JP2013030904A - Directional coupler and wireless communication device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a coupler having flat frequency characteristics of a coupling degree over a wide band.SOLUTION: A directional coupler has: a main line for transmitting a transmission signal; an input port for inputting the transmission signal to the main line; an output port for outputting the transmission signal from the main line; a sub line coupled with the main line in electromagnetic field to extract a part of the transmission signal; a coupling port provided at one end part of the sub line; and an isolation port provided at the other end part of the sub line. A low-pass filter part having a function as a low-pass filter is provided between the sub line and the coupling port.

Description

  The present invention relates to a directional coupler and a wireless communication device, and more particularly to a technique for flattening frequency characteristics of the degree of coupling in a directional coupler.

  Directional couplers (Directional Couplers / hereinafter simply referred to as “couplers”) that can extract a part of electric power propagating on a transmission line are used for cellular phones, wireless LAN communication devices, and Bluetooth standard communication. It is an indispensable part in configuring transmission circuits of various wireless communication devices such as devices.

  Specifically, the coupler constitutes adjustment means for controlling the level of the transmission signal to be constant, and this adjustment means includes a power amplifier (hereinafter referred to as “PA”) capable of controlling the gain, and the transmission signal. And an automatic output control circuit (hereinafter referred to as “APC circuit”). The input transmission signal is amplified by the PA and then output through the coupler. The coupler outputs a monitor signal having a level corresponding to the level of the transmission signal output from the PA to the APC circuit. The APC circuit controls the gain of the PA so that the output of the PA becomes constant according to the level of the monitor signal (that is, the level of the transmission signal). The transmission output is stabilized by such feedback control of the PA.

  The coupler includes a main line and a sub line arranged close to each other so as to be electromagnetically coupled. The main line for transmitting a transmission signal includes an input port at one end and an output port at the other end. The sub-line for detecting the level of each has a coupling port at one end and an isolation port at the other end. A part of the transmission signal transmitted through the main line is taken out by the sub line, and is output to the APC circuit as a monitor signal through the coupling port.

  The main characteristics of the coupler include insertion loss, degree of coupling, isolation, and directionality. The insertion loss is a loss caused by the coupler and is desirably low. The degree of coupling is the ratio of the power propagating in the forward direction to the power extracted at the coupling port. Isolation indicates the degree of low leakage of power propagating in the reverse direction to the coupling port, which is preferably high (leakage is small). Directionality is the difference between isolation and coupling degree, and indicates the ability of the coupler to distinguish between traveling waves and reflected waves. The higher the directionality (the larger the absolute value), the better the coupler, and the smaller the detection error, the better the APC circuit can be formed. For this reason, the directionality is generally required to be 20 dB or more.

  On the other hand, since communication frequency bands of mobile terminals typified by mobile phones and smartphones differ depending on countries and regions, communication devices that can use a plurality of frequency bands have been provided in recent years to flexibly cope with these frequency situations. Yes. For example, there are a dual band method that can use two frequency bands, a triple band method that can use three frequency bands, and a quad band method that can use four frequency bands.

  Further, there are the following patent documents as documents related to such a coupler.

JP 2002-280812 A JP 2011-61440 A JP 2009-27617 A

  By the way, the degree of coupling, which is the ratio between the power propagated as the transmission signal and the power taken out to the coupling port of the coupler, is its frequency characteristic in that high-accuracy control of transmission power (accurate feedback control of PA) is realized. Is generally flat. In general, when the lengths of the main line and the sub-line are set to about ¼ wavelength of the used frequency band, a flat coupling degree can be obtained in the frequency band.

  However, the quarter wavelength of the quasi-microwave band mainly used in mobile wireless devices such as mobile phones is as long as about 25 cm to 3 cm (for example, 3 cm at 10 GHz), and mobile phones that need to be light and thin. It is not practical from the viewpoint of size to provide a coupling line of this length in a coupler used for mobile radio equipment such as the above. In addition, if a long coupled line of several centimeters or more is used, the insertion loss becomes very large, which causes a fatal problem for mobile radio equipment that leads to significant deterioration of battery life. For this reason, a coupler having a coupling line shorter than a quarter wavelength of the used frequency band is usually used. However, the characteristics of such a coupled line generally vary depending on the frequency, and the degree of coupling tends to increase as the frequency increases.

  For this reason, conventionally, when a device having a wide use frequency band is configured, it is necessary to provide a plurality of couplers. For example, FIG. 18 is a block diagram showing an example of a transmission / reception unit of a mobile phone that can use a plurality of frequency bands. As shown in this figure, a conventional multiband mobile phone corresponds to a use frequency band. The transmission circuits 301 and 401 are provided with couplers 311 and 411, respectively. In the figure, reference numeral 101 denotes an antenna, and 102 denotes a switch that distributes radio waves received through the antenna 101 to the reception circuits 103 and 104 and sends out transmission signals input from the transmission circuits 301 and 401 to the antenna 101, respectively. The switch 102 is configured by combining, for example, a diplexer or a high frequency switch.

  On the other hand, in such a multiband communication apparatus, if a common coupler can be used (for example, one coupler), the number of parts in the transmission circuit can be reduced and the manufacturing cost of the apparatus can be reduced. In addition, the mobile communication device can be further reduced in size. Further, flattening the coupling degree of the coupler is preferable in terms of performing control of transmission power more easily and with less detection error. Note that none of the above-mentioned patent documents shows such an idea of the present invention and a solution thereof.

  Accordingly, an object of the present invention is to realize a coupler having a flat coupling degree over a wide band.

  In order to solve the problems and achieve the object, a coupler (directional coupler) according to the present invention includes a main line that transmits a transmission signal, an input port that inputs the transmission signal to the main line, and the main line. An output port for outputting the transmission signal from, a sub line for extracting a part of the transmission signal by electromagnetic coupling with the main line, a coupling port provided at one end of the sub line, A directional coupler having an isolation port provided at the other end, wherein a low pass filter having a function as a low pass filter is provided between the sub line and the coupling port.

  As described in the section of the problem to be solved by the present invention, the present invention has been made while considering the common use of a plurality of couplers conventionally provided, and a main line for transmitting a transmission signal, In a coupler using a sub-line arranged close to the main line so as to be electromagnetically coupled, if a low-pass filter unit (hereinafter referred to as “LPF unit”) is provided between the sub-line and the coupling port, The present inventor has found that the frequency characteristics of the degree of coupling can be flattened, and the present invention is configured based on this finding.

  The LPF section has a function of blocking (attenuating) a high-frequency signal and passing a low-frequency signal, and a function of suppressing the coupling degree of the coupler from fluctuating depending on the frequency (increased as the frequency becomes higher). Fulfill. As a result, the frequency characteristics of the degree of coupling can be flattened over a wide band extending from a low frequency to a high frequency as compared with a conventional coupler. This point will be further described based on the simulation result in the description of the later embodiment.

  As a specific configuration of the LPF unit, for example, an inductor connected in series between the sub-line and the coupling port, a capacitor having one end connected between the sub-line and the coupling port, and the other end connected to the ground, Should be included.

  The coupler of the present invention may further include (1) a capacitor having one end connected between the main line and the input port and the other end connected between the sub line and the coupling port. (2) A capacitor having one end connected between the main line and the input port and the other end connected to the ground may be further provided.

  By providing the LPF portion as described above, it is possible to flatten the degree of coupling, but on the other hand, there is a case where the coupler and the LPF portion are added and the isolation and the directionality are deteriorated. Therefore, as a result of trying to improve the flatness of the coupling degree in such a case, as a result of adopting the configuration of (1) or (2) to which the capacitor is added, isolation and directionality are achieved. It has been found that it is possible to correct the deterioration and improve the above. This point will be described in more detail later in the description of the embodiment.

  Further, in one aspect of the present invention, in addition to the LPF unit (LPF unit connected between the sub-line and the coupling port / hereinafter this may be referred to as “first LPF unit”), it has a function as a low-pass filter. Another LPF section (hereinafter, this may be referred to as “second LPF section”) is further provided between the sub line and the isolation port.

  As described above, according to the aspect including the LPF portions at both ends of the sub line, not only the direction from the input port to the output port of the main line (this direction is referred to as “forward direction”), Conversely, it is possible to flatten the frequency characteristics of the degree of coupling in the direction from the output port of the main line to the input port (this direction is referred to as “reverse direction”). For example, in a wireless communication device, when the reflected wave propagating in the reverse direction is extracted from the isolation port and monitored in order to detect and measure matching with the antenna (reflected power from the antenna), the degree of coupling in the reverse direction According to the aspect that can flatten, it is possible to perform more accurate detection with less error as in the forward direction.

  Moreover, as a configuration of the LPF section in this aspect, for example, a first LPF section (an LPF section provided between the sub line and the coupling port) is connected in series between the sub line and the coupling port, and one end Is connected between the sub-line and the coupling port and the other end is connected to the ground, and the second LPF part (the LPF part provided between the sub-line and the isolation port) is isolated from the sub-line. It is sufficient to include an inductor connected in series between the isolation ports and a capacitor having one end connected between the sub line and the isolation port and the other end connected to the ground.

  Furthermore, in the present invention, even when a capacitor having one end connected between the sub line and the coupling port and the other end connected between the sub line and the isolation port is provided, A coupler having excellent flatness can be formed.

  In the present invention, in any of the couplers of the present invention, an attenuator may be connected to one or both of the sub line and the coupling port and between the sub line and the isolation port. . If an attenuator is added to the sub-line in this way, the influence of impedance fluctuations of other circuit elements connected to the sub-line can be reduced by the attenuator, and the coupler of the present invention is actually incorporated into a device and used. Sometimes it can be made less susceptible to other circuit elements.

  A wireless communication apparatus according to the present invention includes any one of the above couplers.

  The wireless communication device referred to in the present invention is typically a mobile terminal device such as a mobile phone, a smart phone, a PDA (Personal Digital Assistants) or a tablet computer having a wireless communication function, but is not limited thereto. The present invention includes various communication devices capable of wireless communication, such as wireless LAN communication devices and Bluetooth standard communication devices.

  According to the present invention, it is possible to realize a small-sized coupler having a flat coupling over a wide band and excellent isolation and directionality.

  Other objects, features, and advantages of the present invention will become apparent from the following description of embodiments of the present invention described with reference to the drawings. In addition, in each figure, the same code | symbol shows the same or an equivalent part.

FIG. 1 is a diagram showing a coupler according to a first embodiment of the present invention. FIG. 2 is a diagram showing the frequency characteristics of the coupling degree of the coupler according to the first embodiment in comparison with a conventional coupler. FIG. 3 is a diagram showing the frequency characteristics of isolation of the coupler according to the first embodiment in comparison with a conventional coupler. FIG. 4 is a block diagram illustrating a configuration example of a multiband mobile phone including the coupler according to the first embodiment. FIG. 5 is a diagram showing a coupler according to the second embodiment of the present invention. FIG. 6 is a diagram showing a coupler according to the third embodiment of the present invention. FIG. 7 is a diagram showing the frequency characteristics of the coupling degree of the coupler according to the third embodiment in comparison with a conventional coupler. FIG. 8 is a diagram showing the frequency characteristics of isolation of the coupler according to the third embodiment in comparison with a conventional coupler. FIG. 9 is a diagram illustrating a coupler according to a modification of the first embodiment. FIG. 10 is a diagram showing frequency characteristics (coupling degree and isolation) of a coupler to which an LPF unit is added based on the first embodiment. FIG. 11 is a diagram showing frequency characteristics (coupling degree and isolation) of a coupler according to a modification of the first embodiment. FIG. 12 is a diagram illustrating a coupler according to another modification of the first embodiment. FIG. 13 is a diagram showing the frequency characteristics (coupling degree) of a coupler to which an LPF unit is added based on the first embodiment. FIG. 14 is a diagram showing frequency characteristics (isolation) of a coupler to which an LPF unit is added based on the first embodiment. FIG. 15 is a diagram showing frequency characteristics (coupling degree) of a coupler according to another modification of the first embodiment. FIG. 16 is a diagram showing frequency characteristics (isolation) of a coupler according to another modification of the first embodiment. FIG. 17 is a diagram showing a coupler according to the fourth embodiment of the present invention. FIG. 18 is a block diagram illustrating an example of a transmission / reception unit of a conventional multiband mobile phone.

[First Embodiment]
As shown in FIG. 1, a coupler 11 according to the first embodiment of the present invention includes a main line 12 that transmits a transmission signal, and a sub-line 13 that is disposed close to the main line 12 so as to be electromagnetically coupled. Is provided. The main line 12 has an input port P1 at one end and an output port P2 at the other end. Further, the sub-line 13 has a coupling port P3 at one end and an isolation port P4 at the other end. In the following description, the input port may be referred to as “P1”, the output port as “P2”, the coupling port as “P3”, and the isolation port as “P4”.

  In addition, as described above, if the length of the main line and the sub line is set to about a quarter wavelength of the used frequency band, a flat degree of coupling can be obtained, but this makes the main line and the sub line very different. In this embodiment, both the main line and the sub line have a line length shorter than a quarter wavelength of the used frequency band.

  An LPF unit 21 is provided between the sub line 13 and the coupling port P3. The LPF unit 21 is a so-called L-type low-pass filter including an inductor 22 and a capacitor 23, and includes an inductor 22 inserted in series between the sub line 13 and the coupling port P3, and the sub line 13 and the coupling port P3. And a capacitor 23 connected between the transmission line and the ground.

  2 and FIG. 3 compare the simulation results of the frequency characteristics of the coupling degree (FIG. 2) and isolation (FIG. 3) of the coupler 11 according to the present embodiment with those of the conventional coupler (without the LPF unit 21). Each is shown. The inductance of the inductor 22 constituting the LPF unit 21 is 2.7 nH, and the capacitance of the capacitor 23 is 5 pF.

  As shown in FIG. 2, according to the present embodiment in which the coupling port portion includes the LPF portion 21, it can be seen that the coupling degree can be flattened as compared with the conventional case. Further, as shown in FIG. 3, the isolation can be improved as compared with the conventional case.

  Thus, according to the present embodiment, the degree of coupling can be flattened, so that the number of couplers can be reduced.

  For example, when considering the construction of a dual-band mobile phone that can use two communication frequency bands of 800 MHz band and 2 GHz band, the coupling degree greatly varies between the 800 MHz band and the 2 GHz band. For this reason, it is necessary to provide two couplers adjusted so that the degree of coupling is almost equal in each frequency band (800 MHz band and 2 GHz band) in each transmission circuit of each frequency band (see FIG. 18). According to the embodiment, since the coupling degree is almost flat across both frequency bands (800 MHz band and 2 GHz band), it is sufficient to provide one common coupler for each frequency band, and the transmission circuit can be reduced by reducing the number of components. It can be simplified.

  FIG. 4 shows a configuration example of a transmission / reception unit of a multiband mobile phone using the coupler 11 of this embodiment. As shown in this figure, according to this embodiment, it is sufficient to provide one coupler 11 between the antenna 101 and the switch 102, and the transmission circuit 201 can be simplified compared to the conventional case (see FIG. 18). I can do it. The PA 202 is one that can be used over both frequency bands, and the monitor signal (a signal corresponding to the level of the transmission signal) obtained by the coupler 11 is input to the APC circuit 203. The APC circuit 203 The gain of the PA 202 is controlled so that the output of the PA 202 becomes constant according to the level of the signal (that is, the level of the transmission signal). The switch 102 functions to distribute the radio wave received through the antenna 101 to the receiving circuits 103 and 104 and to send out the transmission signal input from the transmission circuit 201 to the antenna 101. For example, a switch is combined with a diplexer or a high-frequency switch. What is necessary is just to comprise.

[Second Embodiment]
As shown in FIG. 5, the coupler according to the second embodiment of the present invention includes an LPF unit (first LPF unit) 31 between the sub line 13 and the coupling port P3, as in the first embodiment. Furthermore, an LPF part (second LPF part) 41 is also provided between the sub line 13 and the isolation port P4.

  In the present embodiment, the LPF units 31 and 41 are so-called π-type low-pass filters including two capacitors 33 and 34; 43 and 44 connected in parallel on both sides of inductors 32 and 42 connected in series. The first LPF unit 31 includes an inductor 32 inserted in series between the sub line 13 and the coupling port P3, and a transmission line between the sub line 13 and the coupling port P3 on both sides of the inductor 32 and the ground. It consists of connected capacitors 33 and 34. The second LPF unit 41 includes an inductor 42 inserted in series between the sub line 13 and the isolation port P4, and a transmission line between the sub line 13 and the isolation port P4 on both sides of the inductor 42 and the ground. Capacitors 43 and 44 connected in between. Each of the LPF units 31 and 41 may be an L-type low-pass filter as in the first embodiment, or two inductors connected in series, and between the two inductors and the ground. A so-called T-type low-pass filter composed of a capacitor connected to the capacitor may be used.

  Thus, according to this embodiment provided with the LPF portions 31 and 41 symmetrically (on both the input side and the output side) with the coupler main body portion composed of the main line 12 and the sub-line 13 interposed therebetween, the first embodiment Not only can the forward degree of coupling be flattened as in the form, but also the frequency characteristic of the backward degree of coupling (power input from the main line output port P2 and output to the isolation port P4). The second LPF unit 41 can perform flattening in the same manner as in the forward direction. Therefore, as described above, for example, when a reflected wave propagating in the reverse direction is extracted from the isolation port P4 and monitored in order to detect and measure matching with the antenna (reflected power from the antenna) in the wireless communication device. In addition, it is possible to perform more accurate detection with less error as in the forward direction.

[Third Embodiment]
As shown in FIG. 6, the coupler according to the third embodiment of the present invention is similar to the coupler of the second embodiment in that both sides of the sub-line 13 (between the coupling port P3 and the sub-line 13 and isolation). LPF portions 51 and 61 are provided respectively between the port P4 and the sub line 13). In addition, a capacitor 65 is connected between the coupling port P3 and the isolation port P4. Each of the LPF units 51 and 61 is an L-type low-pass filter including inductors 52 and 62 and capacitors 53 and 63 similar to those in the first embodiment.

  7 and 8 show the simulation results of the frequency characteristics of the coupling degree (FIG. 7) and the isolation (FIG. 8) of the coupler according to this embodiment, as in the case of FIG. 2 and FIG. , 61 and those without the capacitor 65 between P3-P4). The inductances of the inductors 52 and 62 constituting the LPF portions 51 and 61 are both 5.5 nH, and the capacitances of the capacitors 53 and 63 constituting the LPF portions 51 and 61 are both 2.2 pF. The capacitance of the capacitor 65 is 1.2 pF.

  As shown in FIG. 7, the coupling degree can be flattened as compared with the prior art also in the present embodiment in which the capacitor 65 is provided between P3 and P4.

  Further, in this embodiment, since the LPF part is a second-order L-type low-pass filter, compared with the second embodiment in which the LPF part is a third-order π-type low-pass filter, the number of elements and the routing of conductor lines are reduced. Generation of unnecessary coupling components and resonance can be suppressed, and thereby the frequency characteristic of the degree of coupling can be flattened while improving the isolation characteristic and directionality.

  Furthermore, in this embodiment, by connecting the capacitor 65 between P3 and P4, it is possible to improve the isolation characteristic and directionality, and in the region of 0.7 to 2 GHz as shown in FIG. 7 and FIG. A directionality of 20 dB or more (difference between coupling degree and isolation) required for practical use could be secured. Note that the capacitor 65 between P3 and P4 is considered to perform a phase adjustment function for canceling an unnecessary leakage current generated in the isolation port P4.

[Modification of First Embodiment]
In the coupler 11 of the first embodiment, the coupling degree can be flattened by including the LPF unit 21. However, if the LPF unit 21 is connected in this way, isolation and directionality may be deteriorated. It was.

  That is, the coupler obtains directionality by a combination of the induced current and the displacement current in the sub line, but unnecessary coupling between the coupling port and the output port, a phase shift of the induced current and the displacement current in the sub line, and the like occur. It often causes isolation and directionality degradation. On the other hand, in the above embodiment, the LPF unit 21 is connected to the sub line 13. For this reason, it is considered that the connection of the LPF portion 21 causes the balance of the phase difference between the induced current and the displacement current in the sub-line 13 to be lost, which may cause isolation and directionality deterioration.

  Causes of degradation of isolation and directivity include various other factors (eg, degradation of signal separation due to parasitic capacitance and unnecessary coupling, degradation of directivity due to phase speed difference between even mode and odd mode generated in coupler, reactance element (Impedance mismatch and generation of unnecessary resonance) due to the addition, and depending on the mutual relationship between the connection of the LPF unit 21 and these various factors, the isolation and directionality immediately deteriorate when the LPF unit 21 is connected. Although it cannot be generally stated, when such characteristic deterioration occurs, it has been found that this can be solved by the following method.

  (1) As a first method, a capacitor 71 is connected between the input port P1 and the coupling port P3 in addition to the LPF unit 21 as shown in FIG.

  FIG. 10 shows a simulation showing the frequency characteristics of the degree of coupling (solid line) and isolation (dashed line) of a coupler provided with an LPF unit composed of an L-type low-pass filter between the coupling port and the sub line as in the first embodiment. FIG. 11 is a simulation result showing the frequency characteristics of the coupling degree (solid line) and isolation (dashed line) of the coupler to which the first method is applied. 10 and 11, the inductance of the inductor 22 in the LPF section is 2.7 nH, the capacitance of the capacitor 23 is 5 pF, and the capacitance of the capacitor 71 connected between P1 and P3 is 1 pF.

  As shown in FIG. 10 and FIG. 11, by connecting a capacitor 71 between the input port P1 and the coupling port P3, isolation (and therefore isolation and coupling degree) is maintained while maintaining a substantially equal coupling degree. The directionality that is the difference between the two can be improved.

  (2) As a second method, in addition to the LPF unit 21 as shown in FIG. 12, a capacitor 72 is connected between the input port P1 and the ground.

  FIGS. 13 and 14 show coupling degrees (FIG. 13) and isolations (FIG. 14) of a coupler having an LPF unit composed of an L-type low-pass filter between the coupling port and the sub-line as in the first embodiment. FIG. 15 and FIG. 16 are simulation results showing the frequency characteristics of the coupling degree (FIG. 15) and isolation (FIG. 16) of the coupler to which the second method is applied, respectively. 13 and 14, the inductance 22 of the inductor in the LPF section is 0.3 nH, the capacitance of the capacitor 23 is 5 pF, and the capacitance of the capacitor 72 connected between the input port P1 and the ground is 0.16 pF. It is.

  As shown in FIGS. 13 to 16, by connecting a capacitor 72 between the input port P1 and the ground, isolation (and therefore directivity) can be maintained while maintaining a substantially equal level of coupling. I was able to improve.

  In the first and second methods described above, the added capacitors 71 and 72 cancel out the imbalance of the phase difference between the induced current and the displacement current generated in the sub-line 13, thereby reducing the isolation and directionality. It is thought that it will be improved.

[Fourth Embodiment]
Further, as shown in FIG. 17, the coupler according to the fourth embodiment of the present invention includes LPF portions 51 and 61 on both sides of the sub-line 13, respectively, as in the coupler according to the third embodiment, and P3 -P4, a capacitor 65 is connected, but the attenuator is connected between the sub line 13 (LPF unit 51) and the coupling port P3, and between the sub line 13 (LPF unit 61) and the isolation port P4. 73 and 74 were connected.

  If an attenuator is added to the sub line in this way, the influence of impedance fluctuations of other circuit elements connected to the sub line can be reduced as described above. In each of the couplers according to the first to third embodiments and the modifications (FIGS. 1, 5, 6, 9, and 12), the coupling port P3 and the sub line 13 ( Attenuators may be connected between the LPF units 21, 31, 51) and between the isolation port P4 and the sub line 13 (LPF units 41, 61).

  Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and it will be apparent to those skilled in the art that various modifications can be made within the scope of the claims. is there.

11 Coupler (directional coupler)
12 Main line 13 Sub line 21, 31, 41, 51, 61 LPF part (low-pass filter part)
22, 32, 42, 52, 62 Inductors 23, 33, 34, 43, 44, 53, 63, 65, 71, 72 Capacitor 101 Antenna 102 Switch 103, 104 Reception circuit 201 Transmission circuit 202 PA (power amplifier)
203 APC circuit (automatic output control circuit)
P1 input port P2 output port P3 coupling port P4 isolation port

Claims (9)

A main line for transmitting a transmission signal;
An input port for inputting the transmission signal to the main line;
An output port for outputting the transmission signal from the main line;
A sub line for extracting a part of the transmission signal by electromagnetic coupling with the main line;
A coupling port provided at one end of the sub-line;
A directional coupler having an isolation port provided at the other end of the sub-line,
A directional coupler comprising a low-pass filter section having a function as a low-pass filter between the sub line and the coupling port. The low-pass filter unit is
An inductor connected in series between the sub-line and the coupling port;
The directional coupler according to claim 1, further comprising: a capacitor having one end connected between the sub line and the coupling port and the other end connected to the ground. The directional coupler according to claim 1, further comprising a capacitor having one end connected between the main line and the input port and the other end connected between the sub line and the coupling port. The directional coupler according to claim 1, further comprising a capacitor having one end connected between the main line and the input port and the other end connected to the ground. The directional coupler according to claim 1, further comprising another low-pass filter unit having a function as a low-pass filter in addition to the low-pass filter unit between the sub line and the isolation port. The low pass filter section provided between the sub line and the coupling port is
An inductor connected in series between the sub-line and the coupling port;
A capacitor having one end connected between the sub-line and the coupling port and the other end connected to the ground;
The low pass filter section provided between the sub line and the isolation port is
An inductor connected in series between the sub-line and the isolation port;
The directional coupler according to claim 5, further comprising: a capacitor having one end connected between the sub line and the isolation port and the other end connected to the ground. The directional coupler according to claim 5, further comprising a capacitor having one end connected between the sub line and the coupling port and the other end connected between the sub line and the isolation port. . The attenuator is connected to either one or both between the sub line and the coupling port and between the sub line and the isolation port. Directional coupler.   A wireless communication device including the directional coupler according to any one of claims 1 to 8.

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