Directional coupler

Abstract

This directional coupler comprises 2N (N is a natural number) coupled lines (100) which are arranged and connected in series in the direction of transmission. The coupled lines (100) comprise two first transmission lines (101) and two second transmission lines (102) which are magnetically coupled to one another. The first transmission lines (101) and the second transmission lines (102) have a line length set to a wavelength which equals 1 / 8N of the design wavelength. The coupled lines (100) which are adjacent to one another in the direction of arrangement intersect and are connected via intersecting lines (103). A capacitor (104) is connected between the intersecting lines (103).

Description

Technical Field

[0001] The present invention relates to a directional coupler.

Background Art

[0002] Broadband THz waves (electromagnetic waves of 300 GHz to 30 THz) are considered for applications to ultra-high-speed wireless communications such as next-generation wireless communications (beyond 5G). In particular, the 300-GHz band has less absorption attenuation during atmospheric propagation among the THz bands, and is a frequency band in which a wireless transceiver (TRX) can be realized by electronic devices using SiGe, InP, etc. and electronic devices such as CMOS, and thus research and development are actively underway (Non-Patent Document 1, Non-Patent Document 2).

[0003] Among them, a transistor having excellent high-frequency characteristics using an InP-based semiconductor can realize an amplifier with a high gain of about 20 dB even at 300 GHz (Non-Patent Document 3), and can be said to be a promising device for realizing a high-performance TRX. In such a TRX, in order to perform multi-value modulation such as QAM, generation of orthogonal signals (signals with a 90° phase shift) is essential. Various techniques have been proposed for generating orthogonal signals, such as a method using a polyphase filter (Non-Patent Document 4). Among them, a method using a directional coupler (Non-Patent Document 5) is known for being able to reduce the amplitude difference between broadband orthogonal signals.

[0004] As shown in FIG. 6, the directional coupler is composed of a coupling line 300 formed by a first transmission line 301 and a second transmission line 302 that are electromagnetically coupled. A first port 331 and a second port 332 are connected to the first transmission line 301, and a third port 333 and a fourth port 334 are connected to the second transmission line 302.

[0005] The signal input to the first port 331 is output to the second port 332 and the third port 333 with a 90° phase difference. The fourth port 334 is called an isolation port, and usually, no signal is output. If the coupling degree of this directional coupler (an index representing the strength of power exchange between the coupled transmission lines) is appropriately selected, the absolute value of the amplitude of the signal output from the second port 332 and the absolute value of the amplitude of the signal output from the third port 333 can be made equal. Such a coupling state is called perfect coupling.

[0006] In order to realize a direct conversion TRX or an image rejection TRX, which are modulation / demodulation devices using orthogonal signals, it is important to achieve a perfect coupling state. In these TRXs, even if the phase difference between the orthogonal signals is exactly 90°, if the amplitude error is large, the rejection rate of unwanted signals such as image signals will change significantly, and the signal-to-noise ratio (SNR) when performing wireless communication using these TRXs will be greatly deteriorated. Also, in a directional coupler, if the coupled line length is not appropriately designed (about a quarter wavelength of the design frequency), the phase difference between the orthogonal signals will deviate from 90°. This phase error also causes the same problems as the above amplitude error, so it must be noted in a directional coupler.

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

[0008] However, in the above-described technology, there are the following problems in realizing a directional coupler corresponding to the 300 GHz band using semiconductor devices.

[0009] First, conventional coupling lines that can be fabricated in the manufacturing process of semiconductor devices have a problem that it is difficult to achieve a perfect coupling state required for a directional coupler corresponding to the 300 GHz band due to restrictions caused by the process rules. The two transmission lines of the coupling line are generally composed of metal patterns, but there is a limit to the gap between two metal patterns (transmission lines) that can be fabricated by well-known semiconductor processes. The value of this limit gap can be, for example, 2 μm. A parameter that largely contributes to the coupling degree in a directional coupler is the gap between the transmission lines to be coupled.

[0010] Fig. 7 shows the amplitude of the signal output from the through port (second port) of the directional coupler and the amplitude of the signal output from the coupling port (third port) when the gap is changed from 2.4 μm to 0.3 μm. The line width of the directional coupler is 6 μm, the distance between the line and the ground is 10.5 μm, and the coupling line length is 110 μm, which is a quarter wavelength at 300 GHz. In Fig. 7, the solid line indicates the ratio of the power of the signal output from the through port to the power of the signal input to the input port (first port). The broken line indicates the ratio of the power of the signal output from the coupling port to the power of the signal input to the input port.

[0011] In Fig. 7, the smaller the difference in the vertical axis values between the solid line and the broken line, the higher the coupling degree. As shown in Fig. 7, it can be seen that the coupling degree improves as the gap becomes smaller. As shown in Fig. 7, setting the gap to 0.3 μm is required for perfect coupling. This is a value that cannot be realized in semiconductor processes. As can be seen from this calculation, it is generally difficult to realize a perfect-coupling type directional coupler in semiconductor processes.

[0012] Second, in a conventional directional coupler, even if the length of the coupling line is appropriately selected, it is difficult to maintain a 90° phase difference due to the inherent layout asymmetry of the directional coupler. As shown in FIG. 6, in the directional coupler, the ports where signals with a 90° phase difference are output are the second port (through port) 332 and the third port (coupling port) 333, and these are located at positions asymmetric with respect to the symmetry axis of the directional coupler.

[0013] Therefore, when attempting to use such a directional coupler in a circuit realizable in a semiconductor process, the transmission lines connected to the second port 332 and the transmission lines connected to the third port 333 have an asymmetric layout as shown in FIG. 8. This example applies a directional coupler to the generation of the quadrature LO signal of a quadrature mixer. Due to the layout asymmetry between the second port 332 through port and the third port 333, the layouts of the transmission lines connected to each also become asymmetric, and when connecting these ports to a mixer, a delay line is essential to compensate for the phase error.

[0014] However, in a region where the wavelength is short, such as in the 300 GHz band, the phase rotation becomes large with a very small delay amount, so it is extremely difficult in reality to compensate for the phase amount with such a delay line. If the layout were symmetric, such difficulties would be eliminated, and a simple configuration without a delay line could be realized.

[0015] As described above, in the conventional technology, there were problems such as it not being easy to realize a directional coupler corresponding to the 300 GHz band.

[0016] The present invention has been made to solve the above problems, and aims to realize a directional coupler corresponding to the 300 GHz band.

Means for Solving the Problems

[0017] The directional coupler according to the present invention is configured such that a coupling line composed of two first transmission lines and a second transmission line that are electromagnetically coupled is in series in the transmission direction by 2N (N isTwo or more They are arranged and connected in (a natural number) pieces, the coupling line has a line length of 1 / 8N wavelength of the design wavelength, the coupling lines adjacent in the array direction are connected by a crossing line, and a capacitor for connecting between the crossing lines of the coupling lines adjacent in the array direction is provided. A through-port is connected to the second transmission line of the coupling line at the first end on one end side, and a coupling port is connected to the second transmission line of the coupling line at the second end on the other end side.

Advantages of the Invention

[0018] As described above, according to the present invention, a directional coupler is constituted by 2N (N is a natural number) coupling lines arranged and connected in series in the transmission direction, adjacent coupling lines are cross-connected, and a capacitor is connected between the crossing lines. Therefore, a directional coupler corresponding to the 300 GHz band can be realized.

Brief Description of the Drawings

[0019]

Figure 1A

Figure 1B

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6

Figure 7

Figure 8

[0020] Hereinafter, the directional coupler according to the embodiment of the present invention will be described with reference to FIGS. 1A and 1B. In this directional coupler, 2N (N is a natural number) coupling lines 100 are arranged and connected in series in the transmission direction. In other words, the directional coupler is composed of 2N (an even number) coupling lines 100 connected in series. In FIG. 1, an example in which N is 1 is shown, and FIG. 1B shows an example in which N is 2 or more.

[0021] Each coupling line 100 is composed of two first transmission lines 101 and second transmission lines 102 that are electromagnetically coupled to each other. The first transmission line 101 and the second transmission line 102 are composed of a pattern of a rectangular conductor whose shape in plan view has a long side parallel to the transmission direction. Further, the first transmission line 101 and the second transmission line 102 (coupling line 100) have a line length of 1 / 8N of the design wavelength.

[0022] In addition, in this directional coupler, the coupling lines 100 adjacent to each other in the array direction are connected by the crossover lines 103. Also, a capacitor 104 is connected between the crossover lines 103 of the coupling lines 100 adjacent to each other in the array direction. The number of the crossover lines 103 and the capacitors 104 is 2N - 1.

[0023] In addition, in a directional coupler in which an even number of coupling lines 100 are arranged and connected in series, a first port 131 is connected to the first transmission line 101 of the coupling line 100 at the first end portion 151 which is one end side, and a second port 132 is connected to the second transmission line 102 of the first end portion 151. The first port 131 serves as an input port, and the second port 132 serves as a through port.

[0024] In addition, in a directional coupler in which an even number of coupling lines 100 are arranged and connected in series, a third port 133 is connected to the second transmission line 102 of the coupling line 100 at the second end portion 152 which is the other end side, and a fourth port 134 is connected to the first transmission line 101 of the coupling line 100 at the second end portion 152. The third port 133 serves as a coupling port, and the fourth port 134 serves as an isolation port. Also, a termination resistor 105 is connected to the fourth port 134.

[0025] According to the above-described embodiment, since the capacitor 104 is provided, even if the gap between the first transmission line 101 and the second transmission line 102 cannot be made too small, the coupling degree of the coupling line 100 can be increased. By arranging the capacitor 104, due to the impedance (1 / jωC) of the capacitor 104, coupling of an AC signal occurs between the first transmission line 101 and the second transmission line 102, and an effect of equivalently increasing the coupling degree in the coupling line 100 is obtained.

[0026] Fig. 2 shows the ratio (solid line) of the power of the signal output from the second port 132 to the power of the signal input to the first port 131, and the ratio (dashed line) of the power of the signal output from the third port 133 to the power of the signal input to the first port 131, when the gap between the first transmission line 101 and the second transmission line 102 is 3 μm and the value of the capacitor 104 is changed from 0 fF to 20 fH in steps of 4 fF.

[0027] As the capacitance value of the capacitor 104 increases, the coupling degree improves, and it can be seen that the difference between the two becomes smaller. When the capacitance value of the capacitor 104 is set to 12 fF, perfect coupling occurs at 300 GHz. When the capacitance value of the capacitor 104 is greater than 12 fF, over-coupling (a state where the coupling degree exceeds 1) occurs, where the dashed line is larger than the solid line. In this calculation, the line widths of the first transmission line 101 and the second transmission line 102 are 4 μm, and the one-eighth line length is 55 μm.

[0028] These design parameters and the capacitance value of the capacitor 104 are first given initial values such that the input impedance of each of the first port 131 to the fourth port 134 is 50 Ω when the capacitor 104 does not exist, and then selected to correct the deviation of the port impedance when the capacitor 104 is inserted. By appropriately selecting the capacitance value of the capacitor 104 in this way, perfect coupling can be achieved.

[0029] Further, according to the embodiment, since the adjacent coupling lines 100 are crossed and connected by the crossing line 103, the positional relationship between the second port 132 and the third port 133 can be arranged symmetrically with respect to the symmetry axis. According to the embodiment, the through port and the coupling port are arranged symmetrically with respect to the directional coupler. Note that the symmetry axes include a symmetry axis parallel to the transmission direction of the directional coupler and disposed in the middle of the first transmission line 101 and the second transmission line 102, and a symmetry axis orthogonal to this symmetry axis and passing through the center of the directional coupler. Therefore, for example, when the directional coupler according to the embodiment is used in an orthogonal signal generation circuit for a quadrature mixer, as shown in FIG. 3, the transmission lines connected to the second port 132 and the transmission lines connected to the third port 133 can be arranged in a symmetrical layout. As a result, a delay line becomes unnecessary to compensate for the phase error, and it is easy to maintain a 90° phase difference.

[0030] Next, the capacitor 104 will be described. As shown in FIGS. 4A and 4B, the crossing line 103 can be composed of a lower layer wiring 103a and an upper layer wiring 103b in a device using an InP-based semiconductor. A capacitor 104 using a MIM-type element can be arranged between the crossing portions of the lower layer wiring 103a and the upper layer wiring 103b configured in this way.

[0031] For example, the lower layer wiring 103a and the upper layer wiring 103b are realized by the multilayer wiring technology of a well-known semiconductor device manufacturing process. Among them, contact holes are formed in the interlayer insulating layer formed between the lower layer wiring 103a and the upper layer wiring 103b, and a metal layer, an insulator layer, and a metal layer are laminated in the thickness direction, so that the capacitor 104 using a MIM-type element can be formed.

[0032] In this configuration, since the capacitance value of the capacitor 104 depends on the thickness of the interlayer insulating layer, the area of the intersection, etc., it is difficult to arrange a capacitor 104 with a large capacitance value. However, in an ultra-high frequency band such as the 300 GHz band, the required capacitance value of the capacitor 104 is as small as about 10 - 20 fF (see Fig. 2). Therefore, the present invention can be sufficiently realized even in such a layout. For example, considering a typical MIM type capacitor 104 with a capacitance value of 0.5 fF per square μm, the area of the capacitor 104 required to realize 20 fF is only 40 μm 2 This is achievable with a size of 10 μm square or less and can be arranged within the intersecting line 103.

[0033] Also, as shown in FIGS. 5A and 5B, the capacitor 104a can be configured with two lines at the intersection of the intersecting line 103 as conductors. As described above, the intersecting line 103 can be composed of a lower layer wiring 103a and an upper layer wiring 103b. The lower layer wiring 103a and the upper layer wiring 103b at the intersection of the intersecting line 103 configured in this way are used as the conductors of the capacitor 104a, and the region between the lower layer wiring 103a and the upper layer wiring 103b at the intersection is used as an insulator.

[0034] Generally, as the operating frequency increases, the coupling degree between coupled lines with the same gap increases. This is because the coupling by the capacitor equivalently distributed between the coupled lines increases as the frequency increases. In such a case, the required value (the value required to achieve perfect coupling) of the capacitor 104a arranged at the intersection in the present invention also decreases.

[0035] For example, considering a typical MIM capacitor with a capacitance value of 0.5 fF per square μm, the area required to realize a 1 fF MIM capacitor is only 2 μm 2This is a value that is difficult to achieve due to the constraints of the manufacturing rules of the semiconductor process described above. On the other hand, as shown in FIGS. 5A and 5B, if the lower layer wiring 103a and the upper layer wiring 103b themselves at the intersection are used for the capacitor 104a, the above-described capacitance value can be realized. The capacitor 104a formed by the lower layer wiring 103a and the upper layer wiring 103b at the intersection can have a capacitance value per square μm of about 1 / 20 to 1 / 10 of that of the MIM type element, and a small capacitance value such as 2μm 2 can also be realized without problems in the manufacturing process of known semiconductor devices, and can be realized as a rectangle with a side length of about 10μm in plan view.

[0036] Next, as described with reference to FIG. 1B, the effect of using a plurality of capacitors 104 will be described. By connecting a plurality of coupling lines with different degrees of coupling in cascade (series), the bandwidth of the directional coupler can be broadened (Non-Patent Document 5). Since the degree of coupling can be equivalently varied by the capacitor 104, the bandwidth of the directional coupler can be broadened by using a plurality of capacitors 104 and configuring them as shown below.

[0037] By setting the line lengths of the first transmission line 101 and the second transmission line 102 to 1 / 8 wavelength shorter than 1 / 4 wavelength and connecting the capacitor 104 to each of the cross lines 103 between the adjacent coupling lines 100, the bandwidth of the directional coupler can be equivalently broadened. Although an extremely small capacitance value such as 2 fF is required for the capacitance values of the respective capacitors 104 distributed for bandwidth broadening, it is desirable to use the capacitor 104a described with reference to FIGS. 5A and 5B for such a small capacitance value.

[0038] As described above, according to the present invention, a directional coupler is constituted by 2N (n is a natural number) coupling lines 100 arranged and connected in series in the transmission direction. The adjacent coupling lines 100 are cross-connected, and a capacitor 104 is connected between the cross lines 103. Therefore, the perfect coupling state required for a directional coupler corresponding to the 300 GHz band can be achieved, and the through port and the coupling port can be arranged at positions symmetric with respect to the symmetry axis of the directional coupler. Thus, a directional coupler corresponding to the 300 GHz band can be realized.

[0039] It should be noted that the present invention is not limited to the embodiments described above. It is obvious that within the technical idea of the present invention, many modifications and combinations can be implemented by those with ordinary knowledge in the art.

Explanation of Reference Numerals

[0040] 100... coupling line, 101... first transmission line, 102... second transmission line, 103... cross line, 104... capacitor, 131... first port, 132... second port, 133... third port, 134... fourth port, 151... first end, 152... second end.

Claims

1. A coupled line composed of two first transmission lines and two second transmission lines that are electromagnetically coupled is arranged and connected in series in the transmission direction in 2N (N is a natural number of 2 or more) pieces, The coupled line has a line length of 1 / 8N of the design wavelength, The coupled lines adjacent to each other in the arrangement direction are connected by a cross line, A capacitor is provided that connects between the cross lines of the coupled lines adjacent to each other in the arrangement direction, A through port is connected to the second transmission line of the coupled line at the first end on one end side, and a coupling port is connected to the second transmission line of the coupled line at the second end on the other end side A directional coupler characterized by this.

2. In the directional coupler according to Claim 1, A directional coupler characterized in that the through port and the coupling port are arranged at symmetric positions with respect to the directional coupler.

3. In the directional coupler according to Claim 1 or 2, A directional coupler characterized in that the capacitor is composed of a MIM type element arranged between two lines at the intersection of the cross lines.

4. In the directional coupler according to Claim 1 or 2, A directional coupler characterized in that the capacitor is configured with two lines at the intersection of the cross lines as conductors.

5. In the directional coupler according to any one of Claims 1 to 4, An isolation port connected to the first transmission line of the coupled line at the second end, And a termination resistor connected to the isolation port A directional coupler characterized by including.

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