Low-frequency filter and filter circuit

Abstract

The utility model provides a kind of low-frequency filter and filter circuit, the low-frequency filter includes piezoelectric material ring, first top electrode ring, second top electrode ring and bottom electrode ring, first top electrode ring and second top electrode ring are located at the top of piezoelectric material ring, respectively as signal input end and signal output end, the profile of the outer lateral wall of first top electrode ring and second top electrode ring is similar with the profile of the outer lateral wall of piezoelectric material ring, and the inner lateral wall of first top electrode ring and the outer lateral wall of second top electrode ring are respectively aligned with the inner lateral wall and outer lateral wall of piezoelectric material ring, there is spacing between the inner lateral wall of second top electrode ring and the outer lateral wall of first top electrode ring;Bottom electrode ring is located at the bottom of piezoelectric material ring and is grounded, and bottom electrode ring, second top electrode ring, first top electrode ring and piezoelectric material ring are all concentric similar ring. Among them, the low-frequency filter is simple in structure, and has small insertion loss.

Description

Technical Field

[0001] This utility model relates to the field of piezoelectric devices, and in particular to a low-frequency filter and a filtering circuit. Background Technology

[0002] In the electromagnetic spectrum, low-frequency signals refer to signals with frequencies ranging from 30kHz to 300kHz, corresponding to wavelengths from 10km to 1km, much larger than the dimensions of typical electronic circuits. Therefore, passive filters in this frequency band are typically designed using lumped elements (inductors and capacitors). To generate a lower resonant frequency in the operating band, larger inductors and capacitors are required. However, larger inductors and capacitors often have significant disadvantages such as large size and low Q value. This not only increases the overall size of the filter but also introduces additional insertion loss, resulting in energy loss of the input signal. For example: Please refer to... Figure 1 To construct a first-order Butterworth bandpass filter with a center frequency of 82.5kHz, an inductor with an inductance and Q of 362.6nH and 200Ω, respectively, and a capacitor with a capacitance and Q of 10.26μF and 3000Ω, respectively, are required to be connected in parallel. The inductor exhibits a relatively low Q value.

[0003] Currently, the most common piezoelectric filters on the market are mainly surface acoustic wave (SAW) and bulk acoustic wave (BAW) types, with piezoelectric thin films as their core material. Piezoelectric filters achieve a filtering effect by connecting resonators with different resonant frequencies and input impedances in series or parallel. Both types of piezoelectric filters have relatively complex structural designs and manufacturing processes, and they are only suitable for the frequency range of 10MHz to 10GHz, making them difficult to apply in low-frequency and lower frequency bands.

[0004] Therefore, there is an urgent need for a piezoelectric low-frequency filter with a simple structure and low insertion loss. Utility Model Content

[0005] This invention provides a low-frequency filter and a filtering circuit, which offers a piezoelectric low-frequency filter with a simple structure and low insertion loss.

[0006] According to a first aspect of the present invention, a low-frequency filter is provided, comprising: piezoelectric material ring; The first top electrode ring is located at the top of the piezoelectric material ring to serve as a signal input terminal. The outline of the outer sidewall of the first top electrode ring is similar to the outline of the outer sidewall of the piezoelectric material ring, and the inner sidewall of the first top electrode ring is aligned with the inner sidewall of the piezoelectric material ring. The second top electrode ring is located at the top of the piezoelectric material ring and serves as a signal output terminal. The outline of the outer side wall of the second top electrode ring is similar to that of the outer side wall of the piezoelectric material ring, and the outer side wall of the second top electrode ring is aligned with the outer side wall of the piezoelectric material ring. There is a gap between the inner side wall of the second top electrode ring and the outer side wall of the first top electrode ring. The bottom electrode ring is located at the bottom of the piezoelectric material ring and is grounded. The bottom electrode ring, the second top electrode ring, the first top electrode ring, and the piezoelectric material ring are all concentric similar rings.

[0007] Optionally, the piezoelectric material ring is a circular ring.

[0008] Optionally, the first top electrode ring and the second top electrode ring have the same ring width.

[0009] Optionally, the radius of the outer sidewall of the piezoelectric material ring is in the range of 14mm to 20mm, the radius of the inner sidewall of the piezoelectric material ring is in the range of 3mm to 5mm, and the distance between the inner sidewall of the second top electrode ring and the outer sidewall of the first top electrode ring is in the range of 0.5mm to 4mm.

[0010] Optionally, the thickness of the piezoelectric material ring is 0.8 mm to 2.5 mm.

[0011] Optionally, the piezoelectric material ring is made of piezoelectric ceramic or piezoelectric single crystal material.

[0012] Optionally, the first top electrode, the second top electrode, and the bottom electrode are all made of the same material, and the first top electrode is made of one of aluminum, copper, and silver.

[0013] Optionally, the first top electrode, the second top electrode, and the bottom electrode all have the same thickness, and the thickness of the first top electrode ranges from 18 μm to 35 μm.

[0014] According to a second aspect of the present invention, a filter circuit is provided, comprising: A plurality of filter banks connected in parallel, each filter bank including at least one of the aforementioned low-frequency filters, wherein the signal input terminal of the low-frequency filter is the signal input terminal of the filter bank, and the signal output terminal of the low-frequency filter is the signal output terminal of the filter bank; The circuit signal input terminal is electrically connected to the signal input terminals of several filter banks; The circuit signal output terminal is electrically connected to the signal output terminals of several of the filter banks.

[0015] Optionally, among a plurality of filter banks connected in parallel, at least one of the filter banks includes at least two low-frequency filters connected in series, wherein the signal input terminal of the first low-frequency filter is the signal input terminal of the filter bank, and the signal output terminal of the last low-frequency filter is the signal output terminal of the filter bank.

[0016] Compared with the prior art, the technical solution of this utility model has the following beneficial effects: The present invention provides a low-frequency filter comprising a piezoelectric material ring, a first top electrode ring, a second top electrode ring, and a bottom electrode ring. The first top electrode ring is located at the top of the piezoelectric material ring and serves as the signal input terminal. The outline of the outer wall of the first top electrode ring is similar to that of the outer wall of the piezoelectric material ring, and the inner wall of the first top electrode ring is aligned with the inner wall of the piezoelectric material ring. The second top electrode ring is located at the top of the piezoelectric material ring and serves as the signal output terminal. The outline of the outer wall of the second top electrode ring is similar to that of the outer wall of the piezoelectric material ring, and the outer wall of the second top electrode ring is aligned with the outer wall of the piezoelectric material ring. There is a gap between the inner wall of the second top electrode ring and the outer wall of the first top electrode ring. The bottom electrode ring is located at the bottom of the piezoelectric material ring and is grounded. The bottom electrode ring, the second top electrode ring, the first top electrode ring, and the piezoelectric material ring are all concentric similar rings. This low-frequency filter has a simple structure and utilizes the piezoelectric effect of piezoelectric ceramics and its inherent series and parallel resonant impedance characteristics to filter signals of specific frequencies, thereby achieving the filtering function. In addition, this low-frequency filter has low insertion loss. Attached Figure Description

[0017] Figure 1 It is a Butterworth bandpass filter; Figure 2 This is a top view of the low-frequency filter provided in this embodiment of the utility model; Figure 3 This is a cross-sectional structural diagram of the low-frequency filter provided in this embodiment of the utility model; Figure 4 This utility model provides |S 11 | Curve comparison chart; Figure 5 This utility model provides |S 21 | Curve comparison chart; Figure 6 yes Figure 1 A comparison of the S-parameters of a Butterworth bandpass filter and a low-frequency filter with a 1.5mm pitch in the same frequency band; Figure 7 This is the topology connection of the filter circuit provided by this utility model. Figure 1 ; Figure 8 yes Figure 7The S-parameter curves of the filter circuit; Figure 9 This is the topology connection of the filter circuit provided by this utility model. Figure 2 ; Figure 10 yes Figure 9 The S-parameter curves of the filter circuit.

[0018] Figure label: 1-First top electrode ring; 2-Second top electrode ring; 3-Piezoelectric material ring; 4- Bottom electrode ring. Detailed Implementation

[0019] As described in the background section, low-frequency filters employing lumped elements typically have high insertion loss.

[0020] In view of this, the present invention creatively proposes a low-frequency filter, comprising: a piezoelectric material ring; a first top electrode ring, located at the top of the piezoelectric material ring as a signal input terminal, wherein the outline of the outer sidewall of the first top electrode ring is similar to the outline of the outer sidewall of the piezoelectric material ring, and the inner sidewall of the first top electrode ring is aligned with the inner sidewall of the piezoelectric material ring; a second top electrode ring, located at the top of the piezoelectric material ring as a signal output terminal, wherein the outline of the outer sidewall of the second top electrode ring is similar to the outline of the outer sidewall of the piezoelectric material ring, and the outer sidewall of the second top electrode ring is aligned with the outer sidewall of the piezoelectric material ring, and there is a gap between the inner sidewall of the second top electrode ring and the outer sidewall of the first top electrode ring; and a bottom electrode ring, located at the bottom of the piezoelectric material ring and grounded, wherein the bottom electrode ring, the second top electrode ring, the first top electrode ring, and the piezoelectric material ring are all concentric similar rings.

[0021] This low-frequency filter features a simple structure. It utilizes the piezoelectric effect of piezoelectric ceramics and the inherent series and parallel resonant impedance characteristics to filter signals of specific frequencies. Furthermore, this low-frequency filter exhibits low insertion loss.

[0022] The embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this utility model, and not all of them. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this utility model. The terms "first," "second," "third," etc., in the specification, claims, and accompanying drawings of this utility model are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this utility model described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0023] Figure 2 This is a top view of the low-frequency filter provided in this embodiment of the present invention. Figure 3 This is a cross-sectional structural diagram of the low-frequency filter provided in this embodiment of the present invention.

[0024] Please refer to Figure 2 and Figure 3 This utility model provides a low-frequency filter, which may include a piezoelectric material ring 3, a first top electrode ring 1, a second top electrode ring 2, and a bottom electrode ring 4.

[0025] In this embodiment, the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 are all concentric similar rings. Specifically, the concentric similar rings possess both concentricity and similarity. Concentricity is used to characterize that the centers of the inner and outer contours of the ring structure coincide, that is, the centers of the inner sidewalls of the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 coincide with the centers of the corresponding outer sidewalls. Similarity is used to characterize that the inner and outer contours of the ring structure satisfy similarity characteristics, specifically, the radial dimensions (such as inner diameter, outer diameter, etc.) of the inner and outer contours of the ring structure have a fixed proportional coefficient, that is, the geometric contours of the inner sidewalls and the corresponding outer sidewalls of the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 are similar.

[0026] Specifically, the inner and outer walls of the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 have the same shape and the center of the inner wall coincides with the center of the outer wall. Therefore, the ring width of the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 is uniform.

[0027] In this embodiment, the piezoelectric material ring 3 can be, for example, a circular ring. Therefore, the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 are all circular rings with uniform ring width, and the centers of the inner and outer sidewalls of the bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 coincide.

[0028] In other embodiments, the outer wall shape of the piezoelectric material ring 3 may be a regular quadrilateral or other regular polygon.

[0029] In this embodiment, the thickness t of the piezoelectric material ring 3 p The thickness ranges from 0.8mm to 2.5mm.

[0030] In this embodiment, the material of the piezoelectric material ring 3 can be, for example, a piezoelectric ceramic, specifically a PZT-4 material or a PZT-8 material polarized along the thickness direction.

[0031] In other embodiments, the material of the piezoelectric material ring 3 can be a piezoelectric single crystal material, such as PMN-PT (lead magnesium niobate-lead titanate), PIN-PMN-PT (lead indium niobate-lead magnesium niobate-lead titanate), lithium niobate, lithium tantalate, etc.

[0032] In this embodiment, the mechanical quality factor of the piezoelectric material ring 3 is greater than or equal to 800, and the dielectric loss tangent of the piezoelectric material ring 3 is less than or equal to 0.4%.

[0033] In this embodiment, the radius r2 of the outer sidewall of the piezoelectric material ring 3 ranges from 14mm to 20mm, and the radius r1 of the inner sidewall of the piezoelectric material ring 3 ranges from 3mm to 5mm.

[0034] The first top electrode ring 1 is located on top of the piezoelectric material ring 3, serving as the signal input terminal Port1. The outline of the outer wall of the first top electrode ring 1 is similar to that of the outer wall of the piezoelectric material ring 3, and the inner wall of the first top electrode ring 1 is aligned with the inner wall of the piezoelectric material ring 3. That is to say, the outline of the outer wall of the first top electrode is the same shape as the outline of the outer wall of the piezoelectric material ring 3, and the shapes of the two outer wall outlines are proportional.

[0035] The second top electrode ring 2 is located on top of the piezoelectric material ring 3 as a signal output terminal Port 2. The shape of the outer side wall of the second top electrode ring 2 is the same as that of the outer side wall of the piezoelectric material ring 3, and the outer side wall of the second top electrode ring 2 is aligned with the outer side wall of the piezoelectric material ring 3. There is a gap between the inner side wall of the second top electrode ring 2 and the outer side wall of the first top electrode ring 1.

[0036] In this embodiment, the first top electrode ring 1 and the second top electrode ring 2 have the same ring width, and the ring width of the first top electrode ring 1 is... ,in, Let be the radius of the outer wall of the piezoelectric material ring 3. Let be the radius of the inner wall of the piezoelectric material ring 3. It is the distance between the inner wall of the second top electrode ring 2 and the outer wall of the first top electrode ring 1.

[0037] In this embodiment, the gap between the inner wall of the second top electrode ring 2 and the outer wall of the first top electrode ring 1 ranges from 0.5 mm to 4 mm.

[0038] The bottom electrode ring 4 is located at the bottom of the piezoelectric material ring 3 and is grounded. Specifically, the inner and outer walls of the bottom electrode are aligned with the inner and outer walls of the piezoelectric material ring 3, respectively.

[0039] It should be noted that when applying this low-frequency filter, its inner ring surface (including the inner wall of the piezoelectric material ring 3, the inner wall of the bottom electrode, and the inner wall of the first top electrode) needs to be fixed so that the piezoelectric material ring 3 and the electrodes together form a radial resonance mode.

[0040] In this embodiment, the first top electrode, the second top electrode, and the bottom electrode are all made of the same material, and the first top electrode is made of one of aluminum, copper, and silver.

[0041] In this embodiment, the first top electrode, the second top electrode, and the bottom electrode all have the same thickness, and the thickness t of the first top electrode is... m The range is 18μm~35μm.

[0042] As an example, the piezoelectric material ring 3 is a circular ring made of PZT-8 material polarized along its thickness direction. Its inner wall radius is 4 mm, its outer wall radius is 16 mm, and its thickness is 2 mm. The bottom electrode, the first top electrode, and the second top electrode are made of silver, each with a thickness of 20 μm.

[0043] Please refer to Figure 4 , Figure 4 Low-frequency filters with different spacing gaps are in the same frequency band |S 11 | Curve comparison graph, the horizontal axis of the graph is frequency, and the vertical axis is |S 11 |(dB), the curves in the figure correspond to low-frequency filters with different gaps between the outer wall of the first top electrode and the inner wall of the second top electrode. The gaps corresponding to the curves are 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm and 4mm respectively. From Figure 4As can be seen, as the gap increases, the horizontal axis of the corresponding valley value of the curve gradually increases, that is, the passband of the corresponding low-frequency filter (i.e., near the valley value of the curve) gradually moves towards the high frequency direction. In addition, its vertical axis gradually increases but is still less than -10dB, that is, the return loss is greater than 10dB. Therefore, it is shown that the corresponding low-frequency filter has good impedance matching characteristics.

[0044] Please refer to Figure 5 , Figure 5 Low-frequency filters with different spacing gaps are in the same frequency band |S 21 | Curve comparison graph, the horizontal axis of the graph is frequency, and the vertical axis is |S 21 |(dB), the curves in the figure correspond to low-frequency filters with different gaps between the outer wall of the first top electrode and the inner wall of the second top electrode. The gaps corresponding to the curves are 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm and 4mm respectively. From Figure 5 It can be seen that as the gap increases, the horizontal axis of the corresponding peak value of the curve gradually increases, that is, the passband of the low-frequency filter (i.e., near the peak value of the curve) gradually moves towards the high frequency direction. In addition, the vertical axis of the peak value of each curve is greater than -2dB, that is, the highest insertion loss in the passband is less than 2dB.

[0045] Please refer to Figure 6 , Figure 6 yes Figure 1 The graph compares the S-parameters of a Butterworth bandpass filter and a low-frequency filter with a 1.5mm spacing in the same frequency band. The horizontal axis represents frequency, and the vertical axis represents the S-parameters (including |S|). 11 |and|S 21 |), the four curves in the figure represent the |S| of the Butterworth bandpass filter. 11 |Curve (LC,|S) 11 |)and|S 21 |Curve (LC,|S) 21 |) and the low-frequency filter's |S 11 |Curve (Piezo.,|S 11 |)and|S 21 |Curve (Piezo.,|S 21 |). By Figure 6 It can be seen that the |S| of the Butterworth bandpass filter 21 |Curve (LC,|S) 21 The peak value of |) is lower than that of the low-frequency filter |S 21 |Curve (Piezo.,|S 21 The peak value of |) indicates that the insertion loss of the Butterworth bandpass filter is higher than that of the low-frequency filter.

[0046] In summary, this utility model embodiment provides a low-frequency filter, including a piezoelectric material ring 3, a first top electrode ring 1, a second top electrode ring 2, and a bottom electrode ring 4. The first top electrode ring 1 is located at the top of the piezoelectric material ring 3 as a signal input terminal. The outline of the outer wall of the first top electrode ring 1 is similar to the outline of the outer wall of the piezoelectric material ring 3, and the inner wall of the first top electrode ring 1 is aligned with the inner wall of the piezoelectric material ring 3. The second top electrode ring 2 is located at the top of the piezoelectric material ring 3 as a signal output terminal. The outline of the outer wall of the second top electrode ring 2 is similar to the outline of the outer wall of the piezoelectric material ring 3, and the outer wall of the second top electrode ring 2 is aligned with the outer wall of the piezoelectric material ring 3. There is a gap between the inner wall of the second top electrode ring 2 and the outer wall of the first top electrode ring 1. The bottom electrode ring 4 is located at the bottom of the piezoelectric material ring 3 and is grounded. The bottom electrode ring 4, the second top electrode ring 2, the first top electrode ring 1, and the piezoelectric material ring 3 are all concentric similar rings. This low-frequency filter features a simple structure. It utilizes the piezoelectric effect of piezoelectric ceramics and the inherent series and parallel resonant impedance characteristics to filter signals of specific frequencies. Furthermore, this low-frequency filter exhibits low insertion loss.

[0047] This utility model embodiment also provides a filtering circuit, including a plurality of filter banks connected in parallel, a circuit signal input terminal, and a circuit signal output terminal. Each filter bank includes at least one of the aforementioned low-frequency filters. The signal input terminal of the low-frequency filter is the signal input terminal of the filter bank, and the signal output terminal of the low-frequency filter is the signal output terminal of the filter bank. The circuit signal input terminal is electrically connected to the signal input terminals of the plurality of filter banks, and the circuit signal output terminal is electrically connected to the signal output terminals of the plurality of filter banks.

[0048] Specifically, if the filter bank includes a low-frequency filter, the signal input terminal of the low-frequency filter is the signal input terminal of the filter bank, and the signal output terminal of the low-frequency filter is the signal output terminal of the filter bank. If the filter bank includes at least two low-frequency filters connected in series, the signal input terminal of the first low-frequency filter is the signal input terminal of the filter bank, and the signal output terminal of the last low-frequency filter is the signal output terminal of the filter bank.

[0049] In this embodiment, among a plurality of filter banks connected in parallel, at least one of the filter banks includes at least two low-frequency filters connected in series.

[0050] In this embodiment, in a filter bank including at least two series-connected low-frequency filters, the distance between the inner wall of the second top electrode ring 2 and the outer wall of the first top electrode ring 1 in any two low-frequency filters is not the same.

[0051] Filter circuits formed by different topology connection methods can achieve different functions. The following two examples illustrate this.

[0052] For example, please refer to Figure 7 , Figure 7 It is the topology connection of the filter circuit. Figure 1 The filtering circuit includes low-frequency filters (F1, F2, F3, and F4) connected in parallel. The gaps between the outer sidewall of the first top electrode and the inner sidewall of the second top electrode are 1.5mm, 1.8mm, 2.1mm, and 2.4mm, respectively. The signal input terminals of the four low-frequency filters are connected together as the circuit signal input terminal Port11, and the signal output terminals of the four low-frequency filters are connected together as the circuit signal output terminal Port22. Furthermore, the four low-frequency filters share a common ground. Please refer to [reference needed]. Figure 8 , Figure 8 yes Figure 7 The S-parameter curve of the filter circuit, based on Figure 8 It can be seen that, compared to a single low-frequency filter (see reference...), Figure 4 and Figure 5 The passband of this filter circuit is broadened, and the insertion loss within the passband is within 3dB.

[0053] As another example, please refer to Figure 9 , Figure 9 It is the topology connection of the filter circuit. Figure 2 The filtering circuit includes two filter groups, each with two low-frequency filters connected in series: a first filter group and a second filter group. The first filter group includes two low-frequency filters (F1 and F2) connected in series, with gaps of 1.5mm and 1.8mm between the outer wall of the first top electrode and the inner wall of the second top electrode, respectively. The second filter group includes two low-frequency filters (F3 and F4) connected in series, with gaps of 2.1mm and 2.4mm between the outer wall of the first top electrode and the inner wall of the second top electrode, respectively. The signal input terminals of F1 in the first filter group and F3 in the second filter group are connected together as the circuit signal input terminal Port11. The signal output terminals of F2 in the first filter group and F4 in the second filter group are connected together as the circuit signal output terminal Port22. Please refer to [reference needed]. Figure 10 , Figure 10 yes Figure 9 The S-parameter curve of the filter circuit, based on Figure 10 It can be seen that the filter circuit has two adjacent passbands (i.e., around 82.5kHz and around 83kHz), which can be used to filter out the center frequency component of broadband signals.

[0054] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A low-frequency filter, characterized in that, include: piezoelectric material ring; The first top electrode ring is located at the top of the piezoelectric material ring to serve as a signal input terminal. The outline of the outer sidewall of the first top electrode ring is similar to the outline of the outer sidewall of the piezoelectric material ring, and the inner sidewall of the first top electrode ring is aligned with the inner sidewall of the piezoelectric material ring. The second top electrode ring is located at the top of the piezoelectric material ring and serves as a signal output terminal. The outline of the outer side wall of the second top electrode ring is similar to that of the outer side wall of the piezoelectric material ring, and the outer side wall of the second top electrode ring is aligned with the outer side wall of the piezoelectric material ring. There is a gap between the inner side wall of the second top electrode ring and the outer side wall of the first top electrode ring. The bottom electrode ring is located at the bottom of the piezoelectric material ring and is grounded. The bottom electrode ring, the second top electrode ring, the first top electrode ring, and the piezoelectric material ring are all concentric similar rings.

2. The low-frequency filter according to claim 1, characterized in that, The piezoelectric material ring is a circular ring.

3. The low-frequency filter according to claim 2, characterized in that, The first top electrode ring and the second top electrode ring have the same ring width.

4. The low-frequency filter according to claim 2, characterized in that, The radius of the outer sidewall of the piezoelectric material ring is in the range of 14mm to 20mm, the radius of the inner sidewall of the piezoelectric material ring is in the range of 3mm to 5mm, and the distance between the inner sidewall of the second top electrode ring and the outer sidewall of the first top electrode ring is in the range of 0.5mm to 4mm.

5. The low-frequency filter according to claim 1, characterized in that, The thickness of the piezoelectric material ring is 0.8 mm to 2.5 mm.

6. The low-frequency filter according to claim 1, characterized in that, The piezoelectric material ring is made of piezoelectric ceramic or piezoelectric single crystal material.

7. The low-frequency filter according to claim 1, characterized in that, The first top electrode, the second top electrode, and the bottom electrode are all made of the same material, and the first top electrode is made of one of aluminum, copper, and silver.

8. The low-frequency filter according to claim 1, characterized in that, The first top electrode, the second top electrode, and the bottom electrode all have the same thickness, and the thickness of the first top electrode ranges from 18 μm to 35 μm.

9. A filter circuit, characterized in that, include: A plurality of filter banks connected in parallel, each filter bank comprising at least one low-frequency filter as described in any one of claims 1 to 8; The signal input terminal of the low-frequency filter is the signal input terminal of the filter group, and the signal output terminal of the low-frequency filter is the signal output terminal of the filter group. The circuit signal input terminal is electrically connected to the signal input terminals of several filter banks; The circuit signal output terminal is electrically connected to the signal output terminals of several of the filter banks.

10. The filter circuit according to claim 9, characterized in that, In a plurality of filter banks connected in parallel, at least one of the filter banks includes at least two low-frequency filters connected in series, the signal input terminal of the first low-frequency filter is the signal input terminal of the filter bank, and the signal output terminal of the last low-frequency filter is the signal output terminal of the filter bank.

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