High frequency device

Abstract

To provide a high frequency circuit board capable of obtaining desired characteristics while suppressing interference between filters and generation of an unwanted board mode.SOLUTION: A high frequency circuit board comprises: a support board 101 in which a plane circuit pattern of a metal film is formed on a front face and a rear face ground is included on a rear face; and a rectangular waveguide part 103 covering a portion of the support board in a band shape with a metal which is electrically connected with the rear face ground. By covering the support board in the band shape with the metal, the relevant portion becomes a rectangular waveguide and is functioned not to propagate a mode equal to or lower than a cutoff frequency. Namely, a width restriction of the support board can be mitigated. Further, by covering the support board in the band shape with the metal, a ground which is conventionally present only on the rear face may also be formed on the front face, the number of steps of a filter circuit is reduced, and reduction in size can be attained.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to the structure of a device that handles high frequencies, such as millimeter waves or even terahertz (THz) waves, as carrier frequencies. [Background technology]

[0002] The frequency band ranging from 100 GHz to 10 THz is commonly referred to as the THz band. Historically, technological development in the millimeter wave to submillimeter wave bands (30 GHz to over 300 GHz) has continued since the 1960s, but with the fusion of photonics technology and THz technology in the 1990s, the generation and reception of THz waves became more common than before, and applied research also progressed.

[0003] Currently, THz waves are expected to be applied not only in wireless communications but also in a wide range of fields such as spectroscopy, imaging, and measurement. The highest frequency band used by the fifth generation (5G) of wireless communications is the Ka (28GHz) band, but NTT docomo's white paper "5G Advancements and 6G," published in 2022, states that "radio waves" up to about 300GHz are being considered for 6G, and research and development of THz technology is accelerating. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] I. Maestrojuan et al., “Development of a Sub-Harmonic Mixer Working at 220 GHz,” 2011 IEEEMtt-S Int. Microwave Workshop Series on Millimeter Wave Integration Technologies, 2011 (pp. 78-80). [Non-patent document 2] Hiroshi Ito et al., “Terahertz-wave detector on silicon carbide platform”, Applied Physics Express 15, 2022 (pp. 026501-1~4). Summary of the Invention [Problem to be solved by the invention]

[0005] However, it is difficult to say that the application of THz technology has yet to be fully adopted in industry. One of the reasons for this is thought to be the price of THz transceiver modules and systems. Typical THz module products sold by several European and American companies have a structure in which semiconductor elements are mounted on a quartz substrate using a flip-chip process (see, for example, Non-Patent Document 1).

[0006] NTT Innovative Devices Corporation already sells photodiode modules (waveguide-output THz photomixers) for various frequency bands up to the J-band (220-330 GHz). In this module, the photodiode chip and the waveguide are connected via a microstrip line (MSL)-based waveguide coupler substrate, and wiring is connected to the end of the photodiode chip. In this specification, unless otherwise specified, MSL refers to a component consisting of a support substrate (dielectric layer), a metal film forming a planar circuit pattern formed on the surface of the support substrate, and a ground on the back surface of the support substrate.

[0007] Since all of these existing products require assembly and mounting processes during manufacturing, reducing the variation in characteristics and reducing costs are issues.

[0008] Furthermore, THz circuits are often connected to flanged metal waveguides, which makes them difficult to mass-produce, unlike PCB circuits, which are lightweight and offer greater design freedom, and this also hinders cost reduction.

[0009] Recently, to address the aforementioned issues of characteristic variation and cost reduction, chip technologies have been developed that integrate diode elements and waveguide couplers, eliminating the need for flip-chip processes and wiring for high-frequency signal paths. For example, the diode-integrated THz chip shown in Figure 1 mounts a THz receiving diode 502, a waveguide coupler 503, and filter circuits 504 and 505 on a SiC support substrate 501 (see, for example, Non-Patent Document 2). Here, filter circuit 504 is an IF ground RF open filter, and filter circuit 505 is an RF short IF filter. In this chip, waveguide coupler 503 is located within waveguide port 507. This chip employs a tip-open type waveguide coupler 503 with good bandwidth characteristics, allowing the filter circuits (504, 505) to be mounted in close proximity on a narrow support substrate 501.

[0010] However, mounting the filter circuits close to each other on the narrow support substrate 501 causes the following problems. The filter design follows conventional technology and is based on an MSL with the back surface of support substrate 501 as the ground, which limits the range of characteristic impedance that can be designed. For this reason, in order to ensure the desired filter band characteristics, the filter circuit must be configured with multiple sets of high-impedance and low-impedance sections of the MSL connected in series (multi-stage configuration), as shown in Figure 1, making it difficult to miniaturize the filter circuit (reduce the filter area). Furthermore, as shown in Figure 1, circuit branch 508 is also required to separate the two filter circuits.

[0011] Furthermore, there are difficulties with the size of the filter circuit. When the distance from the receiving diode 502 to the IF output 506 and the distance from the filter circuit 504 to the IF output 506 are designed to match the operating THz frequency, the design values ​​may not necessarily be optimal parameters for the IF frequency. In such cases, the operation may become unstable depending on the input impedance of the amplifier connected to the IF output 506.

[0012] In other words, it is difficult to freely set the size of the MSL filter circuit, and if a filter circuit that meets the requirements is placed on a support substrate 501 with a narrow width, it becomes very difficult to reduce the interference between filters that causes deterioration of characteristics.

[0013] On the other hand, in order to reduce the occurrence of unwanted substrate modes, the width of the support substrate must be limited to λ / 2 or less at the operating frequency, which makes it difficult to reduce interference between filters by widening the support substrate from the perspective of the occurrence of substrate modes.

[0014] In other words, a diode-integrated THz chip equipped with a filter composed of an open-tip type coupler with good bandwidth characteristics and an MSL faces the challenge of being difficult to design a diode-integrated THz chip with the desired characteristics while suppressing interference between filters and the occurrence of unwanted substrate modes.

[0015] Therefore, in order to solve the above problems, an object of the present invention is to provide a high-frequency circuit board that can obtain desired characteristics while suppressing interference between filters and the occurrence of unwanted substrate modes. [Means for solving the problem]

[0016] In order to achieve the above object, the high-frequency circuit board according to the present invention has a support substrate that is partially covered with a metal strip, and elements such as a portion of the signal line, a filter circuit, and a capacitor are formed on the metal strip.

[0017] Specifically, the high-frequency circuit board according to the present invention is characterized by comprising: a support substrate having a planar circuit pattern of a metal film formed on its surface and a back-side ground on its back surface; and a rectangular waveguide portion covering a portion of the support substrate in a band-like manner with metal electrically connected to the back-side ground.

[0018] By covering the support substrate with a metal strip, the corresponding part becomes a rectangular waveguide, which has the function of preventing modes below the cutoff frequency from propagating. Also, by covering the support substrate with a metal strip, the ground, which previously existed only on the back side, can now be formed on the front side as well, which reduces the size of the branch circuit and the number of stages in the filter circuit. It also makes it possible to reduce the impedance of the planar circuit pattern, which also reduces interference between filters.

[0019] As described above, the present invention can reduce the size of the filter circuit, and therefore can provide a high-frequency circuit board that can obtain desired characteristics while suppressing interference between filters and the occurrence of unwanted substrate modes.

[0020] The high-frequency circuit board according to the present invention is characterized in that, when the rectangular waveguide portion is regarded as a TE wave waveguide, the width a of the support substrate is the length of the long side of the opening of the TE wave waveguide, and is a value such that the low cutoff frequency fc of the TE wave waveguide is higher than a desired maximum operating frequency fmax.

[0021] The high-frequency circuit board according to the present invention is characterized in that the planar circuit pattern is also formed on the metal of the rectangular waveguide portion, which is an upper-surface ground on the front side of the support substrate, via a dielectric layer, and the thickness of the dielectric layer varies depending on the characteristics of the planar circuit pattern.

[0022] The high-frequency circuit board according to the present invention comprises: the rectangular waveguide portion has a rectangular cutout portion in which the metal is removed on an upper surface ground on the front side of the support substrate; and When viewed from the front side of the support substrate, the cutout is arranged so that a signal line of the planar circuit pattern having a width narrower than that of the cutout passes through the center of the width of the cutout, and the signal line and the metal around the cutout form a coplanar line. It is characterized by: That is, when viewed from above the support substrate, the coplanar line is composed of a signal line and top-surface ground metal on both sides of the signal line.

[0023] The high-frequency circuit board according to the present invention comprises: further comprising a photodiode, an optical input port, and a high frequency electrical output port; the photodiode is disposed at one end of the coplanar line such that its cathode is connected to the top surface ground and its anode is connected to the signal line; the high frequency electrical output port is a metal waveguide coupler formed on the signal line on the opposite side of the coplanar line from the photodiode; and The optical input port is a light introduction window provided in the rear surface ground. It is characterized by:

[0024] The above inventions can be combined as much as possible. [Effects of the Invention]

[0025] The present invention can provide a high-frequency circuit board that can obtain desired characteristics while suppressing interference between filters and the occurrence of unwanted substrate modes. The high-frequency circuit board according to the present invention can solve the problem of variations in characteristics that has been an issue with typical conventional THz module products, and can also solve the problem of diode-integrated THz chips, which have recently appeared but have issues with interference from filter circuits, making it possible to achieve high-performance THz transmission and reception functions at a low price. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a diagram illustrating the structure of a diode-integrated THz chip. [Figure 2] 1A and 1B are diagrams illustrating a high-frequency circuit board according to the present invention. [Figure 3] 10 is a diagram illustrating the relationship between the frequency fop of a signal and the width a of a support substrate. FIG. [Figure 4]1A and 1B are diagrams illustrating a high-frequency circuit board according to the present invention. [Figure 5] 1A and 1B are diagrams illustrating a high-frequency circuit board according to the present invention. [Figure 6] 1A and 1B are diagrams illustrating a high-frequency circuit board according to the present invention. [Figure 7] 1A and 1B are diagrams illustrating a high-frequency circuit board according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0028] (Embodiment 1) 2 is a diagram illustrating the high-frequency circuit board of this embodiment. This high-frequency circuit board includes a support substrate 101 on the surface of which a planar circuit pattern (such as a signal line 104, a waveguide coupler 106, and an electric signal terminal 107) of a metal film is formed, and a back surface ground is formed on the back surface. a rectangular waveguide portion 103 that covers a part of the support substrate 101 in a strip-like shape with metal that is electrically connected to the back surface ground; The present invention is characterized by comprising:

[0029] This high-frequency circuit board is a rectangular diode-integrated THz chip that includes semiconductor elements, microstrip lines (MSLs) and other planar circuit patterns, functional circuits, input / output ports that are at least one of high-frequency electrical input / output ports and optical input / output ports, and bias terminals for the semiconductor elements, and is configured on an insulating support substrate 101 with a first metal ground plane (backside ground) on part of its bottom surface. This high-frequency circuit board has a rectangular waveguide section 103 surrounded by a strip-shaped metal plane on part of the support substrate 101, and the top surface of the rectangular waveguide section 103 is a second metal ground plane (front surface ground 102).

[0030] This high-frequency circuit board is characterized in that a planar circuit pattern is also formed via a dielectric layer on the top-surface ground 102, which is the metal of the rectangular waveguide section 103 and is located on the front side of the support substrate 101 (this planar circuit pattern is not shown in FIG. 2). In this high-frequency circuit board, the above-mentioned semiconductor elements, signal lines and other planar circuit patterns, and part of the functional circuit are mounted on the top-surface ground 102. In other words, although not shown in FIG. 2, this high-frequency circuit board has matching circuits for the THz receiver, transmitter, etc. formed on the top-surface ground 102 to achieve stable operation of the diode-integrated THz chip. Note that the bias terminals of the semiconductor elements can be arranged on the front-surface ground 102 (the surface of the front-surface ground 102 opposite the back-surface ground).

[0031] Specifically, the support substrate 101 has an electrical signal line 104 based on a microstrip line (MSL), a waveguide coupler 106, and a rectangular waveguide portion 103 that is surrounded by a strip-shaped metal surface. Here, the top surface of the rectangular waveguide portion 103 is a top-surface ground 102. In conventional MSL-based high-frequency circuit boards, the ground (backside ground) exists only on the bottom surface of the support substrate. However, in this structure, by covering the support substrate 101 with a strip-shaped metal, a ground (surface ground 102) also exists on the top surface of the support substrate 101. The presence of the surface ground 102 increases the degree of freedom in circuit design, as will be described later. Here, "increasing the degree of freedom in circuit design" means increasing the degree of freedom in pattern layout (effective use of space) for filter circuits and the like, and increasing the design range for characteristic impedance.

[0032] Here, when the rectangular waveguide portion 103 is regarded as a TE wave waveguide, the width a of the support substrate 101 is the length of the long side of the opening of the TE wave waveguide, and is a value such that the low cutoff frequency fc of the TE wave waveguide is higher than the desired maximum operating frequency fmax.

[0033] The rectangular waveguide section 103 has the following characteristics. When the rectangular waveguide section 103 of the support substrate 101 is regarded as a kind of TE waveguide, its low-frequency cut-off frequency fc is determined by the wavelength λ in vacuum and the dielectric constant and width a of the support substrate 101. The width a of the support substrate 101 is designed such that the low-frequency cut-off frequency fc is higher than the maximum operating frequency fmax of this diode integrated THz chip (fc > fmax).

[0034] More specifically, the rectangular waveguide section 103 formed by covering the periphery of the support substrate 101 with a metal strip can be regarded as a kind of TE waveguide, and its low-frequency cut-off frequency fc exists. Here, it is assumed that a THz signal is incident from the side of the waveguide coupler 106. The signal propagates through the signal line 104 of the MSL and reaches the rectangular waveguide section 103. If the frequency fop of the signal satisfies the relationship fc < fop, the signal can propagate as a mode within the rectangular waveguide section 103 and pass through to the side of the electrical signal terminal 107. On the other hand, if the frequency fop of the signal satisfies the relationship fop < fc, the signal is reflected or propagates through the semiconductor elements, planar circuit patterns, and functional circuits formed within the rectangular waveguide section 103, contributing to the desired operation (receiving operation or transmitting operation) of the diode integrated THz chip.

[0035] Figure 3 is a diagram for explaining the relationship between fop and the width a of the support substrate 101. The support substrate 101 is SiC. In order to operate the diode integrated THz chip as desired, it is necessary to satisfy 2×a < λ(fc). λ(fc) is the wavelength within the support substrate 101 at the low-frequency cut-off frequency fc. Each plot point in Figure 3 corresponds to the maximum frequency on the wideband side of W, F, D, G, and J-band. For example, in the case of J-band, a = 140 μm. Although this substrate width is quite narrow, as long as it is not a complex circuit, there will be no obstacle preventing the arrangement of the planar circuit pattern by using the surface ground.

[0036] (Embodiment 2) Fig. 4 is a diagram illustrating the high-frequency circuit board of this embodiment. This high-frequency circuit board is a chip having the same functions and performance as the diode-integrated THz chip described in Fig. 1, formed using the structure of the present invention. In Fig. 4, the shaded area is the rectangular waveguide portion 103 covered with metal. The shaded area also serves as the surface ground 302. Fig. 5 is an enlarged view of the portion of the high-frequency circuit board in Fig. 4 indicated by the dashed line A.

[0037] A THz receiving diode 310, a waveguide coupler 303, and filter circuits 304 and 305 are mounted on a SiC support substrate 301. Here, filter circuit 304 is an IF ground RF open filter, and filter circuit 305 is an RF short IF filter. In this chip, waveguide coupler 303 is disposed in a waveguide port 307. From waveguide coupler 303, a main signal line 308 of the MSL is connected to IF output 306 via signal line 309 and THz receiving diode 310. In this embodiment, the filter circuits (304, 305) and signal line 309 are formed on surface ground 302 as a planar circuit pattern.

[0038] Here, we will explain the differences between the diode-integrated THz chip in Figure 1 and the high-frequency circuit board of this embodiment. Both are THz receiver integrated chips. Regarding the substrate, there is no difference between them, as they both use SiC as a support substrate. However, in the chip in Figure 1, all of the wiring and filter circuits are designed based on MSL. In this example, the thickness of the support substrate is constant at 32 μm, and the characteristic impedance is in the range of approximately Z0 = 25 (Ws = 95 μm) to 90 Ω (Ws = 5 μm). This narrow Z0 range is not an essential constraint. However, because the support substrate is not necessarily wide enough, the RF short IF filter 505 and the IF ground RF open dc open circuit 504 are close to each other in the chip in Figure 1, causing interference with each other.

[0039] On the other hand, in the chip of this embodiment, a top-surface ground 302 is placed on a support substrate 301, and on top of that are insulating layers (303 and 303B), and on top of that are filter circuits (304, 305), a THz receiving diode 310, and capacitors. Here, an MSL is formed between the top-surface ground 302 and the planar circuit pattern, sandwiching the insulating layers (303 and 303B). Because the thickness of this insulating layer is at most 2 to 3 μm, there is little leakage of the electric field from the MSL to the outside, and interference effects between planar circuit patterns are significantly reduced. In other words, since there is no need to separate circuit patterns, such as between filter circuits or between signal lines and filter circuits, space can be used more efficiently, resulting in advantageous effects for integration.

[0040] The surface ground 302 has another advantage. The thickness of the insulating layer can be varied depending on the characteristics of the planar circuit pattern to be arranged. For example, the thickness of the insulating layer 303 and the thickness of the insulating layer 303B can be varied.

[0041] Because the insulating layer of the MSL used in this chip can be made thin, it is easy to create a main signal line 309 with low characteristic impedance, which results in no need to install a multi-stage filter circuit (saving space). Furthermore, because there is a top-surface ground 302 on the support substrate 301, there is a great advantage in that a large-capacity capacitor (MIM capacitor) can be integrated using the top-surface ground 302.

[0042] 1, even though it is possible to create an RF open filter circuit 504 for the IF ground on the support substrate 501, there is no ground nearby, so it is necessary to connect to the ground with separate wiring, and this wiring will affect the IF characteristics. On the other hand, in the case of this chip, the capacitance of the MIM capacitor that can be formed is large enough to cover frequencies up to 1 GHz or less, for example, and degradation of the IF characteristics can be avoided even with normal wiring.

[0043] (Embodiment 3) FIG. 6 is a diagram illustrating the high-frequency circuit board of this embodiment. FIG. 7 is an enlarged view illustrating a region C of this high-frequency circuit board. This high-frequency circuit board has: The rectangular waveguide portion 403 has a rectangular cutout portion 410A in which metal is removed from the top ground 402 on the front side of the support substrate 401; and When viewed from the front side of the support substrate 401, the cutout 410A is arranged so that a signal line 410B of a planar circuit pattern having a width narrower than that of the cutout 410A passes through the center of the width of the cutout 410A, and the signal line 410B and the metal around the cutout 410A form a coplanar line 410. It is characterized by:

[0044] This high-frequency circuit board is further comprising a photodiode 408, an optical input port 405, and a high frequency electrical output port; The photodiode 408 is disposed at one end of the coplanar line 410 such that the cathode is connected to the top surface ground 402 and the anode is connected to the signal line 409 and the signal line 410B of the coplanar line 410; the high frequency electrical output port is a metal waveguide coupler 407 formed on the signal line 404 on the opposite side of the coplanar line 410 from the photodiode 408; and The optical input port is a light introduction window 405 provided on the rear surface ground. It is characterized by:

[0045] This high-frequency circuit board has a length L CPW The rectangular waveguide portion 403 of the support substrate 401 has a coplanar (CPW) line 410 having a length of 1 / 4 or less of the operating wavelength arranged thereon, a waveguide coupler 407 as a high-frequency electrical input / output port, and a small-diameter light entrance window having a diameter of approximately 1 / 25 or less of the wavelength of the operating frequency arranged on the bottom surface of the rectangular waveguide portion 403 of the support substrate 401 as an optical input / output port 405. Furthermore, this high-frequency circuit board includes an MIM capacitor 411 formed by an upper surface ground 402, a metal layer of a planar circuit pattern, and an insulating layer between them.

[0046] This high-frequency circuit board is a THz transmission integrated chip equipped with a THz photodiode 408. Figs. 6 and 7 are schematic drawings. In addition to the structure of the high-frequency circuit board in Figs. 4 and 5, this high-frequency circuit board is equipped with a light introduction window 405 opened in the ground on the underside of the support substrate 401 and a waveguide coupler 407 for the photodiode to operate. This high-frequency circuit board also has a bias terminal 412 for the photodiode 408 as an alternative to the IF outputs (306, 506) described in Figs. 2, 4 and 5.

[0047] The CPW line 410 is composed of a cutout portion 410A formed by removing a portion of the metal of the top-surface ground 402 and a signal line 410B formed on the support substrate 401. In other words, the CPW line 410 is a coplanar line in which the signal line 410B is sandwiched between the metal of the top-surface ground 402 by the cutout portion 410A. The length L of the CPW line 410 CPW Simulations have confirmed that if the length (length in the direction perpendicular to the width and thickness directions of the support substrate 401) is 1 / 4 or less of the operating wavelength λ, there is no effect on the rectangular waveguide portion 403 (TE waveguide portion). Since the TE waveguide portion has the property of reflecting incident waves rather than attenuating them, no waves can enter from the outside, and the entire portion functions as a ground.

[0048] The output characteristics of the photodiode 408 are optimized by a matching circuit (CPW line 410 and MIM capacitor 411) formed on the top surface ground 402. The photodiode 408 generates a driving current in response to an optical signal introduced through the light introducing window 405, and this current is finally propagated from the waveguide coupler 407 to the waveguide (not shown in FIG. 6) as a THz output. [Industrial Applicability]

[0049] Applications of THz waves have long been explored, but the high cost of THz transmitter / receivers has remained an unresolved issue, hindering industrial progress. By applying the high-frequency circuit board technology of the present invention to integrated THz chips, the characteristic variations that are a problem with typical conventional THz module products can be resolved. Furthermore, it also solves the problems of diode-integrated THz chips, which have recently emerged but have weaknesses in the placement of filter circuits. Therefore, the high-frequency circuit board of the present invention makes it possible to realize low-cost THz transmitter / receiver functions with stable performance. [Explanation of symbols]

[0050] 101: Support board (part of the backside is ground plane) 101a: Width of support substrate (a) 102: Surface ground 103: TE waveguide section 104: Electrical signal line (MSL) 105: MSL-GCPW conversion unit 106: Waveguide coupler 107: Electrical signal terminal 108: Waveguide 301: Support substrate (SiC) 302: Top ground 303: Waveguide coupler 303: First insulating layer insertion region 303B: Second insulating layer insertion region 304:IF ground RF open dc open circuit 305: RF short IF filter 306:IF output 307: Waveguide port (WR-3) 308: Signal line 309: Signal line 310: THz receiving diode 311: MSL-GCPW conversion unit (=108) 401: Optically transparent support substrate 402: Top ground 403: Rectangular waveguide section 404: Main signal line (MSL) 405: Light introduction window opened on the bottom ground 407: Waveguide coupler 408: Semiconductor element (photodiode) 409: Signal line 410: Coplanar Waveguide (CPW matching circuit) 410A: Hollowed out section 410B: Signal line 411: MIM capacitor 412: Bias terminal 501: Support substrate (SiC) 502: THz receiving diode 503: Waveguide coupler 504: IF ground RF open filter dc open circuit 505: RF short IF filter 506:IF output 507: Waveguide port (WR-3) 508: Circuit branch

Claims

1. a support substrate having a planar circuit pattern of a metal film formed on its surface and a rear ground on its rear surface; a rectangular waveguide portion covering a part of the support substrate with a band-like metal that is electrically connected to the back surface ground; A high-frequency circuit board comprising:

2. 2. The high-frequency circuit board according to claim 1, wherein, when the rectangular waveguide portion is regarded as a TE wave waveguide, a width a of the support substrate is a length of a long side of an opening of the TE wave waveguide, and is a value such that a low cutoff frequency fc of the TE wave waveguide is higher than a desired maximum operating frequency fmax.

3. 2. The high-frequency circuit board according to claim 1, wherein the planar circuit pattern is also formed on the metal of the rectangular waveguide portion, which is an upper surface ground on the front side of the support substrate, via a dielectric layer.

4. 4. The high frequency circuit board according to claim 3, wherein the thickness of the dielectric layer varies depending on the characteristics of the planar circuit pattern.

5. the rectangular waveguide portion has a rectangular cutout portion in which the metal is removed on an upper surface ground on the front side of the support substrate; and When viewed from the front side of the support substrate, the cutout is arranged so that a signal line of the planar circuit pattern having a width narrower than that of the cutout passes through the center of the width of the cutout, and the signal line and the metal around the cutout form a coplanar line.

4. The high frequency circuit board according to claim 3, wherein:

6. further comprising a photodiode, an optical input port, and a high frequency electrical output port; the photodiode is disposed at one end of the coplanar line such that its cathode is connected to the top surface ground and its anode is connected to the signal line; the high frequency electrical output port is a metal waveguide coupler formed on the signal line on the opposite side of the coplanar line from the photodiode; and The optical input port is a light introduction window provided in the rear surface ground.

6. The high frequency circuit board according to claim 5,

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