Patent Publication Number: US-2017353056-A1

Title: Electromagnetic resonant coupler including input line, first resonance line, second resonance line, output line, and coupling line, and transmission apparatus including the electromagnetic resonant coupler

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electromagnetic resonant coupler and a transmission apparatus including the electromagnetic resonant coupler. 
     2. Description of the Related Art 
     In a variety of electrical apparatuses, there is a demand that a signal be transmitted while electrical isolation is secured between circuits. Drive-by-Microwave Technology that uses an electromagnetic resonant coupler is being proposed as a transmission system that enables simultaneous and isolated transmission of an electric signal and power (see, for example, Japanese Unexamined Patent Application Publication No. 2008-067012). 
     SUMMARY 
     In one general aspect, the techniques disclosed here feature an electromagnetic resonant coupler that includes an input line to which a transmission signal is input; a first resonance line connected to the input line; a second resonance line opposing the first resonance line, the second resonance line undergoing resonant coupling with the first resonance line to thus wirelessly transmit the transmission signal between the first resonance line and the second resonance line; an output line connected to the second resonance line, the transmission signal being output through the output line; a coupling line that electromagnetically couples with at least one selected from the group consisting of the first resonance line and the second resonance line; and a terminator connected to one end of the coupling line. 
     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  illustrates temperature characteristics of an isolating transmission apparatus; 
         FIG. 2  is a schematic diagram illustrating a configuration of a directional coupler; 
         FIG. 3  is an exploded perspective view of an electromagnetic resonant coupler according to a first embodiment; 
         FIG. 4  is a sectional view of the electromagnetic resonant coupler according to the first embodiment; 
         FIG. 5  is a perspective view illustrating a wiring structure of the electromagnetic resonant coupler according to the first embodiment; 
         FIG. 6  is a top view illustrating a wiring structure of a first resonator and a coupling line included in the electromagnetic resonant coupler according to the first embodiment; 
         FIG. 7  is a schematic diagram for describing an operation of the electromagnetic resonant coupler according to the first embodiment; 
         FIG. 8  illustrates transmission characteristics of an electromagnetic resonant coupler according to a comparative example; 
         FIG. 9  illustrates transmission characteristics of the electromagnetic resonant coupler according to the first embodiment; 
         FIG. 10  is a perspective view illustrating a wiring structure of an electromagnetic resonant coupler according to a second embodiment; 
         FIG. 11  is a top view illustrating a wiring structure of a first resonator and a coupling line included in the electromagnetic resonant coupler according to the second embodiment; 
         FIG. 12  is a perspective view of a transmission apparatus; 
         FIG. 13  illustrates a circuit configuration of the transmission apparatus; 
         FIG. 14  illustrates a circuit configuration of a detection circuit that includes a double voltage rectifier circuit; 
         FIG. 15  is a block diagram of a transmission apparatus that includes a controller; 
         FIG. 16  is a block diagram of a transmission apparatus that includes a controller and an amplifier that amplifies a detection signal; 
         FIG. 17  is a block diagram of a transmission apparatus that includes a controller and an amplifier that amplifies a detection wave; 
         FIG. 18  illustrates a circuit configuration of a transmission apparatus that includes three electromagnetic resonant couplers; and 
         FIG. 19  is a top view illustrating a wiring structure of a first resonator and a coupling line included in an electromagnetic resonant coupler according to a modification of the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Underlying Knowledge Forming Basis of the Present Disclosure 
     In a variety of electrical apparatuses, there is a demand that a signal be transmitted while electrical isolation is secured between circuits. For example, when an electronic apparatus that includes a high-voltage circuit and a low-voltage circuit is put into operation, the ground loop between the circuits may be cut off in order to prevent a malfunction or a failure of the low-voltage circuit. In other words, the circuits may be isolated from each other. Such a configuration can prevent an excess voltage from being applied to the low-voltage circuit from the high-voltage circuit when the high-voltage circuit and the low-voltage circuit become electrically connected to each other. 
     Specifically, for example, a case in which a motor driving circuit that operates at a high voltage of several hundred volts is controlled by a microcomputer, a semiconductor integrated circuit, or the like can be considered. When a high voltage with which the motor driving circuit deals is applied to the microcomputer, the semiconductor integrated circuit, or the like that operates at a low voltage, a malfunction or a failure occurs. In order to suppress an occurrence of such a malfunction or a failure, the microcomputer, the semiconductor integrated circuit, or the like is isolated from the motor driving circuit. 
     A photocoupler is known as a device that transmits a signal while securing isolation between circuits. A photocoupler is a package into which a light-emitting element and a light-receiving element are integrated, and the light-emitting element and the light-receiving element are electrically isolated from each other inside the package member. A photocoupler converts an input electric signal to an optical signal with a light-emitting element, detects the converted optical signal with a light-receiving element, converts the optical signal back to an electric signal, and outputs the electric signal. 
     In recent years, an isolating transmission apparatus that includes an electromagnetic resonant coupler serving as an isolation device is being proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2008-067012). An isolating transmission apparatus that includes an electromagnetic resonant coupler serving as an isolation device modulates a high-frequency signal with a transmission circuit in accordance with an input signal and transmits, in isolation, a modulation signal, which is the modulated high-frequency signal, to a reception circuit with the electromagnetic resonant coupler. The isolating transmission apparatus then demodulates the modulation signal with a rectifier circuit included in the reception circuit. 
     The transmission circuit includes an embedded semiconductor element and thus typically has such temperature characteristics as illustrated in  FIG. 1 .  FIG. 1  illustrates the temperature characteristics of the isolating transmission apparatus. In  FIG. 1 , the prescribed value of the output voltage is indicated by the dashed line. 
     As illustrated in  FIG. 1 , the output voltage output from the isolating transmission apparatus decreases as the temperature increases. Such characteristics are due to that the modulation signal generated and output by the transmission circuit varies depending on the ambient temperature. An output voltage output from the isolating transmission apparatus when the ambient temperature is low is higher than the prescribed value of the output voltage, whereas an output voltage output from the isolating transmission apparatus when the ambient temperature is high is lower than the prescribed value of the output voltage. However, it is desirable that the isolating transmission apparatus output a constant output voltage regardless of the ambient temperature. 
     One of the known typical techniques for keeping the output voltage of an isolating transmission apparatus constant is to carry out feedback control by monitoring the output voltage of a transmission circuit (see, for example, Japanese Unexamined Patent Application Publication No. 2012-257421). However, when an electromagnetic resonant coupler, a transmission circuit, and a reception circuit are integrated into a package, the output voltage of the transmission circuit is not output to the outside of the package member. Therefore, it is difficult to monitor the output voltage of the transmission circuit. 
     A technique in which a directional coupler is used is known as a typical technique for monitoring a high-frequency signal.  FIG. 2  is a schematic diagram illustrating a configuration of a directional coupler. As illustrated in  FIG. 2 , a directional coupler  30  divides a high-frequency signal that has been generated by an oscillator  10  and output from an amplifier  20  into an output signal  40  and a monitor signal  50 . However, the directional coupler  30  typically uses a transmission line having a length of one-quarter the wavelength X of the high-frequency signal, which thus often leads to an increase in size. Therefore, it is often difficult to embed a directional coupler into an isolating transmission apparatus. 
     Accordingly, an electromagnetic resonant coupler according to an aspect of the present disclosure includes an input line to which a transmission signal is input; a first resonance line connected to the input line; a second resonance line opposing the first resonance line, the second resonance line undergoing resonant coupling with the first resonance line to thus wirelessly transmit the transmission signal between the first resonance line and the second resonance line; an output line connected to the second resonance line, the transmission signal being output through the output line; a coupling line that electromagnetically couples with at least one selected from the group consisting of the first resonance line and the second resonance line; and a terminator connected to one end of the coupling line. 
     This configuration makes it possible to obtain a detection wave corresponding to the transmission signal through the coupling line. In other words, the transmission signal can be monitored with ease with the coupling line without a complex device or the like. 
     In addition, as the terminator is connected to the one end of the coupling line, a detection wave can be obtained through another end of the coupling line. 
     In the electromagnetic resonant coupler according to the aspect of the present disclosure, the first resonance line and the coupling line may be disposed in a plane, the second resonance line may oppose the first resonance line in a direction intersecting with the plane, and the coupling line may be disposed along a portion of the first resonance line with a gap provided between the coupling line and the portion of the first resonance line to thus couple with the first resonance line. 
     This configuration makes it possible to obtain a detection wave through the coupling line that undergoes resonant coupling with the first resonance line. 
     In the electromagnetic resonant coupler according to the aspect of the present disclosure, the first resonance line may have an annular shape with a portion of the annular shape being open, and the coupling line may be disposed inside the first resonance line in the plane. 
     This configuration makes it possible to dispose the coupling line without increasing the area dedicated for wiring. 
     In the electromagnetic resonant coupler according to the aspect of the present disclosure, the first resonance line may have an annular shape with a portion of the annular shape being open, and the coupling line may be disposed outside the first resonance line in the plane. 
     This configuration increases the degree of freedom in the wiring gap between the coupling line and the first resonance line, which thus facilitates the adjustment of the degree of coupling. 
     A transmission apparatus according to an aspect of the present disclosure includes an electromagnetic resonant coupler that includes an input line to which a transmission signal is input, a first resonance line connected to the input line, a second resonance line opposing the first resonance line, the second resonance line undergoing resonant coupling with the first resonance line to thus wirelessly transmit the transmission signal between the first resonance line and the second resonance line, an output line connected to the second resonance line, the transmission signal being output through the output line, a coupling line that electromagnetically couples with at least one selected from the group consisting of the first resonance line and the second resonance line and that outputs a detection wave corresponding to the transmission signal, and a terminator connected to one end of the coupling line; a transmission circuit that inputs the transmission signal to the input line; and a detection circuit connected to another end of the coupling line, the detection circuit generating a detection signal by using the detection wave and outputting the detection signal. 
     In this manner, the transmission apparatus can output the detection signal corresponding to the transmission signal. Such a detection signal makes it possible to monitor the transmission signal with ease. 
     The transmission apparatus according to the aspect of the present disclosure may further include a controller that controls the transmission circuit on the basis of the detection signal to thus adjust at least one selected from the group consisting of an amplitude of the transmission signal and a frequency of the transmission signal. 
     In this manner, as the transmission circuit is controlled in accordance with the detection signal, a fluctuation in the amplitude of the transmission signal is suppressed. For example, the control is possible that brings the signal level of the signal output from the transmission apparatus close to being constant regardless of the ambient temperature of the transmission apparatus. 
     In the transmission apparatus according to the aspect of the present disclosure, the transmission circuit may further include an amplifier that adjusts the amplitude of the transmission signal, and the controller may control the amplifier on the basis of the detection signal to thus adjust the amplitude of the transmission signal. 
     In this manner, as the amplifier is controlled in accordance with the detection signal, a fluctuation in the amplitude of the transmission signal is suppressed. For example, the control is possible that brings the signal level of the signal output from the transmission apparatus close to being constant regardless of the ambient temperature of the transmission apparatus. 
     In the transmission apparatus according to the aspect of the present disclosure, the detection circuit may include a rectenna circuit. 
     This configuration enables the detection circuit to generate the detection signal by using the rectenna circuit. 
     In the transmission apparatus according to the aspect of the present disclosure, the detection circuit may include a double voltage rectifier circuit. 
     This configuration enables the detection circuit to generate the detection signal by using the double voltage rectifier circuit. 
     The transmission apparatus according to the aspect of the present disclosure may further include an amplifier that amplifies the detection wave and outputs the detection wave to the detection circuit. 
     This configuration enables the transmission apparatus to amplify the detection wave. 
     The transmission apparatus according to the aspect of the present disclosure may further include an amplifier that amplifies the detection signal output by the detection circuit. 
     This configuration enables the transmission apparatus to amplify the detection signal. 
     The transmission apparatus according to the aspect of the present disclosure may further include a package member that seals the electromagnetic resonant coupler, the transmission circuit, and the detection circuit; and a terminal that is partially exposed through the package member, the detection signal being output through the terminal. 
     This configuration makes it possible to monitor the detection signal with ease through the terminal. 
     In the present disclosure, all or a part of any of circuit, unit, device, part or portion, or any of functional blocks in the block diagrams may be implemented as one or more of electronic circuits including, but not limited to, a semiconductor device, a semiconductor integrated circuit (IC) or a large scale integration (LSI). The LSI or IC can be integrated into one chip, or also can be a combination of plural chips. For example, functional blocks other than a memory may be integrated into one chip. The name used here is LSI or IC, but it may also be called system LSI, very large scale integration (VLSI), or ultra large scale integration (ULSI) depending on the degree of integration. A field programmable gate array (FPGA) that can be programmed after manufacturing an LSI or a reconfigurable logic device that allows reconfiguration of the connection or setup of circuit cells inside the LSI can be used for the same purpose. 
     Further, it is also possible that all or a part of the functions or operations of the circuit, unit, device, part or portion are implemented by executing software. In such a case, the software is recorded on one or more non-transitory recording media such as a read-only memory (ROM), an optical disk, or a hard disk drive, and when the software is executed by a processor, the software causes the processor together with peripheral devices to execute the functions specified in the software. A system or apparatus may include such one or more non-transitory recording media on which the software is recorded and a processor together with necessary hardware devices such as an interface. 
     Hereinafter, embodiments will be described in detail with reference to the drawings. It is to be noted that the embodiments described hereinafter merely illustrate general or specific examples. The numerical values, the shapes, the materials, the constituent elements, the arrangement and positions of the constituent elements, the connection modes of the constituent elements, and so forth indicated in the embodiments hereinafter are examples and are not intended to limit the present disclosure. In addition, among the constituent elements described in the embodiments hereinafter, a constituent element that is not described in an independent claim indicating the broadest concept is described as an optional constituent element. 
     Furthermore, the drawings are schematic diagrams and do not necessarily provide the exact depiction. In the drawings, configurations that are substantially identical are given identical reference characters, and duplicate descriptions thereof may be omitted or simplified. 
     First Embodiment 
     Overall Structure of Electromagnetic Resonant coupler According to First Embodiment 
     Hereinafter, an overall structure of an electromagnetic resonant coupler according to a first embodiment will be described.  FIG. 3  is an exploded perspective view of the electromagnetic resonant coupler according to the first embodiment.  FIG. 4  is a sectional view of the electromagnetic resonant coupler according to the first embodiment.  FIG. 4  is a sectional view of the electromagnetic resonant coupler according to the first embodiment taken along a plane containing a diagonal line of a dielectric layer. 
     An electromagnetic resonant coupler  100  includes a first resonator  115  and a second resonator  125 , which are in electromagnetic resonant coupling, and wirelessly transmits a signal to be transmitted (hereinafter, referred to as a transmission signal) with the use of the first resonator  115  and the second resonator  125 . A transmission signal can be rephrased as a modulated high-frequency signal. For example, upon a transmission signal being input to an input terminal  111   a  by a transmission circuit, this transmission signal is wirelessly transmitted from the first resonator  115  to the second resonator  125  and output through an output terminal  121   a . The output transmission signal is demodulated by a reception circuit, for example. A high-frequency signal is a signal with a frequency of no lower than 1 MHz, for example. 
     The electromagnetic resonant coupler  100  also operates as a so-called directional coupler and includes a coupling line  130  that outputs a detection wave for monitoring a transmission signal as the electromagnetic resonant coupler  100  has a prescribed degree of coupling. 
     The degree of coupling of the electromagnetic resonant coupler  100 , which operates as a directional coupler, is determined by the ratio between a transmission signal input to the first resonator  115  and a detection wave output from the coupling line  130 . The insertion loss of the electromagnetic resonant coupler  100 , which operates as a directional coupler, is determined by the ratio between a transmission signal input to the first resonator  115  and a transmission signal output from the second resonator  125 . 
     As illustrated in  FIGS. 3 and 4 , the electromagnetic resonant coupler  100  has a multilayer structure in which three dielectric layers including a dielectric layer  101 , a dielectric layer  102 , and a dielectric layer  103  are stacked on each other. Sapphire substrates, for example, are used for the dielectric layers  101 ,  102 , and  103 . The dielectric layers  101 ,  102 , and  103  may be formed by a polyphenylene ether resin (PPE resin) filled with an inorganic filler having a high dielectric constant. 
     The first resonator  115  and the coupling line  130  are formed in a plane on the upper surface of the dielectric layer  101 . The first resonator  115  includes a first resonance line  110  and a linear input line  111  electrically connected to the first resonance line  110 . The first resonator  115  may instead be formed on the lower surface of the dielectric layer  103 . 
     The dielectric layer  102  is disposed such that the lower surface of the dielectric layer  101  is on the upper surface of the dielectric layer  102 . The second resonator  125  is formed in a plane on the upper surface of the dielectric layer  102 . The second resonator  125  includes a second resonance line  120  and a linear output line  121  electrically connected to the second resonance line  120 . A second ground shield  105  is provided on substantially the entire lower surface of the dielectric layer  102 . 
     The dielectric layer  103  is disposed such that the lower surface of the dielectric layer  103  is on the upper surface of the dielectric layer  101 . The input terminal  111   a , the output terminal  121   a , a terminal  131   a , a terminal  132   a , a first ground shield  104 , and two receiver-side ground terminals  105   a  are formed in a plane on the upper surface of the dielectric layer  103 . The first ground shield  104  includes two transmitter-side ground terminals  104   a.    
     In this manner, the first resonator  115 , the second resonator  125 , the first ground shield  104 , and the second ground shield  105  are disposed in mutually different planes. The first resonator  115 , the second resonator  125 , the first ground shield  104 , the second ground shield  105 , and the terminals (the input terminal  111   a  and so on) are formed of metal such as copper. 
     The input terminal  111   a  is disposed between the two transmitter-side ground terminals  104   a . The input terminal  111   a  and the two transmitter-side ground terminals  104   a  constitute a ground-signal-ground (G-S-G) pad. The input terminal  111   a  and the two transmitter-side ground terminals  104   a  are used to electrically connect the transmission circuit to the first resonator  115 . 
     The output terminal  121   a  is disposed between the two receiver-side ground terminals  105   a . The output terminal  121   a  and the two receiver-side ground terminals  105   a  constitute a ground-signal-ground (G-S-G) pad. The output terminal  121   a  and the two receiver-side ground terminal  105   a  are used to electrically connect the reception circuit to the second resonator  125 . 
     The terminals  131   a  and  132   a  are used to monitor the transmission signal transmitted by the electromagnetic resonant coupler  100 . The terminal  132   a  is electrically connected to the first ground shield  104  with a terminator  60  provided therebetween. The terminator  60 , for example, is a 50-Ω chip resistor, but another type of resistor may instead be used as the terminator  60 . For example, a so-called component resistor, a metal resistor buried in a semiconductor chip, a resistor formed by an epitaxial layer, or the like may be used as the terminator  60 . 
     The electromagnetic resonant coupler  100  further includes a via that penetrates at least one of the dielectric layers  101 ,  102 , and  103 . Hereinafter, vias included in the electromagnetic resonant coupler  100  will be described with reference to  FIG. 3 . Metal, such as copper, is used for the vias. 
     A first via  111   b  is a conductive via structure that penetrates the dielectric layer  103  at one end portion of the electromagnetic resonant coupler  100 . The first via  111   b  electrically connects the input line  111  to the input terminal  111   a.    
     A second via  121   b  is a conductive via structure that penetrates the dielectric layers  101  and  103  at another end portion of the electromagnetic resonant coupler  100 . The second via  121   b  electrically connects the output line  121  to the output terminal  121   a . The second via  121   b  is located between two third vias  105   b.    
     The third vias  105   b  are conductive via structures that penetrate the dielectric layers  101 ,  102 , and  103  at the other end portion of the electromagnetic resonant coupler  100 . The third vias  105   b  electrically connect the second ground shield  105  to the receiver-side ground terminals  105   a . The electromagnetic resonant coupler  100  includes two third vias  105   b . The second via  121   b  is located between the two third vias  105   b.    
     A fourth via  131   b  is a conductive via structure that penetrates the dielectric layer  103 . The fourth via  131   b  electrically connects an end portion  131  of the coupling line  130  to the terminal  131   a.    
     A fifth via  132   b  is a conductive via structure that penetrates the dielectric layer  103 . The fifth via  132   b  electrically connects an end portion  132  of the coupling line  130  to the terminal  132   a.    
     Wiring Structure of Electromagnetic Resonant Coupler According to First Embodiment 
     Next, the wiring structure of the electromagnetic resonant coupler  100  will be described in further detail with reference to  FIGS. 3, 5, and 6 .  FIG. 5  is a perspective view illustrating the wiring structure of the electromagnetic resonant coupler  100 .  FIG. 6  is a top view illustrating the wiring structure of the first resonator  115  and the coupling line  130  included in the electromagnetic resonant coupler  100 . 
     The shape of the first resonator  115  will be described first. The first resonator  115  includes the first resonance line  110  and the input line  111  electrically connected to the first resonance line  110 . 
     The first resonance line  110  is an annular line with a portion thereof being open at an opening portion. The first resonance line  110  serves as an antenna for a transmission signal. The first resonance line  110  is an annular line with one end portion  112  and another end portion  113  being located close to each other with a predetermined gap provided therebetween. The term “close to” as used herein means that the items are provided in close proximity to each other but are not in contact with each other. 
     It suffices that the first resonance line  110  be annular with a portion thereof being open. The term “annular” means that a given shape is closed if an opening portion is not provided. In other words, a shape that partially winds is also regarded as an annular shape. Examples of such annular shapes include a ring shape and a racetrack-like shape. An annular shape with a polygonal outline and an elliptical shape are also regarded as annular shapes. The line length of the first resonance line  110  is one-half the wavelength of the transmission signal. The line length of the second resonance line  120  can be made one-quarter the wavelength of the transmission signal by connecting one of the end portions  112  and  113  to the first ground shield  104  with a via or the like provided therebetween. 
     The input line  111  is a linear line connected at one end to the first resonance line  110 , and a transmission signal is input to another end of the input line  111  through the input terminal  111   a  and the first via  111   b . The input line  111  is connected, for example, at a position that is one-quarter the line length of the first resonance line  110  from an end included in the end portion  113  of the first resonance line  110 . The position at which the input line  111  is connected is not particularly limited. 
     Although the first resonance line  110  and the input line  111  are aines with a constant line width in the first embodiment, the line width does not need to be constant. For example, the line width of the first resonance line  110  may differ from the line width of the input line  111 , or the line width of the first resonance line  110  may partially vary. 
     Next, the shape of the second resonator  125  will be described. The second resonator  125  includes the second resonance line  120  and the output line  121  electrically connected to the second resonance line  120 . 
     The second resonance line  120  is an annular line with a portion thereof being open at an opening portion. The second resonance line  120  serves as an antenna for a transmission signal. The second resonance line  120  is an annular line with one end portion  122  and another end portion  123  located close to each other with a predetermined gap provided therebetween. The term “close to” as used herein means that the items are provided in close proximity to each other but are not in contact with each other. 
     It suffices that the second resonance line  120  be annular with a portion thereof being open. The term “annular” means that a given shape is closed if an opening portion is not provided. In other words, a shape that partially winds is also regarded as an annular shape. Examples of such annular shapes include a ring shape and a racetrack-like shape. An annular shape with a polygonal outline and an elliptical shape are also regarded as annular shapes. The line length of the second resonance line  120  is one-half the wavelength of the transmission signal. The line length of the second resonance line  120  can be made one-quarter the wavelength of the transmission signal by connecting one of the end portions  122  and  123  to the second ground shield  105  with a via or the like provided therebetween. 
     The output terminal  121   a  is a linear line connected at one end to the second resonance line  120 , and a transmission signal is output from another end of the output line  121  through the second via  121   b  and the output terminal  121   a . The output line  121  is connected, for example, at a position that is one-quarter the line length of the second resonance line  120  from an end included in the end portion  122  of the second resonance line  120 , but the position at which the output line  121  is connected is not particularly limited. 
     Although the second resonance line  120  and the output line  121  are lines with a constant line width in the first embodiment, the line width does not need to be constant. For example, the line width of the second resonance line  120  may differ from the line width of the output line  121 , or the line width of the second resonance line  120  may partially vary. 
     Next, the shape of the coupling line  130  will be described. The coupling line  130  is an annular line with a portion thereof being open at an opening portion. The coupling line  130  can be rephrased as a line that partially constitutes a high-frequency filter. The coupling line  130  is an annular line with the one end portion  131  and the other end portion  132  located close to each other with a predetermined gap provided therebetween. The term “close to” as used herein means that the items are provided in close proximity to each other but are not in contact with each other. 
     It suffices that the coupling line  130  be annular with a portion thereof being open. The term “annular” means that a given shape is closed if an opening portion is not provided. In other words, a shape that partially winds is also regarded as an annular shape. Examples of such annular shapes include a ring shape and a racetrack-like shape. An annular shape with a polygonal outline and an elliptical shape are also regarded as annular shapes. The line length of the coupling line  130  is, for example, no less than 80% nor more than 120% of one-half the wavelength of the transmission signal. The line length of the coupling line  130  is often shorter than that of the first resonance line  110  in the case in which the coupling line  130  is disposed inside the first resonance line  110 . In the case in which the coupling line  130  is disposed outside the first resonance line  110 , the line length of the coupling line  130  may be shorter than that of the first resonance line  110  or may be equal to or longer than that of the first resonance line  110 . 
     As illustrated in  FIG. 3 , the one end portion  131  of the coupling line  130  is connected to the terminal  131   a  with the fourth via  131  b provided therebetween, and the other end portion  132  of the coupling line  130  is connected to the terminal  132   a  with the fifth via  132   b  provided therebetween. In the first embodiment, the terminal  131   a  is used as a terminal for monitoring, and the terminal  132   a  is terminated by the 50-Ω terminator  60 . It suffices that one of the terminals  131   a  and  132   a  be terminated by the 50-Ω terminator  60  and that the other one of the terminals  131   a  and  132   a  be used as a terminal for monitoring. 
     Although the coupling line  130  has a constant line width in the first embodiment, the line width does not need to be constant. For example, the line width of the coupling line  130  may partially vary. The line width of the coupling line  130  may differ from the line width of the first resonance line  110 . 
     Positional Relationship of Wires 
     Next, the positional relationship among the first resonator  115 , the second resonator  125 , and the coupling line  130  will be described. The positional relationship between the first resonator  115  and the second resonator  125  will be described first. 
     The first resonance line  110  in the first resonator  115  is disposed to oppose the second resonance line  120  in the second resonator  125  in the lamination direction. The dielectric layer  101  is present between the first resonance line  110  and the second resonance line  120 . Therefore, the first resonance line  110  and the second resonance line  120  are not in direct contact with each other. 
     The outline of the first resonance line  110  substantially coincides with the outline of the second resonance line  120  when viewed in the direction perpendicular to the principal surface of the dielectric layer  101 , or in other words, when viewed from above. The outline of the first resonance line  110  is defined as follows. 
     Suppose that the opening portion is not provided in the first resonance line  110  and that the first resonance line  110  is a closed annular line, this closed annular line has an inner-peripheral outline that defines a region enclosed by the closed annular line and an outer-peripheral outline that defines the shape of the closed annular line along with the inner-peripheral outline. Of these two outlines, the outline of the first resonance line  110  refers to the outer-peripheral outline. In other words, the inner-peripheral outline and the outer-peripheral outline define the line width of the first resonance line  110 , and the outer-peripheral outline defines the area occupied by the first resonance line  110 . The same definition applies to the outline of the second resonance line  120 . 
     Specifically, in the first embodiment, the outlines of the first resonance line  110  and the second resonance line  120  correspond to the outermost shapes of the first resonance line  110  and the second resonance line  120  and are circular in shape. In this case, that the outlines coincide with each other means that the outlines substantially coincide with each other except for the portions corresponding to the opening portions. 
     That the outlines substantially coincide with each other means that the outlines substantially coincide with each other with taken into account a variation associated with assembling the dielectric layers  101  and  102  and a variation in the sizes of the first resonance line  110  and the second resonance line  120  that could arise in the manufacturing process. In other words, that the outlines substantially coincide with each other does not necessarily mean that the outlines completely coincide with each other. 
     Even in the case in which the outlines of the first resonance line  110  and the second resonance line  120  do not coincide with each other, the electromagnetic resonant coupler  100  is operable. The electromagnetic resonant coupler  100  operates more effectively when the outlines of the first resonance line  110  and the second resonance line  120  coincide with each other. 
     In the first embodiment, the first resonance line  110  and the second resonance line  120  are in the positional relationship of point symmetry or line symmetry when viewed from above. The first resonance line  110  and the second resonance line  120  may be in any desired positional relationship as viewed from above as long as a given positional relationship is within a range in which an electromagnetic resonance phenomenon occurs between the resonance lines. 
     The first resonance line  110  and the second resonance line  120  may be coaxial. Such an arrangement enhances the resonant coupling between the resonance lines and makes it possible to transmit power with high efficiency. 
     The distance between the first resonator  115  and the second resonator  125  in the lamination direction is no more than one-half the operation wavelength, which is the wavelength of a transmission signal. The wavelength in this case is the wavelength that takes into consideration the wavelength compaction ratio by the dielectric layer  101  in contact with the first resonator  115  and the second resonator  125 . Under such a condition, it can be said that the first resonator  115  and the second resonator  125  are in electromagnetic resonant coupling in the near-field range. The distance between the first resonator  115  and the second resonator  125  in the lamination direction corresponds to the thickness of the dielectric layer  101 . 
     The distance between the first resonator  115  and the second resonator  125  in the lamination direction is not limited to one-half the operation wavelength. Even in the case in which the distance between the first resonator  115  and the second resonator  125  in the lamination direction is greater than one-half the operation wavelength, the electromagnetic resonant coupler  100  is operable. However, the electromagnetic resonant coupler  100  operates more effectively when the distance between the first resonator  115  and the second resonator  125  in the lamination direction is no more than one-half the operation wavelength. 
     Next, the positional relationship between the first resonator  115  and the coupling line  130  will be described. 
     Similarly to the first resonance line  110 , the coupling line  130  is formed on the upper surface of the dielectric layer  101 . In other words, the coupling line  130  and the first resonance line  110  are disposed in the same plane. The coupling line  130  is disposed inside the first resonance line  110  along a portion of the first resonance line  110  with a predetermined gap provided between the coupling line  130  and the portion of the first resonance line  110 . This configuration makes it possible to dispose the coupling line  130  without increasing the area dedicated for wiring on the dielectric layer  101 . The coupling line  130  and the first resonance line  110  are not connected with a line and are not in contact with each other. 
     The degree of coupling between the coupling line  130  and the first resonance line  110  is determined by the gap between the coupling line  130  and the first resonance line  110 , the line width of the coupling line  130 , and so on. 
     Thus, as illustrated in  FIG. 19 , the coupling line  130  may be disposed outside the first resonance line  110  along a portion of the first resonance line  110  with a predetermined gap provided between the coupling line  130  and the portion of the first resonance line  110 . Disposing the coupling line  130  outside the first resonance line  110  increases the degree of freedom in the wiring gap between the coupling line  130  and the first resonance line  110 , which thus facilitates the adjustment of the degree of coupling. For example, the degree of coupling can be increased. 
     In the electromagnetic resonant coupler  100 , the coupling line  130  couples with the first resonance line  110 , but the coupling line  130  may couple with the second resonance line  120 . In this case, similarly to the second resonance line  120 , the coupling line  130  is formed on the upper surface of the dielectric layer  102 . In other words, the coupling line  130  and the second resonance line  120  are disposed in the same plane. The term “coupling” as used herein means electromagnetic coupling and does not mean structural coupling. 
     The coupling line  130  may, for example, be disposed inside the second resonance line  120  along a portion of the second resonance line  120  with a predetermined gap provided between the coupling line  130  and the portion of the second resonance line  120 . The coupling line  130  may be disposed outside the second resonance line  120  along a portion of the second resonance line  120  with a predetermined gap provided between the coupling line  130  and the portion of the second resonance line  120 . 
     Operation of Electromagnetic Resonant Coupler 
     An operation of the electromagnetic resonant coupler  100  will be described with reference to  FIG. 7 .  FIG. 7  is a schematic diagram for describing the operation of the electromagnetic resonant coupler  100 . 
     A transmission signal input to the input line  111  is wirelessly transmitted to the second resonance line  120  from the first resonance line  110  through electromagnetic resonant coupling between the first resonance line  110  and the second resonance line  120  and is output through the output line  121 . 
     The first resonance line  110  is shared by the second resonance line  120  and the coupling line  130 . The transmission signal input to the input line  111  is also output to the terminal  131   a  through the end portion  131  of the coupling line  130 . In other words, the terminal  131   a  can be used as a terminal for monitoring the transmission signal. As described above, the end portion  132  of the coupling line  130  is connected to the first ground shield  104  with the terminator  60  provided therebetween. In other words, the end portion  132  of the coupling line  130  is terminated by the terminator  60 . The terminator  60  may be a constituent element of the electromagnetic resonant coupler  100  or may be separate from the electromagnetic resonant coupler  100 . 
     A result of simulating the transmission characteristics of the electromagnetic resonant coupler  100  that operates as described above will be described with reference to  FIGS. 8 and 9 .  FIG. 8  illustrates the transmission characteristics of an electromagnetic resonant coupler according to a comparative example.  FIG. 9  illustrates the transmission characteristics of the electromagnetic resonant coupler  100 . The electromagnetic resonant coupler according to the comparative example is similar to the electromagnetic resonant coupler  100  except in that the former does not include the coupling line  130 . 
     In the simulation, the frequency of the transmission signal is set to 2.4 GHz. The terminator  60  is set to 50Ω. 
     As indicated by the position of m 1  in  FIG. 8 , the insertion loss of the electromagnetic resonant coupler according to the comparative example is 0.96 dB at 2.4 GHz. Meanwhile, as indicated by the position of m 1  in  FIG. 9 , the insertion loss of the electromagnetic resonant coupler  100  is 1.06 dB at 2.4 GHz, which is worse than the insertion loss of the electromagnetic resonant coupler according to the comparative example although the difference is only less than 1%. 
     On the other hand, as indicated by the position of m 2  in  FIG. 9 , the degree of coupling when the electromagnetic resonant coupler  100  is regarded as a directional coupler is −19 dB, which is a sufficiently large degree of coupling. 
     In this manner, in the electromagnetic resonant coupler  100 , the transmission signal can be monitored without any additional, separate component besides the coupling line  130 . 
     As illustrated in  FIG. 9 , the resonant coupling between the first resonance line  110  and the second resonance line  120  is sufficiently stronger than the resonant coupling between the first resonance line  110  and the coupling line  130 . 
     Advantageous Effects of First Embodiment 
     As described thus far, the electromagnetic resonant coupler  100  includes the input line  111  to which a transmission signal is input; the first resonance line  110  connected to the input line  111 ; the second resonance line  120  opposing the first resonance line  110 , the second resonance line  120  undergoing resonant coupling with the first resonance line  110  to wirelessly transmit the transmission signal between the first resonance line  110  and the second resonance line  120 ; the output line  121  connected to the second resonance line  120 , the transmission signal that has been wirelessly transmitted being output through the output line  121 ; and the coupling line  130  that couples with at least one of the first resonance line  110  and the second resonance line  120 . 
     This configuration makes it possible to obtain a detection wave corresponding to the transmission signal through the coupling line  130 . In other words, the transmission signal can be monitored with ease with the coupling line  130  without a complex device or the like. 
     The electromagnetic resonant coupler  100  may further include the terminator  60  connected to the other end portion  132  of the coupling line  130 . The other end portion  132  corresponds to one end of the coupling line. 
     In this manner, connecting the terminator  60  to the other end portion  132  of the coupling line  130  makes it possible to obtain a detection wave through the one end portion  131  of the coupling line  130 . In addition, this configuration renders it unnecessary to externally provide the terminator  60  in a transmission apparatus that will be described later. 
     The first resonance line  110  and the coupling line  130  may be disposed in the same plane, and the second resonance line  120  may oppose the first resonance line  110  in the direction intersecting with the stated plane. The coupling line  130  may be disposed along a portion of the first resonance line  110  with a predetermined gap provided between the coupling line  130  and the portion of the first resonance line  110  and may thus couple with the first resonance line  110 . 
     This configuration makes it possible to obtain a detection wave through the coupling line  130  that couples with the first resonance line  110 . 
     The first resonance line  110  may be annular with a portion thereof being open, and the coupling line  130  may be disposed inside the first resonance line  110  in the same plane. 
     This configuration makes it possible to dispose the coupling line  130  without increasing the area dedicated for wiring. 
     The first resonance line  110  may be annular with a portion thereof being open, and the coupling line  130  may be disposed outside the first resonance line  110  in the same plane. 
     This configuration increases the degree of freedom in the wiring gap between the coupling line  130  and the first resonance line  110 , which thus facilitates the adjustment of the degree of coupling. 
     Second Embodiment 
     Wiring Structure of Electromagnetic Resonant Coupler According to Second Embodiment 
     In a second embodiment, an electromagnetic resonant coupler that can transmit, in isolation, two high-frequency signals independently from each other and that operates as a directional coupler will be described. In the second embodiment described hereinafter, the configurations aside from the wiring structures of a first resonator and a second resonator (for example, the positional relationship between the first resonator and the second resonator) are similar to those of the first embodiment, and thus descriptions of such similar configurations will be omitted. 
       FIG. 10  is a perspective view illustrating the wiring structure of the electromagnetic resonant coupler according to the second embodiment.  FIG. 11  is a top view illustrating the wiring structure of the first resonator and the coupling line included in the electromagnetic resonant coupler according to the second embodiment. The electromagnetic resonant coupler according to the second embodiment includes a first resonator  315 , a second resonator  325 , and a coupling line  330 . 
     The first resonator  315  will be described first. The first resonator  315  includes a first resonance line  310 , a first input line  311 , a second input line  312 , first ground lines  316  and  317 , and a first connection line  318 . 
     The first resonance line  310  is a modified annular line having an opening portion  313 . The first resonance line  310  has two recess portions that are recessed toward the inside as viewed from above, and these two recess portions are close to each other. The opening portion  313  is provided in one of the two recess portions, and a connection portion to which one end of the linear first connection line  318  is connected is provided at the other one of the two recess portions. The connection portion is electrically connected to the first ground line  317  with the first connection line  318  provided therebetween. The first resonator  315  can be seen as two substantially rectangular annular lines being connected at the connection portion. The connection portion may be connected to the first ground shield  104  (not illustrated in  FIGS. 10 and 11 ) with a via provided therebetween instead of being connected to the first ground line  317 . 
     The first input line  311  is a linear line electrically connected to the first resonance line  310 . Specifically, the first input line  311  is electrically connected to one of the aforementioned two substantially rectangular annular lines. A transmission signal input to the first input line  311  is output to a first output line  321  included in the second resonator  325 . 
     The second input line  312  is a linear line electrically connected to the first resonance line  310 . Specifically, the second input line  312  is electrically connected to the other one of the aforementioned two substantially rectangular annular lines. A transmission signal input to the second input line  312  is output to a second output line  322  included in the second resonator  325 . 
     The first ground lines  316  and  317  are lines that serve as a reference potential within the first resonator  315 . The first ground line  316  is bracket-shaped, and the first ground line  317  is linear. The first ground lines  316  and  317  are disposed to surround the first resonance line  310  and function as a so-called coplanar ground. The first ground line  317  is connected to another end of the first connection line  318 . The first ground lines  316  and  317  do not need to be provided, and the first ground shield  104  may instead serve as a reference potential. In that case, the other end of the first connection line  318  is connected to the first ground shield  104  with a via provided therebetween. 
     Next, the second resonator  325  will be described. The second resonator  325  includes a second resonance line  320 , the first output line  321 , the second output line  322 , second ground lines  326  and  327 , and a second connection line  328 . 
     The second resonance line  320  is a modified annular line having an opening portion  323 . The second resonance line  320  has two recess portions that are recessed toward the inside as viewed from above, and these two recess portions are close to each other. The opening portion  323  is provided in one of the two recess portions, and a connection portion to which one end of the linear second connection line  328  is connected is provided at the other one of the two recess portions. The connection portion is electrically connected to the second ground line  327  with the second connection line  328  provided therebetween. The second resonator  325  can be seen as two substantially rectangular annular lines being connected at the connection portion. The connection portion may be connected to the second ground shield  105  (not illustrated in  FIGS. 10 and 11 ) with a via provided therebetween instead of being connected to the second ground line  327 . 
     The first output line  321  is a linear line electrically connected to the second resonance line  320 . Specifically, the first output line  321  is electrically connected to one of the aforementioned two substantially rectangular annular lines. A transmission signal input to the first input line  311  included in the first resonator  315  is output through the first output line  321 . 
     The second output line  322  is a linear line electrically connected to the second resonance line  320 . Specifically, the second output line  322  is electrically connected to the other one of the aforementioned two substantially rectangular annular lines. A transmission signal input to the second input line  312  included in the first resonator  315  is output through the second output line  322 . 
     The second ground lines  326  and  327  are lines that serve as a reference potential within the second resonator  325 . The second ground line  326  is bracket-shaped, and the second ground line  327  is linear. The second ground lines  326  and  327  are disposed to surround the second resonance line  320  and function as a so-called coplanar ground. The second ground line  327  is connected to another end of the second connection line  328 . 
     Next, the coupling line  330  will be described. The coupling line  330  is a bracket-shaped line. The coupling line  330  and the first resonance line  310  are disposed in the same plane. 
     The coupling line  330  is disposed inside the first resonance line  310  along a portion of the first resonance line  310  with a predetermined gap provided between the coupling line  330  and the portion of the first resonance line  310 . To be more specific, the coupling line  330  is disposed inside and along one of the aforementioned two substantially rectangular annular lines to which the first input line  311  is connected. 
     Although not illustrated in  FIGS. 10 and 11 , one end portion  331  of the coupling line  330  is connected to a terminal for monitoring with a via provided therebetween, and another end portion  332  of the coupling line  330  is connected to a terminal terminated by the terminator  60  with a via provided therebetween. 
     In the electromagnetic resonant coupler having such a wiring structure, a transmission signal input to the first input line  311  can be monitored with the coupling line  330 . The electromagnetic resonant coupler may further include another coupling line disposed inside and along the other one of the aforementioned substantially rectangular annular lines to which the second input line  312  is connected. In other words, the electromagnetic resonant coupler may include a plurality of coupling lines with respect to a single first resonance line  310 . Such coupling lines make it possible to further monitor the transmission signal input to the second input line  312 . 
     The coupling line  330  may undergo resonant coupling with the second resonance line  320 . In other words, the coupling line  330  and the second resonance line  320  may be disposed in the same plane. 
     Third Embodiment 
     Structure of Transmission Apparatus According to Third Embodiment 
     In a third embodiment, a transmission apparatus that includes the electromagnetic resonant coupler  100  will be described.  FIG. 12  is a perspective view of the transmission apparatus.  FIG. 13  illustrates a circuit configuration of the transmission apparatus. 
     As illustrated in  FIGS. 12 and 13 , a transmission apparatus  200  includes a transmission circuit  201 , the electromagnetic resonant coupler  100 , a reception circuit  202 , a detection circuit  203 , a first leadframe  204 , a second leadframe  205 , a package member  206 , and terminals (for example, a terminal  207 , a terminal  210 , and a terminal  214 ). A bonding wire  208  is used to electrically connect these devices. 
     The package member  206  is a mold resin that seals the above constituent elements except for the terminals and is indicated by the dashed line in  FIG. 12  in order to show the internal structure of the transmission apparatus  200 . 
     The transmission circuit  201  inputs a transmission signal to the input terminal  111   a  included in the electromagnetic resonant coupler  100 . The transmission circuit  201  is, for example, a semiconductor formed into a chip and is die-bonded on the upper surface of the first leadframe  204 . As illustrated in  FIG. 13 , the transmission circuit  201  includes, for example, an oscillator circuit  211 , a mixing circuit  212 , and an amplifier  213 . 
     The oscillator circuit  211  generates a high-frequency signal, which is a carrier wave of an input signal (for example, a binary digital signal) input to the terminal  214 . A high-frequency signal as used herein means a signal having a frequency higher than that of a signal input to the terminal  214  and is specifically a signal having a frequency of no lower than 1 MHz. 
     The mixing circuit  212  modulates the high-frequency signal output by the oscillator circuit  211  in accordance with the input signal input to the terminal  214  to thus generate a transmission signal. The amplifier  213  amplifies the transmission signal and outputs the amplified transmission signal to the electromagnetic resonant coupler  100 . In addition, the amplifier  213  can amplify or attenuate the transmission signal to thus adjust the amplitude of the transmission signal. 
     The reception circuit  202  demodulates the transmission signal output from the output terminal  121   a  included in the electromagnetic resonant coupler  100 . The demodulated signal is output to the terminal  210 . Specifically, the reception circuit  202  is, but is not particularly limited to, a rectifier circuit that includes a diode, an inductor, and a capacitor. 
     The reception circuit  202  is die-bonded on the upper surface of the second leadframe  205 . The electromagnetic resonant coupler  100  is also die-bonded on the upper surface of the second leadframe  205 . 
     The detection circuit  203  is connected to the terminal  131   a  and acquires a detection wave corresponding to the transmission signal from the coupling line  130 . In addition, the detection circuit  203  generates a detection signal with the use of the detection wave and outputs the generated detection signal to the terminal  207 . The terminal  207  is a terminal through which the detection signal is output and that is exposed to the outside of the package member  206 . The detection circuit  203  is, for example, a semiconductor formed into a chip and is die-bonded on the upper surface of the first leadframe  204 . 
     Hereinafter, the detailed configuration of the detection circuit  203  will be described. The detection circuit  203  converts a transmission signal, which is a high-frequency signal, to a direct current signal. The detection circuit  203  includes, for example, a single-shunt rectenna circuit. The single-shunt rectenna circuit is capable of power conversion of a high-frequency signal into a direct current signal with high efficiency with a simple configuration. The detection circuit  203  includes a diode  203   a , an inductor  203   b , and a capacitor  203   c.    
     In the detection circuit  203 , the anode of the diode  203   a  is connected to the ground, and the cathode of the diode  203   a  is connected to the terminal  131   a  and one end of the inductor  203   b.    
     The inductor  203   b  and the capacitor  203   c  function as a low-pass filter with respect to the fundamental wave of the detection wave. The one end of the inductor  203   b  is connected to the cathode of the diode  203   a  and the terminal  131   a . The one end of the inductor  203   b  is connected to the terminal  207  and one end of the capacitor  203   c . The capacitor  203   c  is connected at one end to the terminal  207  and the other end of the inductor  203   b  and connected at the other end to the ground. 
     When the diode  203   a , the inductor  203   b , and the capacitor  203   c  are connected in this manner, the detection circuit  203  can output a positive direct current voltage to the terminal  207 . The detection circuit  203  can output a negative direct current voltage when the cathode of the diode  203   a  is connected to the ground and the anode of the diode  203   a  is connected to the terminal  131   a  and the one end of the inductor  203   b . The detection circuit  203  operates as follows. 
     Upon a detection wave being input to the detection circuit  203 , a high-frequency signal of half a cycle in which the detection wave has a positive voltage (hereinafter, also referred to as a positive high-frequency signal) is applied to the diode  203   a . At this point, the diode  203   a  enters an OFF state, and the positive high-frequency signal is thus output to the inductor  203   b.    
     The other end of the capacitor  203   c  is connected to the ground, and the one end and the other end of the capacitor  203   c  are short-circuited with respect to the positive high-frequency signal. In other words, the capacitor  203   c  is the fixed end with respect to the positive high-frequency signal. Thus, the positive high-frequency signal is reflected in a reverse phase at the capacitor  203   c , passes through the inductor  203   b  again, and is output to the diode  203   a.    
     The electric wire length of the inductor  203   b  is set to approximately one-quarter the wavelength of the fundamental wave of the detection wave. Thus, the positive high-frequency signal that has been reflected at the capacitor  203   c  and has returned to the diode  203   a  is delayed by half a cycle and is in a reverse phase upon having gone back and forth through the inductor  203   b.    
     Meanwhile, when a high-frequency signal of another half a cycle in which the detection wave is a negative voltage (hereinafter, also referred to as a negative high-frequency signal) is applied to the diode  203   a , the negative high-frequency signal is added, in phase, to the above-described positive high-frequency signal that has been reflected by and has returned from the capacitor  203   c . In this case, the diode  203   a  enters an ON state, and thus the negative high-frequency signal to which the positive high-frequency signal has been added is rectified in a state in which the crest value is higher than that in the case of half-wave rectification. In other words, double voltage rectification is achieved. 
     In this manner, the detection circuit  203  seems like a half-wave rectifier circuit at a glance but is capable of double voltage rectification, and the conversion efficiency equivalent to that of full-wave rectification can be achieved. The rectified signal is smoothed by the capacitor  203   c  to result in a detection signal. The detection signal is a direct current signal of which the signal level varies in accordance with the amplitude of the detection wave. 
     The detection circuit  203  does not need to be such a configuration that includes a single-shunt rectenna circuit. The detection circuit  203  may include a single-series rectenna circuit or may include another rectenna circuit. The transmission apparatus  200  may include, in place of the detection circuit  203 , a detection circuit that includes a circuit other than a rectenna circuit, such as a detection circuit  203   d  that includes a double voltage rectifier circuit as illustrated in  FIG. 14 .  FIG. 14  illustrates a circuit configuration of the detection circuit  203   d  that includes a double voltage rectifier circuit. 
     As described thus far, the transmission apparatus  200  can output, through the terminal  207 , a detection signal of which the signal level varies in accordance with the amplitude of a detection wave corresponding to a transmission signal. As the transmission circuit  201  is controlled in accordance with the detection signal, the fluctuation in the amplitude of the transmission signal is suppressed. For example, the control is possible that brings the signal level of a signal output from the transmission apparatus close to being constant regardless of the ambient temperature of the transmission apparatus  200 . In addition, product inspection, failure analysis, and so on can be carried out with the use of a detection signal. 
     First Modification of Third Embodiment 
     The controller that controls the transmission circuit  201  with the use of the detection signal as described above may be provided externally to the transmission apparatus  200 , or the transmission apparatus  200  may include a controller. In other words, the transmission apparatus  200  may be a device formed into a package including a controller.  FIGS. 15 through 17  are block diagrams of transmission apparatuses that include a controller. 
     A transmission apparatus  200   a  illustrated in  FIG. 15  further includes a controller  400  that controls the transmission circuit  201  on the basis of a detection signal output from the detection circuit  203  to thus adjust the transmission signal. Specifically, the controller  400  controls the amplifier  213  on the basis of the output detection signal to thus adjust the amplitude of the transmission signal. The controller  400 , for example, raises the gain of the amplifier  213  as the signal level of the detection is lower. This configuration suppresses a variation in the amplitude of the signal output from the transmission apparatus  200   a.    
     The controller  400  is implemented, for example, by a circuit but may instead be implemented by a processor and a memory. A processor, for example, is a central processing unit (CPU), a microprocessing unit (MPU), or the like. In this case, the processor may read out and execute a program stored in the memory to thus control the transmission circuit  201 . 
     In the case in which the amplitude of a detection wave is small, a transmission apparatus  200   b  may include an amplifier  501  that amplifies a detection signal output from the detection circuit  203 , as illustrated in  FIG. 16 . Such a transmission apparatus  200   b  can amplify the detection signal. In addition, as illustrated in  FIG. 17 , a transmission apparatus  200   c  may include an amplifier  502  that amplifies a detection wave obtained from the coupling line  130  and outputs the amplified detection wave to the detection circuit  203 . Such a transmission apparatus  200   c  can amplify the detection wave. 
     Second Modification of Third Embodiment 
     For example, when a power switch of large power is driven in a motor driving circuit, a large current is supplied instantaneously to an input terminal of the power switch. Thus, in order to drive a power switch of large power, power is once accumulated in an external capacitor or the like, and the accumulated power is discharged with two or more small-sized switches. Therefore, a transmission apparatus to be used in a motor driving circuit includes two or more pairs of first resonators and second resonators. 
     Thus, a transmission apparatus may include two or more electromagnetic resonant couplers.  FIG. 18  illustrates a circuit configuration of a transmission apparatus that includes three electromagnetic resonant couplers. 
     A transmission apparatus  200   d  illustrated in  FIG. 18  includes three electromagnetic resonant couplers. Specifically, the transmission apparatus  200   d  includes an electromagnetic resonant coupler  100  for adjusting the signal level of a signal output from the transmission apparatus  200   d , an electromagnetic resonant coupler  100   a  for driving a high-side switch, and an electromagnetic resonant coupler  100   b  for driving a low-side switch. The electromagnetic resonant couplers  100   a  and  100   b  have a configuration similar to that of the electromagnetic resonant coupler  100  except in that the electromagnetic resonant couplers  100   a  and  100   b  do not include the coupling line  130 . 
     A transmission circuit  201   a  included in the transmission apparatus  200   d  includes, for example, an oscillator circuit  211   a  having two output terminals, a mixing circuit  212   a  having two output terminals, and an amplifier  213   a.    
     The amplifier  213   a  amplifies a high-frequency signal output from one of the output terminals of the oscillator circuit  211   a  and outputs the amplified high-frequency signal to the electromagnetic resonant coupler  100 . The mixing circuit  212   a  modulates a high-frequency signal output from the other one of the output terminals of the oscillator circuit  211   a  in accordance with an input signal to thus generate a transmission signal and outputs the generated transmission signal to the electromagnetic resonant coupler  100   a . In addition, the mixing circuit  212   a  modulates the high-frequency signal output from the other one of the output terminals of the oscillator circuit  211   a  in accordance with a signal obtained by inverting the logic of the input signal to thus generate a transmission signal and outputs the generated transmission signal to the electromagnetic resonant coupler  100   b.    
     The transmission signal transmitted by the electromagnetic resonant coupler  100   a  is received and demodulated by a reception circuit  202   a . The transmission signal transmitted by the electromagnetic resonant coupler  100   b  is received and demodulated by a reception circuit  202   b . The reception circuits  202   a  and  202   b  are rectifier circuits, for example. For example, a rectifier circuit of which the connection relationship between the anode and the cathode of the diode is reversed from that of the reception circuit  202  is used for the reception circuits  202   a  and  202   b.    
     In this manner, the transmission apparatus  200   d  may include a plurality electromagnetic resonant couplers. Similarly to the transmission apparatus  200 , the transmission apparatus  200   d  can output, through the terminal  207 , a detection signal of which the signal level varies in accordance with the amplitude of the detection signal corresponding to the transmission signal (high-frequency signal). 
     Similarly to the transmission apparatus  200   a , the transmission apparatus  200   d  may include a controller. In other words, the transmission apparatus  200   d  may be a device formed into a package including a controller. 
     Advantageous Effects of Third Embodiment 
     As described thus far, the transmission apparatus  200  includes the electromagnetic resonant coupler  100 ; the transmission circuit  201  that inputs a transmission signal to the input line  111 ; and the detection circuit  203  that is connected to the one end portion  131  of the coupling line  130 , generates a detection signal with the use of a detection wave obtained from the coupling line  130 , and outputs the generated detection signal. The one end portion  131  of the coupling line  130  corresponds to one end of the coupling line  130 . 
     In this manner, the transmission apparatus  200  can output a detection signal corresponding to a transmission signal. The detection signal makes it possible to monitor the transmission signal with ease. 
     In addition, similarly to the transmission apparatus  200   a , the transmission apparatus  200  may further include the controller  400  that controls the transmission circuit  201  on the basis of the output detection signal to thus adjust the transmission signal. 
     In this manner, as the transmission circuit  201  is controlled in accordance with the detection signal, a fluctuation in the amplitude of the transmission signal is suppressed. For example, the control is possible that brings the signal level of the signal output from the transmission apparatus  200  close to being constant regardless of the ambient temperature of the transmission apparatus  200 . 
     Specifically, the transmission circuit  201  may include the amplifier  213  that adjusts the amplitude of the transmission signal, and the controller  400  may control the amplifier  213  on the basis of the output detection signal to thus adjust the amplitude of the transmission signal. 
     In this manner, as the amplifier  213  is controlled in accordance with the detection signal, a fluctuation in the amplitude of the transmission signal is suppressed. For example, the control is possible that brings the signal level of the signal output from the transmission apparatus  200  close to being constant regardless of the ambient temperature of the transmission apparatus  200 . 
     The detection circuit  203  may include a rectenna circuit. 
     This configuration enables the detection circuit  203  to generate the detection signal by using the rectenna circuit. 
     Similarly to the detection circuit  203   d , the detection circuit  203  may include a double voltage rectifier circuit. 
     This configuration enables the detection circuit  203   d  to generate the detection signal by using the double voltage rectifier circuit. 
     Similarly to the transmission apparatus  200   c , the transmission apparatus  200  may further include the amplifier  502  that amplifies the detection wave obtained from the coupling line  130  and outputs the amplified detection wave to the detection circuit  203 . 
     This configuration enables the transmission apparatus  200   c  to amplify the detection wave. 
     Similarly to the transmission apparatus  200   b , the transmission apparatus  200  may further include the amplifier  501  that amplifies the detection signal output from the detection circuit  203 . 
     This configuration enables the transmission apparatus  200   b  to amplify the detection signal. 
     The transmission apparatus  200  may further include the package member  206  that seals the electromagnetic resonant coupler  100 , the transmission circuit  201 , and the detection circuit  203 ; and the terminal  207  through which the detection signal is output and that is exposed through the package member  206 . 
     This configuration makes it possible to monitor the transmission signal with ease through the terminal  207 . 
     Other Embodiments 
     As described thus far, the embodiments have been described to illustrate the techniques disclosed in the present application. However, the present disclosure is not limited to these embodiments and can also be applied to other embodiments that include modifications, replacements, additions, omissions, and so on, as appropriate. In addition, a new embodiment can also be conceived of by combining the constituent elements described in the above embodiments. 
     For example, the circuit configurations described in the first through third embodiments above are merely examples. A different circuit configuration that can implement the functions described in the above first through third embodiments may instead be used. For example, a circuit configuration in which an element such as a switching element, a resistive element, or a capacitative element is connected in series or in parallel to another element within the scope in which the functions similar to those of the circuit configurations described above can be achieved is also included within the present disclosure. In other words, the term “connected” as used in the embodiments described above is not limited to the case in which two terminals (nodes) are connected directly but includes the case in which such two terminals (nodes) are connected with another element interposed therebetween within the scope in which a similar function can be achieved. 
     General or specific embodiments of the present disclosure may be implemented in the form of a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium, such as a CD-ROM. General or specific embodiments of the present disclosure may be implemented through any desired combination of a system, a method, an integrated circuit, a computer program, and a recording medium. For example, the present disclosure may be implemented in the form of a method of adjusting a signal output by a transmission apparatus and a program for causing a computer to execute such a method. 
     Thus far, an electromagnetic resonant coupler and a transmission apparatus according to one or a plurality of aspects have been described on the basis of the embodiments, but the present disclosure is not limited to these embodiments. Unless departing from the spirit of the present disclosure, an embodiment obtained by making various modifications that are conceivable by a person skilled in the art to the present embodiments or an embodiment obtained by combining the constituent elements in different embodiments may also be included within the scope of the one or the plurality of aspects.