and a sixth high frequency signal and a second amplifier that amplifies the second high frequency input signal. The switcher selects the third high frequency signal, the sixth high frequency signal, or the reference signal of the reference signal generator. 
     A detection calibration circuit according to a tenth aspect includes a first amplifier, a first distributor, a second amplifier, a detector, a sensitivity switcher, and a calibration control circuit. The first amplifier amplifies a first high frequency input signal. The first distributor distributes the amplified first high frequency input signal into a second high frequency signal and a third high frequency signal. The second amplifier switches between a saturation operation and a linear operation in accordance with a plurality of different power supply voltages to amplify the third high frequency signal. The detector outputs a detection signal obtained by detecting the amplified third high frequency signal. The sensitivity switcher adjusts an input-output sensitivity for the detection signal. The calibration control circuit adjusts a detection gain of the detector, an input-output sensitivity for the detection signal, and the plurality of different power supply voltages of the second amplifier. 
     A transmission apparatus according to the first aspect includes a detection calibration circuit and a transmission antenna. The detection calibration circuit includes a first distributor, a first amplifier, a second distributor, a reference signal generator, a switcher, a detector, a sensitivity switcher, and a calibration control circuit. The first distributor distributes a first high frequency input signal into a first high frequency signal and a second high frequency signal. The first amplifier amplifies the first high frequency signal. The second distributor distributes the amplified first high frequency signal further into a third high frequency signal and a fourth high frequency signal. The reference signal generator uses the second high frequency signal to generate a reference signal in accordance with a switchable reference voltage. The switcher selects the third high frequency signal or the reference signal of the reference signal generator. The detector outputs a detection signal obtained by detecting the selected signal. The sensitivity switcher adjusts an input-output sensitivity for the detection signal. The calibration control circuit adjusts a detection gain of the detector and an input-output sensitivity for the detection signal. The transmission antenna transmits the fourth high frequency signal. 
     A transmission apparatus according to the second aspect includes a detection calibration circuit, a first transmission antenna, and a second transmission antenna. The detection calibration circuit includes a first distributor, a first amplifier, a second distributor, a reference signal generator, a detector, a sensitivity switcher, a calibration control circuit, a third distributor, a second amplifier, and a switcher. The first distributor distributes a first high frequency input signal into a first high frequency signal and a second high frequency signal. The first amplifier amplifies the first high frequency signal. The second distributor distributes the amplified first high frequency signal further into a third high frequency signal and a fourth high frequency signal. The reference signal generator uses the second high frequency signal to generate a reference signal in accordance with a switchable reference voltage. The detector outputs a detection signal obtained by detecting the selected signal. The sensitivity switcher adjusts an input-output sensitivity for the detection signal. The calibration control circuit adjusts a detection gain of the detector and an input-output sensitivity for the detection signal. The third distributor distributes a second high frequency input signal into a fifth high frequency signal and a sixth high frequency signal. The second amplifier amplifies the second high frequency input signal. The switcher selects the third high frequency signal, the sixth high frequency signal, or the reference signal of the reference signal generator. The first transmission antenna transmits the fourth high frequency signal. The second transmission antenna transmits the fifth high frequency signal. 
     A transmission apparatus according to the third aspect includes a detection calibration circuit and a transmission antenna. The detection calibration circuit includes a first amplifier, a first distributor, a second amplifier, a detector, a sensitivity switcher, and a calibration control circuit. The first amplifier amplifies the first high frequency input signal. The first distributor distributes the amplified first high frequency input signal into a second high frequency signal and a third high frequency signal. The second amplifier switches between a saturation operation and a linear operation in accordance with a plurality of different power supply voltages to amplify the third high frequency signal. The detector outputs a detection signal obtained by detecting the amplified third high frequency signal. The sensitivity switcher switches an input-output sensitivity for the detection signal. The calibration control circuit adjusts a detection gain of the detector, the input-output sensitivity for the detection signal and the plurality of different power supply voltages of the second amplifier. The transmission antenna transmits the second high frequency signal. 
     Various embodiments have been described above with reference to the drawings. Needless to say, however, the present disclosure is not limited to the embodiments described above. It is obvious that the skilled person could have arrived at various variations or modifications within the categories of claims. It is understood that those variations and modifications naturally fall within the technical scope of the present disclosure. Furthermore, each component in the embodiments described above may be optionally combined without departing from the spirits of the disclosures. 
     In each embodiment described above, an example has been described in which hardware is used in the structure. However, software may be used in conjunction with hardware to implement the present disclosure. 
     Furthermore, the functional blocks used in the explanations of various embodiments described above are implemented as LSI, which typically are integrated circuits. These may be made up of one chip individually, or may be integrated into one chip to include some or all. It should be noted that the LSI here may be referred to as IC, system LSI, super LSI, or ultra LSI depending on the degree of integration. 
     Furthermore, the method of circuit integration is not limited to LSI, but circuit integration may be implemented by using a dedicated circuit or a general processor. A field programmable gate array (FPGA) that is programmable after the production of LSI or a reconfigurable processor that can reconfigure a connection or a setting of circuit cells inside an LSI may be used. 
     Furthermore, when a new technique of circuit integration that replaces LSI has emerged due to advances of semiconductor technology and another technique derived therefrom, the technique naturally may be used to integrate the functional blocks. Application of biotechnology and other possibilities are thinkable. 
     The present disclosure is effective as a detection calibration circuit and a transmission apparatus that reduce fluctuations in a detected output voltage of a high frequency signal and suppress deterioration of the detection characteristic even when a temperature fluctuation, a power supply fluctuation, or aging is generated, in a detection circuit detecting the level of the high frequency signal transmitted.