A highly rectangular longitudinally coupled surface acoustic wave filter

Abstract

The present invention belongs to the technical field of filters, and specifically relates to a highly rectangular longitudinally coupled surface acoustic wave filter; the filter comprises: a piezoelectric substrate and metal interdigital electrodes located on the piezoelectric substrate; the metal interdigital electrodes comprise a first reflection grating, first to third interdigital transducers, and a second reflection grating arranged in sequence along a first direction; the first and second reflection gratings are both short-circuit grating structures; the first interdigital transducer comprises two first bus bars and a plurality of electrode fingers periodically and alternately connected to the two first bus bars; the third interdigital transducer comprises two third bus bars and a plurality of electrode fingers periodically and alternately connected to the two third bus bars; the second interdigital transducer comprises two second bus bars and a plurality of electrode fingers periodically and alternately connected to the two second bus bars, and an asymmetric suspended transducer region is provided on the right side of the second interdigital transducer; the present invention achieves a steep transition zone on the left side of the filter, thereby optimizing the rectangularity of the filter.

Description

Technical Field

[0001] The invention belongs to the technical field of filters, and in particular relates to a longitudinally coupled surface acoustic wave filter with high rectangularity. Background Art

[0002] Surface acoustic wave filters are electronic components that utilize acoustic wave signals propagating on the surface of piezoelectric materials to achieve frequency selection. Their operating principle is that an interdigital transducer converts an input electrical signal into surface acoustic waves, which are then converted back into an output electrical signal via the propagating acoustic waves. Currently, surface acoustic wave filters are widely used in wireless communications, RF front-end modules, and other fields.

[0003] A longitudinally coupled surface acoustic wave filter is a surface acoustic wave filter structure that uses multiple interdigital transducers arranged between reflective gratings to concentrate the energy of the surface acoustic wave into the resonant cavity, thereby exciting different acoustic resonance modes. Through the coupling between different resonance modes, the filter achieves lower insertion loss and better out-of-band suppression.

[0004] In traditional longitudinally coupled structures, the performance of the filter's left transition band is generally poor, often exhibiting a small step. To address this issue, a floating electrode is introduced in the filter's middle transducer, creating a steeper transition band on the left side of the filter, thereby improving filter performance. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a highly rectangular longitudinally coupled surface acoustic wave filter, which comprises: a piezoelectric substrate and a metal interdigital electrode located on the piezoelectric substrate; the metal interdigital electrode comprises a first reflection grating, a first interdigital transducer, a second interdigital transducer, a third interdigital transducer and a second reflection grating arranged in sequence along a first direction; the first reflection grating and the second reflection grating are both short-circuit grating structures; the first interdigital transducer comprises two first bus bars and a plurality of electrode fingers periodically and alternately connected to the two first bus bars; the third interdigital transducer comprises a first plurality of .... The interdigital transducer consists of two third bus bars and a plurality of electrode fingers periodically and alternately connected to the two third bus bars; the second interdigital transducer includes two second bus bars and a plurality of electrode fingers periodically and alternately connected to the two second bus bars, and an asymmetric suspended transducer area is provided on the right side of the second interdigital transducer, and the electrode fingers on the left and right sides of the asymmetric suspended transducer area have opposite polarities; the asymmetric suspended transducer area is used to disturb the acoustic wave coupling path; the electrode fingers of the asymmetric suspended transducer are not connected to the bus bars of the second interdigital transducer.

[0006] Preferably, the asymmetric suspended transducer area is an independent suspended structure, a structure with fully connected bus bars on both sides, or a structure with alternately connected bus bars on both sides; wherein, the independent suspended structure is composed of a plurality of periodically arranged electrode fingers; the structure with fully connected bus bars on both sides is composed of bus bars on both sides and a plurality of electrode fingers periodically and simultaneously connected to the two bus bars; the structure with alternately connected bus bars on both sides is composed of bus bars on both sides and a plurality of electrode fingers periodically and alternately connected to the two bus bars.

[0007] Furthermore, the number of electrode fingers in the independent suspension structure, the structure with fully connected bus bars on both sides, or the structure with alternately connected bus bars on both sides is 5 or 7.

[0008] Furthermore, there are at least 7 electrode fingers between the leftmost electrode in the asymmetric suspended transducer area and the middle position of the second interdigital transducer; there are 5 to 7 electrode fingers between the rightmost electrode in the asymmetric suspended transducer area and the rightmost electrode finger of the second interdigital transducer.

[0009] Furthermore, the period of the electrode fingers in the asymmetric suspension transducer region is the same as the period of the electrode fingers in the rest of the second IDT.

[0010] Preferably, all the electrode fingers of the IDTs and the reflective grids are made of copper and have the same thickness.

[0011] Preferably, the electrode finger widths of all IDTs and reflective gratings are 0.4 to 0.5 times the operating wavelength.

[0012] The beneficial effects of the present invention are:

[0013] (1) Optimization of the left side of the passband: By setting an asymmetric suspended transducer structure and adjusting the acoustic wave energy coupling path, the left transition band performance of the filter is significantly optimized, making the frequency response of the low-frequency transition zone steeper, thereby improving the rectangularity of the filter and the out-of-band suppression effect, meeting the design requirements of high-performance filters.

[0014] (2) Simple structure and controllable process: The suspended transducer area design adopted by the present invention has a simple structure, and the materials and sizes of each transducer and reflector are easy to realize. It is also compatible with the existing surface acoustic wave filter manufacturing process, does not require complex additional process steps, and is convenient for large-scale production and process control. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a structural diagram of a longitudinally coupled surface acoustic wave filter when the asymmetric suspended transducer region in the present invention is an independent suspended structure.

[0016] Figure 2This is a structural diagram of a longitudinally coupled surface acoustic wave filter in the present invention when the asymmetric suspension transducer region is a structure in which bus bars on both sides are completely connected.

[0017] Figure 3 This is a structural diagram of a longitudinally coupled surface acoustic wave filter in the present invention when the asymmetric suspension transducer region is a structure in which bus bars on both sides are alternately connected.

[0018] Figure 4 FIG. 4 is a comparison diagram of filter frequency response characteristic curves of an embodiment of the present invention and a traditional filter. DETAILED DESCRIPTION

[0019] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0020] The present invention proposes a highly rectangular longitudinally coupled surface acoustic wave filter, comprising: a piezoelectric substrate and metal interdigital electrodes located on the piezoelectric substrate; the metal interdigital electrodes comprise a first reflection grating, an interdigital transducer group 10, and a second reflection grating arranged in sequence along a first direction, with the arrangement order being along the direction of sound propagation; the first interdigital transducer is adjacent to the right side of the first reflection grating, and the right side is adjacent to the second interdigital transducer, the right side of the second interdigital transducer is adjacent to the third interdigital transducer; and the right side of the third interdigital transducer is adjacent to the second reflection grating.

[0021] The IDT group 10 includes a first IDT 101, a second IDT 102, and a third IDT 103 arranged in sequence along a first direction, and the first IDT and the third IDT have the same number of fingers.

[0022] The finger electrodes of the first reflector and the second reflector are short-circuited to form a short-circuit grid structure; the first interdigital transducer is composed of two first bus bars 1011 and a plurality of electrode fingers 1012 periodically and alternately connected to the two first bus bars; the third interdigital transducer is composed of two third bus bars 1031 and a plurality of electrode fingers 1032 periodically and alternately connected to the two third bus bars; the second interdigital transducer includes two second bus bars 1021 and a plurality of electrode fingers 1022 periodically and alternately connected to the two second bus bars, and an asymmetric suspended transducer area 1023 is provided on the right side of the second interdigital transducer, and the electrode fingers on the left and right sides of the asymmetric suspended transducer area have opposite polarities; the asymmetric suspended transducer area is used to disturb the acoustic wave coupling path; the electrode fingers of the asymmetric suspended transducer are not connected to the bus bars of the second interdigital transducer.

[0023] In some preferred embodiments of the present invention, the asymmetric suspension transducer region can be one of three structures: an independent suspension structure, a structure with bus bars on both sides fully connected, or a structure with bus bars on both sides alternately connected; wherein, Figure 1 As shown in , the independent suspension structure consists of multiple periodically arranged electrode fingers, and all electrode fingers in the suspension area are not connected to each other; Figure 2 As shown, the fully connected busbar structure on both sides is composed of busbars 10231 on both sides and a plurality of electrode fingers 10232 periodically connected to the two busbars at the same time, forming a busbar electrode group. All electrode fingers are not connected to the input and output circuits and are not grounded; Figure 3 As shown, the alternating connection structure of the bus bars on both sides consists of bus bars 10231 on both sides and multiple electrode fingers 10232 periodically and alternately connected to the two bus bars, forming two independent bus electrode groups. All electrode fingers are not connected to the input and output circuits and are not grounded.

[0024] In some preferred embodiments of the present invention, the number of electrode fingers in the independent suspension structure, the structure with fully connected bus bars on both sides, or the structure with alternately connected bus bars on both sides is 5 or 7; when the filter is provided with an asymmetric suspension transducer area, there are no less than 7 electrode fingers between the leftmost electrode in the asymmetric suspension transducer area and the middle position of the second interdigital transducer; there are 5 to 7 electrode fingers between the rightmost electrode in the asymmetric suspension transducer area and the rightmost electrode finger of the second interdigital transducer; and the period of the electrode fingers in the asymmetric suspension transducer area is the same as the period of the electrode fingers in the rest of the second interdigital transducer.

[0025] In some preferred embodiments of the present invention, the filter uses a lithium niobate piezoelectric substrate, and the substrate Euler angle is set to (0°, -75°±5°, 0°); the electrode fingers of all interdigital transducers and reflective gratings are made of copper and have the same thickness; the width of the electrode fingers of all interdigital transducers and reflective gratings is 0.4~0.5 times the operating wavelength.

[0026] The surface acoustic wave filter of the present invention realizes longitudinally coupled surface acoustic wave resonance through three interdigital transducers. Specifically, the first interdigital transducer, the second interdigital transducer and the third interdigital transducer are arranged in sequence to excite and maintain different acoustic wave resonance modes, forming a longitudinally coupled resonant structure. The first reflection grating and the second reflection grating located at both ends of the filter are respectively arranged on the outside of all the interdigital transducers. The main function of the reflection grating is to reflect the surface acoustic waves generated by the interdigital transducers and transmitted along the sound propagation direction, forming a closed resonant cavity and enhancing the resonance effect.

[0027] In traditional longitudinally coupled surface acoustic wave filter structures, the energy distribution and coupling method of the central transducer often limit the performance of the filter's transition band on the low-frequency side (left side), manifesting as an insufficient slope and affecting the filter's rectangularity. This invention introduces an asymmetric suspended transducer region, offset to the right, within the second IDT. This effectively adjusts the acoustic wave coupling path and energy distribution, improving the slope of the low-frequency transition band and achieving a steeper frequency response edge and superior rectangularity.

[0028] The present invention is simulated and verified:

[0029] The comparison of the filter frequency response characteristic curves of the present invention and the traditional filter is shown in the figure Figure 4 As shown in the figure, the black curve represents the filter response of the embodiment of the present invention, and the red curve represents the filter response of the conventional comparative example. As can be seen from the figure, the filter proposed by the present invention adopts a suspended structure design with an asymmetrically offset intermediate transducer. Compared with the comparative filter, the slope of the low-frequency transition band of the filter proposed by the present invention is significantly improved, thereby improving the rectangularity of the signal passband.

[0030] In summary, the highly rectangular longitudinally coupled surface acoustic wave filter designed in the present invention designs an asymmetric suspended transducer region in the second interdigital transducer, thereby achieving a steep transition zone on the left side of the filter, optimizing the rectangularity of the filter, and meeting practical requirements.

[0031] The above embodiments further illustrate the purpose, technical solutions and advantages of the present invention in detail. It should be understood that the above embodiments are only preferred implementation modes of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made to the present invention within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A highly rectangular longitudinally coupled surface acoustic wave filter, characterized in that: include: a piezoelectric substrate and metal interdigital electrodes located on the piezoelectric substrate; The metal interdigital electrode includes a first reflection grid, a first interdigital transducer, a second interdigital transducer, a third interdigital transducer and a second reflection grid arranged in sequence along a first direction; the first reflection grid and the second reflection grid are both short-circuit grid structures; the first interdigital transducer is composed of two first bus bars and a plurality of electrode fingers periodically and alternately connected to the two first bus bars; the third interdigital transducer is composed of two third bus bars and a plurality of electrode fingers periodically and alternately connected to the two third bus bars; the second interdigital transducer includes two second bus bars and a plurality of electrode fingers periodically and alternately connected to the two second bus bars, and an asymmetric suspension transducer area is provided on the right side of the second interdigital transducer, the electrode fingers on the left and right sides of the asymmetric suspension transducer area have opposite polarities, and the asymmetric suspension transducer area is used to disturb the acoustic wave coupling path; the electrode fingers of the asymmetric suspension transducer are not connected to the bus bars of the second interdigital transducer; The asymmetric suspension transducer region is an independent suspension structure, a structure with fully connected busbars on both sides, or a structure with alternately connected busbars on both sides; wherein the independent suspension structure is composed of a plurality of periodically arranged electrode fingers; the structure with fully connected busbars on both sides is composed of busbars on both sides and a plurality of electrode fingers periodically and simultaneously connected to the two busbars; the structure with alternately connected busbars on both sides is composed of busbars on both sides and a plurality of electrode fingers periodically and alternately connected to the two busbars; The period of the electrode fingers in the asymmetric suspension transducer region is the same as the period of the electrode fingers in the rest of the second interdigital transducer.

2. The highly rectangular longitudinally coupled surface acoustic wave filter according to claim 1, characterized in that: The number of electrode fingers in the independent suspension structure, the structure with fully connected bus bars on both sides, or the structure with alternately connected bus bars on both sides is 5 or 7.

3. The highly rectangular longitudinally coupled surface acoustic wave filter according to claim 1, characterized in that: There are at least 7 electrode fingers between the leftmost electrode in the asymmetric suspension transducer area and the middle position of the second interdigital transducer; there are 5 to 7 electrode fingers between the rightmost electrode in the asymmetric suspension transducer area and the rightmost electrode finger of the second interdigital transducer.

4. The highly rectangular longitudinally coupled surface acoustic wave filter according to claim 1, wherein: All the electrode fingers of the IDTs and reflector grids are made of copper and have the same thickness.

5. The highly rectangular longitudinally coupled surface acoustic wave filter according to claim 1, characterized in that: The electrode finger widths of all IDTs and reflector gratings are 0.4 to 0.5 times the operating wavelength.

Images