A flexible ultra-wideband bandpass filter

Abstract

The application discloses a flexible ultra-wideband band-pass filter, which comprises an ultrathin flexible dielectric substrate, a stepped impedance resonator, a half-wavelength short-circuit resonator, an interdigital coupling structure and a defect ground ring structure etched below the interdigital coupling structure. The stepped impedance resonator and the half-wavelength short-circuit resonator are connected with each other in cascade to form a passband of the filter. The interdigital coupling structure and the defect ground ring structure etched below the interdigital coupling structure form an LC parallel resonant circuit, which generates an additional transmission zero point outside the passband, and improves the out-of-band suppression level of the filter. The application has the advantages of ultra-wideband, small size, high out-of-band suppression level and excellent anti-deformation sensitivity, and can be applied to the technical field of microwave transmission and flexible electronic devices.

Description

Technical Field

[0001] This invention relates to a flexible ultra-wideband bandpass filter, belonging to the field of microwave transmission and flexible electronic device technology. Background Technology

[0002] In recent years, flexible electronics technology has developed rapidly, becoming a new cutting-edge technology that research institutions and companies are vying to invest in and develop. Compared with traditional rigid circuits, flexible electronic devices are lightweight, bendable, foldable, stretchable, and easy to bond to complex surfaces. Some more complex structural layouts can also be bent and stretched on flexible substrates, which is unmatched by traditional rigid substrates. Applying flexible electronics technology to traditional rigid circuits to design flexible electronic devices with the same performance as traditional rigid circuits, while also being stretchable, compressible, and bendable, has important applications in fields such as "electronic skin," flexible screens, and wearable devices.

[0003] In modern communication systems, filters are an indispensable module in integrated circuits. Filters can effectively remove redundant frequency bands from the system to obtain signals of specific frequencies. Filters include many types, such as cavity filters, dielectric filters, SAW filters, BAW filters, and microstrip filters. Among them, microstrip filters have advantages such as small size, compact structure, ease of fabrication, and ease of integration with other devices, thus having broad application prospects.

[0004] Ultra-wideband (UWB) technology, as a short-range interconnection technology targeting low complexity, low cost, low power consumption, and high data transmission rates, has experienced rapid development in recent years. As a research area for common and innovative technologies in wireless communication, research on key technologies of UWB wireless communication and their coexistence and compatibility technologies has already reached a considerable scale in China.

[0005] The applicant's review of relevant domestic and international technical literature revealed that many scholars have conducted extensive research on the design of flexible filters and ultra-wideband bandpass filters. Most ultra-wideband bandpass filter structures are fabricated on rigid dielectric substrates, which cannot meet the requirements of applications under bending conditions. While a few ultra-wideband bandpass filters built on flexible substrates meet the requirements for flexible applications, they suffer from poor out-of-band rejection and large size, failing to meet the demands for miniaturized filters with high out-of-band rejection. Therefore, it is necessary to provide a miniaturized flexible ultra-wideband bandpass filter structure based on an ultrathin flexible dielectric substrate, exhibiting good passband performance and high out-of-band rejection. Summary of the Invention

[0006] In view of this, the object of the present invention is to provide a flexible ultra-wideband bandpass filter structure. This object is achieved through the following technical solution:

[0007] Select a suitable dielectric material as the ultrathin flexible dielectric substrate. Typically, the dielectric substrate thickness is less than 0.1 mm and it supports bending deformation.

[0008] This invention utilizes a stepped impedance resonator as the low-pass structure of a filter, which functions as a low-pass filter. Based on the predetermined cutoff frequency and out-of-band rejection level of the low-pass structure, the order of the low-pass structure is determined using the attenuation and normalized frequency graphs of a prototype low-pass filter. Then, according to the component value table for low-pass prototype filters of different orders, the corresponding normalized component values ​​are determined. The electrical length (θ) of the high and low characteristic impedance microstrip lines of the stepped impedance resonator is also considered. h θ l It can be obtained through the following formula:

[0009]

[0010]

[0011] Where R0 is the filter impedance, taken as 50Ω, and L and C are the normalized component values ​​corresponding to the low-pass prototype. Z h With Z l These represent the characteristic impedance values ​​corresponding to high and low characteristic impedance microstrip lines, respectively. Considering the limitations of actual fabrication processes, Z... h With Z l The impedance ratio should be as large as possible. Based on the formula, the electrical length of the microstrip line is calculated from the set impedance value, and the actual length and width of different microstrip lines are ultimately determined using simulation software.

[0012] This invention utilizes a half-wavelength short-circuit resonator as the periodic bandpass structure of a filter. Based on the predetermined center frequency of the ultra-wideband bandpass filter, the electrical length of the resonator is set to half the wavelength at the center frequency of the filter. The actual length of the resonator can be determined using simulation software.

[0013] The passband of the filter in this invention is obtained by cascading and matching a low-pass structure and a periodic bandpass structure. The actual width of the half-wavelength short-circuit resonator will affect the bandwidth of the periodic bandpass structure. Based on the established bandwidth of the ultra-wideband bandpass filter, the actual width of the half-wavelength short-circuit resonator is finally determined through optimization simulation.

[0014] This invention utilizes an LC parallel resonant circuit to generate transmission zeros outside the filter's passband, thereby improving out-of-band rejection. The LC parallel resonant circuit consists of an interdigital coupling structure and a defective ground ring structure. The formula for calculating the frequency position f of the transmission zero is:

[0015]

[0016] The capacitance in the equivalent parallel resonant circuit of the interdigital coupling structure is calculated using the following formula:

[0017]

[0018] Where, ε re denoted as the effective dielectric constant of the microstrip transmission line, N is the number of fingers in the interdigitated coupling structure, and l is the length of the fingers in the interdigitated coupling structure.

[0019]

[0020] in, a = w / 2, b = (w+s) / 2, where w and s are the finger width and gap of the interdigital coupling structure, respectively.

[0021] The inductance in an equivalent LC parallel resonant circuit is affected by the inner radius of the defective ground ring and the difference between the inner and outer radii. In summary, the specific location of the transmission zero point can be adjusted by modifying the number, length, width, and gap of the interdigital coupling structure, as well as the inner radius and the difference between the inner and outer radii of the defective ground ring.

[0022] After cascading the three resonant units, an overall simulation optimization was performed, and the final parameter values ​​of the filter were obtained after achieving the predetermined goal.

[0023] The innovation of this invention lies in the following two aspects:

[0024] First, the present invention utilizes a cascaded low-pass structure and a periodic band-pass structure to form the passband of the filter. Therefore, the relevant parameters of the stepped impedance resonator and the half-wavelength short-circuit resonator can be adjusted to flexibly adjust the center frequency and passband bandwidth of the filter.

[0025] Secondly, this invention utilizes an interdigital coupling structure and a defective ground ring structure to construct an LC parallel resonant circuit, generating a transmission zero outside the band, thereby improving the out-of-band rejection level of the filter. Compared to the traditional method of improving out-of-band rejection by increasing the order of the stepped impedance resonator, the introduction of the LC parallel resonant circuit only increases the filter volume vertically, and indirectly reduces the overall filter volume by etching a defective ground ring in the metal ground layer. The specific frequency position of the transmission zero can be controlled by the various parameters of the LC parallel resonant circuit, offering greater flexibility compared to traditional methods. Attached Figure Description

[0026] Figure 1 This is the overall circuit diagram of the filter of the present invention.

[0027] Figure 2 This is a circuit diagram of the stepped impedance resonator of the present invention.

[0028] Figure 3 This is a simulation result diagram of the stepped impedance resonator of the present invention.

[0029] Figure 4 This is a circuit diagram of the half-wavelength short-circuit resonator of the present invention.

[0030] Figure 5 The figure shows the simulation results of the half-wavelength short-circuit resonator of the present invention.

[0031] Figure 6 This is a circuit diagram of the LC parallel resonant circuit of the present invention.

[0032] Figure 7 This is a simulation result diagram of the LC parallel resonant circuit of the present invention.

[0033] Figure 8 The diagram shows the simulation results of the filter of the present invention under straight and different bending angle conditions.

[0034] Explanation of reference numerals in the attached figures: 1. Ultrathin flexible dielectric substrate, 2. Stepped impedance resonator, 3. Half-wavelength short-circuit resonator, 4. LC parallel resonant circuit. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the present invention. Furthermore, in the following description, descriptions of well-known structures and techniques are omitted to avoid unnecessarily obscuring the concept of the present invention.

[0036] The filter designed in this invention is an ultra-wideband bandpass filter based on an ultrathin flexible dielectric substrate. Its overall circuit diagram is shown in the schematic. Figure 1 In this embodiment, the selected ultrathin flexible dielectric substrate is a flexible polyimide sheet with a dielectric constant of 3.2, a loss tangent of 0.0028, and a thickness of 0.05 mm. The microstrip lines on the upper surface and the ground layer on the lower surface of the ultrathin flexible dielectric substrate are both made of copper, with a thickness of 0.035 mm. The simulation software used in this embodiment is HFSS and ADS.

[0037] In this embodiment, the filter is required to cover a passband range of 3.1-10.6 GHz, with an insertion loss of less than 2 dB within the passband, a return loss better than 10 dB, and an insertion loss greater than 20 dB outside the passband at 14 GHz. Since the filter in this invention is composed of a low-pass structure, a periodic band-pass structure, and a cascaded LC parallel resonant circuit, the design of each of the three resonant units will be described separately.

[0038] In this embodiment, the cutoff frequency of the low-pass structure is set to 12GHz. Considering the overall size of the filter and the out-of-band suppression level, a third-order stepped impedance resonator is selected to design the low-pass structure. Based on the component value table of the 0.5dB equal-ripple low-pass filter prototype, g1 = 1.5963, g2 = 1.0967, and g3 = 1.5963 are determined. The width of the first microstrip line of the stepped impedance resonator is set to 0.1mm, the width of the second microstrip line is set to 0.7mm, and the width of the third microstrip line is set to 0.08mm. Calculations in the simulation software ADS show that the characteristic impedance values ​​of the first, second, and third microstrip lines of the stepped impedance resonator are 50.77Ω, 12.49Ω, and 55.39Ω, respectively. The electrical length of the first microstrip line of the stepped impedance resonator is calculated using the formula:

[0039]

[0040] The electrical length of the second microstrip line in the stepped impedance resonator is:

[0041]

[0042] The electrical length of the third microstrip line in the stepped impedance resonator is:

[0043]

[0044] Calculations using the simulation software ADS show that the actual lengths of the first, second, and third microstrip lines of the stepped impedance resonator are 4.06 mm, 0.71 mm, and 3.72 mm, respectively. (Illustrative diagram) Figure 2 The circuit diagram shows the cascaded two identical stepped impedance resonators. The final simulation result after optimization is shown in the illustration. Figure 3 .

[0045] In this embodiment, the electrical length of the half-wavelength short-circuit resonator is set to half a wavelength at 7.26 GHz. The actual length of the resonator is calculated to be 6.7 mm in the simulation software ADS. The actual width of the resonator affects the bandwidth. In this embodiment, its width is ultimately determined by optimizing the overall filter simulation parameters. The actual width of the resonator is set to 0.2 mm. The circuit diagram and simulation results of the half-wavelength short-circuit resonator are shown in the illustration. Figure 4 and indication Figure 5 .

[0046] In this embodiment, the out-of-band transmission zero generated by the LC parallel resonant circuit is set at 16.6 GHz. By continuously adjusting the finger length, width, gap, and the inner and outer radii difference of the defective ground ring, the transmission zero position can be finally set at 16.6 GHz. In this example, the number of fingers in the interdigital coupling structure is set to 4, the interdigital length is 1.46 mm, the width is 0.12 mm, and the gap is 0.08 mm. At this time, the capacitance value corresponding to the interdigital coupling structure is 0.237 pF. The inner radius of the defective ground ring structure is 1.09 mm, and the difference between the inner and outer radii is 0.1 mm. The circuit diagram and simulation results of the LC parallel resonant circuit are shown in the illustration. Figure 6 and indication Figure 7 .

[0047] After cascading the three resonator units, an overall simulation optimization was performed, based on the schematic diagram. Figure 1 The parameters of the filter, as labeled, were ultimately set to the following values: W1 = 0.1mm, L1 = 4mm, W2 = 0.7mm, L2 = 1mm, W3 = 0.08mm, L3 = 2.14mm, W4 = 0.12mm, L4 = 1.46mm, W5 = 0.72mm, L5 = 0.58mm, W6 = 0.72mm, L6 = 0.6mm, W7 = 0.2mm, L7 = 5.87mm, g = 0.08mm, R1 = 0.99mm, R2 = 1.09mm, A = 0.6mm, r = 0.15mm. The overall dimensions of the filter are 30mm × 17.3mm × 0.12mm. Simulations were performed on the filter under straight, 45° bend, and 90° bend conditions. The final simulation results are shown in the illustration. Figure 8 In its flat state, the filter's center frequency is 7.22 GHz, its passband frequency is 3.09-11.42 GHz, and its relative bandwidth reaches 114%. The insertion loss within the passband is less than 0.97 dB, and the return loss is better than 14 dB. The transmission zero introduced by the LC parallel resonant circuit is located at 16.3 GHz, where the insertion loss reaches 38.6 dB. The filter exhibits ultra-wideband performance, good passband characteristics, high out-of-band rejection, and a small overall size, conforming to the design principles of ultra-wideband bandpass filters. Under bending conditions of 45° and 90°, the filter's center frequency, relative bandwidth, passband characteristics, and out-of-band rejection level show no significant changes, indicating good resistance to deformation sensitivity.

Claims

1. A flexible ultra-wideband bandpass filter, characterized in that, include: Ultrathin flexible dielectric substrate (1), stepped impedance resonator (2), half-wavelength short-circuit resonator (3), LC parallel resonant circuit (4). The upper surface of the ultrathin flexible dielectric substrate (1) is attached with a stepped impedance resonator (2), a half-wavelength short-circuit resonator (3), and an LC parallel resonant circuit (4). The two identical stepped impedance resonators (2) are located on both sides above the half-wavelength short-circuit resonator (3); The LC parallel resonant circuit (4) is located in the center above the two identical stepped impedance resonators (2); The left end of the stepped impedance resonator (2) at the input port and the right end of the stepped impedance resonator (2) at the output port are respectively connected to a 50Ω input feed line and a 50Ω output feed line; The ultrathin flexible dielectric substrate (1) has a thickness of less than 0.1 mm and supports bending deformation; The stepped impedance resonator (2) is a low-pass structure and plays the role of low-pass filtering in the filter. It is composed of high characteristic impedance lines, low characteristic impedance lines and high characteristic impedance lines connected to each other. It works together with the half-wavelength short-circuit resonator (3) to obtain the passband of the flexible ultra-wideband bandpass filter. The half-wavelength short-circuit resonator (3) is a periodic bandpass structure that plays the role of bandpass filtering in the filter. Its actual length is obtained by half the wavelength at the center frequency of the filter passband, and its actual width determines the bandwidth of the bandpass structure. It works together with the stepped impedance resonator (2) to obtain the passband of the flexible ultra-wideband bandpass filter. The LC parallel resonant circuit (4) is composed of an interdigital coupling structure and a defective ground ring structure; this circuit can generate additional transmission zeros outside the band, thereby improving the out-of-band suppression level of the filter.

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