Broadband-pass filter for elastic waves and frequency shift method

Abstract

The invention belongs to the technical field of filters, and particularly relates to a broadband band-pass filter for elastic waves and a frequency shift method, four defect structures are inserted into a periodic fluctuation structure with the total period number of 8n (n = 1, 2, 3,...) according to a specific mode, and the insertion mode enables the filter structure to meet the multi-defect mode coupling condition, so that the frequency shift efficiency is improved. A flat-topped broadband defect mode is formed, a wide pass band with the flat-topped characteristic is shown in a Bragg band gap, elastic wave broadband band-pass filtering with the flat-topped characteristic is achieved, in the optimization scheme, the filter has the advantages of being simple and integrated in structure, and the filter has the advantages of being tunable in filtering frequency band and the like through the frequency shift method.

Description

Technical Field

[0001] This invention belongs to the field of filter technology, specifically relating to a broadband pass filter for elastic waves and a frequency shifting method. Background Technology

[0002] Elastic waves, as mechanical vibrations that propagate relying on the elastic restoring force of a medium, are among the most prevalent physical phenomena in nature. From familiar sound waves and ocean sonar to seismic waves revealing the Earth's internal structure, they are vivid examples. Elastic waves possess many unique physical characteristics. For instance, they can generate a rich variety of wave types in solids, including longitudinal waves (parallel to the propagation direction) and transverse waves (perpendicular to the direction of propagation), as well as surface waves propagating at interfaces, enabling them to carry multi-dimensional information. Furthermore, their propagation characteristics are closely related to the physical properties of the medium, making them sensitive probes for detecting the internal state of a medium. For these reasons, elastic wave technology has been widely applied in many fields such as underwater communication, medical ultrasound imaging, industrial non-destructive testing, geological resource exploration, and seismological research, becoming a powerful tool for humankind to perceive the world and explore the unknown.

[0003] In modern information technology, filters are crucial signal processing units. Their main function is to filter out the desired specific frequency band signal from complex signals containing multiple frequency components and suppress unnecessary interference signals. Their performance directly determines the quality of the communication system, ensuring both the purity of signal reception and the accuracy of transmission. Based on their frequency selectivity characteristics, filters are mainly classified into low-pass, high-pass, band-pass, and band-stop types.

[0004] In practical elastic wave engineering applications, the frequency range covered by elastic waves is often very wide, while the effective information required for a specific scenario is usually only distributed within a specific frequency band. Therefore, filters, as a key component in the signal processing process, play an indispensable role in elastic wave-related technologies. Their performance directly determines whether the system can accurately extract target information and ensure the quality of communication or detection. It can be said that the development of elastic wave filter technology is essential.

[0005] In recent years, many researchers have conducted research on elastic wave filter technology. Chinese patent application CN202510661311.0 discloses an elastic wave filter based on a toroidal cylindrical shell thin-plate structure, which achieves efficient filtering and frequency-selective control of elastic waves by introducing a toroidal cylindrical shell resonant structure. Chinese patent application CN202210569156.6 discloses an elastic wave enhancer based on a heterojunction structure, which enhances specific frequency signals within the elastic wave bandgap by combining two periodic undulation structures with different structural parameters. Chinese patent application CN202211540665.2 discloses a Rayleigh wave filter based on a mirror-symmetric structure, which achieves filtering of specific frequency signals within the elastic wave bandgap by combining a periodic undulation structure with its mirror-symmetric structure.

[0006] The aforementioned research developed an elastic wave filter with a filter passband by designing a filter structure. However, in the practical engineering application scenario of underwater communication systems, the aforementioned annular cylindrical shell thin-plate structure has interfaces such as welding and bonding, which not only makes processing difficult but also poses a higher reliability risk under long-term underwater pressure fluctuations and corrosion environments. In addition, the filter designed based on heterojunction and mirror symmetry structures has a large quality factor in its filter passband. The sharp filter passband means that the filtering effect will be greatly reduced if the required filtering frequency band is slightly shifted. At the same time, underwater communication systems often have drastically different requirements for filtering performance. They not only need a large bandwidth to ensure the information transmission rate but also require the filter signal gain to remain stable to ensure the reliability of information acquisition. Summary of the Invention

[0007] The purpose of this invention is to provide a wideband pass filter and frequency shifting method for elastic waves, so as to achieve a highly reliable and stable wideband filtering function for elastic waves.

[0008] The technical solutions for achieving the above objectives include the following:

[0009] A broadband pass filter for elastic waves includes a planar waveguide with rectangular periodic undulation structures on its upper and lower boundaries, and four defect structures. The total number of periods of the rectangular periodic undulation structures is 8n (n=1, 2, 3, ...). The rectangular periodic undulation structures on the upper and lower boundaries are symmetrical about the horizontal central axis of the filter profile and generate Bragg scattering with the incident elastic wave, forming a Bragg bandgap. The four defect structures are named sequentially as first defect D1, second defect D2, third defect D3, and fourth defect D4. D1, D2, D3, and D4 are inserted after the nth, 3nth, 5nth, and 7nth periodic undulation structures, respectively, and the four defect structures are of equal length.

[0010] Further, the value range of the undulation height α of the rectangular periodic undulation structure is 0 < α < h3 / 2, where h3 is the distance between the upper and lower boundaries.

[0011] Further, the band - pass filter is an integrated structure of the same material. The original elastic wave signal is uniformly incident from the left end of the structure, and the signal received at the right end of the structure is the filtered elastic wave signal.

[0012] Based on the above - mentioned wide - band band - pass filter for elastic waves, the present invention provides a frequency - shift method, which includes the following steps: increasing the length of the defect structure, the center frequency of the filtering pass - band moves from high frequency to low frequency; on the contrary, reducing the length of the defect structure, the center frequency of the filtering pass - band moves from low frequency to high frequency.

[0013] For further optimization, the following steps are adopted: proportionally enlarging the overall size parameters of the filter structure, the overall working frequency band of the filter moves to low frequency; on the contrary, proportionally reducing the overall size parameters of the filter structure, the overall working frequency band of the filter moves to high frequency.

[0014] The wide - band band - pass filter for elastic waves provided by the present invention inserts four defect structures in a specific manner in a periodic undulation structure with a total number of periods of 8n (n = 1, 2, 3,...). This insertion method enables the filter structure to meet the multi - defect mode coupling condition, forming a flat - top wide - band defect mode, which is manifested as a wide pass - band with a flat - top characteristic within the Bragg bandgap, realizing elastic wave wide - band band - pass filtering with a flat - top characteristic. In the optimization scheme, the present invention has the advantages of simple and integrated structure, and the frequency - shift method can make the present invention have advantages such as tunable filtering frequency band.

[0015] Compared with the prior art, the technical advantages of the wide - band band - pass filter for elastic waves of the present invention are as follows: 1. The wide - band band - pass filter for elastic waves of the present invention has a simple and integrated structure, low cost, is easy to implement, does not require encapsulation, and is easy to use.

[0016] 2. Due to multi - defect mode coupling, the filtering pass - band of the wide - band band - pass filter for elastic waves of the present invention is significantly widened compared with the filtering pass - band formed by a single defect mode.

[0017] 3. The filtering pass - band of the wide - band band - pass filter for elastic waves of the present invention has a flat - top characteristic, and the signal gain or attenuation degree within the pass - band is almost the same, making the filtering performance excellent and stable, and enabling continuous filtering.

[0018] 4. The center frequency of the filtering pass - band of the wide - band band - pass filter for elastic waves of the present invention can be regulated by adjusting the defect length l.

[0019] 5. The bandwidth of the filtering pass - band of the wide - band band - pass filter for elastic waves of the present invention can be regulated by adjusting the parameter n.

[0020] 6. The wideband bandpass filter for elastic waves of the present invention can adjust the overall size of the filter structure according to the required filtering working frequency band. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic structural diagram of the wideband bandpass filter for elastic waves in an embodiment; Figure 2 It is a front sectional view of the wideband bandpass filter for elastic waves in an embodiment; Figure 3 It is a comparison diagram of transmission spectra of a four-defect periodic structure and a single-defect periodic structure inserted only corresponding to the embodiment; Figure 4 It is a transmission spectrum diagram of the center frequency of the filtering passband being regulated by the defect length l in an embodiment; Figure 5 It is a transmission spectrum diagram of the Bragg bandgap and the bandwidth of the passband therein being regulated by the parameter n in an embodiment.

[0022] In the figure, 1. upper boundary; 2. lower boundary; 3. rectangular periodic undulating structure; 4. four defect structures DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] The present invention will be specifically described below in conjunction with embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts fall within the scope of protection of the present invention.

[0024] See Figures 1 to 5 , a wideband bandpass filter for elastic waves includes a flat waveguide with a rectangular periodic undulating structure 3 on an upper boundary 1 and a lower boundary 2, and four defect structures 4. The maximum thickness of the flat waveguide, that is, the distance between the upper boundary 1 and the lower boundary 2, is h3. In addition, the minimum thickness and the average thickness of the flat waveguide are h1 and h2 respectively. The rectangular periodic undulating structures 3 on the upper boundary 1 and the lower boundary 2 are symmetric about the horizontal central axis of the filter profile, and the total number of periods is 8n (n = 1, 2, 3,...), the period length is T, and the undulating height is α (0 < α < h3 / 2). The four defect structures 4 are the first defect D1, the second defect D2, the third defect D3, and the fourth defect D4 respectively. The upper and lower thicknesses of the defects are the minimum thickness h1 of the flat waveguide, and the defect lengths are equal and are all l, and they are inserted after the nth, 3nth, 5nth, and 7nth periodic undulating structures in sequence.

[0025] When an elastic wave is incident from the left onto the broadband pass filter for elastic waves of this invention, the rectangular periodic undulation structure 3 induces Bragg scattering. That is, when the frequency satisfies the Bragg condition, the incident fundamental mode and the reflected fundamental mode interfere (resonance occurs between the same transverse modes of the wave), causing the elastic wave near that frequency to be unable to propagate, thus forming a Bragg bandgap. Based on this, the four defect structures 4 inserted in the waveguide interact with the elastic wave within the frequency bandgap, generating four defect modes within the Bragg bandgap. Due to the insertion method, the filter structure satisfies the multi-defect mode coupling condition, causing multiple defect modes to couple with each other to form a flat-top broadband defect mode, which manifests as a wide passband with flat-top characteristics within the Bragg bandgap.

[0026] This filter is an integrated structure made of the same material. The overall dimensions can be scaled up or down proportionally, shifting the overall operating frequency band of the filter towards lower or higher frequencies. The center frequency of the filter passband can be adjusted by changing the defect length *l*. As the defect length increases, the center frequency shifts from high to low frequencies. Furthermore, the filter passband bandwidth can be adjusted by changing the parameter *n*. The four defect structures 4 divide the periodic undulation structure 3 into five parts, and the number of periods (n, 2n, 2n, 2n, n) of each part is adjusted by the same multiple *n*. Increasing the value of *n* increases the number of periods and decreases the filter passband bandwidth. In a preferred embodiment, the overall filter structure is made of 6061 aluminum alloy. The minimum thickness *h1*, average thickness *h2*, and maximum thickness *h3* of the planar waveguide with the rectangular periodic undulation structure 3 on the upper boundary 1 and lower boundary 2 are 2.4 mm, 2.7 mm, and 3 mm, respectively. The total number of periods in the periodic undulation structure 3 is 8 (n=1), the period length *T*=3 mm, and the undulation height *α*=0.3 mm. The defect lengths of the four defect structures 4 are l=0.5T=1.5mm. Based on these parameters, we conduct a simulation study on the filter performance.

[0027] Figure 3 This is a comparison of the transmission spectra of a broadband pass filter for elastic waves using an embodiment, specifically a four-defect periodic structure and a structure with only a single-defect periodic structure. The two structures are identical except for the number of inserted defect structures; the defect structure in the single-defect periodic structure is inserted after the 4nth periodic fluctuation structure. Figure 3 It can be seen that the passband form within the Bragg bandgap changes from a sharp transmission peak in a single-defect periodic structure to a wide passband with flat-top characteristics in a four-defect periodic structure. The -3dB bandwidth of the passband is broadened from 33KHz (0.451MHz-0.484 MHz) to 139KHz (0.396MHz-0.535MHz) while the center frequency of the passband remains unchanged.

[0028] Figure 4This invention relates to a transmission spectrum showing the center frequency of the filter passband of a broadband pass filter for elastic waves, which is adjustable by the defect length *l*. When the defect length *l* = 0.4T, the center frequency of the filter passband is 0.494MHz. Increasing the defect length to 0.5T and 0.6T shifts the center frequency to 0.472MHz and 0.45MHz, respectively. This demonstrates that as the defect length increases, the center frequency of the filter passband shifts from high frequency to low frequency, achieving the goal of adjustable center frequency of the filter passband.

[0029] Figure 5 The transmission spectrum of the broadband passband filter for elastic waves in this embodiment is controlled by parameter n. As the value of parameter n increases from 1 to 3, the -3dB bandwidth of the passband decreases from 139kHz (0.396MHz-0.535MHz) to 30MHz (0.453MHz-0.483MHz), and the attenuation effect of the bandgap also increases, thus achieving the goal of adjustable passband bandwidth.

[0030] In the implementation of the broadband pass filter for elastic waves, the parameters such as defect length, overall structural size, and number of periods can be designed according to the actual required filtering effect.

Claims

1. A broadband pass filter for elastic waves, comprising a planar waveguide with rectangular periodic undulations on its upper and lower boundaries and four defect structures, characterized in that, The total number of periods of the rectangular periodic undulating structure is 8n (n = 1, 2, 3, …). The rectangular periodic undulating structures on the upper and lower boundaries are symmetric about the horizontal central axis of the filter profile and produce Bragg scattering with the incident elastic wave to form a Bragg bandgap. The four defect structures are sequentially named the first defect D1, the second defect D2, the third defect D3, and the fourth defect D4. After D1, D2, D3, and D4 are respectively inserted into the nth, 3nth, 5nth, and 7nth periodic undulating structures, the four defect structures have the same length.

2. The broadband pass filter for elastic waves according to claim 1, characterized in that, The value range of the undulating height α of the rectangular periodic undulating structure is 0 < α < h3 / 2, where h3 is the distance between the upper and lower boundaries.

3. The broadband pass filter for elastic waves according to claim 1, characterized in that, The band-pass filter is an integrated structure of the same material. The original elastic wave signal is uniformly incident from the left end of the structure, and the signal received at the right end of the structure is the filtered elastic wave signal.

4. The frequency shifting method for a broadband pass filter for elastic waves according to any one of claims 1 to 3, characterized in that, It includes the following steps: increasing the length of the defect structure causes the center frequency of the filtering passband to shift from high frequency to low frequency; conversely, decreasing the length of the defect structure causes the center frequency of the filtering passband to shift from low frequency to high frequency.

5. The frequency shifting method for a broadband pass filter for elastic waves according to any one of claims 1 to 3, characterized in that, It includes the following steps: scaling up the overall size parameters of the filter structure proportionally causes the overall working frequency band of the filter to shift to low frequency; conversely, scaling down the overall size parameters of the filter structure proportionally causes the overall working frequency band of the filter to shift to high frequency.

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