An absorption type dual-frequency filter based on N-type coupling and radial stubs

Abstract

This invention discloses an absorptive dual-band filter based on N-type coupling and radial stubs, relating to the field of microwave and radio frequency components. The invention employs an N-type folded coupling structure, utilizing cross-coupling paths and impedance transformation to significantly reduce the requirements for the odd-even mode impedance difference of the microstrip line. The auxiliary absorption network abandons the traditional simple T-type uniform transmission line stubs, introducing a multi-degree-of-freedom composite bandstop network containing open-circuit and lossy stubs. Radial stub-loaded resonators are integrated into the input and output feed lines. The backbone bandpass module, full-band absorption module, and high-frequency low-pass module are highly decoupled in electromagnetic characteristics. The functions of the three network modules do not interfere with each other but rather compensate for each other, significantly improving the isolation between the two passbands and providing an extremely flexible and rapidly reusable topology for future filter customization for different communication frequency bands.

Description

Technical Field

[0001] This invention relates to the field of microwave and radio frequency components technology, specifically to an absorptive dual-frequency filter based on N-type coupling and radial stubs. Background Technology

[0002] With the rapid development of 5G / 6G mobile communication, radar, and satellite communication systems, radio frequency (RF) front-end systems place extremely high demands on the multi-band operation capability and miniaturization of components. Dual-band bandpass filters, as core passive components of the RF front-end, can effectively select and transmit RF signals in two specific frequency bands. However, traditional dual-band filters strongly reflect out-of-band signals back into the RF front-end system within the stopband (non-passband). These reflected signals easily cause nonlinear interference to cascaded sensitive active components (such as power amplifiers and mixers), leading to increased system power consumption and deteriorated signal-to-noise ratio.

[0003] To address the problem of out-of-band signal reflection, academia and industry have proposed the concept of "absorption (reflection-free) dual-frequency filters," which convert out-of-band reflected energy into heat dissipation by introducing an auxiliary absorption network at the filter port. However, current absorption dual-frequency microstrip filter technology still suffers from the following three significant drawbacks and shortcomings:

[0004] First, it is difficult to balance manufacturing yield and miniaturization at a low center frequency ratio (CFR). Most existing dual-frequency backbone networks use conventional parallel-coupled microstrip line structures. According to microwave network theory, when the two passband frequencies are similar (i.e., low CFR, such as less than 2.0), conventional parallel-coupled lines require an extreme difference between odd and even mode impedances. This results in extremely small coupling gaps between microstrip lines (usually exceeding the 0.1mm process limit of conventional PCB manufacturers), which is physically difficult to achieve.

[0005] Second, the broadband absorption capability and independent tuning freedom of the auxiliary absorption stubs are insufficient. Some existing designs use simple T-shaped uniform transmission lines as absorption stubs. Due to their inherent periodic resonance limitations, when facing non-integer multiples of frequencies or broadband absorption requirements, they often suffer from insufficient absorption depth in the high-frequency band and deterioration of insertion loss in the passband. Furthermore, it is difficult to independently adjust the absorption depth of different stopbands.

[0006] Third, it lacks efficient ultra-wideband high-end harmonic suppression capabilities. Distributed parameter microstrip filters inherently suffer from high-order mode recovery problems, easily generating parasitic passbands at high frequencies, severely compromising the system's anti-interference capabilities. Existing solutions typically involve cascading traditional low-pass filters or employing defective ground structures (DGS), but these often significantly increase the physical size of the circuit or compromise the integrity of the ground plane, making it difficult to meet the demands of modern RF front-ends for extreme miniaturization and high integration.

[0007] In summary, there is an urgent need for a compact absorptive dual-frequency filter that can simultaneously solve the challenges of dual-frequency implementation at small frequency ratios, the problem of high-degree-of-freedom non-reflection absorption across the entire frequency band, and the problem of ultra-wideband harmonic suppression without increasing the area. Summary of the Invention

[0008] Therefore, this invention provides an absorptive dual-frequency filter based on N-type coupling and radial stubs to solve the technical problems of existing absorptive dual-frequency microstrip filters being difficult to implement due to physical processing limitations at small center frequency ratios, insufficient full-band absorption depth due to the lack of independent tuning degrees of the auxiliary absorption stubs, and the tendency of traditional microstrip structures to generate high-order parasitic passbands that are difficult to suppress without increasing physical size. This invention simultaneously solves the problems of efficient transmission of low-frequency-ratio dual-frequency signals, deep absorption of out-of-band full-band reflected signals, and broadband suppression of high-end spurious harmonics in an extremely compact size, meeting the stringent requirements of modern highly integrated RF front-end systems for high isolation and strong anti-interference capabilities. By cleverly integrating an N-type microstrip coupler, a multi-segment composite bandstop absorption network, and a radially stub-loaded resonator, multiple functions such as low-frequency-ratio dual-frequency transmission, full-band reflection-free absorption, and ultra-wideband high-end harmonic suppression are achieved in an extremely compact size.

[0009] This invention provides an absorptive dual-band filter based on N-type coupling and radial stubs, comprising: a dielectric substrate, a metal ground plane disposed on the bottom surface of the dielectric substrate, and a microstrip metal structure disposed on the top surface of the dielectric substrate; the microstrip metal structure includes: an input port Prot1, an output port Port2, and a high-frequency resonance suppression network, an auxiliary absorption network, and a backbone dual-band filter network located in the middle, symmetrically connected between the input port and the output port in sequence; the backbone dual-band filter network is connected in series in the backbone transmission path of the radio frequency signal, and includes an N-type coupler composed of two sets of quarter-wavelength microstrip lines ML1, ML2, and ML3, with one end of the quarter-wavelength microstrip line ML1 of the two sets of N-type couplers connected to the input port Prot1 and the output port Port2 respectively, and the other end connected to the quarter-wavelength microstrip line ML2; used to provide the bandpass filtering response of the target dual-band. The auxiliary absorption network includes quarter-wavelength microstrip lines ML4, ML5, ML6, and ML7, and resistor R1. Microstrip line ML5 and resistor R1 are connected in series, and microstrip lines ML6 and ML7 are connected in series, both of which are connected in series with microstrip line ML4. These are connected in parallel at the input and output nodes of the main dual-band filter network, respectively, to guide the out-of-band RF signals reflected by the main dual-band filter network within the stopband to the ground terminal and convert them into heat dissipation. The high-frequency resonance suppression network includes a first pair of symmetrical triangular patch structures and a second pair of symmetrical triangular patch structures connected at their respective symmetrical points. The symmetrical triangular patch structures are connected by microstrip transmission lines. The third pair of triangular patches are connected by extremely narrow microstrip stubs. The microstrip transmission lines are connected at both ends to ports formed by microstrip line structures, serving as input and output ports, respectively. The entire structure is perfectly symmetrical about both the horizontal and vertical lines. It is used to form a low-pass characteristic to suppress higher-order parasitic harmonics.

[0010] Furthermore, the backbone dual-frequency filter network is a centrally symmetric N-type microstrip coupler structure. The two pairs of microstrip lines ML1 and ML3 are used to generate electromagnetic coupling, and microstrip line ML2 is used to provide an uncoupled through-pass microstrip line with additional phase delay. The two pairs of microstrip lines ML1 and ML3 are internally interconnected to form a multipath interference network with two transmission poles. This structure, by introducing cross-coupling and additional phase shift, breaks the extreme requirements of traditional quarter-wavelength coupled lines on odd-even mode impedance, thus achieving a dual-bandpass response with a small center frequency ratio (CFR < 2.0) under the constraints of conventional PCB linewidth and gaps.

[0011] Furthermore, the auxiliary absorption network includes quarter-wavelength microstrip lines ML4, ML5, ML6, and ML7, and a resistor R1; microstrip line ML5 and resistor R1 are connected in series, and microstrip lines ML6 and ML7 are connected in series, both of which are connected in series with microstrip line ML4. These are connected in parallel at the input and output nodes of the main dual-frequency filter network. The lossy band-stop branch is composed of a dual-frequency band-stop filter structure and a lossy absorption stub; wherein, the dual-frequency band-stop filter structure includes a series microstrip transmission line and a parallel open-circuit microstrip stub, used to generate resonant poles at the two passband frequencies of the main filter network, thereby presenting infinite input impedance to the passband signal; the lossy absorption stub is composed of a microstrip phase-shift line and an absorption resistor connected in series, the other end of which is grounded through a metallized via. Compared to the monotonous T-shaped stubs, this composite structure, by independently adjusting the characteristic impedance of the open-circuit stubs and each transmission line segment, can precisely align with two non-integer multiples of the passband at a small center frequency ratio, making the absorption network completely invisible within the passband. Simultaneously, within the stopband frequency range, the network exhibits an input impedance that matches the system's characteristic impedance, efficiently channeling reflected signals from DC to the high-frequency stopband into the absorption resistor and converting them into heat energy. Microstrip lines ML1, ML2, ML3, ML4, ML5, ML6, and ML7 are all quarter-wavelength microstrip lines.

[0012] Furthermore, the high-frequency resonant suppression network is a cascaded radially stubbed resonator low-pass network. It mainly consists of an extremely narrow high-impedance microstrip transmission line and multiple triangular microstrip patches symmetrically loaded on both sides of this high-impedance microstrip transmission line. The high-impedance microstrip transmission line behaves as a series inductor in the equivalent circuit, while the triangular microstrip patches utilize their edge effects to provide broadband parallel capacitance. The combination of these two forms a multi-order LC low-pass filter. The triangular gradient structure effectively avoids the high-frequency parasitic passband caused by traditional rectangular stubs, extending the high-order harmonic suppression range of the upper stopband to more than 10 times the fundamental frequency without increasing the lateral width of the circuit.

[0013] The radio frequency signal fed from the input port Port1 first passes through the high-frequency resonance suppression network on the input side to filter out potential high-frequency spurious interference in the signal. Then the clean signal reaches the input node. For signals within the dual-band passband, the auxiliary absorption network is open-circuited, and the signal passes through the main dual-band filter network with low loss and reaches the output port. For signals within the stopband, the main dual-band filter network reflects them completely. The reflected wave is captured by the auxiliary absorption network connected in parallel at the node and completely converted into heat energy by the absorption resistor at its end, thereby achieving the full-band dual-port reflection-free characteristic.

[0014] The present invention has the following advantages over the prior art:

[0015] 1. This invention breaks through the physical processing limits, significantly reducing manufacturing costs and improving mass production yield: Traditional parallel-coupled-line dual-frequency filters, when facing design requirements with small center frequency ratios (CFR < 2.0), often require extreme odd-even mode impedance differences, resulting in coupling gaps between microstrip lines needing to be less than 0.05 mm. This far exceeds the processing limits of conventional low-cost printed circuit boards (PCBs), necessitating expensive high-precision thin-film lithography processes. By employing an N-type microstrip coupler as the backbone filter network and introducing internal cross-connection paths and additional phase delays, the dependence on strong coupling is significantly reduced. This architecture effectively relaxes the required minimum coupling spacing of microstrip lines to within the range allowed by conventional PCB fabrication processes (e.g., above 0.15 mm). This improvement not only eliminates high special processing costs but also fundamentally avoids the short-circuit risk caused by excessively narrow line spacing, significantly improving the yield and production efficiency of industrial mass production.

[0016] 2. This invention achieves deep, reflection-free absorption across the entire frequency band, significantly improving the reliability of the RF front-end system: Existing filters reflect out-of-band interference signals back to the input within the stopband, easily causing nonlinear distortion or even burnout of the power amplifier (PA) and low-noise amplifier (LNA) in the front end. This invention innovatively connects multiple composite band-stop absorption networks in parallel at the input and output ends. By independently adjusting the characteristic impedance within the network, this structure can efficiently convert out-of-band reflected signals from a wide stopband (from DC to high frequencies) into heat dissipation through grounding resistance without increasing insertion loss within the passband (completely transparent to the main signal). This invention completely eliminates the hidden danger of out-of-band standing wave ratio (VSWR) surges, greatly protecting RF active devices that are highly sensitive to VSWR, making the overall communication system more stable and reliable.

[0017] 3. This invention achieves ultra-wideband harmonic suppression and excellent electromagnetic compatibility (EMC) with zero additional physical area increase: Addressing the persistent problem of high-order parasitic passbands in microwave distributed parameter circuits at high frequencies, conventional solutions typically involve cascading large traditional low-pass filters or employing defective ground structures (DGS). This invention cleverly integrates a radial stub-loaded resonator (RSLS) at the RF feed line. This structure fully utilizes the unused space on both sides of the microstrip transmission line, introducing a strong parallel parasitic capacitance effect without increasing the lateral physical width of the circuit board. It not only extends the harmonic suppression range of the high-end stopband to several times (even 10 times) or more of the fundamental frequency but also maintains the absolute integrity of the underlying ground plane, effectively avoiding electromagnetic leakage and backplane radiation, providing superior EMC performance for high-density integrated microwave communication equipment.

[0018] 4. This invention features highly modular and decoupled components, significantly shortening the product development cycle: The circuit topology of this invention offers extremely high design freedom and modularity. Its main bandpass module, absorption network module, and high-frequency suppression module are highly decoupled in electromagnetic response. Engineers can independently design the N-type main band to determine the dual-band passband, then precisely control the stopband absorption depth by adjusting the absorption branches, and finally smooth out high-frequency spurious signals using triangular branches. This architecture, where each component performs its function without interference, completely changes the traditional microwave device design dilemma of "affecting the whole system with a single change," allowing this patented architecture to be easily and quickly ported and scaled to other different wireless communication frequency bands, significantly shortening the development and iteration cycle of new products. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the overall cross-sectional structure of the present invention;

[0021] Figure 2 This is a schematic diagram of the microstrip metal structure circuit architecture of the present invention;

[0022] Figure 3 This is a schematic diagram of the microstrip metal structure circuit of the present invention;

[0023] Figure 4 This is a schematic diagram of the high-frequency resonance suppression network structure of the present invention;

[0024] Figure 5 This is a schematic diagram illustrating the working principle of the non-reflective filtering generated by the absorption network in this invention.

[0025] Explanation of reference numerals in the attached figures:

[0026] 1. Main dual-frequency filter network; 2. Auxiliary absorption network; 3. High-frequency resonance suppression network; 4. Dielectric substrate; 5. Microstrip metal structure; 6. Metal ground plane; 10. Port; 11. First pair of triangular patches; 12. Second pair of triangular patches; 13. Third pair of triangular patches; 14. Microstrip stub; 15. Microstrip transmission line. Detailed Implementation

[0027] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1

[0029] refer to Figures 1 to 5 , Figure 1 This is a schematic cross-sectional view of the filter of the present invention. Along the thickness direction H, the filter as a whole consists of three main layers from top to bottom: the top layer is etched with the aforementioned... Figure 2 , Figure 3 The microwave radio frequency circuit layout shown includes a microstrip metal structure 5; the intermediate layer is a dielectric substrate 4 that supports and provides the electromagnetic propagation medium (its relative permittivity is...). The bottom layer is a lower metal ground plane 6 that covers the entire bottom surface of the substrate. Components requiring grounding, such as the resistor R1 in the auxiliary absorption network, are stably electrically connected to the bottom metal ground plane 6 through metallized vias penetrating the dielectric substrate 4.

[0030] like Figure 2 As shown, this embodiment of the invention provides a circuit architecture for an absorptive dual-frequency filter based on N-type coupling and radial stubs. The circuit is symmetrically distributed along the vertical centerline and mainly includes: an input port (Port 1), an output port (Port 2), a main dual-frequency filter network 1 located at the center of the circuit, a pair of high-frequency resonance suppression networks 3, and a pair of auxiliary absorption networks 2.

[0031] Specifically, after the in-band / out-of-band signal is fed into the input port (Port 1), it first passes through the high-frequency resonant suppression network 3 to filter out high-frequency spurious signals. Then, the signal reaches the trunk node, where an auxiliary absorption network 2 is connected in parallel. Next, the signal enters the core trunk dual-frequency filter network 1 for frequency band selection. After the main filter, the signal passes again at the output node through the parallel auxiliary absorption network 2 and the series high-frequency resonant suppression network 3, and is finally output from the output port (Port 2). In this configuration, the in-band signal (solid arrow) can pass through the trunk circuit with low loss, while the out-of-band signal (dashed arrow) is reflected by the main filter circuit and guided to the auxiliary absorption network 2 for dissipation, thus achieving a reflection-free characteristic.

[0032] Figure 3 This is a schematic diagram of the specific circuit structure of the present invention. Figure 2As shown, the core dual-backbone dual-frequency filter network 1 adopts an N-type coupled microstrip line structure. Internally, it is mainly composed of interconnected microstrip lines ML1, ML2, and ML3. The vertically parallel microstrip line ML1 forms multi-path coupling and interference through microstrip line ML2 and the folded microstrip line ML3, thereby generating a dual-frequency bandpass filter response at the two desired center frequencies.

[0033] At the input and output nodes of the main dual-frequency filter network 1, an auxiliary absorption network 2 is connected in parallel on each side. This auxiliary absorption network 2 consists of multiple microstrip lines and absorption resistors. Specifically, one end of microstrip line ML4 is connected to the main node, and the other end is connected to a branch node. From this branch node, it splits into two paths: one path consists of microstrip line ML5 and resistor R1 connected in series, with the other end of resistor R1 connected to ground (GND1) through a grounding hole; the other path consists of microstrip lines ML6 and ML7 connected in series, with the end of microstrip line ML7 open-circuited. By adjusting the electrical lengths of ML6 and ML7, the resonant poles of the absorption network can be precisely controlled.

[0034] Figure 4 This is a schematic diagram of the high-frequency resonance suppression network 3 (i.e., the triangular resonance structure) of the present invention. Figure 3 As shown, the structure is centrally symmetrical along the horizontal and vertical lines of symmetry. Its main body is a microstrip transmission line 15 connected in series on the main path, with input / output ports 10 connected to both ends. Multiple sets of triangular metal patches are symmetrically loaded on the upper and lower sides of the microstrip transmission line 15. The vertices of the first pair of triangular patches 11 and the second pair of triangular patches 12 directly intersect and connect to the microstrip transmission line 15; at the centrally symmetrical positions to the left and right of the first pair of triangular patches 11 and the second pair of triangular patches 12, the third pair of triangular patches 13 are connected to the microstrip transmission line 15 through extremely narrow microstrip stubs 14. This triangular patch structure utilizes its broadband gradient characteristics and edge parasitic capacitance effect to construct a multi-order low-pass filter response without increasing the lateral width of the circuit, achieving extremely deep and broadband suppression of higher harmonics.

[0035] Figure 5 The working principle of the present invention, which generates a reflectionless filtering response by loading an auxiliary absorption network, is explained in detail.

[0036] Main filter (left figure): The core backbone dual-frequency filter network 1 itself has typical dual-frequency response characteristics, that is, the transmission coefficient (S) in both passbands is high. 21 The reflectance coefficient (S) is relatively high. 11 The signal is extremely low; however, within the stopband, the signal cannot pass through, resulting in total internal reflection (S0). 11 (Close to 0dB).

[0037] Auxiliary absorption stub (middle figure): The auxiliary absorption network 2 designed in this invention exhibits a high impedance state ("stealth" of in-band signals) at the two passband frequencies, while exhibiting an absorption state that matches the characteristic impedance of the system in the stopband frequency range. Its own reflection curve is exactly complementary to the main filter.

[0038] The main filter is loaded with an auxiliary absorption stub (right figure): After combining the two, in the passband, the in-band signal passes through the main branch without loss; in the stopband, the out-of-band signal reflected back by the main branch enters the auxiliary absorption network along the impedance matching path and is finally converted into heat dissipation by the grounding resistor R1. This achieves S across the entire frequency band. 11 Maintaining a low level of non-reflective (absorption) properties.

[0039] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An absorptive dual-band filter based on N-type coupling and radial stubs, characterized by, include: The dielectric substrate (4) has a microstrip metal structure (5) on its top surface and a metal ground plane (6) on its bottom surface. The microstrip metal structure includes an input port, an output port, and a high-frequency resonance suppression network (3), an auxiliary absorption network (2), and a main dual-frequency filter network (1) connected between the input port and the output port.

2. The N-coupling and radial stubs based absorptive dual-band filter according to claim 1, wherein, The main dual-frequency filter network (1) is connected in series with a high-frequency resonance suppression network (3) on both the left and right sides; an auxiliary absorption network (2) is connected in parallel between the main dual-frequency filter network (1) and the two high-frequency resonance suppression networks (3).

3. The N-coupled and radial branch loaded absorptive dual-band filter according to claim 2, wherein, The main dual-frequency filter network (1) is an N-type coupler composed of quarter-wavelength microstrip line ML1, quarter-wavelength microstrip line ML2 and quarter-wavelength microstrip line ML3. The two sets of N-type couplers are arranged alternately, and one end of the quarter-wavelength microstrip line ML1 in the two sets of N-type couplers is connected to the input port Prot1 and the output port Port2 respectively, and the other end is connected to the quarter-wavelength microstrip line ML2.

4. The N-coupled and radial branch loaded absorptive dual-band filter according to claim 3, wherein, The auxiliary absorption network (2) includes a quarter-wavelength microstrip line ML4, a quarter-wavelength microstrip line ML5, a quarter-wavelength microstrip line ML6, a quarter-wavelength microstrip line ML7, and a resistor R1.

5. The N-coupling and radial stubs based absorptive dual-band filter according to claim 4, wherein, The quarter-wavelength microstrip line ML5 and resistor R1 are connected in series, and the quarter-wavelength microstrip lines ML6 and ML7 are connected in series, and then connected in series with the quarter-wavelength microstrip line ML4. The quarter-wavelength microstrip lines ML4 in the two auxiliary absorption networks (2) are connected in parallel at the input and output nodes of the main dual-frequency filter network (1).

6. The N-coupling and radial stubs based absorptive dual-band filter according to claim 5, wherein, The high-frequency resonance suppression network (3) includes a microstrip transmission line (15) connected in series on the main path, with ports (10) at both ends of the microstrip transmission line (15); multiple sets of triangular metal patches are symmetrically arranged on the upper and lower sides of the microstrip transmission line (15), the vertices of the first pair of triangular patches (11) and the second pair of triangular patches (12) intersect and connect with the microstrip transmission line (15), and the third pair of triangular patches (13) are connected to the microstrip transmission line (15) through microstrip stubs (14).

7. The N-coupling and radial stubs based absorptive dual-band filter according to claim 6, wherein, The main dual-frequency filter network is an N-type microstrip coupler structure. Two pairs of quarter-wavelength microstrip lines ML1 and ML3 are used to generate electromagnetic coupling, and quarter-wavelength microstrip line ML2 is an uncoupled through-line microstrip line used to provide additional phase delay.

8. The N-coupling and radial stubs based absorptive dual-band filter according to claim 7, wherein, Two pairs of quarter-wavelength microstrip lines, ML1 and ML3, are interconnected internally to form a multipath interference network with two transmission poles.

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