Variable Bandpass Filter

Abstract

To provide a variable bandpass filter having a millimeter / submillimeter band and easily manufactured and controlled while obtaining a sufficiently wide variable band width.SOLUTION: A variable bandpass filter 1 includes: a 90-degree hybrid coupler 10 including an input port 12, a branch port 14, a 0-degree output port 16 and a 90-degree output port 18; a first waveguide 21 connected to one of the input port 12 and the branch port 14; a second waveguide 22 connected to one of the 0-degree output port 16 and the 90-degree output port 18; a first wave absorber 31 connected to an end part on the side opposite to the 90-degree hybrid coupler 10 of the second waveguide 22; and a second wave absorber 32 connected to another of the 0 degree output port 16 or the 90-degree output port 18. Inner walls constituting the first and second waveguides 21 and 22 are movable walls movable so as to change the cutoff frequencies of the waveguides.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present disclosure relates to tunable bandpass filters. [Background technology]

[0002] The millimeter wave (wavelength 1-10 mm, frequency 30-300 GHz), submillimeter wave, and terahertz wave (wavelength 0.1 mm-1 mm, frequency 300 GHz-3 THz) electromagnetic wave regions lie between low-frequency (i.e., long-wavelength) "radio waves" that are treated as "waves" and high-frequency (i.e., short-wavelength) "infrared" that are treated as "particles." In other words, the millimeter wave and submillimeter wave bands represent a transitional region from electronics technology, which primarily handles radio waves, to photonics technology, which handles infrared. Because of this, oscillation, transmission, and detection are all difficult, leading to the band being referred to as "the final frontier of electromagnetic waves."

[0003] High-frequency radio waves such as those in the millimeter and submillimeter wave bands are expected to be used in future technologies such as high-capacity communications and safe sensing for the human body, and research into their practical use in a variety of fields, including environmental measurement, information and communications, medicine and biology, non-destructive testing, and security, is being conducted worldwide.

[0004] Under these circumstances, bandpass filters that can appropriately select the frequency band to be used are essential for making effective use of limited radio wave resources. Bandpass filters are essential devices not only for equipment used to transmit and receive radio waves, but also for various measuring instruments used in the development of millimeter-wave and submillimeter-wave band components. In particular, considering versatility and applicability to a variety of uses, it is desirable for bandpass filters to have a tunable (variable) passband. Hereafter, such bandpass filters with a tunable passband are referred to as "tunable bandpass filters."

[0005] Bandpass filters are widely used in the radio wave range at frequencies lower than the millimeter wave and submillimeter wave bands, and various types of frequency-tunable bandpass filters have also been put to practical use. However, unlike low-frequency radio waves that are transmitted using coaxial cables or planar circuits, the millimeter wave and submillimeter wave bands use a three-dimensional transmission path made of metal tubes called waveguides, and the technology required to realize the filters is significantly different. While fixed-frequency waveguide bandpass filters are already in practical use, there have been few reported examples of frequency-tunable bandpass filters. The following four prior art technologies have been reported as tunable bandpass filter technology in the millimeter wave band (here, >100 GHz) frequency range.

[0006] (Waveguide filter bank) A filter bank is a system in which the operating frequency band is divided into multiple bands and fixed-frequency band-pass filters for each band are switched using a switch (see, for example, Non-Patent Document 1). Non-Patent Document 1 discloses the results of prototypes using three frequency bands: 140-190 GHz, 185-260 GHz, and 255-315 GHz. The number of band-pass filters is seven, nine, and eight, respectively. The position of the movable filter section is controlled using an actuator. The passband width of each band-pass filter is fixed at approximately 10 GHz.

[0007] This method has the advantage of good frequency characteristics and a wide relative bandwidth of about 30% because the shape of each bandpass filter is optimized for each frequency. However, the passing frequencies are discrete and cannot be changed continuously. Also, it has some disadvantages, such as the importance of aligning the input and output waveguides with the movable filter section.

[0008] (Fabry-Perot resonator) This method uses two partially reflecting mirrors facing each other inside a waveguide to form a Fabry-Perot resonator, and an actuator is used to change the path length between the mirror surfaces, thereby changing the resonator length and changing the center frequency of the filter's passband (see, for example, Non-Patent Document 1). Patent Document 1 discloses the results of a prototype using two frequency bands, 70-90 GHz and 110-140 GHz. The passband widths in this case are 240-400 MHz and 350-600 MHz, respectively. The achieved fractional bandwidth is approximately 25%.

[0009] This method has drawbacks, such as difficulty in assembly and alignment because the resonator is composed of multiple parts, including moving parts, and the position repeatability of the actuator of the moving mirror, which is ±0.2 μm, causes an error of about ±30 MHz in the center frequency of the filter.

[0010] (piezoelectric thin film resonator) This method involves placing a pair of resonators in a waveguide via an evanescent cavity, and then using an actuator to slightly move (deform) the thin film on one side of the waveguide wall connecting them, thereby changing the center frequency of the passband (see, for example, Non-Patent Document 2).The experimental results in Non-Patent Document 2 disclose that the frequency can be varied within an extremely narrow range of about 10% of the relative bandwidth (approximately 102-112 GHz) relative to the center frequency of 107 GHz.

[0011] (spherical dielectric resonator) This filter method utilizes the phenomenon that when a spherical dielectric is placed on a planar circuit using microstrip lines and the distance between metal plates placed on either side of the dielectric is changed to change the resonant frequency of the circuit, the center frequency of the circuit's passband changes (see, for example, Non-Patent Document 3).The experimental results in Non-Patent Document 3 disclose that the frequency can be varied within an extremely narrow range of about 3% of the relative bandwidth (103.5-106.5 GHz) with respect to the center frequency of 107 GHz. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] Takashi Kawamura and Masanori Machidori, "Introduction of Tunable Filter Technology for the Realization of Millimeter-Wave and Terahertz-Wave Spectrum Analyzers," Anritsu Technical Journal, No. 94, pp. 37-43 (2018.12) [Non-patent document 2] T. Do, Yusha Bey and X. Liu, "A high-Q W band tunable bandpass filter," 2016 IEEE MTT-S International Microwave Symposium (IMS), 2016, pp. 1-4 (2016.8) [Non-patent document 3] U. Dey, JG Marin, and J. Hesselbarth, "Low-loss and tunable millimeterwave filters using spherical dielectric resonators", International Journal of Microwave and Wireless Technologies 13, pp. 751-755 (2020.11) Summary of the Invention [Problem to be solved by the invention]

[0013] As described above, tunable bandpass filters for the millimeter wave band and submillimeter wave band are still in the research and development stage. This disclosure has been made in light of this situation, and its purpose is to provide a tunable bandpass filter for the millimeter wave band and submillimeter wave band that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth. [Means for solving the problem]

[0014] In order to solve the above problems, a tunable bandpass filter according to one embodiment of the present invention comprises: a 90-degree hybrid coupler having an input port, a branch port, a 0-degree output port, and a 90-degree output port; a first waveguide connected to one of the input port or the branch port; a second waveguide connected to one of the 0-degree output port or the 90-degree output port; a first electromagnetic wave absorber connected to the end of the second waveguide opposite to the 90-degree hybrid coupler; and a second electromagnetic wave absorber connected to the other of the 0-degree output port or the 90-degree output port. The inner walls of the first and second waveguides are movable walls that are movable so as to change the cutoff frequencies of the waveguides.

[0015] In an embodiment, the tunable bandpass filter may have a third waveguide connected between the other of the 0-degree output port or the 90-degree output port and the second wave absorber, wherein the inner wall of the third waveguide is a movable wall that is movable so as to change the cutoff frequency of the waveguide.

[0016] In one embodiment, the first, second, and third waveguides may be rectangular waveguides having a rectangular cross section perpendicular to the direction of propagation of the electromagnetic waves through the waveguides, and the movable wall is an inner wall that forms a shorter side of the rectangular cross section.

[0017] In an embodiment, the first waveguide may be connected to an input port.

[0018] In an embodiment, the first waveguide may be connected to a branch port.

[0019] In one embodiment, the input port may receive a signal in the millimeter wave or sub-millimeter wave band.

[0020] Any combination of the above components, and conversion of the present disclosure into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present disclosure. [Effects of the Invention]

[0021] According to the present disclosure, it is possible to provide a tunable bandpass filter for the millimeter wave band and submillimeter wave band that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth. [Brief explanation of the drawings]

[0022] [Figure 1] This is a schematic diagram of a rectangular waveguide with a rectangular cross section. [Figure 2] FIG. 2 is a cross-sectional view of the rectangular waveguide of FIG. 1. [Figure 3] 1 is a graph showing frequency dependence of insertion loss and reflection loss of an electromagnetic wave propagating in a waveguide. [Figure 4] FIG. 2 is a schematic perspective view of a hollow portion of a rectangular waveguide. [Figure 5] 1A and 1B are a front view and a cross-sectional view of a variable high-pass filter; [Figure 6] 6 is a photograph showing the appearance of the variable high-pass filter in FIG. 5. [Figure 7] 10 is a graph showing the propagation intensity of electromagnetic waves in a cavity when the cutoff frequency is changed by moving the inner wall with a micrometer head. [Figure 8] FIG. 1 is a schematic diagram of a 90-degree hybrid coupler. [Figure 9] FIG. 1 is a further schematic diagram of a 90-degree hybrid coupler. [Figure 10] FIG. 1 is a schematic diagram of a variable bandpass filter according to a first embodiment. [Figure 11] FIG. 10 is a schematic diagram of a variable bandpass filter according to a second embodiment. [Figure 12] 10 is a photograph showing the appearance of a variable bandpass filter according to a second embodiment. [Figure 13] FIG. 10 is a schematic diagram of a variable bandpass filter according to a third embodiment. [Figure 14] 10A and 10B are diagrams illustrating the results of an experiment and a simulation of the variable bandpass filter according to the second embodiment. [Figure 15] FIG. 10 is a diagram showing another experimental result of the tunable bandpass filter according to the second embodiment. [Figure 16] FIG. 10 is a schematic perspective view of a rectangular waveguide of a tunable bandpass filter according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present disclosure will be described below based on preferred embodiments with reference to the drawings. In the embodiments and modified examples, identical or equivalent components and members are designated by the same reference numerals, and redundant explanations will be omitted where appropriate. The dimensions of the components in the drawings are enlarged or reduced as appropriate for ease of understanding. Some components that are not important for explaining the embodiments are omitted from the drawings. Terms including ordinal numbers such as "first" and "second" are used to describe various components, but these terms are used only to distinguish one component from another, and do not limit the components.

[0024] Before describing specific embodiments, the basic knowledge will be explained. [Waveguide] A waveguide is a structure used to transmit electromagnetic waves. It is typically a conductive (usually metal) tube with a rectangular (rectangular waveguide) or circular (circular waveguide) cross-section perpendicular to the direction of propagation of the electromagnetic waves through the waveguide. In other words, a waveguide is a tunnel-shaped transmission line surrounded by a conductive inner wall. Electromagnetic waves propagate through this waveguide while forming an electromagnetic field (this behavior is called a propagation mode) that depends on the shape, dimensions, wavelength (frequency), etc. of the tube. Figure 1 is a schematic diagram of a rectangular waveguide with a rectangular cross-section.

[0025] The lowest frequency of electromagnetic waves that can be transmitted through a waveguide is called the cutoff frequency. Figure 2 is a cross-sectional view of the rectangular waveguide shown in Figure 1. When the coordinate system is defined as shown in Figure 2, the cross-sectional shape of this rectangular waveguide is a rectangle with a width (length in the x direction) of a and a height (length in the y direction) of b. Electromagnetic waves propagating in the z direction within this waveguide are broadly divided into TE mode and TM mode. Each mode is further subdivided by the order n in the x direction and the order m in the y direction. The fundamental mode is TE 10The lowest frequency ν of an electromagnetic wave that can propagate in this waveguide in a mode is ν=c / 2a (c is the speed of light). Hereafter, this lowest frequency ν will be called the cutoff frequency. In other words, the cutoff frequency of a rectangular waveguide is defined by the length of the long side of the rectangular cross section of the rectangular waveguide (in this example, a).

[0026] Since electromagnetic waves with frequencies below ν cannot propagate through a waveguide, the waveguide functions as a high-pass filter. Figure 3 is a graph showing the frequency dependence of the insertion loss (signal strength that passes through to the output terminal when a signal is input to the input terminal) and reflection loss (signal strength that is reflected back to the input terminal when a signal is input to the input terminal) of electromagnetic waves propagating through a certain waveguide. Figure 3 shows that this waveguide functions as a high-pass filter with ν (cutoff frequency) = 77 GHz.

[0027] [Tunable high-pass filter using a waveguide] As mentioned above, the cutoff frequency of a rectangular waveguide is determined by the length of the long side of the rectangular cross section of the rectangular waveguide. Therefore, by making the length of this long side variable, it is thought that a high-pass filter with a variable cutoff frequency (variable high-pass filter) can be realized using a waveguide.

[0028] Figure 4 is a schematic perspective view of the hollow portion of a rectangular waveguide. However, this rectangular waveguide is not a simple rectangular parallelepiped; rather, for the purpose of achieving better impedance matching, the tube width (length in the x-axis direction) is stepped toward both ends (both ends in the z-axis direction). As shown in Figure 4, by applying a force in the x-axis direction to the inner wall (inner wall parallel to the yz plane) that constitutes the short side (side parallel to the y-axis) of the rectangular cross section of this waveguide, the inner wall can be moved in the x-axis direction, thereby changing the tube width (length in the x-axis direction).

[0029] FIG. 5 shows the variable high-pass filter 100. The left figure is a front view, and the right figure is a cross-sectional view. FIG. 6 is a photograph of the exterior of the variable high-pass filter 100 in FIG. 5. The variable high-pass filter 100 has a cavity 102 surrounded by a brass block 104. The cavity 102 functions as a waveguide. The cavity 102 has a rectangular cross section. One of the inner walls 106 that forms the short side of this rectangular cross section is connected to a micrometer head 108. When the micrometer head 108 pushes (or pulls) the inner wall 106, the inner wall 106 moves to narrow (or widen) the width of the cavity 102. This changes the cutoff frequency of the waveguide formed by the cavity 102.

[0030] 7 is a graph showing the propagation intensity of the electromagnetic wave in the cavity 102 when the cutoff frequency is changed by moving the inner wall 106 in 50 μm increments using the micrometer head 108. As shown in the figure, a high-pass filter with a variable bandwidth of 75-105 GHz is realized.

[0031] [90-degree hybrid coupler] Next, a 90-degree hybrid coupler (existing technology) will be described. Figures 8 and 9 are schematic diagrams of a typical 90-degree hybrid coupler 200. The 90-degree hybrid coupler 200 includes an input port 202, a branch port 204, a 0-degree output port 206, and a 90-degree output port 208. As shown in Figure 8, an electromagnetic wave input to the input port 202 is separated within the 90-degree hybrid coupler 200 and output from the 0-degree output port 206 and the 90-degree output port 208 (it is not output from the branch port 204). The electromagnetic wave output from the 0-degree output port 206 and the electromagnetic wave output from the 90-degree output port 208 have the same intensity but are out of phase with each other by 90 degrees. On the other hand, as shown in FIG. 9, the electromagnetic waves input to the 0-degree output port 206 and the 90-degree output port 208 are multiplexed within the 90-degree hybrid coupler 200 and output from the branch port 204 (not output from the input port 202).

[0032] The inventors have realized that a tunable bandpass filter can be realized by utilizing the above-mentioned tunable highpass filter and a 90-degree hybrid coupler. The proposed tunable bandpass filter will be described below along with the embodiments.

[0033] [First embodiment] 10 is a schematic diagram of a tunable bandpass filter 1 according to a first embodiment of the present disclosure. The tunable bandpass filter 1 includes a 90-degree hybrid coupler 10, a first waveguide 21, a second waveguide 22, a first wave absorber 31, and a second wave absorber 32.

[0034] The 90-degree hybrid coupler 10 includes an input port 12, a branch port 14, a 0-degree output port 16, and a 90-degree output port 18.

[0035] The first waveguide 21 is connected to the input port 12 of the 90-degree hybrid coupler 10. The second waveguide 22 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10. The first radio wave absorber 31 is connected to the end of the second waveguide 22 opposite to the 90-degree hybrid coupler 10. The second radio wave absorber 32 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10 (i.e., the output port to which the second waveguide 22 is not connected).

[0036] The inner walls of the first waveguide 21 and the second waveguide 22 are movable walls that can be moved to change the cutoff frequencies of the waveguides. For example, the first waveguide 21 and the second waveguide 22 may be realized as a variable high-pass filter of the type shown in Figure 5. However, without being limited thereto, the first waveguide 21 and the second waveguide 22 may be any suitable type of waveguide as long as the inner walls are movable walls that can be moved to change the cutoff frequencies of the waveguides.

[0037] 10, the first waveguide 21 is connected to the input port 12 of the 90-degree hybrid coupler 10. However, as will be described later, the first waveguide 21 may alternatively be connected to the branch port 14 of the 90-degree hybrid coupler 10.

[0038] 10, the second waveguide 22 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10, and the second wave absorber 32 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10. However, the opposite may also be true, where the second waveguide 22 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10, and the second wave absorber 32 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10.

[0039] Hereinafter, the cutoff frequency of the first waveguide 21 is designated as v1, and the cutoff frequency of the second waveguide 22 is designated as v2. As mentioned above, both cutoff frequencies v1 and v2 are variable. In the example described below, the first waveguide 21 and the second waveguide 22 are adjusted so that v1 < v2. When an electromagnetic wave enters the first waveguide 21 from the outside (from the left in FIG. 10), the first waveguide 21 blocks and reflects the electromagnetic wave if its frequency is less than v1, and transmits it if its frequency is v1 or greater. In other words, only electromagnetic waves with frequencies equal to or greater than v1 enter the input port 12 of the 90-degree hybrid coupler 10.

[0040] The electromagnetic wave separated within the 90-degree hybrid coupler 10 and output from the 0-degree output port 16 enters the second waveguide 22. The second waveguide 22 blocks and reflects the electromagnetic wave if its frequency is less than v2, and transmits it if its frequency is v2 or greater.

[0041] The electromagnetic wave with a frequency less than v2 is reflected by the second waveguide 22, then re-inputs into the 0-degree output port 16 of the 90-degree hybrid coupler 10, and is output from the branch port 14. As a result, the frequency of the electromagnetic wave output from the branch port 14 is greater than or equal to v1 and less than v2.

[0042] On the other hand, electromagnetic waves with frequencies equal to or greater than v1 are absorbed by the first wave absorber 31 after passing through the second waveguide 22. Therefore, electromagnetic waves with frequencies equal to or greater than v1 are not re-input into the 0-degree output port 16 of the 90-degree hybrid coupler 10.

[0043] The electromagnetic waves separated within the 90-degree hybrid coupler 10 and output from the 90-degree output port 18 are absorbed by the second wave absorber 32. Therefore, the electromagnetic waves output from the 90-degree output port 18 do not re-input into the 90-degree output port 18 of the 90-degree hybrid coupler 10.

[0044] From the above, the variable bandpass filter 1 operates as a bandpass filter that outputs only the electromagnetic waves input thereto whose frequencies are equal to or greater than v1 and less than v2. Because the cutoff frequencies v1 and v2 are both variable, the center frequency and bandwidth of the electromagnetic waves that are allowed to pass can be freely changed.

[0045] Furthermore, the variable bandpass filter 1 can be constructed using an existing 90-degree hybrid coupler, a simple variable highpass filter as shown in Figure 5, and an existing radio wave absorber, making it easy to manufacture and adjust.

[0046] As described above, according to this embodiment, it is possible to provide a tunable bandpass filter that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth.

[0047] [Second embodiment] 11 is a schematic diagram of a tunable bandpass filter 2 according to the first embodiment of the present disclosure. The tunable bandpass filter 1 includes a 90-degree hybrid coupler 10, a first waveguide 21, a second waveguide 22, a third waveguide 23, a first wave absorber 31, and a second wave absorber 32. That is, the tunable bandpass filter 2 additionally includes a third waveguide 23 in addition to the components of the tunable bandpass filter 1 in FIG. 10. The other configuration of the tunable bandpass filter 2 is the same as that of the tunable bandpass filter 1.

[0048] The first waveguide 21 is connected to the input port 12 of the 90-degree hybrid coupler 10. The second waveguide 22 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10. The third waveguide 23 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10. The first wave absorber 31 is connected to the end of the second waveguide 22 opposite to the 90-degree hybrid coupler 10. The second wave absorber 32 is connected to the end of the third waveguide 23 opposite to the 90-degree hybrid coupler 10.

[0049] The inner walls of first waveguide 21, second waveguide 22, and third waveguide 23 are movable walls that can be moved to change the cutoff frequencies of the waveguides. For example, first waveguide 21, second waveguide 22, and third waveguide 23 may be realized as a tunable high-pass filter of the type shown in Figure 5. However, without being limited thereto, first waveguide 21, second waveguide 22, and third waveguide 23 may be any suitable type of waveguide as long as the inner walls are movable walls that can be moved to change the cutoff frequencies of the waveguides.

[0050] 11, the first waveguide 21 is connected to the input port 12 of the 90-degree hybrid coupler 10. However, as will be described later, the first waveguide 21 may alternatively be connected to the branch port 14 of the 90-degree hybrid coupler 10.

[0051] 11, the second waveguide 22 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10, and the third waveguide 23 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10. However, the opposite may also be true, where the second waveguide 22 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10, and the third waveguide 23 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10.

[0052] Hereinafter, the cutoff frequency of the first waveguide 21 is defined as v1, the cutoff frequency of the second waveguide 22 as v2, and the cutoff frequency of the third waveguide 23 as v2, the same as the second waveguide 22. As mentioned above, the cutoff frequencies v1 and v2 are all variable. In the example described below, the first waveguide 21 and the second waveguide 22 are adjusted so that v1 < v2. When an electromagnetic wave enters the first waveguide 21 from the outside (from the left in FIG. 11), the first waveguide 21 blocks and reflects the electromagnetic wave if its frequency is less than v1, and transmits it if its frequency is v1 or greater. In other words, only electromagnetic waves with frequencies equal to or greater than v1 enter the input port 12 of the 90-degree hybrid coupler 10.

[0053] The electromagnetic wave separated within the 90-degree hybrid coupler 10 and output from the 0-degree output port 16 enters the second waveguide 22. The second waveguide 22 blocks and reflects the electromagnetic wave if its frequency is less than v2, and transmits it if its frequency is v2 or greater.

[0054] The electromagnetic wave with a frequency less than v2 is reflected by the second waveguide 22, then re-inputs into the 0-degree output port 16 of the 90-degree hybrid coupler 10, and is output from the branch port 14. As a result, the frequency of the electromagnetic wave output from the branch port 14 is greater than or equal to v1 and less than v2.

[0055] On the other hand, electromagnetic waves with frequencies equal to or greater than v1 are absorbed by the first wave absorber 31 after passing through the second waveguide 22. Therefore, electromagnetic waves with frequencies equal to or greater than v1 are not re-input into the 0-degree output port 16 of the 90-degree hybrid coupler 10.

[0056] The electromagnetic wave separated within the 90-degree hybrid coupler 10 and output from the 90-degree output port 18 enters the third waveguide 23. The third waveguide 23 blocks and reflects the electromagnetic wave if its frequency is less than v2, and transmits it if its frequency is v2 or greater.

[0057] The electromagnetic wave with a frequency less than v2 is reflected by the third waveguide 23, then re-inputs into the 90-degree output port 18 of the 90-degree hybrid coupler 10, and is output from the branch port 14. As a result, the frequency of the electromagnetic wave output from the branch port 14 is greater than or equal to v1 and less than v2.

[0058] On the other hand, electromagnetic waves with frequencies equal to or greater than v2 are absorbed by the second wave absorber 32 after passing through the third waveguide 23. Therefore, electromagnetic waves with frequencies equal to or greater than v1 are not re-input into the 0-degree output port 16 of the 90-degree hybrid coupler 10.

[0059] In this way, an electromagnetic wave with a frequency equal to or greater than v1 and less than v2 (the electromagnetic wave reflected by the second waveguide 22 and the electromagnetic wave reflected by the third waveguide 23 combined) is output from the branch port 14 of the 90-degree hybrid coupler 10. In other words, the tunable bandpass filter 2 operates as a bandpass filter that outputs only the electromagnetic waves with a frequency equal to or greater than v1 and less than v2 out of the input electromagnetic waves. Because the cutoff frequencies v1 and v2 are both tunable, the center frequency and bandwidth of the transmitted electromagnetic wave can be freely changed.

[0060] 10, the electromagnetic waves branched within the 90-degree hybrid coupler and output from the 90-degree output port 18 are all absorbed by the second wave absorber 32. Therefore, the electromagnetic waves output from the 90-degree output port 18 do not return to the branch port 14, even if they have a frequency below v2. This results in a loss in the filter. On the other hand, in the variable bandpass filter 2, even the electromagnetic waves output from the 90-degree output port 18 with frequencies below v2 are reflected by the third waveguide 23 and return to the branch port 14. In other words, the efficiency of the variable bandpass filter 2 is improved by about two times compared to the variable bandpass filter 1.

[0061] As described above, according to this embodiment, the efficiency of the variable bandpass filter can be improved.

[0062] FIG. 12 shows a photograph of the appearance of a variable bandpass filter 2 configured using the variable highpass filter of the type shown in FIG.

[0063] [Third embodiment] 13 is a schematic diagram of a tunable bandpass filter 3 according to a third embodiment of the present disclosure. The tunable bandpass filter 3 includes a 90-degree hybrid coupler 10, a first waveguide 21, a second waveguide 22, a first wave absorber 31, and a second wave absorber 32.

[0064] The 90-degree hybrid coupler 10 includes an input port 12, a branch port 14, a 0-degree output port 16, and a 90-degree output port 18.

[0065] The first waveguide 21 is connected to the branch port 14 of the 90-degree hybrid coupler 10. The second waveguide 22 is connected to the 0-degree output port 16 of the 90-degree hybrid coupler 10. The first radio wave absorber 31 is connected to the end of the second waveguide 22 opposite to the 90-degree hybrid coupler 10. The second radio wave absorber 32 is connected to the 90-degree output port 18 of the 90-degree hybrid coupler 10 (i.e., the output port to which the second waveguide 22 is not connected).

[0066] The inner walls of the first waveguide 21 and the second waveguide 22 are movable walls that can be moved to change the cutoff frequencies of the waveguides. For example, the first waveguide 21 and the second waveguide 22 may be realized as a variable high-pass filter of the type shown in Figure 5. However, without being limited thereto, the first waveguide 21 and the second waveguide 22 may be any suitable type of waveguide as long as the inner walls are movable walls that can be moved to change the cutoff frequencies of the waveguides.

[0067] Hereinafter, the cutoff frequency of the first waveguide 21 is designated as v11, and the cutoff frequency of the second waveguide 22 is designated as v12. As mentioned above, both cutoff frequencies v11 and v12 are variable. In the example described below, the first waveguide 21 and the second waveguide 22 are adjusted so that v11 < v12. An electromagnetic wave from the outside (from the left in FIG. 13) is input to the input port 11 of the 90-degree hybrid coupler 10. The electromagnetic wave input to the input port 11 is separated within the 90-degree hybrid coupler 10.

[0068] The electromagnetic wave separated within the 90-degree hybrid coupler 10 and output from the 0-degree output port 16 enters the second waveguide 22. The second waveguide 22 blocks and reflects the electromagnetic wave if its frequency is less than v12, and transmits it if its frequency is v12 or greater.

[0069] The electromagnetic wave with a frequency less than v12 is reflected by the second waveguide 22, then re-inputs into the 0-degree output port 16 of the 90-degree hybrid coupler 10, and is output from the branch port 14. At this time, the frequency of the electromagnetic wave output from the branch port 14 is less than v12.

[0070] The electromagnetic waves separated within the 90-degree hybrid coupler 10 and output from the 90-degree output port 18 are absorbed by the second wave absorber 32. Therefore, the electromagnetic waves output from the 90-degree output port 18 do not re-input into the 90-degree output port 18 of the 90-degree hybrid coupler 10.

[0071] Electromagnetic waves with frequencies less than v12 output from branch port 14 are input to first waveguide 21. First waveguide 21 blocks the electromagnetic waves if their frequencies are less than v11, and transmits them if their frequencies are v11 or greater. Therefore, first waveguide 21 outputs only electromagnetic waves with frequencies greater than or equal to v11 and less than v12 to the outside.

[0072] From the above, the variable bandpass filter 3 operates as a bandpass filter that outputs only the electromagnetic waves input thereto whose frequencies are equal to or greater than v11 and less than v12. Because the cutoff frequencies v11 and v12 are both variable, the center frequency and bandwidth of the transmitted electromagnetic waves can be freely changed.

[0073] Furthermore, the variable bandpass filter 3 can be constructed using an existing 90-degree hybrid coupler, a simple variable highpass filter as shown in FIG. 5, and an existing radio wave absorber, making it easy to manufacture and adjust.

[0074] As described above, according to this embodiment, it is possible to provide a tunable bandpass filter that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth.

[0075] In all of the embodiments described above, the waveguide of the component may be a rectangular waveguide having a rectangular cross section perpendicular to the direction in which the electromagnetic wave propagates through the waveguide, and the movable wall is an inner wall that forms the short side of the rectangular cross section.

[0076] By determining the specific configuration of the waveguide in this way, the manufacturing of the tunable bandpass filter can be made easier.

[0077] [Verification experiment] The inventors conducted experiments and simulations to verify the performance of the tunable bandpass filter of the present disclosure. The tunable bandpass filter of the second embodiment described above was used. The results are shown in FIG. 14. The solid line represents the measured values, and the dotted line represents the analysis results from the electromagnetic field simulation. It can be seen that although the measured values ​​show a slight decrease in efficiency and waveform distortion, the results obtained are almost equivalent to those of the simulation. These experiments verified that the tunable bandpass filter of the embodiment can tune its center frequency in the range of 75 to 105 GHz (i.e., its tuning bandwidth is 30 GHz).

[0078] Figure 15 shows the results of another experiment using the variable bandpass filter of the embodiment. In this experiment, a narrower passband width than the experiment in Figure 14 was achieved by adjusting the cutoff frequency of the waveguide. In this way, the variable bandpass filter of the embodiment can freely change not only the center frequency of the electromagnetic wave to be passed, but also the bandwidth. These experiments verified that the variable bandpass filter of the embodiment can achieve a passband width of 2.3 to 25.4 GHz for millimeter waves with a center frequency of 90 GHz, for example.

[0079] [Each aspect of the present disclosure] Each aspect of the present disclosure is summarized below. A tunable bandpass filter according to one aspect of the present disclosure comprises a 90-degree hybrid coupler having an input port, a branch port, a 0-degree output port, and a 90-degree output port, a first waveguide connected to one of the input port or the branch port, a second waveguide connected to one of the 0-degree output port or the 90-degree output port, a first electromagnetic wave absorber connected to the end of the second waveguide opposite the 90-degree hybrid coupler, and a second electromagnetic wave absorber connected to the other of the 0-degree output port or the 90-degree output port. The inner walls of the first and second waveguides are movable walls that can move to change the cutoff frequencies of the waveguides.

[0080] According to this aspect, it is possible to provide a tunable bandpass filter that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth.

[0081] In one aspect, the configurable bandpass filter has a third waveguide connected between the other of the 0-degree output port or the 90-degree output port and the second wave absorber, and the inner wall of the third waveguide is a movable wall that is movable so as to change the cutoff frequency of the waveguide.

[0082] According to this aspect, the efficiency of the variable bandpass filter can be improved.

[0083] In one embodiment, the first, second, and third waveguides may be rectangular waveguides having a rectangular cross section perpendicular to the direction in which the electromagnetic waves propagate through the waveguides, and the movable wall is an inner wall that forms a short side of the rectangular cross section.

[0084] According to this aspect, the specific configuration of the waveguide can be determined, making it easier to manufacture the tunable bandpass filter.

[0085] In one embodiment, the first waveguide is connected to the input port.

[0086] According to this aspect, it is possible to provide a tunable bandpass filter that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth.

[0087] In one embodiment, the first waveguide is connected to a branch port.

[0088] According to this aspect, it is possible to provide a tunable bandpass filter that is easy to manufacture and adjust while ensuring a sufficiently wide tunable bandwidth.

[0089] In one embodiment, a signal in the millimeter wave band or sub-millimeter wave band is input to the input port.

[0090] According to this aspect, it is possible to provide a variable bandpass filter for signals in the millimeter wave band or submillimeter wave band.

[0091] The present disclosure has been described above based on several embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications and changes are possible within the scope of the claims of the present disclosure, and that such modifications and changes also fall within the scope of the claims of the present disclosure. Therefore, the descriptions and drawings in this specification should be treated as illustrative rather than restrictive.

[0092] [Variation 1] In the above embodiment, the waveguide of the component is a rectangular waveguide having a rectangular cross section perpendicular to the direction of propagation of the electromagnetic wave through the waveguide. However, the waveguide is not limited to this and may be any suitable waveguide as long as its inner wall is a movable wall that can be moved to change the cutoff frequency of the waveguide. For example, the waveguide of the component may be a circular waveguide.

[0093] This modification allows for greater freedom in configuration.

[0094] [Variation 2] In the rectangular waveguide of Figure 4, the width of the tube is stepped toward both ends only on the long sides of the rectangular cross section. However, this is not limited to this, and steps may also be provided on the short sides. Figure 16 shows a schematic diagram of a rectangular waveguide with steps on both the long and short sides.

[0095] According to this modification, the provision of steps on both the long and short sides further improves impedance matching, and further improvement in the reflection characteristics of the tunable bandpass filter can be expected.

[0096] [Variation 3] 4 and 16, the pipe width changes in a step-like manner. However, this is not limiting, and the pipe width may change in a tapered manner, for example.

[0097] This modification allows for greater freedom in configuration.

[0098] Any combination of the above-described embodiments and modifications is also useful as an embodiment of the present disclosure. A new embodiment resulting from the combination has the combined effects of the respective embodiments and modifications.

[0099] When understanding the abstract technical ideas of the embodiments and modifications, the technical ideas should not be interpreted as being limited to the contents of the embodiments and modifications. The above-described embodiments and modifications are merely illustrative examples, and many design modifications, such as changes, additions, and deletions of components, are possible. In the embodiments, the contents in which such design modifications are possible are emphasized by adding the notation "embodiment." However, it goes without saying that design modifications are also permitted even in contents not so notated. [Industrial Applicability]

[0100] The sample observation stage and sample measurement device disclosed herein can be used in industrial fields such as millimeter-wave radio telescopes, atmospheric observation devices, environmental measurement devices, information and communications, medicine and biology, non-destructive testing, and security. [Explanation of symbols]

[0101] 1··Tunable bandpass filter, 2. Variable bandpass filter, 3. Variable bandpass filter, 10··90 degree hybrid coupler, 12 input ports, 14··Branch port, 16··0 degree output port, 18··90 degree output port, 21··First waveguide, 22··Second waveguide, 23··Third waveguide, 31··First wave absorber, 33··Second wave absorber, 100·· variable high-pass filter, 102·Cavity, 104··block, 106...inner wall, 108··micrometer head, 200··90 degree hybrid coupler, 202··input port, 204··Branch port, 206··0 degree output port, 208··90 degree output port.

Claims

1. a 90-degree hybrid coupler having an input port, a branch port, a 0-degree output port, and a 90-degree output port; a first waveguide connected to one of the input port and the branch port; a second waveguide connected to one of the 0 degree output port or the 90 degree output port; a first wave absorber connected to an end of the second waveguide opposite to the 90-degree hybrid coupler; a second wave absorber connected to the other of the 0-degree output port and the 90-degree output port; Equipped with the first waveguide and the second waveguide are rectangular waveguides having a rectangular cross section in a direction perpendicular to a direction in which an electromagnetic wave propagates within the waveguide, a tunable bandpass filter, characterized in that inner walls forming the short sides of the rectangular cross sections of the first waveguide and the second waveguide are movable walls that are movable so as to change the cutoff frequencies of the waveguides by varying the lengths of the long sides of the rectangular cross sections.

2. a third waveguide is connected between the other of the 0-degree output port or the 90-degree output port and the second wave absorber, the third waveguide is a rectangular waveguide having a rectangular cross section in a direction perpendicular to a direction in which an electromagnetic wave propagates within the waveguide, 2. The tunable bandpass filter according to claim 1, wherein an inner wall constituting a short side of the rectangular cross section of the third waveguide is a movable wall that is movable so as to change the cutoff frequency of the waveguide by varying the length of the long side of the rectangular cross section.

3. 3. The tunable bandpass filter according to claim 1, wherein the first waveguide is connected to the input port.

4. 3. The tunable bandpass filter according to claim 1, wherein the first waveguide is connected to the branch port.

5. 3. The tunable bandpass filter according to claim 1, wherein a signal in the millimeter wave band or submillimeter wave band is input to the input port.

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