Microstrip band elimination filter with wide upper passband

Abstract

The invention discloses a microstrip band-stop filter with a wide upper passband, and relates to the technical field of radio frequency passive devices, and the microstrip band-stop filter comprises an electromagnetic shielding cavity, a dielectric substrate, a microstrip circuit located on the front surface of the dielectric substrate, and a grounding metal layer located on the back surface of the dielectric substrate. The microstrip circuit is composed of an input feeder line, an output feeder line and a four-stage cascade filter network located between the input feeder line and the output feeder line. The stopband center frequencies of the four filtering units are distributed in a staggered manner so as to broaden the stopband bandwidth; each filtering unit is composed of two U-shaped micro-strips and an interdigital capacitor, and symmetrical rectangular split ring resonators loaded with the interdigital capacitors and double-path arc-shaped parallel micro-strip lines are adopted at the interstage connecting positions of the filtering units respectively. According to the invention, the low insertion loss, high selectivity and miniaturization design of the filter are realized, and the technical problem that the existing band elimination filter is difficult to adapt to a complex and changeable signal environment due to the limited passband width is solved.

Description

Technical Field

[0001] This invention relates to the field of radio frequency passive device technology, specifically a microstrip bandstop filter with a wide upper passband. Background Technology

[0002] As wireless communication systems evolve towards higher frequencies, larger capacities, and higher sensitivity, extremely stringent requirements are being placed on the performance of radio frequency (RF) front-end filters. Especially in cutting-edge applications such as deep space exploration, radio astronomy, and superconducting quantum computing, systems not only need to process weak, effective signals but also must possess the ability to suppress strong interference signals in complex electromagnetic environments.

[0003] Band-stop filters, as a key component in radio frequency systems, are mainly used to suppress strong interference signals within specific frequency bands. Traditional band-stop filters have two inherent drawbacks: first, conductor losses lead to large passband insertion losses, directly affecting the system's signal-to-noise ratio; second, the traditional quarter-wavelength structure generates parasitic stopbands at odd harmonic frequencies, severely disrupting the continuity of the upper passband and limiting its application in broadband systems.

[0004] The emergence of superconducting thin-film technology has provided a solution to the insertion loss problem. Superconducting materials exhibit extremely low surface resistance at low temperatures, enabling microstrip filters made from them to achieve lower insertion losses than ordinary metal filters. However, when applying superconducting technology to band-stop filters, new challenges arise: achieving a wide upper passband typically requires complex multi-resonator structures. Fabricating the circuit on expensive superconducting substrates drastically increases costs, contradicting the trend towards miniaturization and integration in modern electronic systems. Therefore, a core problem urgently needs to be solved in current technology: how to simultaneously achieve the two key characteristics of a wide upper passband and small size while maintaining the low-loss advantage of superconductivity. To address this, we propose a microstrip band-stop filter with a wide upper passband. Summary of the Invention

[0005] The purpose of this invention is to provide a microstrip bandstop filter with a wide upper passband.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a microstrip bandstop filter with a wide upper passband, the microstrip bandstop filter comprising an electromagnetic shielding cavity, a dielectric substrate, a microstrip circuit located on the front side of the dielectric substrate, and a grounding metal layer located on the back side of the dielectric substrate;

[0007] The microstrip circuit includes a 50-ohm input feed line, a 50-ohm output feed line, and a cascaded filter network connected between them. The cascaded filter network consists of a first filter unit, a first interstage coupling structure, a second filter unit, a second interstage coupling structure, a third filter unit, a third interstage coupling structure, and a fourth filter unit, arranged sequentially along the signal transmission direction. The first, second, third, and fourth filter units are each composed of two U-shaped microstrip segments and interdigital capacitors. The first and third interstage coupling structures are symmetrical rectangular open-loop resonators loaded with interdigital capacitors, and the second interstage coupling structure is a dual-path arc-shaped parallel microstrip line structure.

[0008] As a further aspect of the present invention: the relative permittivity of the dielectric substrate is not less than 4, and the thickness is not greater than one-eighth of the wavelength in the dielectric at the highest operating frequency.

[0009] As a further aspect of the present invention: the first filter unit, the second filter unit, the third filter unit, and the fourth filter unit have the same topology. Each of the first filter unit, the second filter unit, the third filter unit, and the fourth filter unit includes a first U-shaped microstrip, a second U-shaped microstrip, and an interdigital capacitor. One end of the first U-shaped microstrip is connected to a 50-ohm main transmission line through a series-loaded interdigital capacitor, and the other end is directly connected to the 50-ohm main transmission line. Both ends of the second U-shaped microstrip are directly connected to the 50-ohm main transmission line. The first U-shaped microstrip branch and the second U-shaped microstrip branch form a parallel transmission path in structure. A specific transmission zero is generated by using an asymmetrical connection method. The transmission zero can enhance the isolation between the stopband and the passband, improve the out-of-band rejection roll-off steepness, and coordinate with the staggered distribution of the stopband center frequency of the four-stage filter unit. Finally, the entire filter stopband (2.45GHz-2.85GHz) and the upper and lower passbands are accurately separated, ensuring the filter's high selectivity and wide upper passband performance.

[0010] As a further aspect of the present invention: the first U-shaped microstrip and the second U-shaped microstrip of the first filter unit, the second filter unit, the third filter unit and the fourth filter unit have different parameters from the interdigital capacitor, and the interdigital capacitor parameters of each filter unit are adapted to the corresponding U-shaped microstrip parameter settings. Through the above parameter differentiation design, the stopband center frequencies of the first to fourth filter units are distributed in a staggered manner, thereby widening the overall stopband bandwidth of the filter.

[0011] As a further aspect of the present invention: the first filter unit, the second filter unit, the third filter unit, and the fourth filter unit are arranged in an alternating manner. In two adjacent filter units, the first U-shaped microstrip branch of the preceding filter unit and the first U-shaped microstrip branch of the following filter unit are alternately located on both sides of the main transmission line along the extension direction of the 50-ohm main transmission line. This alternating arrangement design can optimize the compactness of the circuit layout, reduce electromagnetic coupling interference between adjacent filter units, and coordinate the transmission zero-point characteristics of each filter unit to further improve the out-of-band selectivity of the filter.

[0012] As a further aspect of the present invention: the specific construction of the first-stage inter-coupling structure and the third-stage inter-coupling structure is as follows: a 50-ohm main transmission line extension connecting adjacent filter units, and a pair of resonators symmetrically arranged on both sides of the main transmission line. The width of the main transmission line is different from the width of the adjacent 50-ohm main transmission line. A linearly tapered microstrip is used at the connection point. Both resonators are rectangular open-loop resonators, and a planar interdigital capacitor is loaded at the opening of each rectangular open-loop. The symmetrical resonators are not directly conductively connected to the main transmission line, but are coupled through electromagnetic gaps.

[0013] As a further aspect of the present invention: the specific construction of the second interstage coupling structure is as follows: a dual-path parallel microstrip line connecting the 50-ohm main transmission line at the output end of the second filter unit and the 50-ohm main transmission line at the input end of the third filter unit. The dual-path parallel microstrip line is composed of two physically symmetrical 100-ohm U-shaped microstrip lines. The arc centers of the two U-shaped microstrip lines are connected by a high-impedance line to maintain interstage impedance matching and reduce the lateral length of the circuit.

[0014] As a further aspect of the present invention: the U-shaped microstrip includes a first U-shaped microstrip and a second U-shaped microstrip in a first filter unit, a second filter unit, a third filter unit, and a fourth filter unit, as well as a 100-ohm U-shaped microstrip line in the second-stage inter-coupling structure. The radius of the arc-shaped bending portion of the U-shaped microstrip is greater than three times the width of the corresponding microstrip line. This size design can ensure the impedance continuity of the microstrip bending portion, avoid the problems of reflection and increased loss during signal transmission, and ensure low insertion loss characteristics within the passband of the filter.

[0015] Compared with the prior art, the beneficial effects of the present invention by adopting the above technical solution are as follows:

[0016] This invention significantly reduces dielectric and conductor losses by employing a dielectric substrate with a low-loss tangent and low-loss conductive materials, thereby improving the overall quality factor, achieving lower passband insertion loss and a steeper out-of-band rejection roll-off. At the same time, by designing a fourth-order filter unit and adding an inter-stage coupling structure, a wider upper passband is obtained while ensuring high selectivity. Furthermore, by using a high dielectric constant and a thin dielectric substrate, combined with a fully planar design, a highly compact layout is achieved, significantly reducing manufacturing costs. Attached Figure Description

[0017] Figure 1 A three-dimensional structural schematic diagram of a microstrip bandstop filter with a wide upper passband provided for embodiments of this application;

[0018] Figure 2 An embodiment provided in this application Figure 1 The top view of the band-stop filter shown;

[0019] Figure 3 This is a schematic diagram of the structure of a filtering unit provided in an embodiment of this application;

[0020] Figure 4 A schematic diagram of a symmetrical rectangular open-loop resonator loaded with interdigital capacitors is provided for an embodiment of this application;

[0021] Figure 5 A schematic diagram of a dual-path arc-shaped parallel microstrip line provided in this application embodiment;

[0022] Figure 6 The figure shows the S-parameter simulation results of a microstrip bandstop filter with a wide upper passband without added interstage coupling, as provided in the embodiments of this application.

[0023] Figure 7 The figure shows the S-parameter simulation results of a microstrip bandstop filter with a wide upper passband after adding an interstage coupling structure, which is provided in the embodiment of this application.

[0024] In the diagram: 1. Microstrip circuit; 2. Dielectric substrate; 3. Grounding metal layer; 4. Electromagnetic shielding cavity; 11. Input feed line; 12. Output feed line; 13. First filter unit; 14. Second filter unit; 15. Third filter unit; 16. Fourth filter unit; 17. First stage coupling structure; 18. Second stage coupling structure; 19. Third stage coupling structure; 131. 50-ohm main transmission line; 132. First U-shaped microstrip; 133. Second U-shaped microstrip; 134. Interdigital capacitor A; 171. Linearly tapered microstrip; 172. Main transmission line; 173. Rectangular open-loop resonator; 174. Interdigital capacitor B; 181. 100-ohm U-shaped microstrip line; 182. High impedance line. Detailed Implementation

[0025] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that the description of these embodiments is for the purpose of helping to understand the present invention, but does not constitute a limitation of the present invention.

[0026] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0027] Please see the appendix Figure 1 - Appendix Figure 7 The present invention provides a microstrip bandstop filter with a wide upper passband. The microstrip bandstop filter includes an electromagnetic shielding cavity 4, a dielectric substrate 2, a microstrip circuit 1 located on the front side of the dielectric substrate 2, and a ground metal layer 3 located on the back side of the dielectric substrate 2.

[0028] The microstrip circuit 1 includes a 50-ohm input feed line 11, a 50-ohm output feed line 12, and a cascaded filter network connected between them. The cascaded filter network consists of a first filter unit 13, a first interstage coupling structure 17, a second filter unit 14, a second interstage coupling structure 18, a third filter unit 15, a third interstage coupling structure 19, and a fourth filter unit 16, arranged sequentially along the signal transmission direction. The first filter unit 13, the second filter unit 14, the third filter unit 15, and the fourth filter unit 16 are all composed of two U-shaped microstrip segments and interdigital capacitors A134. The first interstage coupling structure 17 and the third interstage coupling structure 19 are symmetrical rectangular open-loop resonators 173 loaded with interdigital capacitors B174. The second interstage coupling structure 18 is a dual-path arc-shaped parallel microstrip line structure.

[0029] In this embodiment, the dielectric substrate 2 is made of magnesium oxide material with a relative permittivity of 9.8, a loss tangent of 0.0001, a thickness of 0.5 mm, and an area of ​​19.5 mm × 5 mm. This parameter design not only meets the requirements of relative permittivity ≥ 4 and thickness ≤ one-eighth of the wavelength in the dielectric at the highest operating frequency, but also reduces dielectric loss through the low loss tangent characteristic. The microstrip circuit 1 is made of YBCO superconductor with a thickness of 1 μm. The extremely low surface resistance of the superconducting material at low temperature further reduces conductor loss.

[0030] See Figure 3 , Figure 3 This is a schematic diagram of the structure of a filter unit provided in an embodiment of this application. The first to fourth filter units 16 have the same topology. Taking the first filter unit 13 as an example, one end of the first U-shaped microstrip 132 is connected to the 50-ohm main transmission line 131 through a series-loaded interdigital capacitor A134, and the other end is directly connected to the 50-ohm main transmission line 131. Both ends of the second U-shaped microstrip 133 are directly connected to the 50-ohm main transmission line 131.

[0031] In this embodiment, the parameters of the input feed line 11 and the output feed line 12 are the same, with a characteristic impedance of 50Ω and an electrical length of 10deg.

[0032] In this embodiment, the U-shaped microstrip parameters of the first to fourth filter units 16 are different, the capacitance value is 0.8pF, and the stopband center frequency is staggered to broaden the overall stopband bandwidth. The characteristic impedances of the first U-shaped microstrip 132 and the second U-shaped microstrip 133 are Z1 and Z2, respectively, and the electrical lengths are θ1 and θ2, respectively. The characteristic impedances of the two U-shaped microstrips satisfy Z1×Z2 / (Z1+Z2)=50, and the electrical lengths satisfy θ2=k*θ1. The value of k needs to be adjusted according to the actual design requirements. The optimized parameters according to the actual design requirements are shown in Table 1. The values ​​in Table 1 are the final parameter values ​​of each order filter unit. This invention patent protects this structural shape and is not limited to the specific parameter values ​​mentioned above.

[0033] Table 1. U-shaped microstrip parameter values ​​of each order of filter unit in this embodiment.

[0034] Parameter value First filtering unit Second filtering unit Third filtering unit Fourth Filtering Unit <![CDATA[Z1 / Ω]]> 125 131.6 119 131.6 <![CDATA[Z2 / Ω]]> 83.3 80.6 86.2 80.6 <![CDATA[θ1 / deg]]> 19.8 21 22.8 21.6 <![CDATA[θ2 / deg]]> 16.5 17.5 19 18

[0035] In this embodiment, adjacent filter units are arranged in an alternating manner, that is, the first U-shaped microstrip 132 branch of adjacent filter units is located at both ends of the 50-ohm main transmission line 131.

[0036] See Figure 4 , Figure 4 This is a schematic diagram of a resonator provided in an embodiment of this application. The main transmission line 172 is connected to the 50-ohm main transmission line 131 through a linearly tapered microstrip 171. A planar interdigital capacitor B174 is loaded at the opening of the rectangular open-loop to form a rectangular open-loop resonator. The symmetrical rectangular open-loop resonator is coupled to the main transmission line 172 through an electromagnetic gap.

[0037] See Figure 5 , Figure 5 This is a schematic diagram of a dual-path parallel microstrip line provided in an embodiment of this application. Two physically symmetrical 100-ohm U-shaped microstrip lines 181 close on both sides of the central axis to form an elliptical geometric structure. The center of the arc of the two U-shaped microstrips is connected by a high-impedance line 182 to maintain interstage impedance matching and reduce the lateral length of the circuit.

[0038] In this embodiment, the radius of the U-shaped microstrip arc bend is greater than three times the width of the microstrip, ensuring the impedance continuity of the microstrip bend.

[0039] See Figures 6 to 7 , Figure 6 The figure shows the S-parameter simulation results without the addition of inter-stage coupling structure. Figure 7The simulation results of S-parameters after adding the interstage coupling structure are shown. The stopband frequency of the filter is 2.45GHz-2.85GHz, and the center frequency of the stopband is 2.65GHz. By adding the interstage coupling structure and optimizing the parameters, the upper passband is effectively extended to above 14GHz, and the suppression depth in the stopband is also increased from 20dB to more than 30dB.

[0040] Furthermore, the filter has an insertion loss of less than 0.4dB in the passband, a return loss of more than 15dB, and a substrate area of ​​only 19.5mm×5mm. It achieves low insertion loss, high selectivity, and miniaturization, solving the problem of limited passband width in existing band-stop filters.

[0041] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Any variations and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention. Therefore, any modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention, without departing from the scope of the invention, fall within the protection scope defined by the claims of the present invention.

Claims

1. A microstrip bandstop filter with a wide upper passband, characterized in that: The microstrip bandstop filter includes an electromagnetic shielding cavity (4), a dielectric substrate (2), a microstrip circuit (1) located on the front side of the dielectric substrate (2), and a ground metal layer (3) located on the back side of the dielectric substrate (2). The microstrip circuit (1) includes a 50-ohm input feed line (11), a 50-ohm output feed line (12), and a cascaded filter network connected between the two. The cascaded filter network consists of a first filter unit (13), a first interstage coupling structure (17), a second filter unit (14), a second interstage coupling structure (18), a third filter unit (15), a third interstage coupling structure (19), and a fourth filter unit (16) in sequence along the signal transmission direction. The first filter unit (13), the second filter unit (14), the third filter unit (15), and the fourth filter unit (16) are all composed of two U-shaped microstrip segments and interdigitated capacitors A (134). The first interstage coupling structure (17) and the third interstage coupling structure (19) are symmetrical rectangular open-ring resonators (173) loaded with interdigitated capacitors B (174). The second interstage coupling structure (18) is a dual-path arc-shaped parallel microstrip line structure.

2. A microstrip bandstop filter with a wide upper passband according to claim 1, characterized in that: The dielectric substrate (2) has a relative permittivity of not less than 4 and a thickness of not more than one-eighth of the wavelength in the medium at the highest operating frequency.

3. A microstrip bandstop filter with a wide upper passband according to claim 1, characterized in that: The first filter unit (13), the second filter unit (14), the third filter unit (15) and the fourth filter unit (16) have the same topology. The first filter unit (13), the second filter unit (14), the third filter unit (15) and the fourth filter unit (16) each include a first U-shaped microstrip (132), a second U-shaped microstrip (133) and an interdigital capacitor A (134). One end of the first U-shaped microstrip (132) is connected to the 50-ohm main transmission line (131) through the series-loaded interdigital capacitor A (134), and the other end is directly connected to the 50-ohm main transmission line (131). Both ends of the second U-shaped microstrip (133) are directly connected to the 50-ohm main transmission line (131). The first U-shaped microstrip (132) branch and the second U-shaped microstrip (133) branch form a parallel transmission path in structure.

4. A microstrip bandstop filter with a wide upper passband according to claim 3, characterized in that: The first U-shaped microstrip (132), the second U-shaped microstrip (133), the third filter unit (15), and the fourth filter unit (16) have different parameters from the interdigital capacitor A (134). The parameters of the interdigital capacitor A (134) of each filter unit are adapted to the corresponding U-shaped microstrip parameter settings. Through the above parameter differentiation design, the stopband center frequencies of the first to fourth filter units (16) are distributed in a staggered manner, thereby widening the overall stopband bandwidth of the filter.

5. A microstrip bandstop filter with a wide upper passband according to claim 1, characterized in that: The first filter unit (13), the second filter unit (14), the third filter unit (15) and the fourth filter unit (16) are arranged in an alternating manner. In two adjacent filter units, the first U-shaped microstrip (132) branch of the previous filter unit and the first U-shaped microstrip (132) branch of the next filter unit are alternately located on both sides of the main transmission line (172) along the extension direction of the 50-ohm main transmission line (131).

6. A microstrip bandstop filter with a wide upper passband according to claim 1, characterized in that, The specific construction of the first-stage inter-coupling structure (17) and the third-stage inter-coupling structure (19) is as follows: an extension of a 50-ohm main transmission line (131) connecting adjacent filter units, and a pair of resonators symmetrically arranged on both sides of the main transmission line (172). The width of the main transmission line (172) is different from the width of the adjacent 50-ohm main transmission line (131). A linear gradient microstrip (171) is used at the connection. Both of the resonators are rectangular open-loop resonators, and a planar interdigital capacitor B (174) is loaded at the opening of each rectangular open-loop. There is no direct conductive connection between the symmetrical resonators and the main transmission line (172). They are loaded through electromagnetic gap coupling.

7. A microstrip bandstop filter with a wide upper passband according to claim 1, characterized in that, The specific construction of the second-stage inter-coupling structure (18) is as follows: a dual-path parallel microstrip line connecting the 50-ohm main transmission line (131) at the output end of the second filter unit (14) and the 50-ohm main transmission line (131) at the input end of the third filter unit (15). The dual-path parallel microstrip line is composed of two 100-ohm U-shaped microstrip lines (181) with symmetrical physical structure. The arc center of the two U-shaped microstrip lines is connected by a high-impedance line (182).

8. A microstrip bandstop filter with a wide upper passband according to claim 1, characterized in that: The U-shaped microstrip includes a first U-shaped microstrip (132) and a second U-shaped microstrip (133) in a first filter unit (13), a second filter unit (14), a third filter unit (15) and a fourth filter unit (16), and a 100-ohm U-shaped microstrip line (181) in a second-stage inter-coupling structure (18). The radius of the arc bending portion of the U-shaped microstrip is greater than 3 times the width of the corresponding microstrip line.

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