A multilayer board stripline filter

Abstract

The application belongs to the technical field of wireless communication, and particularly relates to a multilayer strip-line filter. The multilayer strip-line filter comprises a first core plate, a first adhesive layer, a second core plate, a second adhesive layer, a third core plate, a third adhesive layer and a fourth core plate which are sequentially stacked; the first core plate comprises a first metal layer and a second metal layer located on opposite sides, the second core plate comprises a third metal layer and a fourth metal layer located on opposite sides, the third core plate comprises a fifth metal layer and a sixth metal layer located on opposite sides, and the fourth core plate comprises a seventh metal layer and an eighth metal layer located on opposite sides; at least two of the second metal layer, the third metal layer, the fourth metal layer, the fifth metal layer, the sixth metal layer and the seventh metal layer form a first planar strip-line filter, a second planar strip-line filter and a third planar strip-line filter in parallel. In the above manner, the performance during use can be enhanced, and the reliability and stability during use can be improved.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, and specifically relates to a multilayer stripline filter. Background Technology

[0002] Microwave passive filters are indispensable in communication systems due to their crucial roles in frequency selection and noise suppression. They are key components of aircraft payloads and account for the majority of the volume and weight of communication system modules. Therefore, miniaturized, highly integrated, high-performance, and highly reliable filters are essential for improving the performance and payload of aircraft communication systems.

[0003] Multilayer printed circuit boards (MPCBs) are suitable for applications such as multilayer line filters due to their mature technology, high design flexibility, small size, low cost, and high integration. However, since radio frequency signals often need to be transmitted between layers through metallized vias, improper design, repeated soldering during substrate processing or application, and non-standard soldering can all cause the vias to break, posing a threat to the product's performance stability and potentially leading to the failure of aircraft communication systems, resulting in significant economic losses.

[0004] Therefore, enhancing the reliability and stability of multilayer board filter structures has gradually become a challenge and key technology in the design of multilayer board filters. Summary of the Invention

[0005] To address the aforementioned problems, the purpose of this invention is to provide a multilayer stripline filter that enhances performance, reliability, and stability during use.

[0006] To achieve the above objectives, the technical solution adopted by the present invention includes: A multilayer stripline filter includes a first core board, a first adhesive layer, a second core board, a second adhesive layer, a third core board, a third adhesive layer, and a fourth core board stacked sequentially. The first core board includes a first metal layer and a second metal layer located on opposite sides of its thickness direction. The second core board includes a third metal layer and a fourth metal layer located on opposite sides of its thickness direction. The third core board includes a fifth metal layer and a sixth metal layer located on opposite sides of its thickness direction. The fourth core board includes a seventh metal layer and an eighth metal layer located on opposite sides of its thickness direction. Three of the second, third, fourth, fifth, sixth, and seventh metal layers respectively form a first planar stripline filter, a second planar stripline filter, and a third planar stripline filter, and these filters are connected in parallel. The first, second, and third planar stripline filters each include a first resonator, a second resonator, a third resonator, a fourth resonator, a fifth resonator, and a sixth resonator. The first resonator and the second resonator, the second resonator and the third resonator, the fourth resonator and the fifth resonator, and the fifth resonator and the sixth resonator all adopt an interdigital structure. The third resonator and the fourth resonator adopt a parallel coupled-line structure and are equipped with interconnecting branches. The first resonators of different layers are connected in parallel, the second resonators of different layers are connected in parallel, the third resonators of different layers are connected in parallel, the fourth resonators of different layers are connected in parallel, the fifth resonators of different layers are connected in parallel, and the sixth resonators of different layers are connected in parallel. A first coupling bridge is introduced between the second resonator and the fifth resonator of the second planar stripline filter. The first coupling bridge is located at the open end of the second resonator and the fifth resonator of the second planar stripline filter. The first coupling bridge and the open end of the second resonator and the fifth resonator of the first planar stripline filter and the third planar stripline filter form a parallel plate capacitor. A second coupling bridge is introduced between the first resonator and the third resonator of the second planar stripline filter. The second coupling bridge is located at the open end of the first resonator and the third resonator of the second planar stripline filter, and the second coupling bridge and the open end of the first resonator and the third resonator of the third planar stripline filter form a parallel plate capacitor. A third coupling bridge is introduced between the fourth and sixth resonators of the second planar stripline filter. The third coupling bridge is located at the open-circuit terminals of the fourth and sixth resonators of the second planar stripline filter, and the third coupling bridge and the open-circuit terminals of the fourth and sixth resonators of the third planar stripline filter form a parallel plate capacitor.

[0007] Preferably, the first metal layer and the eighth metal layer are located on opposite sides of the thickness direction of the multilayer stripline filter, with the first metal layer serving as a ground top plate and the eighth metal layer serving as a ground bottom plate; the second planar stripline filter is located between the first planar stripline filter and the third planar stripline filter, and the first planar stripline filter, the second planar stripline filter, and the third planar stripline filter are respectively connected to the first metal layer and the eighth metal layer.

[0008] Preferably, the first planar stripline filter, the second planar stripline filter, and the third planar stripline filter are multi-order stripline filters.

[0009] Preferably, the first planar stripline filter, the second planar stripline filter, and the third planar stripline filter are all stripline filters; wherein, the first planar stripline filter, the second planar stripline filter, and the third planar stripline filter all include a quarter-wavelength resonator with one end open and the other end short-circuited.

[0010] Preferably, an input port and an output port are respectively provided on opposite sides of the eighth metal layer; a first Z-shaped feed line is mounted on the first resonator of each layer, one end of the first Z-shaped feed line is mounted near the short-circuit end of the first resonator, and the other end of the first Z-shaped feed line corresponds to the input port; a second Z-shaped feed line is mounted on the sixth resonator of each layer, one end of the second Z-shaped feed line is mounted near the short-circuit end of the sixth resonator, and the other end of the second Z-shaped feed line corresponds to the output port; the other ends of the first Z-shaped feed line are respectively connected to the input port, and the other ends of the second Z-shaped feed line are respectively connected to the output port.

[0011] Preferably, at least one through slot is formed on the first metal layer, and the extending direction of the through slot is perpendicular to the line connecting the input port and the output port.

[0012] Preferably, the open-circuit terminals of the first resonators, the second resonators, the third resonators, the fourth resonators, the fifth resonators, and the sixth resonators in each layer are respectively connected by a first conductor; the short-circuit terminals of the first resonators, the second resonators, the third resonators, the fourth resonators, the fifth resonators, and the sixth resonators in each layer are respectively connected by a second conductor, and the second conductor is respectively connected to the first metal layer and the second metal layer; the first Z-shaped feed lines and the second Z-shaped feed lines in each layer are respectively connected by a third conductor.

[0013] Preferably, a fourth conductor is provided between the first Z-shaped feed lines of each layer and between the second Z-shaped feed lines of each layer. The first conductor, the second conductor, the third conductor, and the fourth conductor include at least one of metallized through holes, metallized blind holes, and metallized semi-circular holes.

[0014] Compared with the prior art, the advantages of the present invention are: The multilayer stripline filter of this invention, by employing a multilayer parallel stripline filter structure, significantly improves the unloaded quality factor of the filter compared to a single-layer stripline filter, thereby enhancing the filter's performance. Furthermore, after forming a first planar stripline filter, a second planar stripline filter, and a third planar stripline filter respectively in at least two of the second, third, fourth, fifth, sixth, and seventh metal layers, and by connecting these filters in parallel, the impact of a single or a small number of broken vias at the connection points on the performance of the multilayer stripline filter can be ignored in practical applications, thus enhancing the reliability and stability of the multilayer stripline filter's performance. Attached Figure Description

[0015] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof. In the drawings: Figure 1 A side view of a multilayer stripline filter provided in an embodiment of the present invention; Figure 2 Oblique axonometric view of the internal metal structure of a multilayer stripline filter provided for an embodiment of the invention; Figure 3 An oblique axonometric view of the multi-layer parallel stripline within a multi-layer plate stripline filter provided in an embodiment of the present invention; Figure 4 A top view of a multilayer stripline filter provided in an embodiment of the present invention; Figure 5 The simulation response curve of the multilayer stripline filter provided in the embodiment of the present invention.

[0016] The labels in the diagram represent: Sub1, First core board; PP1, First adhesive layer; Sub2, Second core board; PP2, Second adhesive layer; Sub3, Third core board; PP3, Third adhesive layer; Sub4, Fourth core board; L1, First metal layer; L2, Second metal layer; L3, Third metal layer; L4, Fourth metal layer; L5, Fifth metal layer; L6, Sixth metal layer; L7, Seventh metal layer; L8, Eighth metal layer; F1, First planar stripline filter; F2, Second planar stripline filter; F3, Third planar stripline filter; GND1, Grounding pin Board; GND2, Grounding base plate; R1, First resonator; R2, Second resonator; R3, Third resonator; R4, Fourth resonator; R5, Fifth resonator; R6, Sixth resonator; Rc34, Interconnect stub; Rc25, First coupling bridge; Rc13, Second coupling bridge; Rc46, Third coupling bridge; CPW1, Input port; CPW2, Output port; E1, First Z-shaped feed line; E2, Second Z-shaped feed line; S, Through slot; Rp, First conductor; Rs, Second conductor; Ep, Third conductor; Es1, Fourth conductor; Detailed Implementation The invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of this invention.

[0017] It should be noted that the directional terms mentioned in this document, such as "inner cavity," "inner circumference," "inner wall," and "outer side," are consistent with the specific directions on the paper in the accompanying drawings or the corresponding directions of the space shown in the drawings; all components and devices in this invention, unless otherwise specified, are components and devices known in the prior art.

[0018] like Figure 1 and Figure 2As shown, this embodiment of the invention provides a multilayer stripline filter, comprising a first core board Sub1, a first adhesive layer PP1, a second core board Sub2, a second adhesive layer PP2, a third core board Sub3, a third adhesive layer PP3, and a fourth core board Sub4 stacked sequentially. The first core board Sub1 includes a first metal layer L1 and a second metal layer L2 located on opposite sides in the thickness direction; the second core board Sub2 includes a third metal layer L3 and a fourth metal layer L4 located on opposite sides in the thickness direction; and the third core board Sub3 includes a fifth metal layer L4 located on opposite sides in the thickness direction. The fourth core board Sub4 includes a seventh metal layer L7 and an eighth metal layer L8 located on opposite sides of the thickness direction, with the second metal layer L2, the third metal layer L3, the fourth metal layer L4, the fifth metal layer L5, the sixth metal layer L6, and the seventh metal layer L7 forming a first planar stripline filter F1, a second planar stripline filter F2, and a third planar stripline filter F3, respectively, and the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3 are connected in parallel.

[0019] In this embodiment, the first adhesive layer PP1 can be selected as a prepreg.

[0020] Specifically, the first core board Sub1 includes a first metal layer L1 and a second metal layer L2 located on opposite sides in the thickness direction; the second core board Sub2 includes a third metal layer L3 and a fourth metal layer L4 located on opposite sides in the thickness direction; the third core board Sub3 includes a fifth metal layer L5 and a sixth metal layer L6 located on opposite sides in the thickness direction; and the fourth core board Sub4 includes a seventh metal layer L7 and an eighth metal layer L8 located on opposite sides in the thickness direction. This facilitates the formation of at least two of the second metal layer L2, the third metal layer L3, the fourth metal layer L4, the fifth metal layer L5, the sixth metal layer L6, and the seventh metal layer L7, respectively, to form a first planar stripline filter F1, a second planar stripline filter F2, and a third planar stripline filter F3. For example, as... Figure 1 As shown, the third metal layer L3, the fifth metal layer L5, and the sixth metal layer L6 can be used to form the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3, respectively.

[0021] Furthermore, the first adhesive layer PP1, the second adhesive layer PP2, and the third adhesive layer PP3 can be made from prepreg, whose dielectric constant is similar to that of the core board material, which can reduce signal reflection caused by differences in interlayer dielectric constants. During the lamination process, the prepreg can fully fill the gaps between the first core board Sub1, the second core board Sub2, the third core board Sub3, and the fourth core board Sub4, forming a dense interlayer structure, improving the overall mechanical strength of the multilayer board, and at the same time having good insulation properties, which can effectively avoid short circuit problems in the interlayer metal layers.

[0022] The multilayer stripline filter provided in this application significantly improves the unloaded quality factor compared to a single-layer stripline filter by employing a multilayer parallel stripline filter structure, thereby enhancing the filter's performance. Furthermore, after at least two of the second metal layer L2, third metal layer L3, fourth metal layer L4, fifth metal layer L5, sixth metal layer L6, and seventh metal layer L7 respectively form a first planar stripline filter F1, a second planar stripline filter F2, and a third planar stripline filter F3, and by connecting these filters in parallel, the impact of a single or a small number of broken vias at the connection points on the performance of the multilayer stripline filter can be ignored in practical applications, thus enhancing the reliability and stability of the multilayer stripline filter's performance.

[0023] like Figure 1 and Figure 2 As shown, the first metal layer L1 and the eighth metal layer L8 are located on opposite sides of the thickness direction of the multilayer stripline filter. The first metal layer L1 serves as the grounding top plate GND1, and the eighth metal layer L8 serves as the grounding bottom plate GND2. The second planar stripline filter F2 is located between the first planar stripline filter F1 and the third planar stripline filter F3. The first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3 are connected to the first metal layer L1 and the eighth metal layer L8, respectively.

[0024] Specifically, the first metal layer L1 and the eighth metal layer L8 provide a stable potential reference for the multilayer stripline filter, enabling the internal circuit components to operate in a stable environment. During operation, the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3, connected to the top ground plate GND1 and the bottom ground plate GND2, ensure stable operation of the filter under various conditions such as temperature changes and power fluctuations. Furthermore, the top ground plate GND1 and the bottom ground plate GND2 provide electrostatic protection, facilitating the conduction of static charge to the ground and preventing damage to the multilayer stripline filter, thereby improving its reliability and lifespan.

[0025] In an optional embodiment of this application, the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3 are multi-order stripline filters.

[0026] Specifically, multi-order stripline filters can achieve complex frequency response characteristics through the cascading and coupling of multiple resonators. Each resonator resonates with a signal at a specific frequency. By properly designing the parameters of the resonators and the coupling method between them, the filter can have low insertion loss in the passband to ensure that the signal can pass smoothly, while having high attenuation in the stopband to effectively suppress unwanted frequency signals.

[0027] In an optional embodiment of this application, the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3 have similar structures and are 6th-order stripline filters; wherein, the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3 all include a quarter-wavelength resonator with one end open and the other end short-circuited.

[0028] Specifically, the 6th-order stripline filter, due to the synergistic effect of its multi-order structure and quarter-wavelength resonator, can achieve a steeper transition band, greatly improving the filter's ability to suppress unwanted frequency signals. In communication systems, signals often contain multiple frequency components, some of which may be interference signals. Using a 6th-order stripline filter can effectively suppress these interference signals, improve the signal-to-noise ratio of the communication system, and ensure communication quality.

[0029] like Figure 2 and Figure 3As shown, the first planar stripline filter F1 includes a first resonator R1, a second resonator R2, a third resonator R3, a fourth resonator R4, a fifth resonator R5, and a sixth resonator R6. The first resonator R1 and the second resonator R2 are interdigitated, the second resonator R2 and the third resonator R3 are also interdigitated, the third resonator R3 and the fourth resonator R4 are parallel coupled lines, and an interconnecting stub Rc34 is mounted between the third resonator R3 and the fourth resonator R4. The fourth resonator R4 and the fifth resonator R5 are interdigitated, and the fifth resonator R5 and the sixth resonator R6 are also interdigitated. Among them, the first resonators R1 in each layer are connected in parallel, the second resonators R2 in each layer are connected in parallel, the third resonators R3 in each layer are connected in parallel, the fourth resonators R4 in each layer are connected in parallel, the fifth resonators R5 in each layer are connected in parallel, and the sixth resonators R6 in each layer are connected in parallel.

[0030] Specifically, the first resonator R1 and the second resonator R2, the second resonator R2 and the third resonator R3, the fourth resonator R4 and the fifth resonator R5, and the fifth resonator R5 and the sixth resonator R6 all adopt an interdigital structure. This interdigital structure enhances the coupling strength between adjacent resonators and improves the passband flatness of the filter. The third resonator R3 and the fourth resonator R4 adopt a parallel coupling line structure with interconnecting stubs Rc34, which enhances the coupling strength of this parallel coupling line and allows for flexible adjustment of the coupling coefficient, enabling precise control of the passband bandwidth to meet the bandwidth requirements of different communication scenarios. It should also be noted that the first resonators R1 in each layer are connected in parallel, the second resonators R2 in each layer are connected in parallel, the third resonators R3 in each layer are connected in parallel, the fourth resonators R4 in each layer are connected in parallel, the fifth resonators R5 in each layer are connected in parallel, and the sixth resonators R6 in each layer are connected in parallel, providing more connection points to reduce the impact of a single or a small number of broken metal vias on the performance of the multilayer stripline filter.

[0031] like Figure 3As shown, a first coupling bridge Rc25 is introduced between the second resonator R2 and the fifth resonator R5 of the second planar stripline filter F2. The first coupling bridge Rc25 is located at the open-circuit terminals of the second resonator R2 and the fifth resonator R5 of the second planar stripline filter F2. The first coupling bridge Rc25, together with the open-circuit terminals of the second resonator R2 and the fifth resonator R5 of the first planar stripline filter F1 and the third planar stripline filter F3, forms a parallel plate capacitor. A second coupling bridge Rc13 is introduced between the first resonator R1 and the third resonator R3 of the second planar stripline filter F2. The second coupling bridge Rc13 is located at the open-circuit terminals of the second planar stripline filter F2. The open-circuit terminals of the first resonator R1 and the third resonator R3 of filter F2, and the second coupling bridge Rc13 form a parallel plate capacitor with the open-circuit terminals of the first resonator R1 and the third resonator R3 of the third planar stripline filter F3; a third coupling bridge Rc46 is introduced between the fourth resonator R4 and the sixth resonator R6 of the second planar stripline filter F2. The third coupling bridge Rc46 is located at the open-circuit terminals of the fourth resonator R4 and the sixth resonator R6 of the second planar stripline filter F2, and the third coupling bridge Rc46 forms a parallel plate capacitor with the open-circuit terminals of the fourth resonator R4 and the sixth resonator R6 of the third planar stripline filter F3.

[0032] Specifically, the first coupling bridge Rc25, the second coupling bridge Rc13, and the third coupling bridge Rc46 can all be rectangular patches. By setting the first coupling bridge Rc25, the second coupling bridge Rc13, and the third coupling bridge Rc46, capacitive cross coupling can be introduced, thereby effectively adjusting the position of the transmission zero point and improving the symmetry of the passband.

[0033] like Figure 2 , Figure 3 and Figure 4 As shown, input port CPW1 and output port CPW2 are respectively provided on opposite sides of the eighth metal layer L8; a first Z-shaped feed line E1 is mounted on the first resonator R1 of each layer, one end of the first Z-shaped feed line E1 is mounted near the short-circuit end of the first resonator R1, and the other end of the first Z-shaped feed line E1 corresponds to the input port CPW1; a second Z-shaped feed line E2 is mounted on the sixth resonator R6 of each layer, one end of the second Z-shaped feed line E2 is mounted near the short-circuit end of the sixth resonator R6, and the other end of the second Z-shaped feed line E2 corresponds to the output port CPW2; the other end of the first Z-shaped feed line E1 is connected to the input port CPW1, and the other end of the second Z-shaped feed line E2 is connected to the output port CPW2.

[0034] Using the above configuration, all the first Z-shaped feed lines E1 and the second Z-shaped feed lines E2 can be connected in parallel to simultaneously excite the first planar stripline filter F1, the second planar stripline filter F2, and the third planar stripline filter F3. Furthermore, the input port CPW1 and the output port CPW2 provide bottom-mounted external interfaces for the multilayer board stripline filter, facilitating connection with other devices.

[0035] like Figure 2 As shown, at least one through slot S is provided on the first metal, and the extension direction of the through slot S is perpendicular to the line connecting the input port CPW1 and the output port CPW2.

[0036] Specifically, the through slot S can isolate input and output signals, reduce the coupling between the filter source and the load, improve out-of-band rejection, and help improve the reliability of multilayer board stripline filters.

[0037] like Figure 1 , Figure 2 and Figure 3 As shown, the open-circuit terminals of the first resonator R1, the second resonator R2, the third resonator R3, the fourth resonator R4, the fifth resonator R5, and the sixth resonator R6 in each layer are connected by a first conductor Rp; the short-circuit terminals of the first resonator R1, the second resonator R2, the third resonator R3, the fourth resonator R4, the fifth resonator R5, and the sixth resonator R6 in each layer are connected by a second conductor Rs, and the second conductor Rs is connected to the first metal layer and the second metal layer respectively; the first Z-shaped feed line E1 and the second Z-shaped feed line E2 in each layer are connected by a third conductor Ep.

[0038] Specifically, the connection method of the first conductor Rp, the second conductor Rs, and the third conductor Ep ensures the electrical connection stability and signal transmission reliability of the internal circuit of the multilayer stripline filter.

[0039] In an optional embodiment of this application, a fourth conductor Es1 is further provided between the first Z-shaped feed line E1 of each layer and between the second Z-shaped feed line E2 of each layer. The first conductor Rp, the second conductor Rs, the third conductor Ep and the fourth conductor Es1 include at least one of metallized through holes, metallized blind holes and metallized semi-circular holes.

[0040] Specifically, a metallized via is a conductive via that penetrates through a multilayer board, enabling electrical connections between different layers. In filters, metallized vias are used to connect resonators and feeders on different layers, ensuring smooth signal transmission between them. A metallized blind via, on the other hand, is a conductive via that penetrates only a portion of a layer, establishing electrical connections between specific layers. It is suitable for connection requirements that do not require penetrating the entire multilayer board. A metallized semi-circular via is a conductive via with a special shape that can reduce space occupation and increase layout density while ensuring electrical connections.

[0041] like Figure 5 As shown in the simulation results, the designed filter has a center frequency of 2.0 GHz, a 1 dB bandwidth of 0.4 GHz, a return loss greater than 20 dB in the passband, and a pair of transmission zeros on both sides of the passband located at 1.71 GHz and 2.29 GHz, respectively.

[0042] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0043] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0044] Furthermore, the various implementation methods disclosed in this solution can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content invented by this disclosure.

Claims

1. A multilayer stripline filter, characterized in that, The system comprises a first core board, a first adhesive layer, a second core board, a second adhesive layer, a third core board, a third adhesive layer, and a fourth core board stacked sequentially. The first core board includes a first metal layer and a second metal layer located on opposite sides of its thickness direction. The second core board includes a third metal layer and a fourth metal layer located on opposite sides of its thickness direction. The third core board includes a fifth metal layer and a sixth metal layer located on opposite sides of its thickness direction. The fourth core board includes a seventh metal layer and an eighth metal layer located on opposite sides of its thickness direction. Three of the following metal layers—the second, third, fourth, fifth, sixth, and seventh—form a first planar stripline filter, a second planar stripline filter, and a third planar stripline filter, respectively, and these filters are connected in parallel. The first, second, and third planar stripline filters each include a first resonator, a second resonator, a third resonator, a fourth resonator, a fifth resonator, and a sixth resonator. The first resonator and the second resonator, the second resonator and the third resonator, the fourth resonator and the fifth resonator, and the fifth resonator and the sixth resonator all adopt an interdigital structure. The third resonator and the fourth resonator adopt a parallel coupled-line structure and are equipped with interconnecting branches. The first resonators of different layers are connected in parallel, the second resonators of different layers are connected in parallel, the third resonators of different layers are connected in parallel, the fourth resonators of different layers are connected in parallel, the fifth resonators of different layers are connected in parallel, and the sixth resonators of different layers are connected in parallel. A first coupling bridge is introduced between the second resonator and the fifth resonator of the second planar stripline filter. The first coupling bridge is located at the open end of the second resonator and the fifth resonator of the second planar stripline filter. The first coupling bridge and the open end of the second resonator and the fifth resonator of the first planar stripline filter and the third planar stripline filter form a parallel plate capacitor. A second coupling bridge is introduced between the first resonator and the third resonator of the second planar stripline filter. The second coupling bridge is located at the open end of the first resonator and the third resonator of the second planar stripline filter, and the second coupling bridge and the open end of the first resonator and the third resonator of the third planar stripline filter form a parallel plate capacitor. A third coupling bridge is introduced between the fourth and sixth resonators of the second planar stripline filter. The third coupling bridge is located at the open-circuit terminals of the fourth and sixth resonators of the second planar stripline filter, and the third coupling bridge and the open-circuit terminals of the fourth and sixth resonators of the third planar stripline filter form a parallel plate capacitor.

2. The multilayer stripline filter according to claim 1, characterized in that, The first metal layer and the eighth metal layer are located on opposite sides of the thickness direction of the multilayer stripline filter. The first metal layer serves as the top grounding plate, and the eighth metal layer serves as the bottom grounding plate. The second planar stripline filter is located between the first planar stripline filter and the third planar stripline filter. The first planar stripline filter, the second planar stripline filter, and the third planar stripline filter are respectively connected to the first metal layer and the eighth metal layer.

3. The multilayer stripline filter according to claim 2, characterized in that, The first planar stripline filter, the second planar stripline filter, and the third planar stripline filter are multi-order stripline filters.

4. The multilayer stripline filter according to claim 3, characterized in that, The first planar stripline filter, the second planar stripline filter, and the third planar stripline filter are all stripline filters; wherein, the first planar stripline filter, the second planar stripline filter, and the third planar stripline filter all include a quarter-wavelength resonator with one end open and the other end short-circuited.

5. The multilayer stripline filter according to claim 4, characterized in that, The eighth metal layer has an input port and an output port on opposite sides, respectively; each layer's first resonator is equipped with a first Z-shaped feed line, one end of which is mounted near the short-circuit end of the first resonator, and the other end of which corresponds to the input port; each layer's sixth resonator is equipped with a second Z-shaped feed line, one end of which is mounted near the short-circuit end of the sixth resonator, and the other end of which corresponds to the output port; the other ends of the first Z-shaped feed line are connected to the input port, and the other ends of the second Z-shaped feed line are connected to the output port.

6. The multilayer stripline filter according to claim 5, characterized in that, At least one through slot is formed on the first metal layer, and the extension direction of the through slot is perpendicular to the line connecting the input port and the output port.

7. The multilayer stripline filter according to claim 6, characterized in that, The open-circuit terminals of the first resonators, the second resonators, the third resonators, the fourth resonators, the fifth resonators, and the sixth resonators in each layer are connected by a first conductor. The short-circuit terminals of the first resonators, the second resonators, the third resonators, the fourth resonators, the fifth resonators, and the sixth resonators in each layer are connected by a second conductor, and the second conductor is connected to the first metal layer and the second metal layer, respectively. The first Z-shaped feed lines and the second Z-shaped feed lines in each layer are connected by a third conductor.

8. The multilayer stripline filter according to claim 7, characterized in that, A fourth conductor is also provided between the first Z-shaped feed line and between the second Z-shaped feed line of each layer. The first conductor, the second conductor, the third conductor and the fourth conductor include at least one of metallized through holes, metallized blind holes and metallized semi-circular holes.

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