A Ku-band second harmonic rejection filter
By combining a fifth-order hairpin resonator with open-circuit stubs and staggered slotting design, the problem of balancing high-frequency second harmonic suppression and low insertion loss is solved, achieving strong harmonic suppression and low insertion loss in the high-frequency band, which is suitable for high-frequency radio frequency systems.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHONGQING UNIV OF POSTS & TELECOMM
- Filing Date
- 2026-03-23
- Publication Date
- 2026-06-05
AI Technical Summary
Existing technologies cannot effectively suppress second harmonics at high frequencies while maintaining low insertion loss. They are also highly sensitive to manufacturing tolerances, making them difficult to apply in high-frequency scenarios.
A fifth-order hairpin resonator structure is adopted, combined with the design of open stubs and periodic misaligned slots. Through the synergistic effect of open stubs and misaligned slots, the secondary parasitic passband is shifted to higher frequencies, and the fundamental mode loss and parasitic coupling are reduced by periodic misaligned slots.
In the high-frequency band above 10GHz, the second harmonic suppression depth is ≥18dB, the insertion loss is better than 1dB, and the standing wave ratio is better than 18.9dB, meeting the high-performance filtering requirements of high-frequency radio frequency systems.
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Figure CN122158902A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of communications, and in particular to a Ku-band second harmonic suppression filter. Background Technology
[0002] In modern electronic systems such as wireless communication and radar, the frequency conversion link is the core module for signal frequency conversion. However, it faces a complex electromagnetic environment, and interference from adjacent channels and mirror images can easily lead to a decrease in signal-to-noise ratio and system blockage. Therefore, the pre-selection filter connected in series at the front end of the frequency conversion link is crucial and must meet the requirements of high out-of-band rejection, low insertion loss, and miniaturized integration.
[0003] Traditional preselective filters have obvious limitations: although waveguide filters have excellent out-of-band suppression, they are large in size, difficult to integrate, and unsuitable for large equipment; lumped parameter filters suffer from performance degradation due to parasitic parameters in the high-frequency band, have poor stability, and cannot meet the requirements of high-frequency conversion links.
[0004] Microstrip filters are preferred for high-frequency applications due to their small size, ease of integration, and low cost. Microstrip hairpin filters, with their balanced structure and performance, are particularly well-suited for pre-selective filtering requirements. Their hairpin resonators, through a folding design, shorten the length by approximately 50%, and the coupling coefficient can be controlled by adjusting the arm spacing, flexibly adapting to different bandwidth requirements. They maintain a low insertion loss of 1-3dB at high frequencies, and 3-7 resonant units can achieve out-of-band rejection of over 40dB, effectively filtering out image interference. Furthermore, high-dielectric-constant, low-loss substrates and high-precision photolithography further optimize their performance, making them widely applicable in 5G base stations, portable radar, and other scenarios, becoming a key solution for ensuring signal quality in frequency conversion links.
[0005] The 2022 National Microwave and Millimeter Wave Conference paper, "Design of a Compact Hairpin Bandpass Filter with Second Harmonic Suppression" (Source: Proceedings of the 2022 National Microwave and Millimeter Wave Conference (Volume 1)), reports a C-band narrowband third-order Chebyshev second-fold microstrip hairpin filter. By folding the hairpin filter twice to shift the second harmonic to a higher frequency, and by slotting the sides and bottom of the hairpin filter to improve the phase velocity inconsistency between odd and even modes, a second harmonic suppression of greater than 30 dB is achieved.
[0006] The filter circuit structure is as follows: Figure 1 As shown, the circuit consists of an input / output feeder structure and three double-folded slotted microstrip hairpin resonators. By slotting the sides and bottom of the double-folded hairpin filter, the filter achieves a second harmonic suppression greater than 30dB. The center frequency of this third-order double-folded filter is 5GHz, the relative bandwidth (FBW) is 4%, it uses RO4350B, has a relative permittivity of 3.48, and a dielectric thickness of 1mm. Figure 2 This shows the S-parameter simulation results of this design method. The dashed line represents the simulation results of the ungrooved model.
[0007] Existing technologies can effectively suppress second harmonics (HHMs) by folding the resonant arm of a hairpin filter, but this is difficult to reuse in high-frequency bands. In the mid-to-low frequency band, the wavelengths are longer, the resonator's physical dimensions are sufficient, and the slotting can precisely align with the current antinode of the second-mode corresponding to the HHM, disrupting its resonance condition while avoiding the fundamental mode current concentration area, thus not affecting fundamental mode transmission performance. This results in good HHM suppression, with minimal impact from manufacturing tolerances on performance and strong engineering feasibility. However, in the high-frequency band, the wavelengths are extremely short, and the resonator and slotting dimensions approach manufacturing limits. The slotting cannot precisely match the second-mode field distribution; the parasitic coupling, radiation loss, and equivalent reactance deviations introduced by the folded structure and slotting are significantly amplified at high frequencies, directly deteriorating core performance characteristics such as fundamental mode insertion loss and resonant frequency. Furthermore, even small deviations in size and position can cause harmonic suppression to fail, resulting in extremely high manufacturing tolerance sensitivity and poor mass production consistency. In addition, superior alternative topologies such as SIW and interdigital types exist for the high-frequency band, making this technology difficult to reuse in high-frequency bands. Summary of the Invention
[0008] To address the problems of existing technologies being unable to achieve high-frequency multiplexing and the difficulty in simultaneously suppressing harmonics and reducing insertion loss, this invention proposes a Ku-band second harmonic suppression filter, which, from top to bottom, includes a filter body, a dielectric substrate, and a bottom grounding copper foil. The filter body adopts a fifth-order hairpin resonator, including five U-shaped resonators. Adjacent U-shaped resonators are arranged in a reverse opening layout. Each U-shaped resonator is formed by folding a 1 / 2 wavelength microstrip line, with an arm length of 1 / 4 of the guide wavelength.
[0009] As an optional implementation, a slot is introduced on the third-order U-shaped resonator, and a slot is introduced on the U-shaped resonator adjacent to the third U-shaped resonator that is periodically misaligned with the slot of the third U-shaped resonator.
[0010] As an optional implementation, the single resonant arm is provided with 4 slots, each slot having a size of 0.4mm × 0.1mm, and each slot being 0.4mm apart.
[0011] As an optional implementation, the slot provided on the third U-shaped resonator starts 0.4 mm from the bottom of the third U-shaped resonator, and the slot provided on the adjacent U-shaped resonator of the third U-shaped resonator starts 1 mm from the bottom of the U-shaped resonator.
[0012] As an optional implementation, the fifth-order hairpin resonator adopts a centrally symmetrical arrangement, and includes an input feed port, an input tap, and an input parallel open stub at the input end, and an output feed port, an output tap, and an output parallel open stub at the output end.
[0013] As an optional implementation, the tap has a Z-shaped folded structure with a tap line width of 0.4mm, consisting of a 1.4mm horizontal section connected to the power supply port, a 1.9mm vertical section, and a 0.9mm horizontal section connected to the U-shaped resonator.
[0014] As an optional implementation, the tap has a Z-shaped folded structure with a tap line width of 0.4mm, consisting of a 1.4mm horizontal section connected to the power supply port, a 1.9mm vertical section, and a 0.9mm horizontal section connected to the U-shaped resonator.
[0015] As an optional implementation, the 0.9mm lateral segment is set at the resonant arm 0.5mm from the bottom of the U-shaped resonator.
[0016] As an optional implementation, the parallel open-circuit stub is vertically arranged on the 1.4mm transverse section of the tap, 0.9mm away from the power supply port, and the length of the parallel open-circuit stub is 2.2mm and the width of the stub is 0.2mm.
[0017] As an optional implementation, the dielectric substrate is selected from the Shengyi 6300S high-frequency low-loss epoxy fiberglass substrate, with a thickness of 0.572mm and a size of 5.5mm×15mm.
[0018] To address the problems of existing technologies being unable to achieve high-frequency multiplexing and the difficulty in simultaneously suppressing harmonics and reducing insertion loss, this invention proposes a method that introduces 1 / 8λ. g (Waveguide wavelength) A hairpin filter with slots and periodically staggered arrangement at open stubs and resonant arms. Through the synergistic effect of open stubs and staggered slots, the secondary parasitic passband is shifted to higher frequencies with a suppression level ≥18dB. At the same time, the periodically staggered slots reduce fundamental mode loss, weaken parasitic coupling and radiation loss, and minimize the impact of slots on insertion loss. This design breaks through the high-frequency bottleneck of traditional technology, is suitable for high-frequency scenarios above 10GHz, and simultaneously achieves high harmonic suppression, low insertion loss, and engineering feasibility. Attached Figure Description
[0019] Figure 1 This is a circuit diagram of a compact hairpin bandpass filter circuit with multiple slots for second harmonic suppression in the prior art;
[0020] Figure 2 The S-parameter effect diagram of the existing second harmonic suppression multi-slot compact hairpin bandpass filter;
[0021] Figure 3 This is a top view of a Ku-band second harmonic suppression filter according to the present invention;
[0022] Figure 4 This is an exploded view of a Ku-band second harmonic suppression filter according to the present invention;
[0023] Figure 5This is a simulation diagram of the S-parameters of a Ku-band second harmonic suppression filter according to the present invention. Detailed Implementation
[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.
[0025] This invention proposes a Ku-band second harmonic suppression filter, which includes, from top to bottom, a filter body, a dielectric substrate, and a bottom ground copper foil. The filter body adopts a fifth-order hairpin resonator including five U-shaped resonators. Adjacent U-shaped resonators adopt an opposite opening layout. Each U-shaped resonator is formed by folding a 1 / 2 wavelength microstrip line, and the arm length is 1 / 4 of the guide wavelength.
[0026] To address the issues of existing technologies being unable to achieve high-frequency multiplexing and the difficulty in simultaneously achieving harmonic suppression and low insertion loss, this embodiment proposes a method to introduce... ( This hairpin filter features slots and periodically staggered arrangement at open stubs and resonant arms (based on the fundamental waveguide wavelength). Through the synergistic effect of open stubs and staggered slots, the secondary parasitic passband is shifted to higher frequencies with a suppression level ≥18dB. Simultaneously, the periodically staggered slots reduce fundamental mode loss, weaken parasitic coupling and radiation loss, minimizing the impact of slotting on insertion loss. This design overcomes the high-frequency bottleneck of traditional technologies, adapting to high-frequency scenarios above 10GHz, while simultaneously achieving high harmonic suppression, low insertion loss, and engineering feasibility.
[0027] Specifically, such as Figure 4 The Ku-band second harmonic suppression filter proposed in this embodiment includes, from top to bottom, a filter body, a dielectric substrate, and a bottom ground copper foil. It has a symmetrical structure and combines miniaturization, low insertion loss, and strong harmonic suppression characteristics. Figure 3 The filter body adopts a fifth-order hairpin resonator. The filter body has a centrally symmetrical structure with the center of the third-order filter as the axis of symmetry. From left to right, the filter body includes an input feed port 1, an input tap 2, an input parallel open-circuit stub 3, a first-order U-shaped resonator 4, a second-order U-shaped resonator 5, a third-order U-shaped resonator 7, a fourth-order U-shaped resonator, a fifth-order U-shaped resonator, an output parallel open-circuit stub, an output tap, and an output feed port.
[0028] In this embodiment, each U-shaped resonator is formed by folding a 1 / 2 wavelength microstrip line, with an arm length of 1 / 4 of the guide wavelength; adjacent resonators adopt a reverse opening layout, i.e., as shown below. Figure 3The first-order U-shaped resonator has its opening facing upwards, the second-order U-shaped resonator has its opening facing downwards, the third-order U-shaped resonator has its opening facing upwards, the fourth-order U-shaped resonator has its opening facing downwards, and the fifth-order U-shaped resonator has its opening facing upwards. This arrangement achieves miniaturization while enhancing coupling strength, expanding bandwidth, and suppressing even-order harmonics.
[0029] The present invention introduces periodically staggered slots 6 in the third-order U-shaped resonator located at the center of the filter body and the adjacent second-order U-shaped resonators and fourth-order U-shaped resonators; specifically, the slots 6 are set on the resonant arms on both sides of the third-order U-shaped resonator, and the resonant arms of the second-order U-shaped resonator and fourth-order U-shaped resonators on the side adjacent to the third-order U-shaped resonator are provided with slots that are periodically staggered from the slots of the third-order U-shaped resonator.
[0030] As an optional implementation method, such as Figure 3 Four slots are provided on a single-sided resonant arm, each slot measuring 0.4mm × 0.1mm. The slots are staggered in a step of 0.4mm, meaning that each slot is 0.4mm apart. The slots of the second U-shaped resonator 5 and the fourth U-shaped resonator begin 1mm from the bottom, and the slot of the third U-shaped resonator 7 begins 0.4mm from the bottom. The track-type resonant arm formed by multiple slots forms a distributed band-stop structure in the second harmonic strong current region, suppressing parasitic passbands from the energy path.
[0031] As an optional implementation, the feed port size in this embodiment is 1.2mm × 1.2mm, and the feed port is selected as a 50Ω standard feed input / output port. The feed port is connected to a tap. In this embodiment, the tap is a Z-shaped folded structure with a tap line width of 0.4mm. It consists of a 1.4mm horizontal segment connected to the feed port, a 1.9mm vertical segment, and a 0.9mm horizontal segment connected to the U-shaped resonator. The 0.9mm horizontal segment is connected to the resonant arm of the first-order U-shaped resonator 0.5mm from the bottom, so as to achieve impedance matching between the feed line and the resonator and precisely control the input / output coupling strength to optimize bandwidth and insertion loss.
[0032] In this embodiment, a parallel open-circuit stub is also provided on the 1.4mm transverse section of the tap. The parallel open-circuit stub is perpendicular to the 1.4mm transverse section and is 0.9mm away from the feed port. The parallel open-circuit stub has a line length of 2.2mm (approximately 1 / 8 of the fundamental wavelength) and a line width of 0.2mm. It achieves second harmonic suppression and high-frequency shift through frequency selective impedance characteristics.
[0033] As an optional implementation, in this embodiment, the dielectric substrate 9 is selected from the Shengyi 6300S high-frequency low-loss epoxy fiberglass substrate. The dielectric substrate has a thickness of 0.572mm and a size of 5.5mm×15mm. Its node loss at 10GHz is Df≈0.0015 and dielectric constant is Dk≈3.51, which can significantly reduce filter insertion loss and ensure stable dielectric performance. The bottom ground copper foil 10 is used to form a complete RF loop with the circuit.
[0034] The filtering principle of the Ku-band second harmonic suppression filter of this invention is as follows:
[0035] The quasi-TEM mode signal enters the first-order U-shaped resonator of the fifth-order hairpin resonator via a feeder. The dimensions of the U-shaped resonator are matched with the center operating frequency, forming a series resonance effect. The U-shaped resonator in this invention is essentially a folded half-wavelength (i.e., When the input signal frequency matches the inherent resonant frequency of the U-shaped resonator, the U-shaped resonator undergoes series resonance, minimizing its equivalent impedance. This allows signals near the center frequency to pass through efficiently and be excited to resonate. The signal is then sequentially transmitted to four other U-shaped resonators, each stage further suppressing out-of-band signals and enhancing passband signals. Finally, the pure passband signal, after five stages of filtering, is transmitted to the load through the output feeder. Signals deviating from the center frequency, due to mismatch with the resonator's inherent frequency, experience a sharp increase in the resonator's equivalent impedance, resulting in significant reflection and suppression, thus achieving excellent frequency selectivity.
[0036] This embodiment uses the Ku band as an example, such as Figure 3 The structure shown, and the determination of the dimensions of the U-shaped resonators and the spacing between adjacent U-shaped resonators, were first obtained by looking up normalized element values in a table using a 0.1dB ripple fifth-order Chebyshev low-pass prototype to calculate the coupling coefficient relationship between adjacent U-shaped resonators. The spacing between each resonator was then determined through HFSS simulation. Finally, the waveguide wavelength λ was calculated by combining the center frequency and passband width. g Then, based on the half-wavelength resonance formula, the key dimensions such as the arm length and arm width of the hairpin resonator are determined to ensure that the resonator's inherent resonant frequency is accurately matched to 11.7GHz, thereby achieving low impedance conduction at the center frequency and high impedance suppression at out-of-band frequencies.
[0037] To address the parasitic passband of the second harmonic (2f0, i.e., the second harmonic of the fundamental center frequency f0) inherent in hairpin resonators, this design employs a dual-path collaborative suppression mechanism. The harmonic suppression principle of a Ku-band second harmonic suppression filter of this invention is as follows:
[0038] First, the length of the parallel open-circuit stub provided at the tap in this invention is approximately (Corresponding electrical angle is) This stub exhibits small capacitive reactance to the fundamental frequency, having minimal impact on impedance matching and fundamental mode resonance. For the second harmonic, its electrical length is doubled, equivalent to a 90° open-circuit stub. Specifically, for the second harmonic, the frequency is twice that of the fundamental frequency, and its waveguide length is... (Corresponding electrical angle is) ), equivalent to The open-circuit stub of length presents a short-circuit boundary at the tap end, making the point a voltage node of the second harmonic, which disrupts the half-wavelength resonance condition of the second harmonic, thereby suppressing the parasitic passband of the second harmonic and shifting some of the second harmonic energy to higher frequencies.
[0039] Secondly, periodic misaligned slots are introduced on the resonant arms of the third-order U-shaped resonator and its adjacent second-order and fourth-order U-shaped resonators. The third-order U-shaped resonator in the middle is the main transmission and resonance carrier of the second harmonic energy. The slot length and slot period are designed according to the second harmonic waveguide wavelength, so that the slotted structure resonates at 2f0, exhibiting strong band-stop characteristics. The simultaneous slotting of the second-order, third-order, and fourth-order U-shaped resonators is equivalent to the cascading of three band-stop units, forming a superimposed deep suppression of 2f0. This slotting is a weak perturbation, small-sized, misaligned distribution structure that does not destroy the 1 / 2 wavelength resonance condition of the fundamental mode, so it has little impact on insertion loss. The misaligned structure can destroy the symmetrical resonance boundary condition of the second harmonic, suppress the establishment of the parasitic passband, and weaken the harmonic parasitic coupling between resonators, further improving the suppression effect. While achieving deep suppression of the second harmonic, it maintains low in-band insertion loss characteristics.
[0040] Simulation results of the Ku-band second harmonic suppression filter of this invention are verified as follows: Figure 5 As shown, the filter has a center frequency of 11.7 GHz and a relative bandwidth of approximately 17.1%. Within the passband, it achieves insertion loss better than 1 dB, in-band ripple better than 0.2 dB, and VSWR better than 18.9 dB. Second harmonics appear in the 19 GHz to 23 GHz range, and the second harmonic suppression depth is better than 18 dB. This fully demonstrates the synergistic advantages of this design in miniaturization, low insertion loss, and strong harmonic suppression, meeting the core requirements of high-frequency RF systems for high-performance filtering.
[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims
1. A Ku-band second harmonic suppression filter, characterized in that, From top to bottom, the filter body consists of a filter body, a dielectric substrate, and a bottom grounding copper foil. The filter body uses a fifth-order hairpin resonator, which includes five U-shaped resonators. Adjacent U-shaped resonators are arranged in a reverse opening layout. Each U-shaped resonator is formed by folding a 1 / 2 wavelength microstrip line, with an arm length of 1 / 4 of the guide wavelength.
2. The Ku-band second harmonic suppression filter according to claim 1, characterized in that, A slot is introduced on the third-order U-shaped resonator, and a slot is introduced on the resonator adjacent to the third U-shaped resonator that is periodically misaligned with the slot of the third U-shaped resonator.
3. A Ku-band second harmonic suppression filter according to claim 2, characterized in that, The single resonant arm has four slots, each measuring 0.4mm × 0.1mm, and the slots are spaced 0.4mm apart.
4. A Ku-band second harmonic suppression filter according to claim 2, characterized in that, The slot on the third U-shaped resonator starts 0.4 mm from the bottom of the third U-shaped resonator, and the slot on the adjacent U-shaped resonator starts 1 mm from the bottom of that U-shaped resonator.
5. A Ku-band second harmonic suppression filter according to any one of claims 1 to 4, characterized in that, The fifth-order hairpin U-shaped resonator adopts a centrally symmetrical configuration. At the input end, it also includes an input feed port, an input tap, and an input parallel open-circuit stub. At the output end, it also includes an output feed port, an output tap, and an output parallel open-circuit stub.
6. A Ku-band second harmonic suppression filter according to claim 5, characterized in that, The tap has a Z-shaped folded structure with a tap line width of 0.4mm. It consists of a 1.4mm horizontal section connected to the power supply port, a 1.9mm vertical section, and a 0.9mm horizontal section connected to the U-shaped resonator.
7. A Ku-band second harmonic suppression filter according to claim 6, characterized in that, The tap has a Z-shaped folded structure with a tap line width of 0.4mm. It consists of a 1.4mm horizontal section connected to the power supply port, a 1.9mm vertical section, and a 0.9mm horizontal section connected to the U-shaped resonator.
8. A Ku-band second harmonic suppression filter according to claim 7, characterized in that, The 0.9mm lateral segment is set at the resonant arm 0.5mm from the bottom of the U-shaped resonator.
9. A Ku-band second harmonic suppression filter according to claim 7, characterized in that, The parallel open-circuit stub is vertically installed on the 1.4mm transverse section of the tap, 0.9mm away from the feed port. The length of the parallel open-circuit stub is 2.2mm and the width is 0.2mm.
10. A Ku-band second harmonic suppression filter according to any one of claims 1-4 and 6-9, characterized in that, The dielectric substrate is a Shengyi 6300S high-frequency low-loss epoxy fiberglass substrate with a thickness of 0.572mm and a size of 5.5mm×15mm.