Infrared filtering device comprising ultra-wideband frequency selective absorbing structure

Abstract

A microwave filter arrangement (100), the filter arrangement (100) comprising a housing (110) formed at least partly from an electrically conductive material, the housing (110) defining an inner volume. The filter arrangement (100) further comprises a dielectric substrate (120), the dielectric substrate (120) comprising a first transmission line port (141) and a second transmission line port (142) and a transmission line extending between the first and second transmission line ports (141,142) on a first side of the dielectric substrate (120). The transmission line comprises at least a first microstrip grating (121), the microstrip grating (121) comprising a central microstrip and a plurality of microstrip stubs of a first length (L g ). The stubs are arranged extending outwards from the central microstrip and separated from each other by a first periodic interval (P g ) wherein the first length (L g ) and the first periodic interval (P g ) are determined by a pre-determined device operation frequency. The filter arrangement (100) also comprises an absorbing block (130) arranged parallel to the first side of the dielectric substrate (120) such that a gap (123) is formed between the first side of the dielectric substrate (120) and the absorbing block (130).

Description

[0001] TITLE

[0002] INFRARED FILTERING DEVICE COMPRISING ULTRA- WIDEBAND FREQUENCY

[0003] SELECTIVE ABSORBING STRUCTURE

[0004] TECHNICAL FIELD

[0005] The present disclosure relates to microwave filtering devices, and in particular to filters having low-pass performance.

[0006] BACKGROUND

[0007] Microwave filtering devices are used in electronic systems to pass or reject different signals based e.g. on frequency. Filters with a low-pass performance provide an unperturbed signal transmission at frequencies below some cut-off frequency, while suppressing the transmission at higher frequencies. Such high-frequency rejecting performance extends from the cut-off frequency up to some higher frequency range.

[0008] In certain applications, it is advantageous to have the high-frequency rejecting performance extend over a wide range of frequencies, such as up to the THz frequency range. Such applications are for example cryogenic infrared filtering blocks for quantum computing platforms, space microwave systems for radio instruments, radars, and other terrestrial and satellite communication systems.

[0009] There is therefore a need for low-pass microwave filtering devices that perform well in a wide frequency range.

[0010] SUMMARY

[0011] It is an object of the present disclosure to provide improved low-pass microwave filtering devices, which, i.a., are able to reject high-frequency signals in a frequency interval spread from the cut-off frequency up to and above 300 GHz.

[0012] This object is at least in part obtained by a microwave filter arrangement. The filter arrangement comprises a housing formed at least partly from an electrically conductive material. The housing defines an inner volume in which the other components of the filter arrangements are placed.

[0013] The filter arrangement further comprises a dielectric substrate. The dielectric substrate comprises a first transmission line port and a second transmission line port, and a transmission line extending between the first and second transmission line ports on a first side of the dielectric substrate.

[0014] The transmission line comprises at least a first microstrip grating. The microstrip grating comprises a central microstrip and a plurality of microstrip stubs of a first length. The stubs are arranged extending outwards from the central microstrip and separated from each other by a first periodic interval, wherein the first length and first periodic interval are determined by a pre-determined device operation frequency. The filter arrangement also comprises an absorbing block arranged parallel to the first side of the dielectric substrate such that a gap is formed between the first side of the dielectric substrate and the absorbing block.

[0015] The filter arrangement described above is able to reject high-frequency signals over a wide frequency range, which is an advantage.

[0016] Preferably, the transmission line is a microstrip transmission line formed on the first side of the dielectric substrate. A second side of the dielectric substrate opposite to the first side then comprises a microstrip transmission layer ground plane. The microstrip transmission layer ground plane is then at least partially in contact with an inner surface of the housing. This provides good electrical contact between the ground plane and the housing, which is an advantage.

[0017] According to one example, the housing comprises a cavity formed in an inner surface of the housing facing the second side of the dielectric substrate. Here, the dielectric substrate is arranged such that the microstrip grating is positioned on top of the cavity, and the transmission layer ground plane is etched to form a ground cut.

[0018] According to some aspects, the first periodic interval is determined by the following equation: where pcis a propagation constant of the central microstrip, fcis a cut-off frequency of the low-pass filter and c0is the speed of light in vacuum.

[0019] Furthermore, the first length can be determined as: where fminis a predefined frequency of the first attenuation maximum, is a propagation constant along the microstrip stub, and Wcis the width of the central microstrip.

[0020] As an example, the transmission line can comprise at least a second microstrip grating. The second microstrip grating comprises a plurality of microstrip stubs of a second length, the stubs being arranged extending outwards from the central microstrip and separated by a second periodic interval. The second length and the second periodic interval are 20-50 % smaller compared to the first length and the first periodic interval respectively. As an example, the second length and the second periodic interval may be 30 % smaller compared to the first length and periodic interval, or they may be 40 % smaller.

[0021] In this example, the filter arrangement will comprise two different microstrip gratings with different stub lengths and periodic intervals. However, the filter arrangement may also comprise three or more gratings. Advantageously, using multiple gratings makes it possible to achieve a high stopband signal attenuation in a wide frequency range.

[0022] There is also herein disclosed use of a filter arrangement as described above in a cryogenic infrared filtering block for a quantum computing platform.

[0023] Furthermore, there is herein disclosed a method for producing a low-pass microwave filter arrangement. The method comprises producing a dielectric substrate. The dielectric substrate comprises a first transmission line port and a second transmission line port and a transmission line extending between the first and second transmission line ports on a first side of the dielectric substrate. The transmission line comprises at least a first microstrip grating, the microstrip grating comprising a central microstrip and a plurality of microstrip stubs of a first length. The stubs being arranged extending outwards from the central microstrip and separated from each other by a first periodic interval (Pg), wherein the first length (Lfl) and the first periodic interval (Pg) are determined by a pre-determined device operation frequency.

[0024] The method further comprises arranging the dielectric substrate in a housing formed at least partly from an electrically conductive material, the housing defining an inner volume, and arranging an absorbing block parallel to the first side of the dielectric substrate such that a gap is formed between the first side of the dielectric substrate and the absorbing block. The method may also comprise affixing the dielectric substrate in the inner volume of the housing by soldering or by means of a conductive adhesive.

[0025] The methods disclosed herein are associated with the same advantages as discussed above in connection to the different apparatuses.

[0026] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present disclosure will now be described in more detail with reference to the appended drawings, where:

[0029] Figure 1 shows the 3-D view of the filtering device (the housing parts are shown in wireframe);

[0030] Figure 2A gives the side view of the longitudinal cross-section of the filtering device;

[0031] Figure 2B is a schematic drawing of a longitudinal cross-section of the filtering device;

[0032] Figure 3A depicts the top view of the filtering device (the housing lid is removed);

[0033] Figure 3B is a schematic drawing of a top view of the filtering device with the housing lid removed;

[0034] Figure 4 demonstrates the complex electric field magnitude distribution in the lateral cross-section of the grating structure at 1 GHz;

[0035] Figure 5 presents the complex electric field magnitude distribution in the lateral crosssection of the grating structure at 30 GHz; Figure 6 depicts the magnitudes of scattering parameters (S-parameters) for the filtering device having a 1 GHz cut-off frequency; and

[0036] Figure 7 is a flow chart illustrating methods.

[0037] DETAILED DESCRIPTION

[0038] Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

[0039] The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0040] The disclosed filtering device, as depicted in Figure 1 , is based on a dielectric substrate structure placed in a metal enclosure (housing), realizing the inner device volume shielded from the outer space, and a microwave-absorbing block interacting with the dielectric substrate through a gap, as shown in Fig. 2A and 2B, between them. With reference to Figures 2A and 2B, it should be noted that 2B is a simplified schematic where the size of some components, such as the gap 123 and the ground 122, are exaggerated.

[0041] The dielectric substrate can be a printed circuit board (PCB). The PCB can be made of any PCB material, including FR4 material as well as high-frequency PCB laminates, alumina, or other ceramic material. The dielectric substrate can also be a semiconductor substrate such as a silicon or GaAs substrate. The dielectric substrate may also comprise a foam layer, a dielectric polymer I plastic material, or any other suitable dielectric material that can be manufactured in the shape of a planar structure or a layer and that can support electrically conductive traces.

[0042] The gap 123 between the dielectric substrate and the absorbing block may be an air gap, or it may be partially or completely filled with a dielectric spacer. If the gap is at least partially occupied by a dielectric spacer, the spacer should have a permittivity lower than that of the dielectric substrate. The dielectric spacer can comprise a foam material such as Rohacell foam or polyurethane foam, a perforated slab such as a Teflon perforated slab, or any other suitable dielectric material. Here, Rohacell is a brand name referring to a polymethacrylimide-based structural foam. Similar structural foams from other brands may also be used. Likewise, Teflon is the brand name for a polytetrafluoroethylene compound. Other compounds with similar properties can also be used.

[0043] When the gap 123 is an air gap, the absorber block may be mounted on an inner surface of the housing opposite to the inner surface on which the dielectric substrate is mounted. The mounting may be performed using an adhesive or any other suitable means of attaching the absorber block. If the gap is at least partially occupied by a dielectric spacer, the absorber block may rest on the dielectric spacer. In this case, the absorber block can be attached to a surface of the dielectric spacer by means of an adhesive or other suitable means.

[0044] In other words, the dielectric substrate and the absorber block are separated by a spacer with dielectric properties such that the spacer has a lower permittivity compared to the dielectric substrate. The spacer may be an air gap.

[0045] A microwave signal is injected into the structure through one of the device ports. In the preferred device implementation, each port represents a microstrip transmission line (MTL), which is then interfaced with the conducting dielectric substrate topology, in the following referred to as a microstrip grating, between two ports implementing the main filtering function of the device. Both MTL and grating parts are located on the top dielectric substrate layer, while the dielectric substrate has two conducting layers, i.e. the top and bottom layers. The MTL is formed by creating a metal strip on the top dielectric substrate layer. The microstrip grating is composed of the central MTL having the width l / l / cand loaded with periodically (period Pg) allocated microstrip stubs having the length Lgand width l / l / g(Fig. 3). In general, several gratings with different grating periods Pg, and stub sizes (Lg,and IZI / g / ) can be cascaded forming a combined microstrip grating structure, as depicted in Fig.1 - Fig.3 for the exemplary filtering device with three different gratings. The bottom dielectric substrate layer accommodates the dielectric substrate ground that is in galvanic contact with the housing metal.

[0046] The housing generally can be composed of two interfacing parts: the housing base and the housing lid. The housing parts can be made of any conducting material such as copper, brass, aluminum, or metallized plastic. The dielectric substrate I PCB can be mounted on the housing base using soldering or gluing with a conductive adhesive.

[0047] The microwave-absorbing block preferably has the shape of a rectangular brick of height / 7a, length La, and width 1 / 14. The absorbing block can be mounted by gluing on the inner surface of the housing lid and placed right above the microstrip grating structure after the device assembly. Any magnetic or dielectric absorbing material can be used to implement the absorbing block, e.g., a carbonyl-iron-filled epoxy, a carbon- loaded foam or plastic, a copper-powder-filled epoxy, etc. Preferably, the absorbing block material should have a relative permittivity £a2-3 times higher than the dielectric substrate relative permittivity EPCB, and permeability pain the range of 1-3. Nevertheless, £acan also be similar or close to EPCB keeping the device functional, however with a reducing signal attenuation in the stopband. The width I / I4 is chosen to be approximately the width of the inner housing volume; the length Lais chosen to be of the same length as the full grating structure or slightly (5-10%) longer; the height Hahas a crucial impact on the device performance and is selected to provide a specific gap size / 7gapbetween the dielectric substrate and the absorbing block.

[0048] Optionally, an air-filled cavity can be allocated in the housing base symmetrically underneath the microstrip grating by removing a portion of metal from the housing base. The shape of the cavity is preferably rectangular with width I / I4av, depth / 7cav, and length Lcav. The cavity is used to increase the stopband signal attenuation by decoupling the electromagnetic field from the dielectric substrate substrate and sending it to the absorber material. To provide grating structure interaction with the cavity, a portion of the dielectric substrate bottom ground is removed to form the dielectric substrate ground cut, typically having the same transverse sizes (width and length) as the housing cavity. In the preferred case, / 7cavis below half of the dielectric substrate thickness, I / I4av is below half the dielectric substrate width, and Lcavis approximately equal to La.

[0049] The device can be interfaced with radio-frequency connectors, e.g., SMA, SMPM, SMP, 3.5 mm, 2.92mm, or 2.4 mm coaxial connectors, on each MTL port or can be directly integrated with other circuitry in the common housing and on a common dielectric substrate. The device operation principle is based on the frequency-dependent transformation of the electromagnetic field propagating through the microstrip grating. At lower frequencies, below the cut-off frequency fc, the electromagnetic signal is tightly bonded to a low-loss dielectric substrate, when most of the signal energy does not interact with the absorbing block. The electric field distribution for this situation is shown in Fig. 4 for the exemplary filtering device having fc= 1 GHz operating with a 1 GHz harmonic signal. In this case, £a= 18, pa= 3, EPCB = 3.5, / 7gap= 0.17 mm. The absorbing material is a magnetic-type absorber with a magnetic loss tangent of 0.8. The same structure operating at 30 GHz, i.e. in the stopband, is depicted in Fig. 5, where a large portion of the signal energy is captured by the absorbing block. This results in a very fast signal attenuation during the propagation through the microstrip grating structure. For this exemplary device, three grating periods, allocated in a mirrored arrangement, have been used. In general, when a multi-grating structure is used in such a mirrored arrangement, the gratings having smaller Pgand Lgshould be placed on the other grating structure regions (closer to the device input / output MTL). That provides better impedance-matching performance.

[0050] In other words, an example filtering device with cut-off frequency fc= 1 GHz can have the parameters £a= 18, pa= 3, EPCB = 3.5, / 7gap= 0.17 mm. The absorbing material is a magnetic-type absorber with a magnetic loss tangent of 0.8. The device may comprise three grating periods, allocated in a mirrored arrangement. The gratings having smaller Pgand Lgare placed closer to the ends of the device, while the grating with larger Pgand Lgis placed in the center. This improves the impedancematching performance. Below the cut-off frequency fc, the electromagnetic signal is tightly bonded to a low-loss dielectric substrate, when most of the signal energy does not interact with the absorbing block. In contrast, for a signal in the stopband, e.g. at 30 GHz, a large portion of the signal energy is captured by the absorbing block, resulting in a fast signal attenuation during the propagation through the microstrip grating structure.

[0051] Fig. 6 illustrates the scattering parameters (S-parameters) of the abovepresented exemplary filtering device. As seen the device has a very small signal attenuation < 0.5 dB (a high modulus of transmission coefficient S21) and great impedance matching (a low modulus of reflection coefficient S11) below 1 GHz. At the same time, for frequencies > 1 GHz, the attenuation starts gradually and monotonically increasing, while |Sn| is well below -10 dB level. The IS21I remains < minus 40 dB up to and above the 100 GHz frequency. This provides evidence for the ultra-wideband low-pass performance of the disclosed filtering device. Also, a device of this type can be classified as an absorbing low-pass filter since the stopband reflection coefficient is notably below -10 dB level.

[0052] The following design relations are used to synthesize the filtering device.

[0053] - The cut-off frequency fcis approximately defined by the largest grating period Pg max through the following relation: where / 3Cis the propagation constant of the central MTL of the grating structure, which depends on frequency; Co is the speed of light in vacuum. However, in practice fcis 20-30 % lower compared with the value found from (1), which is defined by the absorber material parameters and can be found through numerical electromagnetic simulations.

[0054] - The microstrip grating stub length Lgis defined based on the expected stopband attenuation maxima positions. The grating with the largest period should have the longest stub length Lg max which is defined by the predefined frequencymin of the first attenuation maximum as follows: where / 3gis the propagation constant of the along the microstrip stub of the largest grating.

[0055] If a multi-grating structure is used, any smaller grating structure should have a period and length selected as 20-50% smaller compared with the period and length of the previous larger grating structure.

[0056] - The width l / l / gof any grating is selected as < l / l / c / 2.

[0057] - The gap height / 7gapis selected in the range (0.1 - 0.5) of the substrate thickness, that is, between 10 % and 50 % of the substrate thickness. A lower gap provides a higher signal attenuation in the stopband.

[0058] - The width of the central grating MTL l / l / cis selected to provide a good impedance matching in the passband by equating an average characteristic impedance of the grating structure to the characteristic impedance of the port MTLs, the latter typically being 50 Ohm. This optimization is done numerically taking into account the absorbing block and the housing base cavity.

[0059] The proposed device is enclosed in a conducting (metal) housing, which may have a base with sidewalls and a top lid, interfacing with the base to form an inner air-filled volume. The housing contains a printed circuit board (dielectric substrate) having two transmission line ports (input / output). In the preferred implementation, the transmission line is the microstrip transmission line (MTL) formed on the top dielectric substrate layer, where the bottom layer is used for an MTL ground plane. Also, the bottom layer interfaces with the housing base to provide good electrical contact. In between the MTL ports, the dielectric substrate contains a specific conducting topology, also denoted below as the microstrip grating. The microstrip grating is formed of a central MTL with periodically allocated MTL stubs, where the stubs have a specific length and width depending on the device operation frequency. In general, several gratings with different grating periods and stub sizes can be cascaded to create a combined microstrip grating structure. The device also contains a microwaveabsorbing block mounted on the top housing lid and placed above the microstrip grating structure with a gap between the dielectric substrate and the absorbing block. The microwave absorbing block can be made of any absorbing material, e.g., a carbonyl-iron-filled epoxy or a carbon-loaded plastic, which is then shaped to a specific form by machining, casting, 3-D printing, or any other suitable method. The device may optionally have an air cavity formed in the housing base underneath the microstrip grating. In this case, the dielectric substrate ground plane is etched to form a ground cut, thus coupling to the housing cavity.

[0060] The filtering performance is obtained owing to differences in the electromagnetic field distribution at low, i.e. below a cut-off frequency fc, and high frequencies. In the former case, the electromagnetic field, injected into the grating structure from the MTL port, is effectively concentrated below the grating inside the dielectric substrate. This results in lossless signal propagation through the device. At the same time, at high frequencies, the electromagnetic field, perturbed by the grating, couples to the absorbing block through the gap. This way, while propagating through the device, the input electromagnetic signal experiences attenuation due to its power dissipation in the absorbing block. Eventually, signal transmission through the device at high frequency is suppressed. The transition between the device’s passband and stopband is determined by the material properties of both the substrate and the absorbing block, as well as by the topological parameters of the microstrip grating structure. When the combined microstrip grating structure, having 2 or more different grating periods, is used a very high stopband signal attenuation (above 40 dB) can be achieved in an ultra-wideband frequency bandwidth (potentially in an infinite bandwidth having no limit at high frequencies).

[0061] In other words, what is described herein with reference to Figure 1 is a microwave filter arrangement 100 comprising a housing 110 formed at least partly from an electrically conductive material. The housing 110 defines an inner volume.

[0062] Herein, an electrically conductive material is a material that has a high electric conductivity. A high electric conductivity could be an electric conductivity similar to that of a metal such as aluminum, iron, or copper, or it could be an electric conductivity of more than 100 (Qm)’1.

[0063] The filter arrangement 100 further comprises a dielectric substrate 120. The dielectric substrate 120 comprises a first transmission line port 141 and a second transmission line port 142, and a transmission line extending between the first and second transmission line ports 141 , 142 on a first side of the dielectric substrate 120.

[0064] The dielectric substrate may preferably be in the form of a layer or plate. A layer or plate is a planar structure extended in two dimensions and comparatively thin in the third dimension. That is, it has two sides, or faces, and is associated with a thickness in the third dimension. The thickness is much smaller than the dimension of the sides or faces so that the element has two surfaces opposite of one another that are larger than other surfaces of the element. The first side of the dielectric substrate 120, on which the transmission line is positioned, is then taken to be one of the large surfaces. A layer, plate, or planar element can be flat or approximately flat, e.g. as in an arcuate shape.

[0065] The dielectric substrate can be a printed circuit board (PCB). The PCB can be made of any PCB material, including FR4 material as well as high-frequency PCB laminates, alumina, or other ceramic material. The dielectric substrate can also be a semiconductor substrate such as a silicon or GaAs substrate. The dielectric substrate may also comprise a foam layer, a dielectric polymer I plastic material, or any other suitable dielectric material that can be manufactured in the shape of a planar structure, a plate, or a layer and that can support electrically conductive traces. The transmission line comprises at least a first microstrip grating 121. The microstrip grating 121 comprises a central microstrip and a plurality of microstrip stubs of a first length Lg. The stubs are arranged extending outwards from the central microstrip. A stub is separated from neighboring stubs by a first periodic interval Pg. The first length Lgand first periodic interval Pgare determined by a pre-determined device operation frequency.

[0066] Here, a device operation frequency may be a cut-off frequency of the device, i.e. , a frequency below which the device transmits an electromagnetic signal and above which the device filters out a signal. However, other device operation frequencies may also be used, such as a frequency corresponding to an attenuation maximum.

[0067] That the microstrip stubs are arranged extending outwards from the central microstrip is herein taken to mean that they extend away from the central microstrip rather than being parallel to it. The stubs may form an angle of at least 45° with the central microstrip, or preferably an angle of at least 80°. According to some preferred examples, the stubs extend at right angles to the central microstrip, or form an angle in the interval of 88°-92° with the central microstrip.

[0068] The filter arrangement 100 also comprises an absorbing block 130 arranged parallel to the first side of the dielectric substrate 120 such that a gap 123 is formed between the first side of the dielectric substrate 120 and the absorbing block 130.

[0069] The gap 123 between the dielectric substrate and the absorbing block 130 may be an air gap, or it may be partially or completely filled with a dielectric spacer. If the gap is at least partially occupied by a dielectric spacer, the spacer should have a permittivity lower than that of the dielectric substrate 120. The dielectric spacer can comprise a foam material such as Rohacell foam or polyurethane foam, a perforated slab such as a Teflon perforated slab, or any other suitable dielectric material.

[0070] When the gap 123 is an air gap, the absorber block 130 may be mounted on an inner surface of the housing 110 opposite to the inner surface on which the dielectric substrate 120 is mounted. The mounting may be performed using an adhesive or any other suitable means of attaching the absorber block. If the gap 123 is at least partially occupied by a dielectric spacer, the absorber block may rest on the dielectric spacer. In this case, the absorber block can be attached to a surface of the dielectric spacer by means of an adhesive or other suitable means. Preferably, the filter arrangement is a filter with a low-pass performance, i.e. a low- pass filter. Filters with a low-pass performance provide an unperturbed signal transmission at frequencies below some cut-off frequency, while suppressing the transmission of any other higher frequencies. Such high-frequency rejecting performance extends from the cut-off frequency up to some higher frequency range.

[0071] Preferably, the transmission line is a microstrip transmission line formed on the first side of the dielectric substrate 120. A second side of the dielectric substrate 120 opposite to the first side then comprises a microstrip transmission layer ground plane 122. This is illustrated in Figures 2A and 2B. In order to achieve good electrical contact between the ground plane and the housing, the microstrip transmission layer ground plane is at least partially in contact with an inner surface of the housing 110.

[0072] Figures 2A and 2B also show the housing 110 comprising an air cavity 124 formed in an inner surface of the housing 110 facing the second side of the dielectric substrate 120. The dielectric substrate 120 is arranged such that the microstrip grating 121 is positioned on top of the air cavity 124, and the transmission layer ground plane is etched to form a ground cut.

[0073] Here, that the microstrip grating 121 is positioned on top of the air cavity 124 is taken to mean that the microstrip grating 121 and the air cavity 124 occupy a similar position in the x and y directions shown in Figures 1-3, but that the microstrip grating 121 is positioned further along the z axis compared to the air cavity 124.

[0074] The first periodic interval may be given by: where pcis a propagation constant of the central microstrip, fcis a cut-off frequency of the low-pass filter and c0is the speed of light in vacuum.

[0075] The first length may be given by: where fminis a predefined frequency of the first attenuation pole, is a propagation constant along the microstrip stub, and Wcis the width of the central microstrip. The first periodic interval, the first length, and the width of the central microstrip are indicated in Figures 3A and 3B. According to some aspects, the transmission line comprises at least a second microstrip grating 121. The second microstrip grating comprises a plurality of microstrip stubs of a second length, the stubs being arranged extending outwards from the central microstrip and separated from each other I neighboring stubs by a second periodic interval. The second length and the second periodic interval are 20- 50 % smaller compared to the first length and the first periodic interval respectively. For example, the second length and second periodic interval may be 30 % smaller, or 40 % smaller.

[0076] There is herein disclosed use of a filter arrangement 100 according to any previous claim in a cryogenic infrared filtering block for a quantum computing platform.

[0077] There is furthermore herein disclosed a method for producing a low-pass microwave filter arrangement 100. The method comprises producing S1 a printed circuit board, the printed circuit board comprising a dielectric substrate 120. The dielectric substrate 120 comprises a first transmission line port 141 and a second transmission line port 142 and a transmission line extending between the first and second transmission line ports 141 , 142 on a first side of the dielectric substrate 120. The transmission line comprises at least a first microstrip grating 121 , the microstrip grating 121 comprising a central microstrip and a plurality of microstrip stubs of a first length Lg. The stubs are arranged extending outwards from the central microstrip and separated from each other by a first periodic interval Pg. The first length Lgand first periodic interval Pgare determined by a pre-determined device operation frequency.

[0078] The method also comprises arranging S2 the dielectric substrate 120 in a housing 110 formed at least partly from an electrically conductive material. The housing 110 defines an inner volume. Further, the method comprises arranging S3 an absorbing block 130 parallel to the first side of the dielectric substrate 120 such that a gap 123 is formed between the first side of the dielectric substrate 120 and the absorbing block 130.

[0079] According to an example, the method also comprises affixing the dielectric substrate 120 in the inner volume of the housing 110 by soldering or by use of a conductive adhesive.

Claims

CLAIMS1. A microwave filter arrangement (100), the filter arrangement (100) comprising a housing (110) formed at least partly from an electrically conductive material, the housing (110) defining an inner volume, the filter arrangement (100) further comprising a dielectric substrate (120), the dielectric substrate (120) comprising a first transmission line port (141) and a second transmission line port (142) and a transmission line extending between the first and second transmission line ports (141 , 142) on a first side of the dielectric substrate (120), the transmission line comprising at least a first microstrip grating (121), the microstrip grating (121) comprising a central microstrip and a plurality of microstrip stubs of a first length (Lfl), the stubs being arranged extending outwards from the central microstrip and separated from each other by a first periodic interval (Pg), wherein the first length (Lfl) and first periodic interval (Pg) are determined by a pre-determined device operation frequency, the filter arrangement (100) also comprising an absorbing block (130) arranged parallel to the first side of the dielectric substrate (120) such that a gap (123) is formed between the first side of the dielectric substrate (120) and the absorbing block (130).

2. The filter arrangement (100) according to claim 1 , wherein the transmission line is a microstrip transmission line formed on the first side of the dielectric substrate (120), and wherein a second side of the dielectric substrate (120) opposite to the first side comprises a microstrip transmission layer ground plane.

3. The filter arrangement (100) according to claim 2, wherein the microstrip transmission layer ground plane is at least partially in contact with an inner surface of the housing (110).

4. The filter arrangement (100) according to claim 2, wherein the housing (110) comprises a cavity (124) formed in an inner surface of the housing (110) facing the second side of the dielectric substrate (120), wherein the dielectric substrate (120) is arranged such that the microstrip grating (121) is positioned on top of the cavity (124), and wherein the transmission layer ground plane is etched to form a ground cut.

5. The filter arrangement (100) according to any previous claim, wherein the first periodic interval iswhere pcis a propagation constant of the central microstrip, fcis a cut-off frequency of the low-pass filter and c0is lightspeed in vacuum.

6. The filter arrangement (100) according to any previous claim, wherein the first length iswhere fminis a predefined frequency of a first attenuation maximum,is a propagation constant along the microstrip stub, and Wcis the width of the central microstrip.

7. The filter arrangement (100) according to any previous claim, wherein the transmission line comprises at least a second microstrip grating (121), the second microstrip grating comprising a plurality of microstrip stubs of a second length, the stubs being arranged extending outwards from the central microstrip and separated by a second periodic interval, wherein the second length and the second periodic interval are 20-50 % smaller compared to the first length and the first periodic interval respectively.

8. Use of a filter arrangement (100) according to any previous claim in a cryogenic infrared filtering block for a quantum computing platform.

9. Method for producing a low-pass microwave filter arrangement (100), the method comprising; producing (S1) a printed circuit board, the printed circuit board comprising a dielectric substrate (120), the dielectric substrate (120) comprising a first transmission line port (141) and a second transmission line port (142) and a transmission line extending between the first and second transmission line ports (141 , 142) on a first side of the dielectric substrate (120), the transmission line comprising at least a first microstrip grating (121), the microstrip grating (121) comprising a central microstrip and a plurality of microstrip stubs of a first length (Lfl), the stubs being arranged extending outwards from the central microstrip and separated from each other by a first periodic interval (Pg), wherein the first length (Lfl) and first periodic interval (Pg) are determined by a pre-determined device operation frequencyarranging (S2) the dielectric substrate (120) in a housing (110) formed at least partly from an electrically conductive material, the housing (110) defining an inner volume, and arranging (S3) an absorbing block (130) parallel to the first side of the dielectric substrate (120) such that a gap (123) is formed between the first side of the dielectric substrate (120) and the absorbing block (130).

10. The method according to claim 9, comprising affixing the dielectric substrate (120) in the inner volume of the housing (110) by soldering or by means of a conductive adhesive.

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