A defect-based branch-loaded t-shaped band filter

Abstract

The application belongs to the technical field of radio frequency transmission devices, and particularly relates to a branch-loaded three-passband filter based on a defect ground structure, which comprises a microstrip substrate, the microstrip substrate has a front surface and a back surface, the front surface of the microstrip substrate is provided with a first asymmetric branch-loaded resonator unit and a second asymmetric branch-loaded resonator unit, the first asymmetric branch-loaded resonator unit and the second asymmetric branch-loaded resonator unit form a filter main body through an electric coupling mode, the back surface of the microstrip substrate is provided with a first defect ground structure and a second defect ground structure, and the front surface filter main body and the back surface defect ground structure synergistically act to realize decoupling design of a multi-passband resonant mode. The filter main body is regarded as a half-wavelength stepped impedance folded resonator loaded with asymmetric short-circuit branches and symmetric open-circuit branches, so that the three-passband center frequencies and bandwidths can be flexibly regulated, and the design and engineering debugging difficulty of the filter is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of radio frequency transmission device technology, and specifically relates to a stub-loaded three-pass filter based on a defective ground structure. Background Technology

[0002] With the rapid evolution of 5G / 6G mobile communication, satellite internet, and IoT technologies, wireless communication systems are developing towards multi-band collaboration, high-frequency band reuse, and miniaturized integration. Taking typical application scenarios as examples, the 5G Sub-6GHz band needs to simultaneously support the coexistence of multiple standards, including n78 (3.3–3.8GHz), n41 (2.496–2.69GHz), and WLAN 5.8GHz (5.725–5.875GHz); satellite IoT terminals need to be compatible with L-band (1.2–1.6GHz), S-band (2.0–2.3GHz), and C-band (4.0–4.5GHz) signal transmission; and industrial wireless sensor networks need to operate stably in the 2.4GHz ISM band, 5GHz ISM band, and dedicated control bands. This places increasingly higher demands on the reliability and integration of communication transceiver systems. As an indispensable and crucial component of the RF front-end, filters face even more stringent requirements: higher integration, multi-passband response, low insertion loss, and high out-of-band rejection.

[0003] As a core component of the radio frequency front end, filters are used to filter out out-of-band noise and signal interference from adjacent and different frequency channels to the maximum extent. Therefore, their performance directly determines the working stability and overall performance of the wireless communication system. In the context of increasingly scarce spectrum resources, how to achieve efficient reuse of spectrum resources and support the compatible operation of communication terminals under multiple communication protocols and multiple working modes is one of the key technical problems that the wireless communication field is currently trying to overcome.

[0004] The mainstream implementation methods for three-pass microstrip filters can be divided into three categories. While each approach has its advantages, they also face technical bottlenecks and performance limitations that are difficult to balance. Among them:

[0005] The first type of scheme is a multi-resonator cascaded topology. This scheme uses multiple half-wavelength or quarter-wavelength resonators with different resonant frequencies, all fed by a common feeder. Three independent signal transmission paths are constructed through electromagnetic coupling between the resonators, ultimately achieving a single-input, single-output three-passband filter response. A typical implementation is a three-stage stepped-impedance resonator (SIR) cascade structure, with the three resonant units corresponding to three target operating frequency bands. The three-passband filter characteristics are generated by the coupling effect between adjacent resonators. The core advantages of this scheme lie in its passband performance and out-of-band rejection: low in-band insertion loss, typically controllable to within 0.5 dB, and in-band amplitude fluctuation less than 0.2 dB, while exhibiting excellent out-of-band rejection capability, with stopband attenuation exceeding 40 dB. However, this topology has two inherent shortcomings that are difficult to avoid: First, the cascaded layout of multi-stage resonators will significantly increase the overall size of the circuit, which cannot meet the stringent requirements of wireless communication terminals for device miniaturization and high integration. Second, the coupling and mutual interference between cascaded resonant units are serious. When adjusting a certain passband parameter, the center frequency, bandwidth and other key indicators of the other passbands will be changed simultaneously, making it extremely difficult to achieve independent tuning of multiple passbands.

[0006] The second category is a multi-passband implementation scheme based on multi-mode stepped impedance resonators (SIRs). This scheme excites multiple resonant modes of the resonator by adjusting the high-low impedance ratio of the SIR and the length ratio of each transmission line segment, thereby achieving a multi-passband filtering response. For example, a three-section SIR structure can excite the fundamental, second-order, and third-order modes of the resonator by setting the impedance ratio, so that the three resonant modes correspond to three target passbands. Compared with the cascaded scheme, this scheme has a simpler topology and the overall circuit size can be reduced by about 20%. However, it also has insurmountable technical limitations: on the one hand, the higher-order resonant modes excited by the SIR are susceptible to parasitic radiation, and the out-of-band rejection performance decays rapidly with increasing operating frequency. In the high-frequency band above 5GHz, the stopband rejection level is usually below 30dB; on the other hand, the center frequency of each passband is highly bound to the impedance ratio and length ratio of the SIR, and the bandwidth and center frequency of the passband are difficult to adjust independently, severely limiting the design freedom.

[0007] The third type is the traditional stub-loaded resonator scheme. This scheme introduces additional resonant modes by loading open-circuit or short-circuit stubs onto the main resonator, thereby expanding the number of passbands. In a typical design, loading an open-circuit stub at the symmetry line of a half-wavelength resonator can excite a second resonant mode to achieve a dual-passband response; adding a short-circuit stub on top of this can further introduce a third resonant mode, theoretically achieving three-passband filtering characteristics. The core advantage of this scheme is that the stub-loaded structure can introduce new resonant modes without significantly increasing the circuit size, possessing significant potential for miniaturization design; however, existing traditional stub-loaded schemes still have many performance shortcomings. This type of scheme relies entirely on electrical coupling to achieve mode control, and the coupling relationship between each resonant mode is complex, resulting in poor isolation between passbands. The passband isolation of conventional designs is generally low; at the same time, this scheme has insufficient suppression capability for high-order harmonics, easily generating harmonic interference to adjacent communication frequency bands; in addition, the loading position and physical size of the stub are highly sensitive to processing accuracy, resulting in poor performance consistency in mass production of devices.

[0008] In the performance optimization research of microstrip filters, defective ground structures (DGS) are widely used for harmonic suppression, bandwidth extension, and device miniaturization design. This structure modifies the current distribution of the ground plane and the electromagnetic field distribution of the transmission lines by etching specific topological groove structures, such as H-shape, dumbbell shape, and spiral shape, onto the ground metal plate of the microstrip substrate, thereby controlling the equivalent circuit parameters and resonant characteristics of the device. Existing DGS-based filter research mainly focuses on single-passband or dual-passband scenarios. In single-passband filter design, the resonant characteristics of DGS can be used to suppress higher harmonics other than the fundamental mode, achieving a narrowband, high-suppression filtering response. A typical 2.4GHz WLAN band DGS filter can achieve an out-of-band suppression level of over 50dB, but this cannot meet the application requirements of multi-band communication. In dual-passband filter design, a dual-DGS unit and a front resonator are often used to work together to achieve a typical dual-band filtering response. However, there are still many technical challenges in the design of DGS filters for three-passband scenarios, including complex multi-mode coupling relationships and poor independent controllability of the passband. Currently, there are few three-passband filter schemes that deeply coordinate the design of DGS and stub-loaded resonators. Summary of the Invention

[0009] The purpose of this invention is to address the technical deficiencies of existing three-pass microstrip filters by proposing a stub-loaded three-pass filter based on a defective ground structure. This invention employs a front-side multi-stub-loaded resonator and a back-side defective ground structure in a collaborative architecture. By utilizing the deep electromagnetic synergy between the two, the device can be miniaturized. It supports independent adjustment of the three-pass center frequency and bandwidth, and has the advantages of low insertion loss, high in-band selectivity, strong out-of-band suppression, and easy fabrication and integration.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A stub-loaded three-pass band filter based on a defective ground structure includes a microstrip substrate having a front side and a back side.

[0012] The front side of the microstrip substrate is provided with a first asymmetric stub-loaded resonator unit and a second asymmetric stub-loaded resonator unit.

[0013] The first asymmetric stub-loaded resonator unit includes a first half-wavelength stepped impedance resonator, a first short-circuit loaded stub, a first open-circuit stepped impedance loaded stub, and a first feed line; wherein:

[0014] The first half-wavelength stepped impedance resonator includes a first vertical arm, an upper horizontal arm, and a lower horizontal arm; one end of the upper horizontal arm and the lower horizontal arm are respectively connected to both ends of the first vertical arm to form a U-shaped structure, the other end of the upper horizontal arm is bent downward to form an upper open-circuit stub, and the other end of the lower horizontal arm is bent upward to form a lower open-circuit stub. The upper open-circuit stub and the lower open-circuit stub are arranged opposite to each other and do not contact each other, so that a first coupling window is formed between them.

[0015] The side of the first vertical arm facing the inner region enclosed by the upper and lower horizontal arms is defined as the inner side, and the side facing away from the inner region is defined as the outer side; one end of the first short-circuit loading branch is connected to the lower inner end of the first vertical arm, and the other end is provided with a metal via for grounding;

[0016] The first feed line is used for signal input / output. The first feed line is connected to the lower end of the outer side of the first vertical arm, and its position corresponds to the first short-circuit loaded stub in the height direction of the first vertical arm. One end of the first open-circuit stepped impedance loaded stub is connected to the middle position of the first vertical arm, and the other end extends towards the first coupling window and is spaced apart from the first coupling window.

[0017] The second asymmetric stub-loaded resonator unit has the same structure as the first asymmetric stub-loaded resonator unit, and the two are centrally symmetrical about the geometric center of the microstrip substrate; and the open stubs of the two are interwoven to achieve cascade coupling.

[0018] The back side of the microstrip substrate is provided with a first defect ground structure and a second defect ground structure. The first defect ground structure and the second defect ground structure have the same structure and are symmetrically distributed about the geometric center of the microstrip substrate.

[0019] Furthermore, the first short-circuit loaded stub is folded into an L-shape, and the first open-circuit stepped impedance loaded stub is T-shaped.

[0020] Furthermore, both the first defective ground structure and the second defective ground structure are H-shaped.

[0021] Furthermore, the folded first short-circuit loaded stub bends toward the side where the first open-circuit stepped impedance loaded stub is located, and the T-shape of the first open-circuit stepped impedance loaded stub is a horizontally placed T-shape.

[0022] Furthermore, the back side of the microstrip substrate, except for the areas where the first defect ground structure and the second defect ground structure are located, is a grounded metal plate.

[0023] This invention provides a stub-loaded three-passband filter based on a defective ground structure. A first asymmetric stub-loaded resonator unit and a second asymmetric stub-loaded resonator unit are disposed on the front side of a microstrip substrate, forming the filter body through electrical coupling. A first defective ground structure and a second defective ground structure are disposed on the back side of the microstrip substrate. Through the synergistic effect of the front filter body and the back defective ground structure, decoupling of multiple passband resonant modes is achieved. In the filter body, the first and second asymmetric stub-loaded resonator units are centrally symmetrically distributed, while the first and second asymmetric stub-loaded resonators themselves are asymmetric structures. Therefore, the filter body can be regarded as a half-wavelength stepped impedance folded resonator loaded with asymmetric short-circuited stubs and symmetrical open-circuited stubs, allowing for more flexible control of the center frequencies and bandwidths of the three passbands, significantly reducing the design and engineering debugging difficulty of the filter.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. An architecture combining multi-stub loaded resonators and defective ground structures is adopted to achieve a three-pass band filter response while taking into account low insertion loss and excellent in-band frequency selectivity.

[0026] 2. The center frequencies of the first, second, and third passbands of this filter are mainly affected by the geometric parameters of the short-circuit loaded stub, the half-wavelength main resonator, the open-circuit stub, and the defective ground structure. Each passband can be independently and flexibly adjusted within the design range. The bandwidth of each passband can be matched and optimized by adjusting the width of the corresponding stub and the slot width of the defective ground structure, resulting in a high degree of design freedom.

[0027] 3. By using the folded design of the first half-wavelength stepped impedance resonator and the second half-wavelength stepped impedance resonator with the loading stub, the overall physical size of the filter is greatly reduced. At the same time, the overall structure of the device is simple, the manufacturing process is mature, and there is no need for a complex multi-layer three-dimensional structure. It is easy to integrate with planar RF circuits and perfectly meets the miniaturization integration requirements of wireless communication terminal equipment.

[0028] 4. The double H-shaped defect ground structure can effectively suppress high-order harmonic interference of devices, improve the out-of-band rejection capability of filters, effectively avoid interference from adjacent channels and different frequency signals, and ensure the working stability of wireless communication systems. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the multi-branch loaded stepped impedance resonator unit in this invention;

[0030] Figure 2 This is a front view of the stub-loaded three-pass band filter based on the defective ground structure of the present invention;

[0031] Figure 3 This is a reverse schematic diagram of the stub-loaded three-pass band filter based on the defective ground structure of the present invention;

[0032] Figure 4 This is the frequency response simulation curve of the stub-loaded three-pass band filter based on the defective ground structure of this invention;

[0033] Reference numerals in the attached figures: 1 is the first half-wavelength stepped impedance resonator, 2 is the second half-wavelength stepped impedance resonator, 3 is the first short-circuit loaded stub, 4 is the second short-circuit loaded stub, 5 is the first open-circuit stepped impedance loaded stub, 6 is the second open-circuit stepped impedance loaded stub, 7 is the first asymmetric stub loaded resonator unit, 8 is the second asymmetric stub loaded resonator unit, 9 is the first input / output feed line, 10 is the second input / output feed line, 11 is the first defective ground structure, and 12 is the second defective ground structure. Detailed Implementation

[0034] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0035] like Figures 1 to 3 As shown, this embodiment provides a stub-loaded three-pass band filter based on a defective ground structure, including a microstrip substrate having a front side and a back side.

[0036] like Figure 1 and Figure 2 As shown, the front side of the microstrip substrate is provided with a first asymmetric stub-loaded resonator unit 7 and a second asymmetric stub-loaded resonator unit 8.

[0037] The first asymmetric stub-loaded resonator unit 7 includes a first half-wavelength stepped impedance resonator 1, a first short-circuit loaded stub 3, a first open-circuit stepped impedance loaded stub 5, and a first feed line 9. The first half-wavelength stepped impedance resonator 1 includes a first vertical arm, an upper horizontal arm, and a lower horizontal arm. One end of the upper and lower horizontal arms is connected to both ends of the first vertical arm to form a U-shaped structure. The other end of the upper horizontal arm is bent downwards to form an upper open-circuit stub, and the other end of the lower horizontal arm is bent upwards to form a lower open-circuit stub. The upper and lower open-circuit stubs are arranged opposite each other and do not contact each other, so that a first coupling window is formed between them. The side of the first vertical arm facing the internal region enclosed by the upper and lower horizontal arms is defined as the inner side, and the side facing away from the internal region is defined as the outer side. The first short-circuit loaded stub 3 is folded into an L-shape, with one end connected to the lower inner end of the first vertical arm, and the other end having a metal via for grounding. The first feed line 9 is used for signal input / output. It is connected to the lower end of the outer side of the first vertical arm, and its position corresponds to the first short-circuit loaded stub 3 in the height direction of the first vertical arm. The first open-circuit stepped impedance loaded stub 5 is T-shaped, with one end connected to the middle position of the first vertical arm and the other end extending towards the first coupling window, spaced apart from it. It should be noted that the "middle position" mentioned in this embodiment refers to the approximate middle region along the length of the resonator stub. Its specific position can be adjusted within a reasonable range according to the actual passband frequency requirements, and is not limited to a strict halfway point.

[0038] The second asymmetric stub-loaded resonator unit 8 has the same structure as the first asymmetric stub-loaded resonator unit 7, and the two are centrally symmetrical about the geometric center of the microstrip substrate. Specifically, the second asymmetric stub-loaded resonator unit 8 includes a second half-wavelength stepped impedance resonator 2, a second short-circuit loaded stub 4, a second open-circuit stepped impedance loaded stub 6, and a second feed line 10. The relative position and connection relationship of the above components in the second asymmetric stub-loaded resonator unit 8 are consistent with the corresponding components in the first asymmetric stub-loaded resonator unit 7. The upper open-circuit stubs and the lower open-circuit stubs of the two are interwoven to achieve cascaded coupling.

[0039] like Figure 3 As shown, the back side of the microstrip substrate is provided with a first defect ground structure 11 and a second defect ground structure 12. The first defect ground structure 11 and the second defect ground structure 12 have the same structure, both being H-shaped, and both are symmetrically distributed about the geometric center of the microstrip substrate. Except for the areas where the first defect ground structure 11 and the second defect ground structure 12 are located, the remaining part of the back side of the microstrip substrate is a ground metal plate.

[0040] In some other specific examples, the folded first short-circuit loaded stub 3 bends toward the side where the first open-circuit stepped impedance loaded stub is located, and the T-shape of the first open-circuit stepped impedance loaded stub 5 is a horizontally placed T-shape.

[0041] In this embodiment, the center frequency of the first passband of the stub-loaded three-passband filter is around 2.15 GHz, generated by the first short-circuit loaded stub 3 and the second short-circuit loaded stub 4; the center frequency of the second passband is around 3.88 GHz, generated by the first open-circuit stepped impedance loaded stub 5 and the second open-circuit stepped impedance loaded stub 6; and the center frequency of the third passband is around 5.3 GHz, generated by the upper open-circuit stub and lower coupling in the first asymmetric stub-loaded resonator unit 7 and the first asymmetric stub-loaded resonator unit 8. By adjusting the geometric parameters of the upper and lower open-circuit stubs in the two asymmetric stub-loaded resonator units, the center frequency position of the third passband can be adjusted, for example, by increasing the length of the open-circuit stub, so that the center frequency of the third passband can be moved towards a lower frequency direction, while the frequency characteristics of the first and second passbands remain basically unchanged. The bandwidth of the first and second passbands of the filter can be controlled by adjusting the physical width of the short-circuit loaded stubs and the open-circuit loaded stubs. Adjusting the dimensional parameters such as the slot width of the double H-shaped defect ground structure can help adjust the overall passband performance, thereby optimizing the filter's in-band selectivity and out-of-band suppression performance.

[0042] The quality factor can be calculated using the following formula, where f0 is the center frequency and BW is the base frequency. 3dB This is a 3dB bandwidth. For the passband, a higher Q value represents greater selectivity. Here, the center frequency of the third passband is 5.3GHz, the 3dB bandwidth is 0.28GHz, and the Q value is 18.9.

[0043]

[0044] In this embodiment, the microstrip substrate used is a Taconi high-frequency microwave substrate with a relative permittivity of 2.55, a substrate thickness of 0.8 mm, and a loss tangent of 0.0019.

[0045] The core design principle of the aforementioned filter lies in the following: the first and second asymmetric stub-loaded resonator units on the front side are centrally symmetrically distributed, while the first and second asymmetric stub-loaded resonators themselves are asymmetric structures. This allows them to be equivalent to a half-wavelength stepped impedance folded resonator loaded with asymmetric short-circuited stubs and symmetrical open-circuited stubs. Based on this, the double H-shaped defect ground structure on the back of the microstrip substrate assists in adjusting the passband performance, significantly simplifying the design process of multi-passband filters. It allows for intuitive and independent analysis of the influence of each parameter on the passband characteristics. Simultaneously, by using stepped impedance resonators instead of uniform impedance resonators for the main resonators (the first and second half-wavelength stepped impedance resonators), and employing a folded design for the main resonators and loaded stubs, the physical area occupied by the circuit is significantly reduced, achieving device miniaturization. Furthermore, the double H-shaped defect ground structure effectively suppresses high-order harmonics in the filter and optimizes insertion loss within the passband, ensuring efficient signal transmission. The following simulation experiments verify this effect:

[0046] The stub-loaded three-pass filter based on the defective ground structure in this embodiment was subjected to full-wave electromagnetic simulation using the high-frequency electromagnetic simulation software Ansoft HFSS 2023R1. The frequency response characteristics obtained from the simulation include the transmission coefficient S21 (insertion loss) and the reflection coefficient S11 (return loss).

[0047] like Figure 4 As shown, the horizontal axis of the simulation results represents the frequency component in GHz, and the vertical axis represents the amplitude variable in dB. The simulation results show that the filter of this invention forms three independent filter passbands at 2.15 GHz, 3.88 GHz, and 5.3 GHz. The insertion loss in all three passbands is less than 0.8 dB, and the return loss is greater than 20 dB. Simultaneously, the filter generates three transmission zeros at 2.42 GHz, 4.26 GHz, and 5.5 GHz, achieving excellent in-band selectivity and out-of-band suppression capabilities. In the stopband below 1.5 GHz and above 6.2 GHz, the suppression level is greater than 30 dB.

Claims

1. A stub-loaded three-passband filter based on a defective ground structure, characterized in that, Includes a microstrip substrate having a front side and a back side; The front side of the microstrip substrate is provided with a first asymmetric stub load resonator unit (7) and a second asymmetric stub load resonator unit (8). The first asymmetric stub-loaded resonator unit (7) includes a first half-wavelength stepped impedance resonator (1), a first short-circuit loaded stub (3), a first open-circuit stepped impedance loaded stub (5), and a first feed line (9); wherein: The first half-wavelength stepped impedance resonator (1) includes a first vertical arm, an upper horizontal arm and a lower horizontal arm; one end of the upper horizontal arm and the lower horizontal arm are respectively connected to the two ends of the first vertical arm to form a U-shaped structure, the other end of the upper horizontal arm is bent downward to form an upper open-circuit stub, and the other end of the lower horizontal arm is bent upward to form a lower open-circuit stub. The upper open-circuit stub and the lower open-circuit stub are arranged opposite to each other and do not contact each other, so that a first coupling window is formed between them. The side of the first vertical arm facing the inner region enclosed by the upper and lower horizontal arms is defined as the inner side, and the side facing away from the inner region is defined as the outer side; one end of the first short-circuit loading branch is connected to the lower inner end of the first vertical arm, and the other end is provided with a metal via for grounding; The first feed line (9) is used for signal input / output. The first feed line (9) is connected to the lower end of the outer side of the first vertical arm. Its position corresponds to the first short-circuit loading stub (3) in the height direction of the first vertical arm. One end of the first open-circuit stepped impedance loading stub (5) is connected to the middle position of the first vertical arm, and the other end extends towards the first coupling window and is spaced apart from the first coupling window. The second asymmetric stub-loaded resonator unit (8) has the same structure as the first asymmetric stub-loaded resonator unit (7), and the two are centrally symmetrical about the geometric center of the microstrip substrate; and the open stubs of the two interweave to achieve cascade coupling. The back side of the microstrip substrate is provided with a first defect ground structure (11) and a second defect ground structure (12). The first defect ground structure (11) and the second defect ground structure (12) have the same structure and are symmetrically distributed about the geometric center of the microstrip substrate.

2. The stub-loaded three-passband filter based on a defective ground structure according to claim 1, characterized in that: The first short-circuit loaded stub (3) is folded into an L-shape, and the first open-circuit stepped impedance loaded stub (5) is T-shaped.

3. A stub-loaded three-passband filter based on a defective ground structure according to claim 1, characterized in that: Both the first defective ground structure (11) and the second defective ground structure (12) are H-shaped.

4. A stub-loaded three-pass filter based on a defective ground structure according to claim 2 or 3, characterized in that: The first short-circuit loaded branch (3) after folding bends toward the side where the first open-circuit stepped impedance loaded branch is located, and the T-shape of the first open-circuit stepped impedance loaded branch (5) is a horizontally placed T-shape.

5. A stub-loaded three-passband filter based on a defective ground structure according to claim 1, characterized in that: Except for the areas where the first defect ground structure (11) and the second defect ground structure (12) are located, the remaining part of the back side of the microstrip substrate is a grounded metal plate.

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