A low insertion loss high frequency surface acoustic wave filter

Abstract

The present invention claims protection for a high-frequency low-insertion-loss surface acoustic wave filter, belonging to the technical field of surface acoustic wave filters. It includes the structural design of the filter and an optimization method for reducing in-band ripple, reducing insertion loss, and improving out-of-band rejection performance. The structural design of the filter includes four parallel resonators P1 - P4 with the same structural parameters and four series resonators S1 - S4 with different structural parameters, forming a "T + π" structure to improve out-of-band rejection and optimize the rectangularity coefficient. The optimization method for improving performance includes using a thick metal film interdigital electrode structure for each resonator unit to suppress the Love wave velocity and increase the Rayleigh wave intensity; increasing the duty cycle of all series resonators and reducing the reflection grating period to reduce transverse mode interference; by adjusting the number of interdigital fingers of the series resonators, changing the position of the transverse mode frequency point of the resonator, so that when cascading different resonators, the loss misalignment introduced by the transverse mode is superposed, thereby reducing in-band ripple and insertion loss while improving out-of-band rejection.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface acoustic wave filters, and particularly relates to the design of high-frequency low-loss filters above GHz. Background Art

[0002] At present, surface acoustic wave (SAW) filters have been increasingly widely used in line with the development of mobile communication technology due to their characteristics such as small volume, high frequency and low insertion loss. Usually, piezoelectric materials with high sound velocity and high electromechanical coupling coefficient are used to increase the center frequency of the filter and reduce the insertion loss, or a multi-layer film structure containing diamond is adopted, and its purpose is also to increase the wave velocity of the surface acoustic wave. However, the manufacturing process of the multi-layer film structure is relatively complex, and a high electromechanical coupling coefficient often enhances the intensity of other spurious mode while enhancing the intensity of the required surface acoustic wave mode, which will undoubtedly increase the insertion loss of the device.

[0003] The SAW filter with a ladder structure has a more flexible bandwidth design compared to the longitudinal coupling resonator filter (LCRF), and has lower insertion loss than the interdigital dual-track filter (IIDT). Moreover, the out-of-band rejection can be improved by adjusting the cascading method. However, the cascading of the same resonators will cause the superposition of the spurious mode intensity in the passband and a large rectangularity. Therefore, in this paper, a single-layer film and a ladder structure are cascaded in a "T + π" type and different series resonators are cascaded. The cascading of different series resonators suppresses the spurious mode, mainly referring to the transverse mode. The number of finger pairs is adjusted to change the superposition of the spurious mode at the same frequency point. For the structure in this paper, when its structure parameters are determined, the wave velocity of the corresponding transverse mode can also be determined. Then, by changing the structure, such as different numbers of finger pairs, the transverse mode will also change. It is known from the simulation results that changing the number of finger pairs will change the resonance frequency point of the transverse mode and suppress the superposition of the spurious mode intensity at the same frequency point in the passband. Summary of the Invention

[0004] The present invention aims to solve the above problems in the prior art. A high-frequency surface acoustic wave filter with low insertion loss is proposed. The technical solution of the present invention is as follows:

[0005] A low insertion loss high-frequency surface acoustic wave filter, comprising: four resonators S1, S2, S3, and S4 connected in series in sequence, and four parallel resonators P1-P4 with the same structural parameters. The grounding ports of the parallel resonators P1-P4 are connected to each other and then grounded. One port of P1 is connected to port 1 of the series resonator S1, serving as port 1 of the surface acoustic wave filter. The other port of P4 is connected to port 2 of the series resonator S4, serving as port 2 of the surface acoustic wave filter; the other ports of P2 and P3 are both arranged between the series-connected S2 and S3. Port 2 of S1 is connected to port 1 of S2, and port 2 of S3 is connected to port 1 of S4. The series resonators S1, S2 and the parallel resonators P1, P2 form a "π" structure. The series resonators S3, S4 and the parallel resonators P3, P4 form a "π" structure. The parallel resonators P2, P3 and the series resonators S1, S2, S3, S4 form a "T" structure, and the overall is cascaded and combined into a "T+π" structure.

[0006] Furthermore, the resonators S1, S2, S3, S4, and the four parallel resonators all use a thick metal film interdigital electrode structure.

[0007] Furthermore, the electrode material of the thick metal film interdigital electrode structure is aluminum, and the relative electrode thickness is set between 33% and 36% to improve the electromechanical coupling coefficient and Rayleigh wave intensity and weaken the Love wave velocity.

[0008] Furthermore, by increasing the duty cycle of the resonators S1, S2, S3, S4 and reducing the reflection grating period; adjusting the number of interdigital fingers of the series resonator to change the position of the resonator transverse mode frequency point.

[0009] Furthermore, increase the duty cycle of the series resonators S1-S4 (Equation 1, a is the interdigital finger width, b is the interdigital finger gap) to 5:1, and their reflection grating periods are all less than 0.5 times the wavelength corresponding to the center frequency.

[0010]

[0011] Furthermore, the piezoelectric material of the thick metal film interdigital electrode structure is 128°Y-X LiNbO3. The wavelength λ of the series resonator is 1.1 μm, the wavelength of the parallel resonator is 1.2 μm, the interdigital finger thicknesses of the series and parallel resonators are both 0.4 μm, and the relative wavelength thicknesses are 36.4% and 33.3% respectively.

[0012] Furthermore, by adjusting the number of interdigital fingers of the series resonators S1-S4, change the resonance points of the transverse mode in the passband, so that when the resonators are cascaded, the transverse modes can be misaligned and superimposed to reduce the in-band ripple and insertion loss. The parallel resonators all adopt a uniform finger structure.

[0013] The advantages and beneficial effects of the present invention are as follows:

[0014] The present invention adopts a single-layer film structure, and the process is simpler. The trapezoidal structure is cascaded in a "T + π" type, which improves the out-of-band suppression. At the same time, the rectangularity of the filter is optimized to be closer to 1. The "T + π" type structure makes its rectangularity closer to 1. Breaking the conventional thin electrode thinking that the electrode thickness only accounts for 3-10% of the wavelength, a thick metal film electrode structure is adopted to increase the electromechanical coupling coefficient. By increasing the duty cycle of the series resonator and reducing its reflection grating period, the interference of the transverse mode is reduced. By changing the number of finger pairs of the series resonator, the superposition of the losses introduced by the transverse mode during the cascaded resonators at the same frequency point is suppressed, thereby reducing the in-band ripple and insertion loss while improving the out-of-band suppression. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the overall (T + π) structure of the filter provided by the preferred embodiment of the present invention.

[0016] Figure 2 It is a two-dimensional schematic diagram of a series resonator with a high duty cycle and a thick film finger structure.

[0017] Figure 3 It is a frequency response simulation diagram of the designed high-frequency low-insertion-loss surface acoustic wave filter.

[0018] Figure 4 is Figure 3 A partial enlarged view at the center frequency. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Next, the technical solutions in the embodiments of the present invention will be clearly and detailedly described in conjunction with the accompanying drawings in the embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention.

[0020] The technical solution for the present invention to solve the above technical problems is:

[0021] The design of a high-frequency low-insertion-loss surface acoustic wave filter includes the structural design of the filter and an optimization method for reducing in-band ripple, reducing insertion loss, and improving out-of-band suppression performance. Among them, the structural design of the filter consists of 4 parallel resonators with the same structural parameters and 4 series resonators with different structural parameters to form a "T + π" type structure. The optimization method for improving performance includes using a thick metal film finger electrode structure for each resonator unit; increasing the duty cycle of all series resonators and reducing the reflection grating period; adjusting the number of finger pairs of the series resonators to change the position of the transverse mode frequency point of the resonator.

[0022] Such as Figure 1As shown, the two resonators S1 and S2 connected in series on the left and the two resonators S3 and S4 connected in series on the right are respectively connected in parallel with the two parallel resonators P2 and P3 on the left and right sides of their series combination structures to form two "π" - shaped structures. The two parallel resonators P2 and P3 in the middle and the four series resonators S1 - S4 form a "T" - shaped structure, and the overall is cascaded to form a "T + π" - shaped structure.

[0023] As Figure 2 shown, the electrode material is metallic aluminum, the piezoelectric material is 128°Y - X LiNbO3, the wavelength (λ) of the series resonator is 1.1 μm, and the wavelength of the parallel resonator is 1.2 μm. In order to improve the electromechanical coupling coefficient and Rayleigh wave intensity and weaken the Love wave velocity, a thick metal film interdigital structure is adopted: the interdigital thickness of both the series and parallel resonators is 0.4 μm, and the relative wavelength thicknesses are 36.4% and 33.3% respectively. As Figure 2 shown, the duty cycle of all series resonators is 5:1

[0024]

[0025] That is, the ratio of the interdigital width (a) to the interdigital gap (b) is 5:1, and the reflection grating period (λ g ) is 0.6 μm. A high duty cycle of 5:1 is also adopted to reduce the influence of the transverse mode on the in - band loss.

[0026] By adjusting the number of interdigital pairs of the series resonators S1 - S4 to be 61.5 pairs, 59.5 pairs, 63.5 pairs, and 57.5 pairs in sequence, the resonance points of the transverse mode in the passband are changed, so that when the resonators are cascaded, the transverse modes can be misaligned and superimposed, thereby reducing the in - band fluctuation and insertion loss.

[0027] Figure 3 This is the simulated frequency response result of a surface acoustic wave filter with a "T + π" structure and a working frequency of 2.517 GHz. The simulation results show that after adopting this structure and performance optimization method, the - 3dB bandwidth of the filter is 99 MHz, reaching 3.9%, the center insertion loss and in - band fluctuation are less than 1 dB, the rectangularity is 1.3, and the out - of - band rejection reaches - 32 dB, meeting the design requirements of high - frequency and low - insertion loss.

[0028] It should also be noted that the term "comprising", "including" or any other variant thereof is intended to cover non - exclusive inclusion, so that a process, method, commodity or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, commodity or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, method, commodity or device including the said element.

[0029] The above embodiments should be understood as only for illustrating the present invention and not for limiting the protection scope of the present invention. After reading the content described in the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A low insertion loss high-frequency surface acoustic wave filter, characterized in that, Comprising: Four resonators S1, S2, S3, and S4 connected in series in sequence, and four parallel resonators P1 - P4 with the same structural parameters. The grounded ports of the parallel resonators P1 - P4 are connected to each other and then grounded. One port of P1 is connected to port 1 of the series resonator S1, serving as port 1 of the surface acoustic wave filter. One port of P4 is connected to port 2 of the series resonator S4, serving as port 2 of the surface acoustic wave filter; the other ports of P2 and P3 are both set between the series S2 and S3. Port 2 of S1 is connected to port 1 of S2, and port 2 of S3 is connected to port 1 of S4; the series resonators S1, S2 and the parallel resonators P1, P2 form a "π" structure, the series resonators S3, S4 and the parallel resonators P3, P4 form a "π" structure, and the parallel resonators P2, P3 and the series resonators S1, S2, S3, S4 form a "T" structure, and are integrally cascaded to form a "T + π" structure; By increasing the duty cycle of the resonators S1, S2, S3, S4, η = a / b, where a is the finger width and b is the finger gap, and reducing the reflection grating period; the duty cycle is the finger width divided by the finger gap. For uniform fingers, the duty cycle is 1, and the reflection grating period is half of the finger period. Here, it is also adjusted downward according to 0.5 times of the finger period λ. It can be seen from the simulation results that as the reflection grating period becomes smaller, the interference becomes smaller. Adjust the number of finger pairs of the series resonator to change the position of the resonance frequency point of the transverse mode; the transverse mode corresponds to different mode SAW modes and has different wave velocities. The transverse mode is just some interference modes and is a spurious mode signal. Changing the number of finger pairs will change the resonance frequency point of the transverse mode; Increase the duty cycle of the series resonators S1 - S4 to 5:1, and their reflection grating periods are all less than 0.5 times of the wavelength corresponding to the center frequency.

2. The low insertion loss high frequency surface acoustic wave filter according to claim 1, characterized in that All resonators use a thick metal film finger electrode structure.

3. The low insertion loss high frequency surface acoustic wave filter according to claim 2, wherein The electrode material of the thick metal film finger electrode structure is aluminum, and the relative electrode thickness is set between 33% and 36% to increase the electromechanical coupling coefficient and the Rayleigh wave intensity and weaken the Love wave velocity.

4. A low insertion loss high frequency surface acoustic wave filter according to claim 3, characterized in that, The piezoelectric material of the thick metal film finger electrode structure is 128°Y - X LiNbO3. The wavelength λ of the series resonator is 1.1 μm, the wavelength of the parallel resonator is 1.2 μm, and the finger thicknesses of the series and parallel resonators are both 0.4 μm, and the relative wavelength thicknesses are 36.4% and 33.3% respectively.

5. The low insertion loss high frequency surface acoustic wave filter according to claim 4, characterized in that, By adjusting the number of finger pairs of the series resonators S1 - S4 to change the resonance points of the transverse mode in the passband, so that when cascading the resonators, the transverse modes can be misaligned and superimposed to reduce the in - band ripple and insertion loss. The parallel resonators all adopt a uniform finger structure.

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