Abstract:
From an antenna provided in a transmitter for transmitting a radio signal, the radio signal is transmitted in such a manner of: detecting transmission power of a transmission signal outputted to the antenna; detecting reflection power of a reflection signal reflected from the antenna; integrating the difference between the transmission power and the reflection power at a timing of transmitting a preamble signal of the radio signal; comparing the difference value obtained by the integration with a predetermined threshold value, and outputting an alarm in the case where the result of the comparison is that the difference value obtained by the integration is smaller than the threshold value.

Description:
This application is the National Phase of PCT/JP2009/068766, filed Nov. 2, 2009, which claims the benefit of priority from Japanese Patent Application No. 2008-318516 filed in Japan on Dec. 15, 2008, the entire content of which is hereby incorporated by reference in the application and claims of the present application. 
     TECHNICAL FIELD 
     The present invention relates to a power detection circuit, a transmitter, and a power detection method for detecting electric power. 
     BACKGROUND ART 
     In a transmitter for transmitting a radio signal from an antenna, in order to detect a case where an abnormality, such as a failure or unconnected state of the antenna occurs in the antenna, a technique has been proposed which measures the reflection power reflected from the antenna and which detects the abnormality on the basis of the magnitude of the measured reflection power (see, for example, Patent Literature 1). 
       FIG. 1  is a diagram showing one form of a radio apparatus using a common power detection circuit. 
     The radio apparatus shown in  FIG. 1  includes transmission signal output section  1000 , power detection circuit  10 , and antenna  4000 . Transmission signal output section  1000  is an amplifier which outputs to antenna  4000  a transmission signal to cause a radio signal to be transmitted from antenna  4000 . 
     Power detection circuit  10  detects the reflection power of the reflection signal reflected from antenna  4000 . 
     Antenna  4000  transmits, as a radio signal, the transmission signal outputted from transmission signal output section  1000 . 
     Further, power detection circuit  10  is configured by reflection power detection section  3000  and comparison section  7000 . Further, reflection power detection section  3000  is configured by coupler  3001  and detector  3002 . 
     Coupler  3001  branches the reflection signal reflected from antenna  4000 , so as to output the branched signal to detector  3002 . 
     Detector  3002  converts to a DC voltage the reflection power of the reflection signal outputted from coupler  3001 . Further, detector  3002  outputs the converted voltage to comparison section  7000 . 
     Comparison section  7000  compares the voltage outputted from detector  3002  with a preset threshold value. Further, when the comparison result is one in which the voltage outputted from detector  3002  is larger than the preset reference value (threshold value), comparison section  7000  outputs an alarm (ALM). 
     In the radio apparatus configured in this way, when the return loss (reflection power) from antenna  4000  is increased, the reflection power taken out (branched) by coupler  3001  is increased. The reflection power is detected by detector  3002 , and then, in comparison section  7000 , the detected reflection power is compared with the reference value to determine whether or not an alarm is to be issued. 
     Here, the reflection power is proportional to the transmission power of the transmission signal outputted from transmission signal output section  1000 . Therefore, when the transmission power is small, it is not possible to determine the abnormality of antenna  4000 . In the case where the transmission power is too large, even when the return loss (reflection power) of antenna  4000  is in a normal range, there is a case where, since the absolute value of the reflection power is large, the state of antenna  4000  is determined to be abnormal. 
     In this way, since an accurate determination cannot be made according to the change in the transmission power, there is a problem that, when the signal level is successively changed as in the case of a burst signal, a correct determination cannot he made. 
     Thus, in addition to the configuration shown in  FIG. 1 , a technique has been proposed in which the transmission power transmitted from the amplifier to the antenna is also detected so that the abnormality is detected on the basis of the difference between the detected transmission power and the reflection power (see, for example, Patent Literature 2). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP2008-085849A 
         Patent Literature 2: JP2003-525455A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the technique described in Patent Literature 2, there is a problem that, when the transmission power and the reflection power are small, the voltage characteristics detected by the diode are varied. In this way, when the detected voltage is varied, an error may be caused in the calculation of return loss (reflection power) so that accurate determination cannot be made. 
     An object of the present invention is to provide a power detection circuit, a transmitter, and a power detection method for solving the above-described problems. 
     Solution to Problem 
     A power detection circuit according to the present invention includes: 
     a transmission power detection section that detects transmission power of a transmission signal which, in order to cause a radio signal to be transmitted from an antenna provided in a transmitter for transmitting the radio signal, is outputted to the antenna; 
     a reflection power detection section that detects reflection power of a reflection signal reflected from the antenna; 
     an integration section that integrates the difference between the transmission power and the reflection power at a timing of transmitting a preamble signal of the radio signal; and 
     a comparison section that compares the difference value obtained by the integration by the integration section with a predetermined threshold value and that outputs an alarm in the case where the result of the comparison is that the difference value obtained by the integration by the integration section is smaller than the predetermined threshold value. 
     Further, a transmitter according to the present invention includes: 
     the power detection circuit; 
     the antenna; and 
     a transmission signal output section that outputs a transmission signal to the antenna in order to cause the radio signal to be transmitted from the antenna. 
     Further, a power detection method according to the present invention includes: 
     a process that detects transmission power of a transmission signal which, in order to cause a radio signal to be transmitted from an antenna provided in a transmitter for transmitting the radio signal, is outputted to the antenna; 
     a process that detects reflection power of a reflection signal reflected from the antenna; 
     a process that integrates the difference between the transmission power and the reflection power at a timing of transmitting a preamble signal of the radio signal; 
     a process that compares the difference value obtained by the integration with a predetermined threshold value; and 
     a process that outputs an alarm in the case where the result of the comparison is that the difference value obtained by the integration is smaller than the threshold value. 
     Advantageous Effects of Invention 
     As described above, the present invention is configured for: detecting transmission power of a transmission signal which, in order to cause a radio signal to be transmitted from an antenna provided in a transmitter for transmitting the radio signal, is outputted to the antenna; detecting reflection power of a reflection signal reflected from the antenna; integrating the difference between the transmission power and the reflection power at a timing of transmitting a preamble signal of the radio signal; comparing the difference value obtained by the integration with a predetermined threshold value; and outputting an alarm in the case where the result of the comparison is that the difference value obtained by the integration is smaller than the threshold value. Thereby, it is possible to accurately detect an abnormal state of the antenna port irrespective of the magnitude of the transmission power. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing one form of a radio apparatus using a common power detection circuit. 
         FIG. 2  is a diagram showing a first exemplary embodiment of a power detection circuit according to the present invention. 
         FIG. 3  is a diagram showing the voltage conversion characteristics of the detector shown in  FIG. 2 . 
         FIG. 4  is a diagram showing a characteristic of transmission power (reflection power) logarithmically converted by the logarithmic amplifier shown in  FIG. 2  with respect to detection (DC) voltage. 
         FIG. 5  is a diagram showing temporal changes of the output (power) level of a signal transmitted by using a time division duplex system. 
         FIG. 6  is a diagram showing a state in which an integration period is generated in the integration period generation section shown in  FIG. 2 . 
         FIG. 7  is a diagram showing characteristics in which the detection voltage is varied in an area of small input power. 
         FIG. 8  is a diagram showing characteristics in which the logarithmically amplified detection voltage is varied in an area of small input power. 
         FIG. 9  is a diagram showing temporal changes of an FWD (transmission) voltage, an REV (reflection) voltage, and the difference output between the FWD voltage and the REV voltage in the case where a burst signal is transmitted. 
         FIG. 10  is a diagram showing a relationship between the gate signal and the difference signal during integration period t generated in the integration period generation section shown in  FIG. 2 . 
         FIG. 11  is a diagram showing a second exemplary embodiment of a power detection circuit according to the present invention. 
         FIG. 12  a diagram showing a third exemplary embodiment of a power detection circuit according to the present invention. 
         FIG. 13  is a diagram showing a fourth exemplary embodiment of a power detection circuit according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, exemplary embodiments will be described with reference to the accompanying drawings. 
     First Exemplary Embodiment 
       FIG. 2  is a diagram showing a first exemplary embodiment of a power detection circuit according to the present invention. 
     As shown in  FIG. 2 , the exemplary embodiment is configured by transmission signal output section  100 , power detection circuit  1  according to the present invention, and antenna  110 . 
     Transmission signal output section  100  is an amplifier which outputs to antenna  110  a transmission signal for causing a radio signal to be transmitted from antenna  110 . 
     Power detection circuit  1  detects transmission power of the transmission signal outputted from transmission signal output section  100  to antenna  110 . Further, power detection circuit  1  detects reflection power of a reflection signal reflected from antenna  110 . 
     Antenna  110  transmits, as a radio signal, the transmission signal outputted from transmission signal output section  100 . 
     Further, power detection circuit  1  is configured by transmission power detection section  200 , reflection power detection section  300 , differential amplifier section  400 , integration period generation section  500 , integration section  600 , and comparison section  700 . 
     Transmission power detection section  200  detects transmission power of the transmission signal outputted from transmission signal output section  100  to antenna  110 . 
     Further, transmission power detection section  200  is configured by coupler  201 , detector  202 , and logarithmic amplifier  203 . 
     Coupler  201  is a first directional coupler for branching the transmission signal outputted from transmission signal output section  100  to antenna  110  and for outputting the branched signal to detector  202 . 
     Detector  202  is a first detector for converting to a first DC voltage the transmission power of the transmission signal outputted from coupler  201 . Further, detector  202  outputs the first DC voltage to logarithmic amplifier  203 . 
     Logarithmic amplifier  203  is a first logarithmic amplifier for logarithmically converting the DC voltage outputted from detector  202  to obtain a transmission voltage corresponding to the transmission power. Further, logarithmic amplifier  203  outputs the transmission voltage to differential amplifier section  400 . 
     Reflection power detection section  300  detects reflection power of a reflection signal reflected from antenna  110 . 
     Further, reflection power detection section  300  is configured by coupler  301 , detector  302 , and logarithmic amplifier  303 . 
     Coupler  301  is a second directional coupler for branching the reflection signal reflected from antenna  110  and for outputting the branched signal to detector  302 . 
     Detector  302  is a second detector for converting to a second DC voltage the reflection power of the reflection signal outputted from coupler  301 . Further, detector  302  outputs the second DC voltage to logarithmic amplifier  303 . 
     Logarithmic amplifier  303  is a second logarithmic amplifier for logarithmically converting the second DC voltage outputted from detector  302  to obtain a reflection voltage corresponding to the reflection power. Further, logarithmic amplifier  303  outputs the reflection voltage to differential amplifier section  400 . 
     Differential amplifier section  400  takes the difference between the transmission voltage outputted from logarithmic amplifier  203  and the reflection voltage outputted from logarithmic amplifier  303 , and outputs the difference to integration section  600 . 
     Integration period generation section  500  generates an integration period on the basis of an operation clock and a burst timing signal. The burst timing signal will be described below. Integration period generation section  500  outputs the generated integration period to integration section  600 . 
     Integration section  600  integrates the difference outputted from differential amplifier section  400  during the integration period outputted from integration period generation section  500 . Further, integration section  600  outputs the integration value obtained by the integration to comparison section  700 . 
     Comparison section  700  compares the integration value outputted from integration section  600  with a preset threshold value. Further, when the result of the comparison is that the integration value outputted from integration section  600  is smaller than the preset threshold value, comparison section  700  outputs an alarm (ALM). 
     The operation in the configuration shown in  FIG. 2  will be described below. 
     A transmission signal for causing a radio signal to be transmitted from antenna  110  is amplified to a desired output and outputted by transmission signal output section  100 . 
     The transmission signal outputted from transmission signal output section  100  is branched by coupler  201 . That is, a part of the transmission signal is extracted by coupler  201 . 
     On the other hand, when mismatching occurs between the output and the antenna, the transmission signal inputted to antenna  110  is reflected by antenna  110 , so that a reflection signal is returned toward transmission signal output section  100 . 
     The reflection signal is branched by coupler  301 . That is, a part of the reflection signal is extracted by coupler  301 . 
     Thereafter, the output of the coupling port of coupler  201  is outputted to detector  202 . Further, the output of the coupling port of coupler  301  is outputted to detector  302 . 
     Then, the transmission power outputted from coupler  201  is converted (detected) by detector  202  to a DC voltage. Further, the reflection power outputted from coupler  301  is converted (detected) by detector  302  to a DC voltage. 
     Here, the voltage conversion characteristics of detector  202  and  302  are described. 
       FIG. 3  is a diagram showing the voltage conversion characteristics of detectors  202  and  302  shown in  FIG. 2 . 
     As shown in  FIG. 3 , the voltage conversion characteristics of detectors  202  and  302  are exponential function characteristics. This is because a diode is usually used in each of detectors  202  and  302  and because the voltage conversion characteristics of detectors  202  and  302  depend on the characteristics of the diode. 
     The DC voltage resulting from the conversion by detector  202  is logarithmically converted by logarithmic amplifier  203 . Further, the DC voltage resulting from the conversion by detector  302  is logarithmically converted by logarithmic amplifier  303 . 
       FIG. 4  is a diagram showing a characteristic of the transmission power (reflection power) logarithmically converted by each of logarithmic amplifier  203  and  303  shown in  FIG. 2  with respect to the detection (DC) voltage. 
     As shown in  FIG. 4 , the detection voltage exhibits substantially a linear characteristic with respect to the transmission power (reflection power). 
     When the logarithmically converted transmission voltage is outputted from logarithmic amplifier  203 , and when the logarithmically converted reflection voltage is outputted from logarithmic amplifier  303 , the difference between the transmission voltage outputted from logarithmic amplifier  203  and the reflection voltage outputted from logarithmic amplifier  303  is calculated by differential amplifier section  400 . When an abnormal state (in other words, degradation of return loss in the antenna port), such as an unconnected state of antenna  110  and a disconnection state of cable to antenna  110 , occurs, the reflection voltage is increased. The transmission voltage and the reflection voltage, which are respectively proportional to the transmission power and the reflection power, are outputted from logarithmic amplifier  203  and  303 , respectively. For this reason, when the difference between the transmission voltage and the reflection voltage is calculated, the abnormal state (degradation of return loss in the antenna port) can be detected irrespective of the magnitude of the transmission signal, that is, the magnitude of the transmission power. In differential amplifier section  400 , the difference between the transmission voltage and the reflection voltage is calculated, and the difference (voltage) proportional to the return loss of the antenna port is outputted. 
     Here, a signal transmitted by using a time division duplex (TDD) system is considered. 
       FIG. 5  is a diagram showing temporal changes of the output (power) level of a signal transmitted by using the time division duplex system. 
     As shown in  FIG. 5 , the signal transmitted by using the time division duplex system is formed as a burst-type (burst wave) signal. Further, the output (power) level of the signal is changed with time. The burst wave signal includes a preamble signal used to effect the synchronization of a modem of a terminal, and hence the portion corresponding to the preamble signal is configured so that a signal having power of a fixed level or more is surely outputted. 
     In the exemplary embodiment, the degradation of return loss in the antenna port is not detected in all (time) periods, but the degradation of return loss in the antenna port is detected only in the period of the preamble signal. 
     The output of differential amplifier section  400  is inputted to integration section  600 , while the integration time (integration period) is determined by integration period generation section  500 . 
       FIG. 6  is a diagram showing a state in which an integration period is generated in integration period generation section  500  shown in  FIG. 2 . 
     As shown in  FIG. 6 , a burst timing signal synchronized with a burst period, and a clock (CLK) are inputted to integration period generation section  500 , so that a control signal (gate signal) with preset integration period (t) is generated by counting the clock pulses from the burst timing signal. 
     In integration section  600 , an integration value is obtained during a fixed period corresponding to the integration period generated by integration period generation section  500 . That is, an integration value of the difference value between the transmission voltage corresponding to the transmission power and the reflection voltage corresponding to the reflection power is obtained by integration section  600  during the preamble time of the burst wave, and the obtained integration value is outputted from integration section  600 . 
     Then, the integration value outputted from integration section  600  is compared with the preset threshold value (reference value) in comparison section  700 . When the result of the comparison is that the integration value is smaller than the reference value (when the difference between transmission voltage and the reflection voltage becomes small, that is, the return loss is degraded), an alarm (ALM) is outputted from comparison section  700 . 
     If, when the alarm (ALM) is outputted, the operation of transmission signal output section  100  is controlled to be turned off, it is possible to prevent transmission signal output section  100  from being damaged when the return loss is degraded, such as when the antenna is not connected. 
     As described above, in the circuit for outputting a signal from transmission signal output section  100  to antenna  110 , not only the power in the reflection direction but also the power in the propagation direction are used, and the logarithmic values of the detected voltages of the power in the respective directions are compared with each other, so that an abnormal state of the antenna port can be detected irrespective of the magnitude of the transmission power. This is based on the principle that, since the return loss is degraded in an abnormal state of the antenna port, the return loss can be calculated by obtaining the ratio between the transmission power (voltage) and the reflection power (voltage). 
     Further, when the input power is small, the variation of the detection voltage is increased due to the characteristics of the diode. 
       FIG. 7  is a diagram showing characteristics in which the detection voltage is varied in an area of small input power.  FIG. 8  is a diagram showing characteristics in which the logarithmically amplified detection voltage is varied in an area of small input power. 
     As shown in  FIG. 7  and  FIG. 8 , when the signal is small, the detection voltage is varied, and hence an error may be caused in the calculation of return loss so that an accurate determination cannot be made. 
     For this reason, in the present invention, an erroneous determination is avoided in such a manner that only the preamble period of the burst signal is used for the determination, and that the small signal portion is not used in the calculation of return loss. 
       FIG. 9  is a diagram showing temporal changes of an FWD (transmission) voltage, an REV (reflection) voltage, and the difference output between the FWD voltage and the REV voltage in the case where a burst signal is transmitted. The diagram of the upper stage in  FIG. 9  is a diagram showing the temporal changes of the FWD (transmission) voltage. Further, the diagram of the middle stage in  FIG. 9  is a diagram showing the temporal changes of the REV (reflection) voltage. Further, the diagram of the lower stage in  FIG. 9  is a diagram showing the temporal changes of the difference output. 
     As shown in the diagram of the lower stage in  FIG. 9 , when the level of the burst signal is low, variations as shown by the broken lines are caused in the difference voltage due to the variation in the detection characteristics. 
       FIG. 10  is a diagram showing a relationship between the gate signal and the difference signal during integration period t generated in integration period generation section  500  shown in  FIG. 2 . 
     As shown in  FIG. 10 , the influence of the variation in the detection voltage is eliminated by determining the integration period by the gate signal generated in integration period generation section  500 . 
     Second Exemplary Embodiment 
       FIG. 11  is a diagram showing a second exemplary embodiment of a power detection circuit according to the present invention. 
     As shown in  FIG. 11 , in power detection circuit  2  according to the exemplary embodiment, circulator  800  is provided between transmission power detection section  200  and antenna  110 . Further, coupler  301  and terminator  311  are provided in reflection power detection section  310  connected to the side of circulator  800 , which side is seen as an output port side from antenna  110 . The same effect as that of the first exemplary embodiment can be obtained because the reflection signal from antenna  110  can be taken out by coupler  301 . 
     Third Exemplary Embodiment 
       FIG. 12  is a diagram showing a third exemplary embodiment of a power detection circuit according to the present invention. 
     As shown in  FIG. 12 , power detection circuit  3  according to the exemplary embodiment is an example in which, in place of detectors  202  and  302  and logarithmic amplifiers  203  and  303  in the first exemplary embodiment, LOG detectors  221  and  321  having logarithmic characteristics are used in transmission power detection section  220  and reflection power detection section  320 , respectively. Many devices (ICs) having logarithmic characteristics have been used in recent years. The circuit can be simplified by using the devices having logarithmic characteristics. 
     Fourth Exemplary Embodiment 
       FIG. 13  is a diagram showing a fourth exemplary embodiment of a power detection circuit according to the present invention. 
     As shown in  FIG. 13 , power detection circuit  4  according to the exemplary embodiment is an example in which the processing of logarithmic amplifiers  203  and  303 , differential amplifier section  400 , integration period generation section  500 , and comparison section  700  is realized by using firmware of CPU  900 . The same effect can be obtained as that of the first exemplary embodiment in such a manner that the detection voltages are respectively converted into digital data by AID converters  231  and  331  respectively provided in transmission power detection section  230  and reflection power detection section  330 , so as to be taken in CPU  900 , and thereafter the alarm determination is performed by numerical calculation. 
     Note that the present invention is particularly useful for application to an apparatus, such as a transmission section of a base station used in a WiMAX (Worldwide Interoperability for Microwave Access) system, which uses a time division duplex system and transmits a burst signal. 
     Although the invention of the present application has been described with reference to exemplary embodiments, the invention of the present application is not limited to the above described exemplary embodiments. The constitution and details of the invention of the present application are open to various modifications within the scope of the invention of the present application that will be clear to anyone of ordinary skill in the art.