Patent Publication Number: US-7907032-B2

Title: Directional coupler

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a directional coupler having a main line and a coupled line. 
     2. Background Art 
     It is common for a wireless terminal to include a directional coupler to monitor the level of its transmission power.  FIG. 36  shows a typical configuration of a directional coupler. Referring to  FIG. 36 , a main line  500  is a line for transmitting transmission power and is connected between an input port (# 1 ) and an output port (# 2 ). On the other hand, a coupled line  502  is provided to couple out a portion of the transmission power in the main line  500  and is connected between a coupled port (# 3 ) and an isolated port (# 4 ). It should be noted that some directional couplers are formed in a spiral shape to reduce their overall dimensions (see  FIG. 37 ). The performance of a directional coupler is measured by its directivity, which is defined as the ratio of its coupling to isolation. The higher the directivity, the less the influence of the reflected wave from the output port when the power transmitted from the input port to the output port is coupled out to the coupled port. The coupling and isolation of a directional coupler are often frequency dependent, e.g., as shown in  FIG. 38  in which the symbol Dir indicates directivity. 
     A directional coupler is inserted, e.g., between a transmit power amplifier and an antenna, and used, e.g., in a cellular phone unit as shown in  FIG. 39 . In  FIG. 39 , the BB-LSI is the core component of the cellular phone unit and exchanges voice and data with an external device and performs signal processing. Further, the RF/IF-IC shown in  FIG. 39  is an IC, and receives the transmission signal from the BB-LSI, frequency converts it to a high frequency signal, and supplies the high frequency signal to an amplifier (PA). The RF/IF-IC also receives the received signal from an antenna (ANT), converts it to an intermediate frequency signal, and supplies the intermediate frequency signal to the BB-LSI. The directional coupler is series connected in the transmission line for the transmission signal. The signal appearing on the coupled port of the directional coupler is delivered through a capacitor Cc to a detector DET. This signal is further delivered from the detector to the BB-LSI and provides information for monitoring and controlling the output level of the amplifier. 
     Thus, since the directional coupler is used to monitor the output level (or output power) of the amplifier, it is desired that the coupled out signal from the coupled port accurately reflect the output level of the amplifier without error.  FIG. 40  is a graph showing the relationship between the directivity of the directional coupler and the error in the power measurement by the detector. Generally, a directional coupler must have a directivity of approximately 20 dB or higher to ensure a measurement error of 0.5 dB or less. 
     For example, Japanese Utility Model Laid-Open Patent Publication No. 02-098534 (1990) discloses a directional coupler with improved directivity. Specifically, this directional coupler includes a wave combiner in which the multiple reflected wave component included in the transmission wave is cancelled out with a wave obtained by phase adjusting the reflected wave, thereby improving the directivity. 
     However, the configuration disclosed in this patent publication does not permit miniaturization of the directional coupler (i.e., does not allow for a reduction in the circuit size). Another way to improve the directivity of a directional coupler is to make the coupling length between the main line and the coupled line equal to one-quarter wavelength (λ/4) of the operating frequency. However, for example, cellular phone units use 0.8-5 GHz bands. Such low frequencies mean large values of λ/4, making it impossible to reduce the size of the directional coupler if the coupling length between the main line and the coupled line is made equal to λ/4. Further, in the case of directional couplers using a relatively expensive substrate, such as a GaAs substrate, which provides for improved characteristics, there is great need to reduce the size of the couplers in order to reduce the manufacturing cost. This means that even if they use frequency bands higher than the above 0.8-5 GHz bands, it may not be possible to achieve a coupling length of λ/4, resulting in insufficient directivity. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems. It is, therefore, an object of the present invention to provide a small compact directional coupler in which the coupling length between the main line and the coupled line is shorter than λ/4 of the operating frequency of the coupler, yet which has high directivity. 
     According to one aspect of the present invention, A directional coupler includes a main line formed on a substrate and connected at one end to an input port and at the other end to an output port, a coupled line formed on the substrate and extending along the main line, the coupled line being connected at one end to a coupled port and at the other end to an isolated port, the one end of the coupled line being located at the same side of the directional coupler as the input port, the other end of the coupled line being located at the same side of the directional coupler as the output port, and a phase shifter connected at one end to the isolated port and at the other end to the coupled port. The coupling length between the main line and the coupled line is shorter than one-quarter wavelength of the frequency of power transmitted from the input port to the output port. The phase shifter phase shifts a second reflected wave component such that the second reflected wave component is opposite in phase to a first reflected wave component, the second reflected wave component traveling from the output port to the coupled port through the isolated port and the phase shifter, the first reflected wave component traveling from the output port to the coupled port through the coupled line. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a directional coupler of the first embodiment; 
         FIG. 2  is a diagram illustrating the potential Va at one end of the inductor and the potential Vb at one end of the capacitor; 
         FIG. 3  is a diagram showing currents and voltages at selected locations in a directional coupler of the first embodiment; 
         FIG. 4  shows that the voltage Vo of the reflected wave component traveling through the phase shifter is substantially opposite in phase to the voltage Va at the isolated port; 
         FIG. 5  shows that the voltage Vo of the reflected wave component traveling through the phase shifter is substantially opposite in phase to the voltage Va at the isolated port; 
         FIG. 6  shows the voltage Va at the isolated port when the directional coupler does not have the phase shifter; 
         FIG. 7  is a combination of  FIGS. 5 and 6  and shows that Vo and Vo′ are substantially opposite in phase to each other; 
         FIG. 8  is a graph illustrating various characteristics of the directional coupler of the first embodiment as a function of frequency; 
         FIG. 9  shows a directional coupler including two phase shifters adapted for transmission in opposite directions; 
         FIG. 10  shows a directional coupler having a construction which facilitates its design; 
         FIG. 11  is a diagram illustrating a directional coupler of the second embodiment; 
         FIG. 12  is a graph illustrating various characteristics of the directional coupler of  FIG. 11  as a function of frequency; 
         FIG. 13  shows a directional coupler including a single variable capacitor; 
         FIG. 14  is a diagram illustrating a directional coupler of the third embodiment; 
         FIG. 15  shows the most generalized circuit diagram of the directional coupler of the third embodiment; 
         FIG. 16  shows a directional coupler including a phase inverting transformer; 
         FIG. 17  is a diagram illustrating a directional coupler of the fourth embodiment; 
         FIG. 18  is a graph illustrating various characteristics of the directional coupler of the fourth embodiment as a function of frequency; 
         FIG. 19  shows a generalized circuit diagram of the directional coupler of the fourth embodiment; 
         FIG. 20  is a diagram illustrating a directional coupler of the fifth embodiment; 
         FIG. 21  shows an exemplary method of controlling the directional coupler of the fifth embodiment; 
         FIG. 22  is a diagram illustrating a directional coupler of the sixth embodiment; 
         FIG. 23  is a diagram illustrating the directional coupler of the seventh embodiment; 
         FIG. 24  is a diagram illustrating a variation of the seventh embodiment; 
         FIG. 25  is a diagram illustrating another variation of the seventh embodiment; 
         FIG. 26  is a circuit diagram illustrating the configuration of the phase inverting amplifier; 
         FIG. 27  is a diagram illustrating the configuration of the variable phase shifter; 
         FIG. 28  is a diagram illustrating another variation of the seventh embodiment; 
         FIG. 29  is a diagram illustrating the directional coupler of the eighth embodiment; 
         FIG. 30  is a graph illustrating various characteristics of the directional coupler of the eighth embodiment as a function of frequency; 
         FIG. 31  is a diagram illustrating the directional coupler of the ninth embodiment; 
         FIG. 32  is a graph illustrating various characteristics of the directional coupler of the ninth embodiment as a function of frequency; 
         FIG. 33  is a diagram illustrating the directional coupler of the tenth embodiment; 
         FIG. 34  is an enlarged view of the portion of  FIG. 33  within the dashed circle; 
         FIG. 35  shows the difference in coupling (S 31 ) between when the main line and the coupled line have a comb-shaped portion and when they do not have a comb-shaped portion; 
         FIG. 36  shows a typical configuration of a directional coupler; 
         FIG. 37  shows directional coupler having spiral shape; 
         FIG. 38  is a graph illustrating various characteristics of the general directional coupler as a function of frequency; 
         FIG. 39  shows a directional coupler inserted between a transmit power amplifier and an antenna, and used in a cellular phone unit; and 
         FIG. 40  is a graph showing the relationship between the directivity of the directional coupler and the error in the power measurement by the detector. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention will be described with reference to  FIGS. 1 to 10 . It should be noted that throughout the description of the first embodiment, like numerals represent like materials or like or corresponding components, and these materials and components may be described only once. This also applies to other embodiments of the invention subsequently described. 
       FIG. 1  is a diagram illustrating a directional coupler of the present embodiment. This directional coupler and those of other embodiments described below are loosely laterally coupled directional couplers. Referring to  FIG. 1 , the directional coupler  10  of the present embodiment includes a main line  14  formed on a substrate. One end of the main line  14  is connected to an input port  12 , and the other end is connected to an output port  16 . The main line  14  transmits transmission power (a forward wave) from the input port  12  to the output port  16 . A coupled line  20  is formed on the substrate and extends along the main line  14 . One end of the coupled line  20  is connected to a coupled port  18 , and the other end is connected to an isolated port  22 . The coupled line  20  is used to couple out a portion of the power transmitted in the main line  14 . 
     As shown in  FIG. 1 , the input port  12  and the coupled port  18  are disposed at the same side of the directional coupler. Further, the output port  16  and the isolated port  22  are disposed at the side of the directional coupler opposite the input port  12  and the coupled port  18 . In  FIG. 1 , reference numeral Lcp 1  denotes the coupling length between the main line  14  and the coupled line  20 . According to the present embodiment, the coupling length Lcp 1  is relatively short, namely, one tenth ( 1/10) to one twentieth ( 1/20) of λ/4, where λ is the wavelength of the frequency of the power transmitted through the main line of the directional coupler  10 . 
     The directional coupler  10  of the present embodiment also includes a phase shifter  24  connected at one end to the isolated port  22  and connected at the other end to the coupled port  18  through a resistance  30 . The phase shifter  24  substantially inverts the phase of the reflected wave from the output port  16  and supplies the inverted wave to the coupled port  18 , as described later. The phase shifter  24  includes a inductor  26  and a capacitor  28 . One end of the inductor  26  is connected to the isolated port  22 , and the other end is connected to the coupled port  18  through the resistance  30 . One end of the capacitor  28  is connected to the other end of the inductor  26 , and the other end of the capacitor  28  is grounded. This completes the description of the configuration of the directional coupler  10  of the present embodiment. 
     The phase shifter  24  phase shifts the reflected wave component traveling from the output port  16  to the coupled port  18  through the isolated port  22  and the phase shifter  24  such that this reflected wave component is opposite in phase to the reflected wave component traveling from the output port  16  to the coupled port  18  through the coupled line  20 . (The former reflected wave component is referred to herein as the “second reflected wave component,” and the latter is referred to herein as the “first reflected wave component.”) This phase shift is caused by the resonance of the phase shifter  24 . 
       FIG. 2  is a diagram illustrating the potential Va at one end of the inductor  26  and the potential Vb at one end of the capacitor  28 , i.e., at the other end of the inductor  26 , as a function of frequency (see also  FIG. 1 ). As shown in  FIG. 2 , the LC circuit of the phase shifter  24  is designed to resonate at f 0 -Δf 0 , where f 0  is the frequency of the power transmitted through the main line and Δf 0  is a frequency shift determined so that the second reflected wave is opposite in phase to the first reflected wave. More specifically, in the present embodiment, a phase difference of approximately 5-10° occurs between the potentials at the isolated port  22  and the coupled port  18  since the coupling length Lcp 1  is short. Therefore, the resonant frequency of the phase shifter  24  is shifted from the frequency of the transmitted power by Δf 0  to compensate for this phase difference. The value of Δf 0  is around 5-10°. 
     With reference to  FIGS. 3 to 7 , the following describes how the phase shifter  24  functions to make the second reflected wave opposite in phase to the first reflected wave.  FIG. 3  is a diagram showing currents and voltages at selected locations in a directional coupler of the present embodiment, wherein these currents and voltages are indicated by different symbols. The directional coupler shown in  FIG. 3  is similar to the directional coupler  10  shown in  FIG. 1 , except that it additionally includes a terminating resistance  31  (approximately 50Ω) connected to the coupled port  18 . 
       FIGS. 4 to 7  are vector diagrams showing the currents and voltages indicated by symbols in  FIG. 3 .  FIGS. 4 and 5  show that the voltage Vo of the reflected wave component traveling through the phase shifter  24 , as measured at the coupled port  18 , is substantially opposite in phase to the voltage Va at the isolated port  22 . It should be noted that this reflected wave component corresponds to the second reflected wave component described above. 
       FIG. 6  shows the voltage Va at the isolated port  22  when the directional coupler does not have the phase shifter  24 , and also shows the voltage Vo′ of the reflected wave component traveling through the coupled line  20  to the coupled port  18  (without passage through the phase shifter  24 ) as it appears at the coupled port  18 . This reflected wave component corresponds to the first reflected wave component described above. The voltages Va and Vo′ have a phase difference of approximately 5-10°, as described above. 
       FIG. 7  is a combination of  FIGS. 5 and 6  and shows that Vo and Vo′ are substantially opposite in phase to each other. Thus, the circuit constants of the phase shifter  24  can be adjusted so that Vo (the second reflected wave component) is opposite in phase to Vo′ (the first reflected wave component). Further, Vo and Vo′ preferably have equal amplitudes in order to ensure that the directional coupler has high directivity. According to the present embodiment, the resistance  30  acts to reduce the voltage Vb at the ungrounded end of the capacitor  28  so that Vo and Vo′ have equal amplitudes. Specifically, Vo is equal to Vb minus the voltage Vr across the resistance  30 , as can be seen from the vector diagram of  FIG. 7 . Thus, Vo can be adjusted such that Vo and Vo′ have equal amplitudes and cancel out each other, as shown in  FIG. 7 . 
     Generally, the performance of a directional coupler is determined by its coupling, isolation, and directivity. In the case of the directional couplers shown in  FIGS. 1 and 3 , the coupling means the degree to which the coupled port  18  is coupled to the input port  12 . More specifically, the coupling is the signal input to the coupled port  18  divided by the signal input to the input port  12  and is typically approximately −10 dB to −20 dB. The isolation means the degree to which the reflected wave from the output port  16  is coupled to the coupled port  18 . Specifically, the isolation is the signal power of the reflected wave input to the coupled port  18  divided by the power of the reflected wave output from the output port  16  and is typically approximately −15 dB to −30 dB. The directivity is the ratio of the coupling to the isolation. 
     The higher the directivity, the less the influence of the reflected wave from the output port  16  and hence the less the error the directional coupler makes in detecting the transmission power. That is, the wave detecting circuit can accurately monitor the transmission power (or forward wave power) even under load variations. As a result, the error in the detected voltage due to the reflected wave is reduced, thereby reducing the distortion components generated when the amplifier (PA) produces excessive transmission power in response to load variations. 
     However, there is a need to reduce the size of directional couplers. If, in order to satisfy this need, the coupling length between the main line and the coupled line in prior art directional couplers is reduced to less than λ/4, a reduction in the directivity results. 
     On the other hand, the present embodiment allows a directional coupler to have high directivity even if its coupling length is shorter than λ/4.  FIG. 8  is a graph illustrating various characteristics of the directional coupler of the present embodiment as a function of frequency. In  FIG. 8 , reference numeral S 31  indicates the coupling vs frequency characteristic, S 32  indicates the isolation vs frequency characteristic, and S 32 -S 31  indicates the directivity vs frequency characteristic. Since, as described above, the phase shifter  24  phase shifts the second reflected wave such that this reflected wave is opposite in phase to the first reflected wave, the isolation (dB) is high over a frequency range around 2 GHz, as indicated by the isolation vs frequency characteristic S 32 . More specifically, the directional coupler of the present embodiment has high directivity (namely, approximately 30 dB) at frequencies around 2 GHz. 
     Thus the directional coupler of the present embodiment has high directivity at frequencies around 2 GHz, which makes it suitable for use in devices for narrow band communications such as radio communications. Various alterations may be made to the directional coupler of the present embodiment. Several variations of the directional coupler of the present embodiment will now be described with reference to  FIGS. 9 and 10 . 
       FIG. 9  shows a directional coupler including two phase shifters adapted for transmission in opposite directions. The directional coupler shown in  FIG. 9  is characterized in that power can be transmitted both from the input port to the output port and from the output port to the input port (i.e. bidirectional transmission). Referring to  FIG. 9 , one end of a first phase shifter  40  is connected to the isolated port  54  through a first transistor  51 , and the other end of the first phase shifter  40  is connected to the coupled port  50  through a resistance  44  and a second transistor  48 . On the other hand, one end of a second phase shifter  42  is connected to the coupled port  50  through a third transistor  52 , and the other end of second phase shifter  42  is connected to the isolated port  54  through a resistance  46  and a fourth transistor  55 . When power is transmitted from the input port  12  to the output port  16 , the first and second transistors  51  and  48  are turned on and the third and fourth transistors  52  and  55  are turned off. When power is transmitted from the output port  16  to the input port  12 , on the other hand, the first and second transistors  51  and  48  are turned off and the third and fourth transistors  52  and  55  are turned on. Such control of the phase shifters allows the directional coupler to accommodate bidirectional power transmission while achieving the advantages of the present embodiment using a phase shifter. It should be noted that the configurations of the first and second phase shifters  40  and  42  are similar to that of the phase shifter  24  described above in connection with the present embodiment, and therefore these phase shifters will not be described in detail herein. 
       FIG. 10  shows a directional coupler having a construction which facilitates its design. Referring to  FIG. 10 , the phase shifter  60  in the directional coupler has substantially the same function as the phase shifter  24  of  FIG. 1  described above. However, the phase shifter  60  includes a first phase shifting portion made up of a first inductor  62  and a first capacitor  64  and a second phase shifting portion made up of a second inductor  66  and a second capacitor  68 . This arrangement facilitates design of the directional coupler, since an LC circuit is usually used to provide a phase shift of 90°. Furthermore, it is possible to increase the frequency range over which the directivity is high. 
     Various other alterations may be made to the directional coupler of the present embodiment. For example, although in the present embodiment the directional coupler has high directivity (namely, approximately 30 dB) at frequencies around 2 GHz, it is to be understood that the frequency range over which the coupler has high directivity can be varied arbitrarily by varying the circuit constants (or resonant frequency) of the phase shifter  24  described with reference to  FIG. 2 . Further, as described above, the resistance  30  is provided primarily to make the voltages Vo and Vo′ equal in amplitude. This means that the resistance  30  may be omitted if these voltages Vo and Vo′ have equal or only slightly different amplitudes without the resistance  30 . Further, the coupling length may be any length shorter than one-quarter wavelength of the operating frequency (λ/4), since in such a case the present embodiment enables the directional coupler to have improved directivity. 
     Second Embodiment 
     A second embodiment of the present invention relates to a directional coupler that includes a phase shifter using a variable capacitor. The present embodiment will be described with reference to  FIGS. 11 to 13 . The directional coupler of the present embodiment is substantially similar to that of the first embodiment, except that it includes a different phase shifter  80  which will be described below with reference in  FIG. 11 . 
     The phase shifter  80  includes a capacitor  82  connected at one end to the inductor  26  and at the other end to ground. The phase shifter  80  also includes a capacitor  84  connected at one end to the one end of the capacitor  82  and at the other end to a diode  86  (described below). The phase shifter  80  also includes the diode  86  connected at its anode to ground and at its cathode to the other end of the capacitor  84 . A voltage source is connected through a resistance  88  to the cathode of the diode  86  to supply a control voltage Vc thereto. 
     The diode  86  of the directional coupler can be regarded as a combination of a resistance and a variable capacitor. According to the present embodiment, the control voltage Vc is varied to vary the capacitance of the diode  86  (acting as a variable capacitor), thereby adjusting the resonant frequency of the phase shifter  80 . That is, the frequency range over which the directional coupler has high directivity can be shifted by varying the control voltage Vc. 
       FIG. 12  is a graph illustrating various characteristics of the directional coupler of  FIG. 11  as a function of frequency. The resonant frequency of the phase shifter may be varied to vary the frequency range over which the directional coupler has high directivity, as indicated by the arrow in  FIG. 12 . This may be accomplished by varying the control voltage Vc and thereby adjusting the capacitance of the diode  86 , as described above. This directional coupler is especially suitable as a multiband directional coupler (i.e., a directional coupler for use at a plurality of different frequencies). The control voltage applying means for applying the control voltage Vc to the cathode of the diode  86  may be connected to a circuit outside the directional coupler, which circuit sets the frequency of the power transmitted through the main line. With this arrangement, the control voltage Vc may be adjusted in accordance with the frequency of the transmission power, thereby optimizing the directivity of the directional coupler. In  FIG. 11 , the control voltage applying means is represented simply by a port Vc. 
     The variable capacitor of the present embodiment is not limited to the configuration shown in  FIG. 11 . Specifically, the present embodiment is characterized in that the value of a capacitance in the phase shifter is varied such that the directional coupler has high directivity at the current operating frequency. Therefore, the variable capacitor of the present embodiment may be made up of a single variable capacitor  90 , as shown in  FIG. 13 , while retaining the advantages described above in connection with the present embodiment. 
     Third Embodiment 
     A third embodiment of the present invention relates to a directional coupler that includes a phase shifter using a variable inductor. The present embodiment will be described with reference to  FIGS. 14 to 16 . The directional coupler of the present embodiment is substantially similar to that of the first embodiment, except that it includes a different phase shifter  106  which will be described below with reference to  FIG. 14 . 
     An inductor  100  of the present embodiment includes a spiral line. The inductor  100  also includes a transistor  102  connected at its source to a point on the spiral line and at its drain to another point on the spiral line. In this example, the transistor  102  is an FET. However, the present invention is not limited to this particular device. The gate of the transistor  102  is controlled by a control voltage Vc. With this arrangement, the inductance of the inductor  100  can be varied by varying the control voltage Vc. This makes it possible to shift the frequency range over which the directional coupler has high directivity, as in the second embodiment. As in the second embodiment, the control voltage applying means for applying the control voltage Vc may be connected to an appropriate control circuit outside the directional coupler in order to make the coupler suitable for use as a multiband directional coupler. The details of such an arrangement will not be further described herein. It should be noted that in addition to the transistor  102  another transistor may be connected to the inductor to allow the directional coupler to be used in a plurality of frequency bands.  FIG. 15  shows the most generalized circuit diagram of the directional coupler of the present embodiment. As indicated by this figure, the present embodiment is characterized in that the value of the inductance in the phase shifter is varied. Therefore, the inductor may be implemented by a phase inverting transformer  110 , as shown in  FIG. 16 . 
     Fourth Embodiment 
     A fourth embodiment of the present invention relates to a directional coupler in which a variable resistance is connected between the coupled port and the phase shifter. The present embodiment will be described with reference to  FIGS. 17 to 19 . The directional coupler of the present embodiment is substantially similar to that of the first embodiment, except that it includes, instead of the fixed resistance  30 , a variable resistance  120  which will be described below with reference to  FIG. 17 . 
     The resistance  120  includes a transistor  126  connected between the phase shifter  24  and the coupled port  18 . The channel resistance of the transistor  126  (an FET) is controlled by the control voltage Vc applied to its gate. The control voltage Vc may be varied to vary the directivity of the directional coupler, as indicated by the arrows in  FIG. 18 , which shows the directivity vs frequency characteristic, etc. As can be seen from  FIG. 17 , the resistance  122  connected in parallel to the transistor  126  allows the second reflected wave component to pass to the coupled port  18  when the transistor  126  is turned off.  FIG. 19  shows a generalized circuit diagram of the directional coupler of the present embodiment. 
     Thus since the whole resistance  120  functions as a variable resistance, the value of the resistance  120  may be varied to make the first and second reflected waves equal in amplitude, or compensate for the difference in amplitude between these reflected waves due to manufacturing variations, even after the manufacture of the directional coupler. 
     Fifth Embodiment 
     A fifth embodiment of the present invention relates to a directional coupler for use at a plurality of different frequencies in which the degree of coupling can be varied. The present embodiment will be described with reference to  FIGS. 20 and 21 . The directional coupler of the present embodiment is substantially similar to that of the first embodiment, except for the following features. The main line  14  is connected to the coupled line  20  by a first field effect transistor  130  and a second field effect transistor  132 , that is, the source-drain path of each field effect transistor is connected between these lines. Further, the directional coupler of the present embodiment includes a phase shifter  134  using a variable capacitor  90 . 
     The directional coupler is provided with means for applying control voltages Vc 1  and Vc 2  to the gates of the first and second field effect transistors  130  and  132 , respectively, to control these gates. In the directional coupler of the present embodiment, the degree of coupling between the main and coupled lines can be varied by controlling Vc 1  and Vc 2 , i.e., by utilizing the variable capacitance characteristics of the first and second field effect transistors  130  and  132 . More specifically, Vc 1  and Vc 2  are controlled to equalize the coupling of the directional coupler at different operating frequencies. 
       FIG. 21  shows an exemplary method of controlling the directional coupler of the present embodiment. In  FIG. 21 , the directional coupler is operated at two frequencies Band 1  and Band 2 . The first field effect transistor  130  (denoted by F 1  in  FIG. 21 ) and the second field effect transistor  132  (denoted by F 2  in  FIG. 21 ) are controlled as follows. When the directional coupler is operated at the frequency Band 1 , F 1  is turned on and F 2  is turned off; when the directional coupler is operated at the frequency Band 2 , F 1  is turned off and F 2  is turned on. This control equalizes the coupling of the directional coupler at Band 1  and Band 2 . 
     Generally, when, as in the present embodiment, a directional coupler is used at a plurality of different frequencies, it is preferable to equalize the coupling of the coupler at these frequencies. For example, if the coupling of the directional coupler is increased at one of these frequencies, the power coupled out to the coupled port increases and the output to the antenna decreases at that frequency, which is not desirable. That is, increasing the coupling of a directional coupler improves its directivity but increases the loss. Therefore, the coupling should preferably be lower than a certain level. Further, since the detector for detecting the output from the coupled port is designed to receive a substantially constant voltage, it is not desired that the coupling varies significantly with the frequency at which the directional coupler is operated. The present embodiment solves these problems by including a circuit for varying the coupling of the directional coupler, and equalizing the coupling at the different operating frequencies using this circuit. 
     When the coupling is increased, e.g., from 20 dB to 15 dB, as by increasing the drain-source capacitance of the first or second field effect transistor, the isolation decreases and as a result the directivity significantly decreases. However, the present embodiment allows this decrease in the directivity to be compensated for in a plurality of frequency bands since the phase shifter  134  includes the variable capacitor  90 . 
     Although the present embodiment has been described as including two field effect transistors, it may include one or three or more field effect transistors while retaining the advantages of the present embodiment described above. 
     Sixth Embodiment 
     A sixth embodiment of the present invention relates to a directional coupler for use in at least a low band and a high band higher in frequency than the low band in which the coupling length can be varied. The present embodiment will be described with reference to  FIG. 22 . The directional coupler of the present embodiment is substantially similar to that of the first embodiment, except for the following features. The directional coupler of the present embodiment includes, instead of the coupled line  20 , a first coupled line  142  and a second coupled line  144  connected to each other through the source-drain path of a first switching device  140 . Further, the directional coupler includes a phase shifter  134  using a variable capacitor  90 . 
     One end of the first coupled line  142  is connected to the isolated port  22 , and the other end is connected to one end of the first switching device  140 . One end of the second coupled line  144  is connected to the other end of the first switching device  140 , and the other end of the second coupled line  144  is connected to the coupled port  18 . Further, one end of the phase shifter  134  is connected to the one end of the second coupled line  144  through a second switching device  146 . 
     This completes the description of the directional coupler of the present embodiment. When a directional coupler is used in a plurality of bands, it is preferable to equalize the coupling of the coupler in these bands, as described in connection with the fifth embodiment. In the case of the directional coupler of the present embodiment, which is used in at least a low band and a high band, its coupling length may be changed to equalize the coupling in these bands. According to the present embodiment, when the directional coupler is used in the low band, the first switching device  140  is turned on and the second switching device  146  is turned off. When the directional coupler is used in the high band, on the other hand, the first switching device  140  is turned off and the second switching device  146  is turned on. It should be noted that the lengths of the first and second coupled lines  142  and  144  are such that the coupling of the directional coupler in the low and high bands can be equalized by the above switching of the first and second switching devices  140  and  146 . Thus, the present embodiment allows the coupling of the directional coupler to be equalized in the low and high bands, as in the fifth embodiment. 
     The above switching control for equalizing the coupling of the directional coupler at a plurality of operating frequencies is accomplished by applying a voltage signal Vc and its inverse to the gates of the first and second switching devices  140  and  146 . In  FIG. 22 , the ports connected to the gates of the first and second switching devices  140  and  146  represent the means for controlling Vc and its inverse. 
     Seventh Embodiment 
     A seventh embodiment of the present invention relates to a directional coupler that includes a phase shifter using a phase inverting amplifier (an active device). The present embodiment will be described with reference to  FIGS. 23 to 28 . The directional coupler of the present embodiment is substantially similar to that of the first embodiment, except that the phase shifter is made up of a phase inverting amplifier.  FIG. 23  is a diagram illustrating the directional coupler of the present embodiment. The phase shifter,  200 , of the present embodiment differs from those of the first to sixth embodiments in that it uses a phase inverting amplifier  202  (an active device). This phase inverting amplifier  202  does not amplify the input signal but attenuates it and supplies the attenuated signal to the coupled port. 
     Generally, a phase inverting amplifier can provide a phase inversion over a wide frequency range. Therefore, like the phase shifters of the embodiments described above, the phase shifter  200  of the present embodiment can phase shift the second reflected wave component such that this reflected wave component is opposite in phase to the first reflected wave component. An important point to note when using an amplifier as the phase shifter is that the amplifier must be designed so as not to cause signal distortion, since excess input tends to result in signal distortion. However, when the directional coupler of the present embodiment is incorporated in a transmission module, the phase inverting amplifier  202  operates at a much lower current than the amplifier (PA) in the preceding stage. Therefore, the chances are low that the current consumption of the phase inverting amplifier will degrade the module characteristics. 
     The use of a phase inverting amplifier ( 202 ) as the phase shifter, as in the present embodiment, is advantageous in reducing the circuit dimensions of the phase shifter. The reason for this is that since the phase inverting amplifier ( 202 ) is typically made up of transistors and resistances, it is smaller than the phase shifters of the first to sixth embodiments, which include an inductor and a capacitor. 
       FIG. 24  is a diagram illustrating a variation of the present embodiment. The directional coupler shown in  FIG. 24  includes a variable gain phase inverting amplifier  212 . Therefore, the gain of the phase inverting amplifier  212  may be varied to attenuate the second reflected wave component such that the first and second reflected wave components have equal amplitudes. That is, this simple configuration of the directional coupler achieves the same advantages as described above in connection with the fourth embodiment. 
       FIG. 25  is a diagram illustrating another variation of the present embodiment. This variation provides a directional coupler for use at a plurality of different frequencies. Specifically, the phase shifter  220  in this directional coupler shown in  FIG. 25  includes a variable gain phase inverting amplifier  212  and a variable phase shifter  214 . One end of the phase inverting amplifier  212  is connected to the isolated port  22 , and the other end is connected to one end of the variable phase shifter  214 . The other end of the variable phase shifter  214  is connected to the coupled port  18  through a resistance  30 . The variable phase shifter  214  is controlled to shift the frequency range over which the directional coupler has high directivity such that the directional coupler has high directivity at the current operating frequency, thus achieving the same advantages as described above in connection with the second embodiment. 
       FIG. 26  is a circuit diagram illustrating the configuration of the phase inverting amplifier  212 . In  FIG. 26 , reference numerals Tr 1  and TrREF denote HBTs (heterojunction bipolar transistors), F 1  denotes an FET (field effect transistor), and Rc 1  denotes a load resistance. Reference numerals R FB1  and R FB2  denote resistances, and C FB1  denotes a capacitance. The resistances R FB1  and R FB2 , the capacitance C FB1 , and the FET F 1  form a feedback circuit connected between the base and collector of Tr 1 . The phase inverting amplifier  212  is a variable gain circuit having attenuation characteristics, as described above. The feedback circuit serves to widen the band and decrease the gain of the phase inverting amplifier  212 . The gate voltage V GC1  of the FET F 1  in the feedback circuit may be controlled to adjust the on resistance of F 1  and thereby adjust the amount of feedback and hence the gain of the amplifier. It should be noted that reference numerals R IN1  and R 01  denote gain reducing resistances of the phase inverting amplifier  212 . The values of these resistances may be such that the phase inverting amplifier  212  has attenuation characteristics that enable the directional coupler to have high directivity. 
     The HBTs Tr 1  and TrREF form a current mirror. The bias current to Tr 1  can be controlled by V REF . Since the conductance (gm) of Tr 1  is proportional to this bias current, the gain (or the amount of attenuation) of the amplifier can be adjusted by adjusting this bias current. 
       FIG. 27  is a diagram illustrating the configuration of the variable phase shifter  214  shown in  FIG. 25 . The variable phase shifter  214  shown in  FIG. 27  differs from the phase shifter of  FIG. 10  described in connection with the first embodiment in that it includes variable capacitors instead of fixed value capacitors. The use of variable capacitors in the phase shifter ( 214 ) and its advantages over fixed value capacitors are the same as those described above in connection with the second embodiment. 
       FIG. 28  is a diagram illustrating another variation of the present embodiment. This variation provides a directional coupler adapted for bidirectional power transmission yet having high directivity. The directional coupler shown in  FIG. 28  is characterized in that it includes a variable gain phase inverting amplifier  230  and a variable gain phase inverting amplifier  232  serving as phase shifters. This directional coupler is similar to that of  FIG. 9  described above in connection with the first embodiment, except that the LC phase shifters are replaced by active devices. Therefore, this simple configuration of the directional coupler achieves the same advantages as described above in connection with the directional coupler shown in  FIG. 9 . 
     Eighth Embodiment 
     An eighth embodiment of the present invention relates to a directional coupler for use in at least a low band and a high band higher in frequency than the low bad in which the coupling can be equalized at different operating frequencies. The present embodiment will be described with reference to  FIGS. 29 and 30 . 
       FIG. 29  is a diagram illustrating the directional coupler of the present embodiment. In this directional coupler, different coupled lines are used for different operating frequencies. As shown in  FIG. 29 , a main line  14  connected between an input port  12  and an output port  16  is sandwiched along its length between a high band coupled line  300  and a low band coupled line  302 . 
     One end of the low band coupled line  302  is connected to a coupled port  18  through a first switching device  312 , and the other end is connected to a first isolated port  317  through a third switching device  314 . On the other hand, one end of the high band coupled line  300  is connected to the coupled port  18  through a second switching device  308 , and the other end is connected to a second isolated port  315  through a fourth switching device  310 . 
     Further, a series connection of a first phase shifter  306  and a resistance  318  is connected in parallel with the low band coupled line  302 . A series connection of a second phase shifter  304  and a resistance  316  is connected in parallel with the high band coupled line  300 . The first and second phase shifters  306  and  304  each include a variable capacitor. The configurations of the first and second phase shifters  306  and  304  are the same as that of the phase shifter described above in connection with the second embodiment. 
     When the directional coupler of the present embodiment is used in the low band, the first and third switching devices  312  and  314  are turned on and the second and fourth switching devices  308  and  310  are turned off. When the directional coupler is used in the high band, on the other hand, the first and third switching devices  312  and  314  are turned off and the second and fourth switching devices  308  and  310  are turned on. This on-off control, i.e., the turning on and off of these switching devices, is done by the voltage applying means provided inside or outside the directional coupler. The directional coupler of the present embodiment includes at least voltage applying ports (denoted by Vc 1  and Vc 2  in  FIG. 29 ) as switching control means for these switching devices. 
     The low band coupled line  302  is spaced a shorter distance from the main line  14  than is the high band coupled line  300 . That is, a relatively small distance is provided between the main line  14  and the low band coupled line  302  to ensure sufficient coupling therebetween when the directional coupler is used in the low band. On the other hand, there is a relatively large distance between the main line  14  and the high band coupled line  300  to compensate for an increase in the coupling between these lines when the directional coupler is used in the high band. Thus, according to the present embodiment, the low and high band coupled lines  302  and  300  are spaced from the main line  14  such that the coupling of the directional coupler is substantially equalized in the low and high frequency bands. Generally, the power detected by the detector (in a subsequent stage) connected to the coupled port is preferably within a predetermined range regardless of the operating frequency in order to ensure sufficient detection accuracy. The present embodiment achieves this by equalizing the coupling of the directional coupler at a plurality of frequencies, thus achieving the advantages described above. 
     Further, the resonant frequencies (or circuit constants) of the phase shifters  306  and  304  for the low and high bands, respectively, are such that the directional coupler has high directivity in both bands. Thus the present embodiment allows for increasing the directivity of a directional coupler for use at a plurality of frequencies, regardless of the operating frequency, as shown in  FIG. 30 . 
     Ninth Embodiment 
     A ninth embodiment of the present invention relates to a directional coupler for use in at least a low band and a high band higher in frequency than the low band in which the coupling can be equalized at different operating frequencies. The present embodiment will be described with reference to  FIGS. 31 and 32 . 
       FIG. 31  is a diagram illustrating the directional coupler of the present embodiment. In this directional coupler, different main lines are used for different operating frequencies. As shown in  FIG. 31 , a coupled line  20  connected between a coupled port  18  and an isolated port  22  is sandwiched along its length between a high band main line  400  and a low band main line  402 . 
     One end of the high band main line  400  is connected to a high band input port  404 , and the other end is connected to a high band output port  406 . On the other hand, one end of the low band main line  402  is connected to a low band input port  408 , and the other end is connected to a low band output port  410 . 
     The directional coupler of the present embodiment, like that of the first embodiment, includes a phase shifter connected at one end to the coupled port  18  and at the other end to the isolated port  22 . This phase shifter includes a high band phase shifter  450  and a low band phase shifter  452 . One end of the high band phase shifter  450  is connected to the isolated port  22  through a third switching device  430 , and the other end is connected to the coupled port  18  through a resistance  30  and a first switching device  428 . On the other hand, one end of the low band phase shifter  452  is connected to the isolated port  22  through a fourth switching device  434 , and the other end is connected to the coupled port  18  through another resistance  30  and a second switching device  432 . 
     When the directional coupler of the present embodiment is used in the low band, the second and fourth switching devices  432  and  434  are turned on and the first and third switching devices  428  and  430  are turned off. When the directional coupler is used in the high band, on the other hand, the first and third switching devices  428  and  430  are turned on and the second and fourth switching devices  432  and  434  are turned off. This on-off control, i.e., the turning on and off of these switching devices, is done by the voltage applying means provided inside or outside the directional coupler. The directional coupler of the present embodiment includes at least voltage applying ports (denoted by Vc 1  and Vc 2  in  FIG. 31 ) as switching control means for these switching devices. 
     The low band main line  402  is spaced a shorter distance from the coupled line  20  than is the high band main line  400 . That is, the distances between the coupled line and these main lines are adjusted to equalize the coupling of the directional coupler in the low and high bands. Further, the directional coupler of the present embodiment also includes a low band phase shifter  452  and a high band phase shifter  450  each for a different operating frequency. The use of these phase shifters allows the directional coupler to have high directivity regardless of the operating frequency (see  FIG. 32 ). Thus the present embodiment achieves the same advantages as described above in connection with the eighth embodiment. Since the directional coupler of the present embodiment includes two main lines, it is suitable for use in GSM (Global-System-for-Mobile-communications) terminals, or GSM transmission modules, which include two PAs (amplifiers) in a stage preceding the directional coupler and also include two output lines. 
     Tenth Embodiment 
     A tenth embodiment of the present invention relates to a directional coupler which includes a phase shifter and which is adapted to compensate for the reduction in the coupling due to the incorporation of the phase shifter. The present embodiment will be described with reference to  FIGS. 33 to 35 .  FIG. 33  is a diagram illustrating the directional coupler of the present embodiment. As shown in  FIG. 33 , the main line  504  of the present embodiment is formed in a spiral shape. One end of the main line  504  is connected to an input port  500 , and the other end is connected to an output port  502 . A coupled line  508  of a spiral shape is formed along the spiral main line  504 . The coupled line  508  is connected at one end to a coupled port  506  and at the other end to an isolated port  507 , as in the first embodiment. 
     The main line  504  and the coupled line  508  each have a comb-shaped portion, shown encircled by dashed line in  FIG. 33 .  FIG. 34  is an enlarged view of the portion of  FIG. 33  within the dashed circle. The comb-shaped portions of the main line  504  and the coupled line  508  are interdigitated with and spaced apart from each other, as shown in  FIG. 34 . The main line  504  and the coupled line  508  are spaced a predetermined distance from each other. 
     The directional coupler of the present embodiment includes a phase shifter  24 . The configuration of the phase shifter  24  is the same as in the first embodiment. One end of the phase shifter  24  is connected to the isolated port  507 , and the other end is connected to the coupled port  506  through a resistance  30 . 
     The incorporation of a phase shifter (such as the phase shifter  24 ) into a directional coupler may result in reduced coupling and hence reduced directivity. The present inventors have found, through experiments, that a directional coupler without a phase shifter has a coupling of approximately −20 dB, and the same directional coupler has a coupling of approximately −23 dB when provided with a phase shifter. According to the present embodiment, the main line  504  and the coupled line  508  have a comb-shaped portion at which the electric field is concentrated, making it possible to increase the coupling of the directional coupler without increasing its size. Further, since the main line  504  and the coupled line  508  are of a spiral shape, the coupling length can be increased without increasing the size of the directional coupler. Therefore, the present embodiment allows compensation for the reduction in the coupling of the directional coupling due to the incorporation of the phase shifter, thereby maintaining the directivity at a high level.  FIG. 35  shows the difference in coupling (S 31 ) between when the main line  504  and the coupled line  508  have a comb-shaped portion and when they do not have a comb-shaped portion. 
     Although in the present embodiment the facing portions of the main line  504  and the coupled line  508  are partially formed in a comb shape, it is to be understood that the entire facing portions may be formed in a comb shape in order to increase the coupling. On the other hand, only small portions of the facing portions may be formed in a comb shape if this still provides sufficient coupling. 
     Thus the present invention enables the manufacture of a directional coupler of small size yet having high directivity. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of a Japanese Patent Application No. 2009-000874, filed on Jan. 6, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.