Wideband balanced bandpass filter based on stub-loaded resonators

Abstract

The application provides a wideband balanced band-pass filter based on a stub-loaded resonator, and solves the problems of strong common-mode interference, narrow passband, low selectivity and large size. The filter comprises two U-shaped microstrip lines, an asymmetric first stepped impedance microstrip line and an asymmetric second stepped impedance microstrip line printed on the upper surface of a dielectric substrate, and a stepped impedance gap line and a rectangular gap line etched on a metal ground plate. A metal through hole penetrates through the whole substrate. The application separates common-mode and differential-mode by using a U-shaped microstrip-to-rectangular gap line structure, realizes good common-mode suppression, introduces a transmission zero point by using a path difference of an L-shaped rotary microstrip stub, and improves selectivity and reduces size. The application has the characteristics of super bandwidth, small size, high selectivity and good common-mode suppression, and is applied to the fields of electromagnetic microwave and radio frequency circuit and system.

Description

Technical Field

[0001] This invention belongs to the field of microwave and radio frequency technology, and mainly relates to the structure and design of bandpass filters. Specifically, it is a broadband balanced bandpass filter based on a stub load resonator, which can be applied to the radio frequency front end of a wireless communication system. Background Technology

[0002] In many existing wireless communication systems, higher demands are being placed on the bandwidth and transmission rate of microwave components. In recent years, with the rapid development of wireless communication technology, filters, as a key frequency selection device, have played an increasingly important role, and their performance often directly affects the quality of the entire communication system. Microstrip bandpass filters are widely used due to their advantages such as small size, light weight, ability to generate arbitrary transmission zero distribution, and asymmetric suppression characteristics. On the other hand, with the increasing complexity of the modern electromagnetic environment, the requirements for common-mode rejection capability are becoming increasingly stringent, thus differential circuits have been proposed and have received widespread attention. Differential circuits are widely used due to their excellent common-mode (CM) interference rejection capability and improved system dynamic range. The design of differential filters with common-mode noise suppression is of great significance in balanced circuits, where the suppression of environmental noise, interference, and crosstalk is a key advantage of single-ended circuits. Based on these ideas, medium-, narrow-band, dual-band, and ultra-wideband (UWB) balanced filters have been reported. Most of these common-mode suppression balanced filters are based on distributed components, and filter optimization requires parametric analysis. For example, in UWB balanced filters, common-mode rejection is achieved by connecting the branch portion of the open-circuit stub along the plane of symmetry, or by using an open parallel coupling line. Microstrip-slot (MS) transition structures are widely used in differential passive devices due to their inherent immunity to CM interference and independent differential-mode (DM) response.

[0003] To improve the performance of balanced bandpass filters, including in-band notch filtering, out-of-band rejection, out-of-band selectivity, and common-mode rejection, research on balanced bandpass filters has attracted increasing attention from scholars both domestically and internationally. For example, in their paper "A Novel Compact Capacitive Loaded Differential Bandpass Filter," published at the 2013 Asia-Pacific Microwave Conference Proceedings (2013, pp. 933-935), Hui Wang et al. proposed a compact differential bandpass filter (BPF) with a parallel coupled resonator and a capacitive load. By properly designing the coupled resonator and load capacitor, differential mode passband response and common-mode rejection can be obtained. In common-mode operation, the load capacitor network provides a broadband attenuation response to suppress common-mode signals.

[0004] The paper proposes a coupled resonator with a load capacitance. By controlling two parts of the filter—part A, which provides the series capacitance, and part B, which provides the inductance—the center frequency and bandwidth of the passband can be adjusted. Furthermore, when a common-mode input occurs, the inductance generated by part B is replaced by a capacitor, while part A continues to provide the series capacitance, forming a pure capacitive attenuation network with broadband hysteresis performance, thus achieving excellent common-mode rejection.

[0005] However, since the inductive resonant coupling method cannot sensitively adjust the notch position, it cannot introduce a transmission zero point to enhance out-of-band rejection characteristics and thus improve passband selectivity. In addition, single-ended signal transmission has the disadvantage of poor common-mode rejection characteristics. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of the prior art by proposing a balanced ultrawideband bandpass filter based on a multimode slot line resonator with high selectivity and good common-mode suppression.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a broadband balanced bandpass filter based on a stub load resonator, including a dielectric substrate, wherein the origin of the XOY rectangular coordinate system is set at the center of the dielectric substrate, and a metal ground plane is printed on the lower surface of the dielectric substrate and a bandpass filter structure is printed on the upper surface. The lower surface of the metal floor is etched with a slit line structure; The bandpass filter structure includes two U-shaped microstrip lines 2 with openings facing away from each other; the opening of each U-shaped microstrip line 2 coincides with the edge of the dielectric substrate 1; The bandpass filter structure also includes two L-shaped impedance microstrip lines 9 that are mirror-symmetric about the Y-axis, an asymmetric first-step impedance microstrip line stub 7, and an asymmetric second-step impedance microstrip line stub 8. Both the asymmetric first-step impedance microstrip line stub 7 and the asymmetric second-step impedance microstrip line stub 8 are L-shaped cycloid stubs. The inner surface of the L-shaped impedance microstrip line 9 is distributed with respect to the origin of the XOY coordinate system. The slot line structure includes a pair of stepped impedance slot lines 5 and a pair of rectangular low impedance slot lines 6, each mirror-symmetrical about the Y-axis. The stepped impedance slot lines 5 and the rectangular low impedance slot lines 6 on the same side of the Y-axis are connected. The stepped impedance slot lines 5 are located below the U-shaped microstrip line. The dielectric substrate 1 is also etched with two through-holes 3, each of which is connected to the L-shaped impedance microstrip line 9 and the stepped impedance slot line 5 located on the same side of the Y-axis.

[0008] In some embodiments, the present invention further includes the following technical features: The two U-shaped microstrip lines 2 are mirror-symmetric about the Y-axis of the XOY rectangular coordinate system.

[0009] Each of the U-shaped microstrip lines 2 includes a rectangular microstrip base 2-2 and two rectangular microstrip arms 2-1 connected to both ends of the rectangular microstrip base. Both U-shaped microstrip lines can be used as input or output microstrip lines and are mirror-symmetric about the Y-axis in the XOY coordinate system.

[0010] The rectangular microstrip base is perpendicular to the X-axis, and the rectangular microstrip arm is parallel to the X-axis.

[0011] The linewidths of the rectangular microstrip base and the rectangular microstrip arm are equal.

[0012] The starting edges of the L-shaped spiral stubs of the asymmetric first-step impedance microstrip line stub 7 and the asymmetric second-step impedance microstrip line stub 8 are both parallel to the Y-axis; the asymmetric first-step impedance microstrip line stub 7 spirals counterclockwise 90° along the X direction, and the asymmetric second-step impedance microstrip line stub 8 spirals clockwise along the Y-axis, each time spiraling 90°.

[0013] The asymmetric first-step impedance microstrip line stub 7 includes a low-impedance L-shaped microstrip stub 7-1 and a single-line high-impedance microstrip line 7-2 extending along the positive X-axis direction and connected to the center of the short side inside the low-impedance L-shaped microstrip stub; the asymmetric second-step impedance microstrip line stub 8 is an L-shaped spiral microstrip stub extending along the negative X-axis direction, with the spiral L-shaped stub located on the upper side of the X-axis and the end stubs distributed along the negative X-axis direction. The low-impedance L-shaped microstrip stub and the spiral L-shaped stub in the asymmetric second-step impedance microstrip line stub 8 are parallel about the X-axis.

[0014] The stepped impedance slot line 5 is a straight stepped impedance slot line, including a low-impedance rectangular slot line 5-1 and a high-impedance linear slot line 5-2 connected to each other. The end of the high-impedance linear slot line is connected to the short rectangular side of the rectangular low-impedance slot line 6. The rectangular low-impedance slot line 6 is rotated 90° relative to the high-impedance linear slot line.

[0015] Each of the U-shaped microstrip lines 2 includes a rectangular microstrip base and two rectangular microstrip arms connected to both ends of the rectangular microstrip base. The low-impedance rectangular slot line is etched at the middle position below the rectangular microstrip base and is parallel to the edge of the rectangular microstrip base.

[0016] The upper end of the metal through-hole 3 is connected to the L-shaped starting end of the L-shaped impedance microstrip line 9, and the lower end is connected to the end of the high-impedance linear slot line in the stepped impedance slot line 5.

[0017] This invention solves the technical problems of small size, narrow bandwidth, low selectivity and strong common-mode interference.

[0018] Compared with the prior art, the present invention has the following advantages: 1. Multiple resonant points achieve good out-of-band suppression and high selectivity: The present invention uses two asymmetric first-step impedance microstrip line stubs and two asymmetric second-step impedance microstrip line stubs printed on the dielectric substrate. By changing the position of the stubs, two transmission poles are introduced in the passband, achieving good out-of-band suppression characteristics in the passband.

[0019] 2. Small size: The present invention uses two asymmetric first-step impedance microstrip line stubs and asymmetric second-step impedance microstrip line stubs printed on the dielectric substrate. By placing the stubs of different lengths in an L-shape and adding metal vias, the size of the filter can be effectively reduced, making the present invention competitive among similar filters. The structure is simple and flexible and does not increase the size of the filter element.

[0020] 3. Good common-mode suppression effect: The present invention uses a U-shaped microstrip line structure on the upper surface of the dielectric substrate, and two stepped impedance slot lines mirror-symmetric about the Y-axis of the XOY plane rectangular coordinate system are etched on the metal floor. Together, they form a microstrip-to-slot structure, which makes the differential mode signal independent of the common mode signal, achieving good common-mode suppression. In addition, the U-shaped microstrip line structure effectively reduces the size and realizes the miniaturization of the board material. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of the present invention; Figure 2 This is a diagram showing the interrelationship of the structures on the upper and lower surfaces of the dielectric substrate of the present invention; Figure 3 This is a diagram showing the dimensions of the various structures on the upper surface of the dielectric substrate of the present invention; Figure 4 This is a diagram showing the dimensions of the various structures on the lower surface of the dielectric substrate of the present invention. Figure 5 The measured S-parameters of the differential-mode return response and differential-mode insertion loss of this invention are shown. Figure 6 The above are measured S-parameter graphs of the common-mode return response and common-mode insertion loss of this invention.

[0022] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Detailed Implementation

[0023] Microwave filters are a crucial component of modern communication systems, and their performance directly impacts the overall communication quality. Existing microwave filters are mostly large in size, and with the increasing complexity of the modern electromagnetic environment, there is significant out-of-band noise interference. Traditional microstrip ultra-wideband multimode filters suffer from drawbacks such as narrow stopband, poor out-of-band selectivity, and large size, failing to meet the high-performance requirements of ultra-wideband filters. Therefore, researching broadband filters with good out-of-band suppression characteristics and miniaturization is of great significance and practical necessity. This invention proposes a broadband balanced bandpass filter based on a stub-loaded resonator.

[0024] Reference Figure 1 and Figure 2 The present invention includes a rectangular dielectric substrate 1 and a metal ground plane 4 stacked on top of each other. The dielectric substrate 1 is made of rectangular Rogers RT / duroid 5880 material with a relative permittivity of 2.2, a loss tangent of 0.0009, a size of 82.8mm*29.8mm, and a thickness of 0.8mm. The line connecting the centers of the two short sides of the dielectric substrate 1 coincides with the X-axis.

[0025] Reference Figure 3 The dielectric substrate 1 of this invention has two U-shaped microstrip lines 2 with opposite openings and two metal vias 3 printed on its upper surface. The two U-shaped microstrip lines 2 and the two metal vias 3 are mirror-symmetrical about the Y-axis of a Cartesian coordinate system XOY distributed on the upper surface of the dielectric substrate 1. The openings of the U-shaped microstrip lines 2 coincide with the edges of the dielectric substrate 1. The widths of the rectangular microstrip arms parallel to the X-axis and the rectangular microstrip base perpendicular to the X-axis in the U-shaped microstrip lines 2 are equal. The line connecting the midpoints of the short sides of the two metal vias 3 coincides with the X-axis of the Cartesian coordinate system XOY. The length of the microstrip base is L1 = 16 mm, and the width is W1 = 2.4 mm. The length of the microstrip arm is L0 = 10 mm, and the width is W0 = 2.4 mm.

[0026] Reference Figure 3The upper surface of the dielectric substrate 1 of the present invention is printed with an L-shaped impedance microstrip line 9, an asymmetric first-step impedance microstrip line stub 7, and an asymmetric second-step impedance microstrip line stub 8. The L-shaped impedance microstrip line 9 has an L-shaped side length of Lr2 = 6 mm and a width of Wr2 = 1 mm, and an L-shaped base length of Lr3 = 5 mm and a width of Wr3 = 1 mm. The asymmetric first-step impedance microstrip line stub 7 includes a low-impedance L... The system consists of a low-impedance L-shaped microstrip stub and a single-line high-impedance microstrip line extending along the X-axis from the center of the short side inside the low-impedance L-shaped microstrip stub. The L-shaped base of the low-impedance L-shaped microstrip stub has a length of Lrr3 = 2 mm and a width of Wrr3 = 6 mm. The L-shaped side of the low-impedance L-shaped microstrip stub has a length of Lrr4 = 6 mm and a width of Wrr4 = 28.5 mm. The single-line high-impedance microstrip line has a length of Lrr2 = 40 mm and a width of Wrr2 = 0.3 mm. The asymmetric second-step impedance microstrip line stub 8 is an L-shaped spiral microstrip stub extending along the negative X-axis. The spiraling L-shaped stub is located on the upper side of the X-axis, and the terminal L-shaped stubs are distributed along the positive X-axis. The base length of the L-shaped spiral stub is Lrr5 = 14.5 mm, and the width is Wrr5 = 2.4 mm. The side length of the L-shaped spiral stub is Lrr6 = 40 mm, and the width is Wrr6 = 2.4 mm. The terminal L-shaped stub spirals along the negative X-axis. The L-shaped base has a length of Lrr7 = 2.5 mm and a width of Wrr7 = 2.4 mm. The L-shaped side of the terminal L-shaped stub has a length of Lrr8 = 2.4 mm and a width of Wrr8 = 12 mm. The low-impedance L-shaped microstrip stub in the asymmetric first-step impedance microstrip line stub 7 and the L-shaped stub in the spiral direction of the asymmetric second-step impedance microstrip line stub 8 are parallel about the X-axis. The L-shaped impedance microstrip line 9 is mirror-symmetric about the Y-axis, and the inner surface of the L-shaped stub is distributed with respect to the origin of the XOY coordinate system. Reference Figure 3 and Figure 4 The end of the high-impedance linear slot in the stepped impedance slot line 5 is connected to the short rectangular side of the rectangular low-impedance slot line 6. The metal via 3 is located at the end of the high-impedance linear slot and the beginning of the L-shaped impedance microstrip line 9, penetrating the entire substrate. The diameter of the metal via 3 is Lr1=0.6mm. Reference Figure 4The metal floor 4 is etched with stepped impedance slot lines 5 and rectangular low-impedance slot lines 6. Stepped impedance slot line 5 is a straight segment comprising a low-impedance rectangular slot line and a high-impedance linear slot line. The low-impedance rectangular slot line has a length of Ls1 = 11 mm and a width of Ws1 = 8 mm, while the high-impedance linear slot line has a length of Ls2 = 7.4 mm and a width of Ws2 = 0.1 mm. The end of the high-impedance linear slot line in stepped impedance slot line 5 intersects with the rectangular low-impedance slot line. The short rectangular sides of the anti-slot line 6 are connected; the rectangular low-impedance slot line 6 is rotated 90° counterclockwise from the high-impedance linear slot line in the stepped impedance slot line 5; the length of the rectangular low-impedance slot line 6 is Ls4=10mm and the width is Ws4=4mm; the low-impedance rectangular slot line in the stepped impedance slot line 5 is etched to correspond to the middle position below the bottom of the rectangular microstrip in the U-shaped microstrip line 2, and is parallel to the bottom edge of the rectangular microstrip; the stepped impedance slot line 5 and the rectangular low-impedance slot line 6 are mirror-symmetrical about the Y-axis. Traditional microstrip broadband bandpass filters suffer from poor out-of-band selectivity, poor common-mode rejection, and large size, failing to meet the high-performance requirements of ultra-wideband filters. This invention addresses these problems by utilizing an L-shaped swirl stub structure with different paths, a U-shaped microstrip-to-slot structure, and synthesizing a balanced broadband bandpass filter based on a multimode slot-line resonator.

[0027] In this invention, the signal is input through a U-shaped microstrip line on one side. Differential-mode and common-mode signals enter simultaneously. The differential-mode signal enters the stepped impedance slot line on the same side, exciting its electric field. The common-mode signal cannot enter the stepped impedance slot line structure on the same side below, thus separating the differential-mode and common-mode signals and suppressing the common-mode signal. The differential-mode signal is transmitted from the stepped impedance slot line to the rectangular low-impedance slot line on the same side, and coupled to the upper L-shaped impedance microstrip line through a metal via, before being transmitted into the upper L-shaped cyclotron microstrip stub. When the signal enters the upper L-shaped cyclotron microstrip stub, a portion of the signal enters the L-shaped low-impedance microstrip stub within it, coupling to the end of the lower L-shaped cyclotron stub. At this point, this signal and the signal entering the linear high-impedance microstrip line from the upper L-shaped impedance microstrip line have different paths, creating a transmission signal difference and introducing two transmission zeros. This results in good out-of-band suppression characteristics. By changing the position and length of the L-shaped cyclotron stub, the position and number of introduced zeros can be changed. Furthermore, swirl of the L-shaped microstrip stub can effectively reduce the physical size of the filter. Finally, the signal is converged to the L-shaped impedance microstrip line at the other end, coupled to the lower rectangular low-impedance slot line through a metal via, and then transmitted to the stepped impedance slot line, thereby coupling to the upper U-shaped microstrip line and outputting the signal, realizing a broadband balanced bandpass filter based on a stub-loaded resonator.

[0028] This invention presents an overall technical solution for a broadband balanced bandpass filter based on a stub load resonator. It improves the common-mode rejection characteristics by using a microstrip-to-slot structure feeding method that replaces the impedance slot line with a U-shaped microstrip line for impedance slot line feeding. It introduces transmission zeros by utilizing the path difference of the L-shaped microstrip swirl stubs on the upper and lower sides, thereby improving the filter's out-of-band rejection characteristics and selectivity. Furthermore, it effectively reduces the physical size of the filter by introducing metal vias and the stub swirl method.

[0029] Currently proposed bandpass filters all require load matching at the port. This invention sets the linewidth of the microstrip arm to a 50Ω impedance linewidth, which can achieve good load matching and is easy to process.

[0030] This invention is a broadband balanced bandpass filter based on a stub load resonator. This filter has the advantages of high common-mode rejection, small size, and high out-of-band signal rejection.

[0031] The technical effects of this invention will be further explained below based on actual test results:

[0032] Measurement experiments one and two were conducted on the broadband balanced bandpass filter based on the stub load resonator in this invention using a vector network analyzer N5230A. The input signals were common-mode and differential-mode signals, and the output signals were the common-mode and differential-mode signals after being filtered by this invention.

[0033] Experiment 1 tests the differential-mode echo response of the balanced ultrawideband bandpass filter of this invention. Sum and difference mode insertion loss The experimental results are shown in Figure 5 , Figure 5 The above are measured S-parameter graphs of the differential mode return response and differential mode insertion loss of this invention.

[0034] Experiment 2 tested the common-mode echo response of the balanced ultrawideband bandpass filter. and common-mode insertion loss The experimental results are shown in Figure 6 , Figure 6 The above are measured S-parameter graphs of the common-mode return response and common-mode insertion loss of this invention.

[0035] See Figure 5 , Figure 5 The differential-mode echo response of the broadband balanced bandpass filter based on a stubby load resonator is presented in this invention. Sum and difference mode insertion loss Measured S-parameter graphs Figure 5 The horizontal axis represents the response frequency, in GHz. Figure 5 The ordinate represents the differential mode echo response. Intermediate mode insertion loss The numerical value is expressed in dB, where the curve with the solid black square icon represents the differential mode echo response. This means that the signal is input from the port and reflected back from the same port; the curve with the hollow circle icon represents the differential insertion loss. This means that after a signal enters from one port, it is reflected back from the other port. In this embodiment, the 3dB bandwidth of the broadband bandpass filter ranges from 2.5 to 4.5 GHz, with a relative bandwidth of approximately 57%. The center frequency within the passband is 3.5 GHz, generating two transmission zeros at 2.3 GHz and 4.4 GHz to improve out-of-band selectivity. The attenuation exceeds 37 dB at 2.3 GHz and exceeds 46 dB at 4.4 GHz. The maximum differential-mode echo response within the passband... The minimum differential insertion loss is 18.0 dB. 1.1dB; from Figure 5 As can be seen, the out-of-band suppression performance of the filter of this invention is excellent.

[0036] See Figure 6 , Figure 6 This invention relates to the common-mode echo response of a broadband balanced bandpass filter based on a stubby load resonator. and common-mode insertion loss Measured S-parameter graphs Figure 6 The horizontal axis represents the response frequency, in GHz. Figure 6 The ordinate represents the common-mode echo response. and common-mode insertion loss The numerical value is expressed in dB, where the curve with the black square icon represents the common-mode echo response. The curve with the hollow circle icon represents the common-mode insertion loss. . Figure 6 Common-mode echo response The common-mode insertion loss is less than or equal to 0.37 dB within the operating frequency range. The filter exhibits a stability of ≥37dB across the entire operating frequency band, with an extreme value of 60dB within the operating band. This demonstrates that the filter of this invention is relatively stable throughout the entire operating frequency band. Figure 6 The experimental data verified the significant common-mode suppression effect of the balanced ultrawideband bandpass filter of the present invention.

[0037] In summary, the broadband balanced bandpass filter based on a stub load resonator of this invention solves the technical problems of strong common-mode interference, narrow passband, low selectivity, and large size of filters currently on the market. The filter of this invention includes two U-shaped microstrip lines, two L-shaped impedance microstrip lines, and stubs of an asymmetric first-step impedance microstrip line and an asymmetric second-step impedance microstrip line, all printed on the upper surface of a dielectric substrate. The asymmetric first-step impedance microstrip line stub includes a low-impedance L-shaped microstrip stub and a single-line high-impedance microstrip line extending along the X-axis from the center of the short side inside the low-impedance L-shaped microstrip stub. By introducing two transmission zeros through the path difference between the asymmetric first-step impedance microstrip line and the asymmetric second-step impedance microstrip line, the out-of-band rejection characteristics and selectivity are improved. A metal ground plane is laid on the lower surface of the dielectric substrate. Two stepped impedance slot lines and two linear high-impedance slot lines are etched on the metal ground plane. The stepped impedance slot lines and rectangular low-impedance slot lines are also present. The low-impedance rectangular slot line within the stepped impedance slot lines is etched to the middle position below the rectangular microstrip bottom of the U-shaped microstrip line and is parallel to the bottom edge of the rectangular microstrip. This microstrip-to-slot line feeding structure effectively improves common-mode rejection characteristics. By introducing a metal via 3 that runs through the entire dielectric substrate, connecting the upper microstrip line and the lower slot lines, the physical size of the filter can be effectively reduced. This invention has the advantages of wide bandwidth, small size, high selectivity, and good common-mode rejection characteristics, and is widely used in the fields of electromagnetic microwave and radio frequency circuits and systems.

Claims

1. A broadband balanced bandpass filter based on a stub load resonator, comprising a dielectric substrate, wherein the origin of an XOY rectangular coordinate system is located at the center of the dielectric substrate, characterized in that, The dielectric substrate has a metal ground plane printed on its lower surface and a bandpass filter structure printed on its upper surface. The lower surface of the metal floor is etched with a slit line structure; The bandpass filter structure includes two U-shaped microstrip lines (2) with openings facing away from each other; the opening of each U-shaped microstrip line (2) coincides with the edge of the dielectric substrate (1); The bandpass filter structure also includes two L-shaped impedance microstrip lines (9) that are mirror-symmetric about the Y-axis, an asymmetric first-step impedance microstrip line stub (7), and an asymmetric second-step impedance microstrip line stub (8). The asymmetric first-step impedance microstrip line stub (7) and the asymmetric second-step impedance microstrip line stub (8) are both L-shaped cycloid stubs. The inner surface of the L-shaped impedance microstrip line (9) is distributed with respect to the origin of the XOY coordinate system. The slot line structure includes a pair of stepped impedance slot lines (5) and a pair of rectangular low impedance slot lines (6) that are mirror-symmetrical about the Y-axis. The stepped impedance slot lines (5) and the rectangular low impedance slot lines (6) on the same side of the Y-axis are connected. The stepped impedance slot lines (5) are located below the U-shaped microstrip line. The dielectric substrate (1) is also etched with two through-holes (3) through the dielectric substrate (1), each of the through-holes (3) being connected to the L-shaped impedance microstrip line (9) and the stepped impedance slot line (5) on the same side of the Y-axis. The asymmetric first-step impedance microstrip line stub (7) includes a low-impedance L-shaped microstrip stub (7-1) and a single-line high-impedance microstrip line (7-2) connected to the center of the short side inside the low-impedance L-shaped microstrip stub and extending along the positive X-axis; the asymmetric second-step impedance microstrip line stub (8) is an L-shaped spiral microstrip stub extending along the negative X-axis, the spiral L-shaped stub is located on the upper side of the X-axis, and the end stub is distributed along the negative X-axis. The low-impedance L-shaped microstrip stub and the spiral L-shaped stub in the asymmetric second-step impedance microstrip line stub (8) are parallel to the X-axis.

2. A broadband balanced bandpass filter based on a stub load resonator according to claim 1, characterized in that, The two U-shaped microstrip lines (2) are mirror-symmetric about the Y-axis of the XOY rectangular coordinate system.

3. A broadband balanced bandpass filter based on a stub load resonator according to claim 1, characterized in that, Each of the U-shaped microstrip lines (2) includes a rectangular microstrip base (2-2) and two rectangular microstrip arms (2-1) connected to both ends of the rectangular microstrip base. Both U-shaped microstrip lines can be used as input or output microstrip lines and are mirror-symmetric about the Y-axis in the XOY coordinate system.

4. A broadband balanced bandpass filter based on a stub load resonator according to claim 3, characterized in that, The rectangular microstrip base is perpendicular to the X-axis, and the rectangular microstrip arm is parallel to the X-axis.

5. A broadband balanced bandpass filter based on a stub load resonator according to claim 3, characterized in that, The linewidths of the rectangular microstrip base and the rectangular microstrip arm are equal.

6. A broadband balanced bandpass filter based on a stub load resonator according to claim 1, characterized in that, The starting edges of the L-shaped spiral stubs of the asymmetric first-step impedance microstrip line (7) and the asymmetric second-step impedance microstrip line (8) are both parallel to the Y-axis; the asymmetric first-step impedance microstrip line stub (7) spirals counterclockwise 90° along the X direction, and the asymmetric second-step impedance microstrip line stub (8) spirals clockwise along the Y-axis, each time spiraling 90°.

7. A broadband balanced bandpass filter based on a stub load resonator according to claim 1, characterized in that, The stepped impedance slot line (5) is a straight stepped impedance slot line, including a low impedance rectangular slot line (5-1) and a high impedance linear slot line (5-2) connected to each other. The end of the high impedance linear slot line is connected to the short rectangular side of the rectangular low impedance slot line (6). The rectangular low impedance slot line (6) is rotated 90° relative to the high impedance linear slot line.

8. A broadband balanced bandpass filter based on a stub load resonator according to claim 7, characterized in that, Each of the U-shaped microstrip lines (2) includes a rectangular microstrip base and two rectangular microstrip arms connected to both ends of the rectangular microstrip base. The low-impedance rectangular slot line is etched at the middle position below the rectangular microstrip base and is parallel to the edge of the rectangular microstrip base.

9. A broadband balanced bandpass filter based on a stub load resonator according to claim 7, characterized in that, The upper end of the metal through hole (3) is connected to the L-shaped starting end of the L-shaped impedance microstrip line (9), and the lower end is connected to the end of the high-impedance linear slot line in the stepped impedance slot line (5).

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