Suboctave equalization structure and in-band flatness optimization method based on it

Abstract

This invention discloses a suboctave band equalization structure and a method for optimizing in-band flatness based on it, relating to the technical field of microwave circuits. The suboctave band equalization structure consists of a 50-ohm microstrip line, an absorption resistor, and a high-impedance line. The high-impedance line employs an S-shaped bend structure to reduce volume, and the absorption resistor's geometric dimensions are adjusted to precisely control the equalization depth. This invention utilizes the transmission characteristics of a resonator exhibiting high insertion loss at the center of the band and gradually decreasing insertion loss at both sides when the fundamental frequency reaches the second resonant frequency. This complements the transmission characteristics of a filter, which has low insertion loss in the center of the passband and gradually increasing insertion loss at both sides. This application can significantly improve in-band flatness by physically canceling passband edge losses through passive microstrip circuits while ensuring high matching performance with return loss better than -20dB. It has advantages such as simple structure, easy integration, and strong engineering applicability.

Description

Technical Field

[0001] This invention belongs to the field of microwave circuit technology, specifically, it relates to a suboctave equalization structure and an in-band flatness optimization method based thereon. Background Technology

[0002] In microwave communication systems, filters play a crucial role. Their core function is to allow signals of specific frequencies to pass through while significantly attenuating unwanted frequency signals. Filters are widely used in various communication devices. For example, in receiver systems, input filters effectively filter out unwanted out-of-band signals, preventing interference from entering subsequent receiving links, thereby improving the system's anti-interference capability and attenuating large out-of-band signals to protect sensitive receiver components from damage. In transmitting systems, output filters ensure that the final transmitted signal is in the required frequency band, preventing unnecessary signal interference to other communication systems.

[0003] Although filters play an irreplaceable role in communication systems, their inherent characteristics also bring some technical problems. Existing filters can be classified into cavity filters, LC filters, microstrip filters, and dielectric filters based on their structure. Due to the limitations imposed by the quality factor (Q value) of the selected materials, the loss characteristics of filters are essentially fixed. In practical applications, due to the lack of ideal superconducting materials, filters made of various materials all exhibit a transition band, which is the frequency range in the filter's frequency response where attenuation gradually changes from the passband to the suppression stopband. The existence of this transition band changes the originally ideal passband loss from a flat state to a transmission characteristic where the insertion loss is low in the middle of the passband and gradually increases towards the sides. When used in communication systems, this characteristic causes severe unevenness within the passband, affecting the performance indicators of the receiver and transmitter systems.

[0004] Furthermore, when optimizing for such problems, traditional equalization methods often struggle to balance transmission characteristics and system matching performance. If the equalization structure is improperly chosen, while it may compensate for transmission characteristics with high insertion loss in the middle of the passband and gradually decreasing insertion loss on both sides within certain frequency bands, its return loss characteristics may be poor, leading to severe mismatch problems when cascaded with filters or other subsystems. This mismatch not only fails to effectively improve in-band flatness but may also cause sideband collapse, further deteriorating overall system performance. Therefore, designing an optimization scheme that is simple in structure, can fully utilize resonant characteristics to achieve sub-octave flatness equalization, and possesses good matching capabilities is a crucial issue currently facing the microwave circuit field. Summary of the Invention

[0005] The purpose of this invention is to provide a suboctave equalization structure and an in-band flatness optimization method based thereon, which mainly solves the technical defects of in-band flatness degradation and sideband collapse caused by the physical limitation of material quality factor (Q value) of bandpass filters in existing microwave communication systems.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A suboctave band equalization structure is disclosed, employing a microstrip circuit. The microstrip circuit includes a first 50-ohm microstrip transmission line, a first absorption resistor, a first high-impedance line, a second 50-ohm microstrip transmission line, a second high-impedance line, a third high-impedance line, and a second absorption resistor. The first and second 50-ohm microstrip transmission lines are respectively disposed at the signal input and signal output terminals of the suboctave band equalization structure. The first and second high-impedance lines are electrically connected through the third high-impedance line. The first absorption resistor is connected between the first and third high-impedance lines, and the second absorption resistor is connected between the second and third high-impedance lines.

[0008] Furthermore, in this invention, both the first high-resistance line and the second high-resistance line adopt an S-shaped bending structure.

[0009] Furthermore, in this invention, the microstrip circuit is disposed on the surface of an alumina ceramic substrate.

[0010] Furthermore, in this invention, the first absorption resistor and the second absorption resistor are thin-film sheet resistors, and their resistance values ​​are set by adjusting the ratio of the length to the width of the resistor body.

[0011] Furthermore, in this invention, the linewidth of the first 50-ohm microstrip transmission line and the second 50-ohm microstrip transmission line is 0.243 mm, and the total length is 0.8 mm.

[0012] Furthermore, in this invention, the first high-resistivity line and the second high-resistivity line are high-resistivity lines with a line width of 0.05 mm and a total length of 6.55 mm.

[0013] Furthermore, in this invention, the third high-resistance line is a high-resistance line with a line width of 0.23 mm and a total length of 2 mm.

[0014] This invention also provides an in-band flatness optimization method, based on the above-mentioned suboctave equalization structure, comprising the following steps:

[0015] S1, a bandpass filter is cascaded at the signal output of the suboctave equalizer structure;

[0016] S2, the signal first enters the suboctave equalization structure, so that when the fundamental frequency of the signal reaches the second resonant frequency, it has the transmission characteristics of large insertion loss in the center of the band and gradually smaller insertion loss on both sides.

[0017] S3, the signal enters the bandpass filter, and by utilizing the inherent transmission characteristics of the bandpass filter, the signal has the transmission characteristics of low insertion loss in the middle of the passband and gradually increasing insertion loss on both sides.

[0018] S4 utilizes the transmission characteristics of the suboctave equalization structure and the inherent transmission characteristics of the bandpass filter to make the signal have opposite slope trends on the frequency axis. The superposition of the two achieves the constant total loss in the passband.

[0019] Furthermore, in this invention, the passband frequency curve of the bandpass filter exhibits a transmission characteristic where the insertion loss is large at the center of the band between the fundamental frequency and the second resonant frequency, and the insertion loss gradually decreases on both sides, and the passband frequency satisfies f1≤2*f0; where f0 is the starting frequency and f1 is the cutoff frequency.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) This invention precisely compensates for the transmission characteristics of a bandpass filter, which have a large insertion loss in the center and gradually decreasing insertion loss on both sides of the fundamental frequency reaching the second resonant frequency through a suboctave equalization structure. This reduces the unevenness in the passband of the filter from the ±1.05dB level to the ±0.2dB level. This physical-level complementary characteristic solves the long-standing sideband collapse problem in microwave communication systems and improves the fidelity of signal transmission.

[0022] (2) This invention selects a suboctave band with a return loss better than -20dB by carefully screening the resonant frequency response band. This high matching characteristic ensures that the equalization structure will not induce additional standing wave fluctuations when cascaded with various bandpass filters, thus maintaining the overall return loss index of the system and avoiding gain fluctuations caused by cascade mismatch.

[0023] (3) The equalization amount of the equalization structure of the present invention can be precisely controlled by simply changing the geometric dimensions of the thin film resistor, without involving complex semiconductor devices or active bias circuits. This purely passive implementation not only reduces production costs, but also improves the system's working stability under harsh environments such as high temperature and high pressure.

[0024] (4) The suboctave equalization structure of the present invention is not only applicable to bandpass filters with high rectangular coefficients, but also to any microwave component or subsystem that exhibits loss fluctuations in the suboctave range. Flatness optimization can be achieved by adjusting the resonant frequency and resistance parameters in this scheme. Attached Figure Description

[0025] Figure 1 This is a diagram of the microstrip topology of the suboctave band equalization structure of the present invention.

[0026] Figure 2 This is a schematic diagram of the simulation curve of the suboctave band equalization structure of the present invention.

[0027] Figure 3 This is a schematic diagram of the simulation curve of the suboctave band equalization structure of the present invention.

[0028] Figure 4 This is a topology diagram of a high rectangular coefficient bandpass filter.

[0029] Figure 5 Schematic diagram of simulation curve for a high rectangular coefficient bandpass filter.

[0030] Figure 6 Schematic diagram of simulation curve for a high rectangular coefficient bandpass filter.

[0031] Figure 7 This is a diagram of the cascaded topology of a suboctave equalizer and a high rectangular coefficient bandpass filter.

[0032] Figure 8 Schematic diagram of simulation curves for a suboctave equalizer structure cascaded with a high rectangular coefficient bandpass filter.

[0033] Figure 9 Schematic diagram of simulation curves for a suboctave band equalizer structure cascaded with a high rectangular coefficient bandpass filter.

[0034] The names corresponding to the reference numerals in the attached figures are as follows:

[0035] 1. First 50-ohm microstrip transmission line; 2. First absorption resistor; 3. First high-impedance line; 4. Second 50-ohm microstrip transmission line; 5. Second high-impedance line; 6. Third high-impedance line; 7. Second absorption resistor; 8. First high-impedance resonator; 9. First low-impedance resonator; 10. Second low-impedance resonator; 11. Second high-impedance resonator, A-Equalization structure, B-Bandpass filter. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0037] This invention discloses a suboctave equalization structure, which is primarily constructed on a high-performance microstrip circuit substrate. In this embodiment, the circuit substrate is an Al2O3 ceramic substrate with a dielectric constant of 9.8 and a thickness of 0.254 mm. This substrate possesses excellent physicochemical stability and an extremely low loss tangent, ensuring good transmission performance in high-frequency bands above 40 GHz. Furthermore, its stable temperature characteristics guarantee reliable operation of the circuit under harsh environments.

[0038] like Figure 1 As shown, the physical components of this suboctave band equalization structure include a first 50-ohm microstrip transmission line 1, a first absorption resistor 2, a first high-impedance line 3, a second 50-ohm microstrip transmission line 4, a second high-impedance line 5, a third high-impedance line 6, and a second absorption resistor 7. The first 50-ohm microstrip transmission line 1 and the second 50-ohm microstrip transmission line 4 perform the crucial function of impedance matching in the circuit. In microwave RF communication standards, 50Ω is a common characteristic impedance standard. In this embodiment, to maintain the continuity of system impedance, the linewidth of the first 50-ohm microstrip transmission line 1 and the second 50-ohm microstrip transmission line 4 are precisely set to 0.243mm, and the total length is set to 0.8mm. This dimensional setting scheme ensures that the suboctave band equalization structure is in a standard 50-ohm matching state both internally and externally, preventing severe signal reflections at the ports.

[0039] The first absorption resistor 2 and the second absorption resistor 7 are the core components for adjusting the equalization depth. These two resistors are directly attached to the ceramic substrate using a thin-film sputtering process to form a thin-film sheet resistor structure. The resistance value of the sheet resistor is determined by the aspect ratio of the resistor element and the inherent sheet resistance of the material. In this embodiment, a resistor element with a sheet resistance of 50Ω per square is selected. The first absorption resistor 2 and the second absorption resistor 7 play a coupling and absorption role in the circuit, and their specific resistance values ​​directly determine the amount of sag in the equalization structure. For example, when the ratio of the length L to the width W of the resistor element is set to 2.6, the corresponding resistance value is 130Ω. By changing the length and width geometry of the resistors, technicians can achieve precise customization of the equalization amount. Figure 3 As shown, different resistance values ​​of the square resistor (such as 100Ω, 130Ω, 150Ω) correspond to different equalization depths, which provides flexible matching redundancy for filters with different loss characteristics.

[0040] The first high-impedance line 3 and the second high-impedance line 5 act as notch filter generators, generating a resonant zero at a specific frequency through their extremely high characteristic impedance. To reduce the physical size of the circuit while ensuring sufficient electrical length, both the first high-impedance line 3 and the second high-impedance line 5 employ an "S"-shaped bending structure design. In this embodiment, the linewidth of these two high-impedance lines is only 0.05 mm, with a total length of 6.55 mm. This high aspect ratio and bending layout significantly reduce the overall envelope size of the suboctave band equalization structure without altering the electromagnetic transmission characteristics, which is beneficial for the miniaturization and integration of the system.

[0041] The third high-impedance line 6 is located in the central region of the circuit and serves as a signal transmission and node connection. Its linewidth is set to 0.23 mm, and its total length is 2 mm. The third high-impedance line 6 is responsible for accommodating and matching the connections of the first 50-ohm microstrip transmission line 1, the first absorption resistor 2, the first high-impedance line 3, the second 50-ohm microstrip transmission line 4, the second high-impedance line 5, and the second absorption resistor 7. Through optimization of the geometric parameters of the third high-impedance line 6, the phase relationship between the passive components is ensured to conform to the preset physical model of suboctave equalization.

[0042] The working principle of this scheme is based on a deep understanding of the fundamental and harmonic characteristics of the resonator. For example... Figure 2 As shown ( Figure 2 The data on the right side of the graph represents the horizontal and vertical coordinates of the corresponding numerical labels (the subsequent curves will be expressed in the same way). Electromagnetic simulation analysis shows that the resonator formed by this suboctave equalization structure has the lowest insertion loss near frequencies such as 9GHz, 18GHz, and 27GHz. A key innovation of this invention lies in the ingenious extraction of the transmission characteristic segment of the resonator between the primary resonant frequency f0 (9GHz) and the secondary resonant frequency 2f0 (18GHz).

[0043] Within the specific frequency band of 9GHz to 18GHz, the insertion loss curve of the suboctave equalized structure exhibits a perfect transmission characteristic of high insertion loss at the center of the band and gradually decreasing insertion loss at both sides, while its return loss (such as...) Figure 2 The return loss of ports 1 and 2 (as shown in the diagram) is better than -22dB across the entire frequency band. This high matching characteristic ensures that the equalization structure does not introduce additional mismatch loss when cascaded. In contrast, although a similar loss trend exists in the frequency band from DC to above 9GHz to 18GHz, its return loss performance is poor (only about -10dB), which easily leads to the deterioration of the standing wave ratio of the cascaded system. Therefore, selecting the suboctave band between the fundamental frequency and the second resonance as the operating bandwidth is the optimal physical choice for achieving high-performance equalization.

[0044] like Figure 4 and Figure 5As shown, this embodiment introduces a high rectangular coefficient bandpass filter B as an example of the equalized object. This bandpass filter B employs a specific resonator layout, including a first high-impedance resonator 8, a first low-impedance resonator 9, a second low-impedance resonator 10, and a second high-impedance resonator 11. By transforming the traditional resonant structure into this combination of high and low-impedance resonators, and combining this with the bending treatment of the high-impedance lines, a high rectangular coefficient and miniaturized size of the filter are achieved.

[0045] like Figure 6 As shown, the original insertion loss curve of bandpass filter B in the passband range of 9.5 GHz to 17.5 GHz exhibits a typical transmission characteristic where the insertion loss is low in the middle of the passband and gradually increases towards the edges. Based on simulation data, the maximum insertion loss of this filter is -3.9 dB, the minimum insertion loss is -1.8 dB, and its in-band flatness is calculated to be ±1.05 dB. This significant edge collapse phenomenon severely degrades the signal quality of the communication system.

[0046] The complete implementation process of the present invention follows the following logical steps:

[0047] Determine the target frequency band: Based on the operating passband of bandpass filter B (9.5GHz to 17.5GHz), set the resonant frequency parameters of the suboctave equalization structure. By adjusting the electrical lengths of the first high-impedance line 3, the second high-impedance line 5, and the third high-impedance line 6, the fundamental frequency of the resonator is positioned near 9GHz, ensuring that the target passband falls entirely within the equalization window between the fundamental and second resonant frequencies.

[0048] Microstrip circuit fabrication. On an alumina ceramic substrate with a thickness of 0.254 mm, a first 50-ohm microstrip transmission line 1, a second 50-ohm microstrip transmission line 4, and various high-resistivity line structures were formed using photolithography. Linewidth accuracy was ensured to be controlled within ±0.01 mm to maintain impedance precision.

[0049] Customized resistor values: The first absorption resistor 2 and the second absorption resistor 7 are fabricated using a thin-film sputtering process. Based on the approximately 2.1 dB in-band ripple of the bandpass filter B, the required compensation depth is calculated. In this embodiment, the sheet resistor value is set to 130 Ω. The quality factor of the resonant circuit is adjusted through the thermal dissipation effect of the resistor, thereby obtaining a balanced transmission characteristic curve with controlled slope, characterized by low insertion loss in the middle of the passband and gradually increasing insertion loss on both sides.

[0050] Cascaded simulation and optimization: such as Figure 7 and Figure 8As shown, a suboctave equalizer structure is physically cascaded with a bandpass filter B. The overall cascaded structure is then jointly simulated using 3D electromagnetic simulation software. During this process, the impedance matching at the cascade point is optimized by fine-tuning the bending distance between the first high-impedance line 3 and the second high-impedance line 5.

[0051] Final flatness verification: After cascaded debugging, the signal first undergoes pre-emphasis processing through a sub-octave equalizer, which moderately suppresses the passband center frequency while maintaining low loss at the passband edge frequencies. The signal then enters bandpass filter B, where the filter's inherent edge loss and the pre-emphasis effect of the equalizer cancel each other out through vector superposition.

[0052] like Figure 9 As shown, the cascaded system, after optimization of the sub-octave equalization structure, exhibits a significantly improved frequency response curve. While maintaining high suppression and low insertion loss characteristics, the in-band return loss remains at an excellent level, ensuring good system matching. Most importantly, the original in-band flatness of ±1.05 dB has been successfully reduced to ±0.2 dB after optimization using this scheme. This technological advancement directly eliminates the risk of sideband collapse in the filter, significantly improving the signal transmission stability of the communication system in the sub-octave range.

[0053] In specific applications, such as radar front-end receivers or high-precision frequency source systems, the in-band flatness optimization method provided by this solution demonstrates extremely high engineering value. Due to the use of a purely passive microstrip structure, this solution does not generate additional power consumption and eliminates the nonlinear distortion caused by active devices. Furthermore, by changing the output directions and bending methods of the first high-impedance resonator 8, the first low-impedance resonator 9, the second low-impedance resonator 10, and the second high-impedance resonator 11, the bandpass filter B and the equalization structure A achieve an integrated, miniaturized layout, enabling direct embedding into high-density microwave integrated modules.

[0054] Furthermore, the resistance adjustment mechanism in this solution offers exceptional flexibility. In actual production batches, if process deviations cause a shift in the filter's loss curve, engineers can simply adjust the resistance value of the first absorption resistor 2 or the second absorption resistor 7 using laser trimming technology to achieve a secondary correction of flatness without altering the circuit layout. This compensation mechanism, based on physical structure, effectively reduces the debugging difficulty and production cost of microwave circuits.

[0055] In this embodiment, the "S"-shaped bends of the first high-resistance line 3 and the second high-resistance line 5 not only reduce the size but also fine-tune the group delay characteristics of the resonator through the electromagnetic coupling effect at the bend. This multi-dimensional adjustment of physical parameters enables this scheme to optimize flatness while also improving the phase linearity within the passband, which is crucial for the transmission of broadband vector signals.

[0056] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. A suboctave equalization structure, characterized in that, The microstrip circuit is configured as follows: a first 50-ohm microstrip transmission line (1), a first absorption resistor (2), a first high-impedance line (3), a second 50-ohm microstrip transmission line (4), a second high-impedance line (5), a third high-impedance line (6), and a second absorption resistor (7). The first 50-ohm microstrip transmission line (1) and the second 50-ohm microstrip transmission line (4) are respectively disposed at the signal input end and the signal output end of the suboctave equalization structure. The first high-impedance line (3) and the second high-impedance line (5) are electrically connected through the third high-impedance line (6). The first absorption resistor (2) is connected between the first high-impedance line (3) and the third high-impedance line (6), and the second absorption resistor (7) is connected between the second high-impedance line (5) and the third high-impedance line (6).

2. The suboctave band equalization structure according to claim 1, characterized in that, Both the first high-resistance line (3) and the second high-resistance line (5) adopt an S-shaped bending structure.

3. The suboctave band equalization structure according to claim 2, characterized in that, The microstrip circuit is disposed on the surface of an alumina ceramic substrate.

4. The suboctave band equalization structure according to claim 3, characterized in that, The first absorption resistor (2) and the second absorption resistor (7) are thin-film sheet resistors, and their resistance values ​​are set by adjusting the ratio of the length to the width of the resistor body.

5. The suboctave band equalization structure according to claim 4, characterized in that, The linewidth of the first 50-ohm microstrip transmission line (1) and the second 50-ohm microstrip transmission line (4) is 0.243 mm, and the total length is 0.8 mm.

6. The suboctave equalization structure according to claim 5, characterized in that, The first high-resistance line (3) and the second high-resistance line (5) are high-resistance lines with a line width of 0.05 mm and a total length of 6.55 mm.

7. A suboctave band equalization structure according to claim 6, characterized in that, The third high-resistivity line (6) is a high-resistivity line with a line width of 0.23 mm and a total length of 2 mm.

8. A method for optimizing in-band flatness, characterized in that, Based on the suboctave equalization structure described in claim 7. Includes the following steps: S1, a bandpass filter is cascaded at the signal output of the suboctave equalizer structure; S2, the signal first enters the suboctave equalization structure, so that when the fundamental frequency of the signal reaches the second resonant frequency, it has the transmission characteristics of large insertion loss in the center of the band and gradually smaller insertion loss on both sides. S3, the signal enters the bandpass filter, and by utilizing the inherent transmission characteristics of the bandpass filter, the signal has the transmission characteristics of low insertion loss in the middle of the passband and gradually increasing insertion loss on both sides. S4 utilizes the transmission characteristics of the suboctave equalization structure and the inherent transmission characteristics of the bandpass filter to make the signal have opposite slope trends on the frequency axis. The superposition of the two achieves the constant total loss in the passband.

9. The method for optimizing in-band flatness according to claim 8, characterized in that, The passband frequency curve of the bandpass filter exhibits a transmission characteristic where the insertion loss is large at the center of the band between the fundamental frequency and the second resonant frequency, and the insertion loss gradually decreases on both sides. The passband frequency satisfies f1≤2*f0, where f0 is the starting frequency and f1 is the cutoff frequency.

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