Directional coupler

A directional coupler capable of improving a directionality of a directional coupler body including four terminals. The directional coupler includes a directional coupler body including the four terminals of an input port, an output port, a coupling port, and an isolation port; and a combiner for combining powers of an output signal of the coupling port and an output signal of the isolation port of the directional coupler body; and a directionality improving circuit for amplifying or attenuating at least one of the output signal of the coupling port and the output signal of the isolation port before outputting the same, and the combiner combines powers of the output signals amplified or attenuated by the directionality improving circuit.

TECHNICAL FIELD

The present invention relates to a directional coupler. In particular, the present invention relates to a directional coupler including a circuit for improving a directionality of an existing directional coupler.

BACKGROUND ART

A directional coupler is used as a microwave circuit for separating a progressive wave from a reflected wave in various fields. In an ideal directional coupler, the progressive wave and the reflected wave are completely separated from each other, and hence only the progressive wave appears at a coupling port while only the reflected wave is generated at the isolation port. Therefore, a directionality that is a power ratio between the progressive wave and the reflected wave becomes infinite.

If a high directionality is desired to be realized, it is necessary to match phase speeds of even and odd modes. However, a microstrip line that is widely used as a micro circuit is a heterogeneous medium line, and hence a difference in wavelength shortening ratio occurs between the even mode and the odd mode. Therefore, the reflected wave may leak out to the coupling port, which causes a problem that the directionality expressed by a difference between a power generated at the coupling port and a leakage power at the isolation port is deteriorated.

Therefore, some methods have been proposed for improving the directionality, which includes a method of providing a feedback line (see, for example, Non-patent Document 1), a method of processing portions of a main line and a coupling line facing each other (see, for example, Patent Document 1), a method of providing a floating conductor to a coupling portion of the coupling line (see, for example, Non-patent Document 2), and the like.

FIG. 10is a schematic diagram of a conventional directional coupler in Non-patent Document 1. The directional coupler in Non-patent Document 1 includes feedback lines103and104, which are respectively disposed between input and output terminals of a main line101, and between the isolation port108and the coupling port107, and hence the directionality is improved.

More specifically, the improvement of the directionality is intended as follows. The main line101and a coupling line102facing each other are connected to the feedback lines103and104, respectively. An RF signal received from an input terminal105is led to an output terminal106via the main line101. Further, the main line101is coupled with the coupling line102, and hence the signal received from the input terminal105is led to the coupling port107.

Then, the phase speeds of the even mode and the odd mode are matched with each other by the feedback lines103and104. Thus, the signal is not led to an isolation port108, and hence the improvement of the directionality is intended.

In addition,FIG. 11is a schematic diagram of a conventional directional coupler in Patent Document 1. This directional coupler of Patent Document 1 improves the directionality by disposing portions115and116at both ends of the coupling portion of a coupling line110so as to increase capacitance of a main line119, and by letting facing portions117of the main line119and the coupling line110have inductances.

More specifically, the improvement of the directionality is intended as follows. A coupling portion118of the main line119and the coupling line110has portions115and116for increasing capacitance of the main line119at both ends of the coupling line110, and a portion for having inductance at the facing portions117of the main line119and the coupling line110. An RF signal received from an input terminal111is led to an output terminal112via the main line119.

Further, the main line119is coupled with the coupling line110, and hence the signal received from the input terminal111is led to a coupling port113. The directional coupler of Patent Document 1 makes phase speeds of the even mode and the odd mode match with each other by means of the portions115and116that increase the capacitance of the main line119and are disposed on both ends of the coupling portion118of the coupling line110, and the facing portions117of the main line119and the coupling line110having inductance. Thus, the signal is prevented from being led to an isolation port114, and hence improvement of the directionality is intended.

In addition,FIG. 12is a schematic diagram of a conventional directional coupler in Non-patent Document 2. This directional coupler in Non-patent Document 2 compensates for a phase difference between the even mode and the odd mode by providing floating conductors127to a coupling portion of a coupling line126, and hence the directionality is improved.

More specifically, the improvement of the directionality is intended as follows. The floating conductors127are disposed in a coupling portion of a main line125and the coupling line126. An RF signal received from an input terminal121is led to an output terminal122through the main line125. Further, the main line125is coupled with the coupling line126, and hence the signal received from the input terminal121is led to a coupling port123.

Then, periodical slits provided to the coupling portion increase a distributed inductance of the odd mode mainly. In addition, the floating conductor inserted in the coupling portion affects almost only distributed capacitance of the odd mode. Therefore, the phase speeds of the even mode and the odd mode can be matched with each other by adjusting sizes of the slit and the floating conductors127. Thus, it is possible to prevent the signal from being led to an isolation port124, and hence the improvement of the directionality is intended.

PATENT DOCUMENT 1

JP 56-138302 A

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, the conventional techniques have following problems.

Each of the conventional directional couplers described above is required to be designed so that the directionality becomes optimal at a predetermined frequency when the directional coupler is designed. As a result, there is a problem that if the directionality is deteriorated because of a manufacturing fluctuation of the substrate, expansion of the substrate due to temperature, an error in designing accuracy, or the like, correction thereof is impossible. In addition, there is a problem that it is necessary to change the design of the directional coupler body in the designing stage, and hence the directionality of an existing directional coupler cannot be improved.

The present invention has been made to solve the problems described above, and an object thereof is to obtain a directional coupler that is capable of improving a directionality of a directional coupler body including four terminals.

Means for Solving the Problems

The present invention provides a directional coupler comprising: a directional coupler body including four terminals of an input port, an output port, a coupling port, and an isolation port; and a combiner for combining powers of an output signal of the coupling port and an output signal of the isolation port of the directional coupler body, in which: the directional coupler further comprises a directionality improving circuit for amplifying or attenuating at least one of the output signal of the coupling port and the output signal of the isolation port before outputting the same; and the combiner combines powers of the output signals amplified or attenuated by the directionality improving circuit.

EFFECT OF THE INVENTION

According to the present invention, a directionality improving circuit is provided for amplifying or attenuating at least one of the output signal of the coupling port and the output signal of the isolation port, and hence it is possible to obtain the directional coupler that is capable of improving the directionality of the directional coupler body including four terminals.

BEST MODES FOR CARRYING OUT THE INVENTION

Preferred embodiments of a directional coupler according to the present invention are described below with reference to the drawings. The directional coupler according to the present invention has a technical feature that the directional coupler includes a directionality improving circuit is connected to one or both of a coupling port and an isolation port of the directional coupler body.

FIG. 1is a schematic diagram of the directional coupler according to Embodiment 1 of the present invention. In addition,FIG. 2are operation explanatory diagrams of the directional coupler according to Embodiment 1 of the present invention. The directional coupler according to Embodiment 1 includes a directional coupler body10, an attenuator20athat corresponds to a directionality improving circuit, and a combiner30.

In addition, the directional coupler body10includes four terminals11to14. The terminal11corresponds to an input terminal and is denoted by Port1inFIG. 1. In addition, the terminal12corresponds to an output terminal and is denoted by Port2inFIG. 1. Further, the terminal13corresponds to a coupling port, and the terminal14corresponds to an isolation port.

On the other hand, the combiner30includes three terminals31to33, so as to combine powers of signals received from the terminal32and the terminal33, respectively. The combined signal is outputted from the terminal31(denoted by Port3inFIG. 1).

The coupling port13of the directional coupler body10is connected to the terminal32of the combiner30. On the other hand, the isolation port14of the directional coupler body10is connected to the terminal33of the combiner30via the attenuator20a.

Next, an operation of the directional coupler in Embodiment 1 is described. Note that the directional coupler body10in Embodiment 1 is a ¼ wavelength directional coupler.

First, a case of receiving the signal from the terminal11is described with reference toFIG. 2(A). The RF signal (Pi (dB)) received from the terminal11passes through the directional coupler body10and afterward is led to the terminal12. On this occasion, when a coupling amount is denoted by C (dB), and the directionality is denoted by D (dB), a coupling power (Pi−C (dB)) is generated at the coupling port13, and a power that is smaller than the power generated at the coupling port13by the directionality and has the opposite phase (Pi−C−D (dB)) leaks out to the isolation port14.

The signal leaking out to the isolation port14is attenuated by the attenuator20athat is set to have an amplitude value (D (dB)) that is the same as the directionality of the directional coupler body10, and afterward its power is combined with a power of the signal from the coupling port13in the combiner30. Here, one of the input signal of the combiner30is the signal outputted from the coupling port13(Pi−C (dB)), and the other is the signal that is outputted from the isolation port14and has passed through the attenuator20a(Pi−C−2D (dB)).

Powers of the both signals are combined in opposite phases and different amplitudes by the combiner30. Therefore, if a Wilkinson distributor is used as the combiner30, the power P (dB) that is decreased to be lower than the power at the coupling port13and is expressed by the following expression (1) is generated at Port3of the combiner30.
[Expression 1]
P=Pi−C−3+20 log(1−10−D/10)  (1)

Next, the case where a signal outputted from the terminal12is reflected by an antenna or the like and returns to the terminal12is described with reference toFIG. 2(B). The reflected wave (Pr (dB)) that returns to the terminal12passes through the directional coupler body10and afterward is led to the terminal11. On this occasion, a coupling power (Pr−C (dB)) is generated at the isolation port14, and a power that is smaller than the power generated at the isolation port14by the directionality and has the opposite phase (Pr−C−D (dB)) leaks out to the coupling port13.

The signal generated at the isolation port14is attenuated by the attenuator20ahaving an attenuation amount that is set to be the same amplitude value (D (dB)) as the directionality of the directional coupler body10(Pi−C−D (dB)), and its power is combined with a power of the signal from the coupling port13(Pr−C−D (dB)) in the combiner30. Here, the both signals have opposite phases and the same amplitude. Therefore, as for the reflected wave, no power is generated at the Port3of the combiner30.

As a result, as to the power generated at the Port3, it is understood that there is a large difference in power between the case where the signal enters from the terminal11and the case where the signal enters from the terminal12. Therefore, by using the directional coupler of Embodiment 1 including the attenuator20athat works as the directionality improving circuit, the directionality of the directional coupler body10can be improved.

In order to confirm the improvement of the directionality, a prototype of the directional coupler of Embodiment 1 was manufactured, and the improvement effect of the directionality was measured. In the experiment, the attenuator20ahaving an amplitude value as the attenuation amount that is the same as the directionality (D (dB)) of the directional coupler body10was connected to the isolation port14. Further, in addition to this, attenuators having the same attenuation amount were inserted in both the coupling port13and the isolation port14in order to prevent multiple reflection due to impedance mismatch between the directional coupler body10and the combiner30.

FIG. 3is a circuit schematic diagram of the prototype directional coupler according to Embodiment 1 of the present invention.FIG. 3illustrates an example case where the attenuators are respectively connected to the isolation port14and the coupling port13of the directional coupler body10, and the outputs thereof are combined by the Wilkinson distributor corresponding to the combiner30.

FIG. 4are diagrams illustrating a result of measurement of the directionality of the prototype directional coupler according to Embodiment 1 of the present invention.FIG. 4(A)illustrates a result of measurement of the directionality in the case with directionality improving circuit, andFIG. 4(B)illustrates a result of measurement of the directionality in the case without the directionality improving circuit. For instance, the directionality at a frequency of 2 GHz is improved from 8.5 dB inFIG. 4(B)to 21.8 dB inFIG. 4(A), and hence the effectiveness of the directionality improving circuit can be confirmed.

As described above, according to Embodiment 1, the directionality of the directional coupler body can be improved by combining the signals outputted from the coupling port and the isolation port of the directional coupler body via the directionality improving circuit.

In particular, according to Embodiment 1, the attenuator is used as the directionality improving circuit, and hence it is easy to readjust the directionality to obtain an optimal directionality by adjusting the attenuator. In addition, it is also easy to change the frequency. Further, it is possible to improve the directionality of the existing directional coupler as well by adding the directionality improving circuit that is the technical feature of the present invention to the same.

FIG. 5is a schematic diagram of the directional coupler according to Embodiment 2 of the present invention. In Embodiment 1 described above, the attenuator20ais connected to the isolation port14as the directionality improving circuit. In contrast, Embodiment 2 is different from Embodiment 1 in that an amplifier20bis connected to the coupling port13as the directionality improving circuit. This amplifier20bhas a gain of the amplitude value (D (dB)) that is the same as the directionality of the directional coupler body10.

Further, powers of the signal outputted from the coupling port13via the amplifier20band the output of the isolation port14are combined with each other by the combiner30.

In Embodiment 1 described above, the output of the isolation port14is attenuated by the attenuator20aand afterward is used for the combining. In contrast, according to Embodiment 2, the output of the coupling port13is amplified by the amplifier20band afterward is used for the combining. Although the directionality improving circuits are different, the basic operation of the directional coupler in Embodiment 2 is the same as that of Embodiment 1 described above.

Next, an operation of the directional coupler in Embodiment 2 is described. Note that the directional coupler body10in Embodiment 2 is also a ¼ wavelength directional coupler similarly to Embodiment 1 described above.

FIG. 6are operation explanatory diagrams of the directional coupler in Embodiment 2 of the present invention. First, the case where the signal enters from the terminal11is described with reference toFIG. 6(A). The RF signal (Pi(dB)) received from the terminal11passes through the directional coupler body10and afterward is led to the terminal12. In this case, if the coupling amount is denoted by C (dB), and the directionality is denoted by D (dB), then the coupling power (Pi−C (dB)) is generated at the coupling port13, and a power that is smaller than the power generated at the coupling port13by the directionality and has the opposite phase (Pi−C−D (dB)) leaks out to the isolation port14.

The signal of the coupling power generated at the coupling port13is amplified by the amplifier20bhaving a gain of the amplitude value (D (dB)) that is the same as the directionality of the directional coupler body10(Pi−C+D (dB)), and afterward a power thereof is combined with a power of the signal from the isolation port14at the combiner30.

Here, one of the signals entering the combiner30is the signal outputted from the coupling port13and passes through the amplifier20b(Pi−C+D (dB)), and the other is the signal outputted from the isolation port14(Pi−C−D (dB)).

Powers of the both signals are combined by the combiner30in opposite phases and different amplitudes. Therefore, if the Wilkinson distributor is used as the combiner30, the power P (dB) that is decreased to be lower than the power (Pi−C+D (dB)) outputted from the amplifier20aand is expressed by the following expression (2) is generated at the Port3of the combiner30.
[Expression 2]
P=Pi−C+D−3+20 log(1−10−D/10)  (2)

Next, the case where the signal outputted from the terminal12is reflected by the antenna or the like and returns to the terminal12is described with reference toFIG. 6(B). The reflected wave (Pr (dB)) that returns to the terminal12passes through the directional coupler body10and afterward is led to the terminal11. On this occasion, a coupling power (Pr−C (dB)) is generated at the isolation port14, and a power that is smaller than the power generated at the isolation port14by the directionality and has the opposite phase (Pr−C−D (dB)) leaks out to the coupling port13.

The signal generated at the coupling port13is amplified by the amplifier20bhaving a gain set to be the amplitude value (D (dB)) that is the same as the directionality of the directional coupler body10(Pi−C (dB)), and a power thereof is combined with a power of the signal from the isolation port14(Pr−C (dB)) in the combiner30. Here, the both signals have opposite phases and the same amplitude. Therefore, as for the reflected wave, no power is generated at the Port3of the combiner30.

As a result, as to the power generated at the Port3, it is understood that there is a large difference in power between the case where the signal enters from the terminal11and the case where the signal enters from the terminal12. Therefore, by using the directional coupler of Embodiment 2 including the amplifier20bthat works as the directionality improving circuit, the directionality of the directional coupler body10can be improved similarly to the case of Embodiment 1 described above.

As described above, according to Embodiment 2, the directionality of the directional coupler body can be improved by combining the signals outputted from the coupling port and the isolation port of the directional coupler body via the directionality improving circuit.

In particular, the amplifier is used as the directionality improving circuit in Embodiment 2 so that the signal outputted from the coupling port is amplified by the amplifier, and hence it is possible to produce a power larger than that of Embodiment 1 described above by the gain of the amplifier. As a result, there is a merit that even when the coupling amount of the directional coupler body10is small, the power that appears at the terminal31does not become too small.

Further, there is a merit that a signal of good quality can be obtained because it is hardly interfered even when an unnecessary wave is generated in the case housing the directional coupler due to a large power applied to the terminal11or when a signal is induced in the substrate. Further, by using a variable gain amplifier as the amplifier, the frequency for improving the directionality can be adjusted easily.

FIG. 7is a schematic diagram of the directional coupler according to Embodiment 3 of the present invention. In Embodiment 1 described above, the attenuator20ais connected to the isolation port14as the directionality improving circuit. Further, in Embodiment 2 described above, the amplifier20bis connected to the coupling port13as the directionality improving circuit.

In contrast, according to Embodiment 3, the outputs of the isolation port and the coupling port are connected to a combiner30afor combining powers thereof by a combining ratio of the amplitude value (D (dB)) that is the same as the directionality of the directional coupler, so as to improve the directionality of the directional coupler body10. In other words, the combiner30acorresponds to a combiner with a directionality improving circuit, which includes two input terminals having different combining ratios and has a function of the attenuator as well that works as the directionality improving circuit.

The directional coupler of Embodiment 3 includes the directionality improving circuit incorporated in the combiner30a, but the basic operation is the same as those of Embodiments 1 and 2 described above, and hence description thereof is omitted.

As described above, according to Embodiment 3, the directionality of the directional coupler body can be improved by combining the signals outputted from the coupling port and the isolation port of the directional coupler body via the directionality improving circuit.

In particular, Embodiment 3 uses the combiner having different combining ratios, and hence there is a merit that the combiner itself has the both functions of the attenuator and the combiner, thereby eliminating the need for an independent attenuator. As a result, there is a merit that the structure can be simplified.

Note that the directional coupler body that is used in Embodiments 1 to 3 described above can be constituted of microstrip lines. Thus, the directional coupler body and the directionality improving circuit can be formed on the same plane.

In addition, though Embodiments 1 to 3 described above exemplify the case where the ¼ wavelength directional coupler is used as the directional coupler body, it is possible to use a ½ wavelength directional coupler instead. If a coupling line length is an integral multiple of the ¼ wavelength, a large coupling amount can be obtained. In addition, if it is the ½ wavelength directional coupler, it is possible to improve the directionality of the directional coupler body without the directionality improving circuit.

In addition, the directional coupler body in Embodiments 1 to 3 described above may be one including an isolator connected to at least one of the coupling port and the isolation port. Thus, it is possible to suppress multiple reflection due to impedance mismatch between the directional coupler body and the combiner.

Further, it is possible to connect additionally a phase adjustment line to at least one of the coupling port and the isolation port as the directional coupler body of Embodiments 1 to 3 described above, so as to adjust a phase difference between a phase between the input terminals of the coupling port and the combiner, and a phase between the input terminals of the isolation port and the combiner.

Thus, a passing amplitude difference between the coupling port side and the isolation port side can be adjusted. Thus, even when the phases of the input signals at the input terminals of the combiner30do not have opposite, the passing amplitude difference can be adjusted.

Further, the attenuator having the attenuation amount as the amplitude value that is the same as the directionality of the directional coupler body of Embodiments 1 to 3 described above can include a plurality of attenuators.

If a common available resistor is used for constituting the attenuator, there is a problem that it is difficult to realize a desired attenuation amount accurately with one attenuator because resistance values are discrete values. Therefore, it is possible to combine a plurality of attenuators so that a desired attenuation amount can be realized accurately.

Thus, a passing amplitude difference between the coupling port side and the isolation port side can be adjusted more precisely. Thus, even when the amplitude of the input signals at the input terminals of the combiner30do not have the same amplitude, the passing amplitude difference can be adjusted.

FIG. 8is a diagram illustrating cancelled amounts in the combiner30according to Embodiment 3 of the present invention. More specifically,FIG. 8illustrates cancelled amounts in the combiner with respect to the amplitude difference and the phase difference between the coupling port side and the isolation port side, in which curves indicate cancelled amounts corresponding to −10 dB, −15 dB, −20 dB, −25 dB, and −30 dB.

By the adjustment of the attenuation amount or the adjustment of the phase adjustment line length, the amplitude difference and the phase difference are decreased, and hence the cancelled amount can be improved. Thus, the directionality can be improved more, and a frequency band in which the directionality is improved can be widened.

In addition, as the directional coupler body of Embodiments 1 to 3 described above, a filter circuit may be connected additionally to at least one of the coupling port and the isolation port. In this filter circuit, a passing amplitude or a passing phase changes in accordance with a frequency. Therefore, such the filter circuit can compensate for the amplitude difference and the phase difference between the coupling port and the isolation port of the directional coupler body, which change in accordance with a frequency.

As a result, it is possible to realize a constant amplitude difference and a constant phase difference over a wide frequency range. Finally, in the directional coupler of Embodiments 1 to 3, it is possible to widen the frequency band in which a good directionality can be obtained.

In addition, Embodiments 1 to 3 described above exemplify the case where the ¼ wavelength directional coupler is used as the directional coupler body. However, a directional coupler of a wavelength shorter than or equal to the ¼ wavelength may be used as the directional coupler body.

FIG. 9are diagrams illustrating a result of computation of the directionality and the phase difference of the directional coupler in Embodiment 3 of the present invention. More specifically,FIG. 9(a) illustrates a graph indicating a result of computation of the amplitude difference (i.e., the directionality) and the phase difference between the coupling port and the isolation port with respect to a length of the directional coupler body for each of the ¼ wavelength directional coupler and the 1/32 wavelength directional coupler.

In addition,FIG. 9(b) is a table showing values of a result of the graph illustrated inFIG. 9(a). It is clear fromFIGS. 9(a) and9(b) that frequency dependencies of the amplitude difference and the phase difference are decreased by using the directional coupler of a wavelength shorter than or equal to the ¼ wavelength. Thus, in the directional coupler of Embodiments 1 to 3, it is possible to widen the frequency band in which a good directionality can be obtained.

In addition, as exemplified in Embodiments 1 and 2, the combiner and the directionality improving circuit can be formed on the same plane by using the Wilkinson distributor as the combiner.

In addition, as the combiner used in Embodiments 1 to 3 described above, a branch line hybrid circuit may be used, in which the ¼ wavelength line is added to the coupling port side. Thus, the combiner and the directionality improving circuit can be formed on the same plane.

Further, it is possible to connect attenuators having the same attenuation amount additionally to both the coupling port and the isolation port of the directional coupler body. Thus, it is possible to suppress multiple reflection due to impedance mismatch between the directional coupler body and the combiner.

In addition, it is possible to connect amplifiers having the same gain additionally to both the coupling port and the isolation port of the directional coupler body. Thus, it is possible to suppress multiple reflection due to impedance mismatch between the directional coupler body and the combiner.