Surface acoustic wave filter capable of reducing spurious signals

Abstract

The invention discloses a surface acoustic wave filter capable of reducing spurious signals, and the surface acoustic wave filter employs an impedance element structure, and is a single-ended input / output structure. The surface acoustic wave filter comprises a plurality of surface acoustic wave resonators, and the plurality of surface acoustic wave resonators are connected together in series and in parallel at the same time. And the Rayleigh wave resonant frequency of one surface acoustic wave resonator in the two surface acoustic wave resonators at the symmetrical positions is equal to the Rayleigh wave anti-resonant frequency of the other surface acoustic wave resonator. According to the invention, the half wavelengths of the two parallel resonators at the symmetrical positions are finely tuned, so that the frequencies of the two resonators are finely tuned and staggered, the frequency of one resonator just falls on the anti-resonator frequency of the other resonator, and spurious parasitic signals are effectively suppressed through positive and negative superposition. Therefore, the low-end out-of-band rejection of the surface acoustic wave filter is improved.

Description

Technical Field

[0001] The present application relates to the technical field of surface acoustic wave filters, and in particular to a surface acoustic wave filter capable of reducing stray signals. Background Art

[0002] Surface acoustic wave devices are devices that use surface acoustic waves to simulate electrical signals. They mainly include surface acoustic wave resonators and surface acoustic wave filters and other related devices. Among them, the relative bandwidth of the surface acoustic wave filter is limited by the substrate material and design structure. In order to obtain a relatively wide bandwidth, 41° lithium niobate (LN41) is generally used as the substrate material. In addition to being able to excite strong leakage waves, the interdigital transducer with 41° lithium niobate as the substrate material can also excite relatively weak Rayleigh waves. When the surface acoustic wave filter is designed with LN41 and uses a trapezoidal impedance element (IEF) design structure, a spurious signal will be generated at the low end of the frequency response due to Rayleigh waves. Summary of the invention

[0003] To this end, the present application provides a surface acoustic wave filter capable of reducing stray signals, so as to solve the problem of the surface acoustic wave filter existing in the prior art having stray signals at the low end of the frequency response.

[0004] In order to achieve the above objectives, this application provides the following technical solutions:

[0005] A surface acoustic wave filter capable of reducing stray signals, the surface acoustic wave filter adopts an impedance element structure and is a single-ended input and output structure;

[0006] The surface acoustic wave filter comprises a plurality of surface acoustic wave resonators, wherein the plurality of surface acoustic wave resonators are connected together in series and in parallel at the same time, and in the plurality of surface acoustic wave resonators connected in parallel, the Rayleigh wave resonance frequency of one surface acoustic wave resonator in two surface acoustic wave resonators located at symmetrical positions is equal to the Rayleigh wave anti-resonance frequency of the other surface acoustic wave resonator;

[0007] The surface acoustic wave resonator includes an interdigital transducer, a first reflection grating and a second reflection grating, wherein the interdigital transducer, the first reflection grating and the second reflection grating are all deposited on the surface of a piezoelectric substrate, wherein the first reflection grating is located on the left side of the interdigital transducer, and the second reflection grating is located on the right side of the interdigital transducer.

[0008] Preferably, the interdigital transducer includes a first metal interdigital bar, a second metal interdigital bar, a first bus bar and a second bus bar, the first bus bar and the second bus bar are deposited parallel to each other on the surface of the piezoelectric substrate, the first metal interdigital bar and the second metal interdigital bar are cross-deposited on the surface of the piezoelectric substrate, and are located between the first bus bar and the second bus bar.

[0009] Preferably, the surface acoustic wave filter comprises a first surface acoustic wave resonator, a second surface acoustic wave resonator, a third surface acoustic wave resonator, a fourth surface acoustic wave resonator, a fifth surface acoustic wave resonator and a sixth surface acoustic wave resonator, wherein the first surface acoustic wave resonator, the second surface acoustic wave resonator, the third surface acoustic wave resonator and the fourth surface acoustic wave resonator are sequentially connected in series between an input end and an output end, one end of the fifth surface acoustic wave resonator is connected in parallel between the first surface acoustic wave resonator and the second surface acoustic wave resonator, and the other end is grounded, one end of the sixth surface acoustic wave resonator is connected in parallel between the third surface acoustic wave resonator and the fourth surface acoustic wave resonator, and the other end is grounded, wherein the first surface acoustic wave resonator and the fourth surface acoustic wave resonator have the same structure, the second surface acoustic wave resonator and the third surface acoustic wave resonator have the same structure, and the fifth surface acoustic wave resonator and the sixth surface acoustic wave resonator have the same structure;

[0010] The Rayleigh wave resonance frequency of the fifth surface acoustic wave resonator is the Rayleigh wave anti-resonance frequency of the sixth surface acoustic wave resonator or the Rayleigh wave anti-resonance frequency of the fifth surface acoustic wave resonator is the Rayleigh wave resonance frequency of the sixth surface acoustic wave resonator.

[0011] Preferably, the half wavelength of the fifth surface acoustic wave resonator and the half wavelength of the sixth surface acoustic wave resonator deviate by 0.13%, so that the Rayleigh wave resonance frequency of the fifth surface acoustic wave resonator is the Rayleigh wave anti-resonance frequency of the sixth surface acoustic wave resonator.

[0012] Preferably, the IDT is made of one or more of aluminum, aluminum-copper alloy, copper or gold.

[0013] Preferably, the interdigital transducer is deposited on the surface of the piezoelectric substrate by coating and photolithography methods.

[0014] Preferably, the IDT is a single-layer structure or a multi-layer structure.

[0015] Preferably, the material of the piezoelectric substrate is lithium niobate, lithium tantalate, quartz, depyroelectric lithium niobate, lithium tantalate wafer or composite sheet.

[0016] Compared with the prior art, this application has at least the following beneficial effects:

[0017] The present application provides a surface acoustic wave filter capable of reducing stray signals, the surface acoustic wave filter adopts an impedance element structure and is a single-ended input and output structure; the surface acoustic wave filter includes a plurality of surface acoustic wave resonators, the plurality of surface acoustic wave resonators are simultaneously connected together in series and in parallel, and in the plurality of parallel surface acoustic wave resonators, the Rayleigh wave resonance frequency of one surface acoustic wave resonator in two surface acoustic wave resonators located at symmetrical positions is equal to the Rayleigh wave anti-resonance frequency of the other surface acoustic wave resonator. In order to suppress stray signals caused by Rayleigh waves excited by parallel resonators, the present application fine-tunes the half wavelength of the two parallel resonators at symmetrical positions and then fine-tunes the frequencies of the two to stagger, so that the frequency of one of the resonators falls exactly on the anti-resonance frequency of the other resonator, and the positive and negative superposition is used to reduce stray signals, effectively suppressing stray parasitic signals, thereby improving the low-end out-of-band suppression of the surface acoustic wave filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more intuitively illustrate the prior art and the present application, exemplary drawings are given below. It should be understood that the specific shapes and structures shown in the drawings should not generally be regarded as limiting conditions for implementing the present application; for example, those skilled in the art are capable of easily making conventional adjustments or further optimizations to the addition / reduction / attribution division, specific shapes, positional relationships, connection methods, dimensional ratios, etc. of certain units (components) based on the technical concepts and exemplary drawings disclosed in the present application.

[0019] Figure 1 A schematic diagram of the structure of the surface acoustic wave resonator provided in this application;

[0020] Figure 2 A schematic diagram of the structure of the simplest impedance element filter provided in this application;

[0021] Figure 3 A schematic diagram of the structure of a surface acoustic wave filter capable of reducing stray signals provided in an embodiment of the present application;

[0022] Figure 4 Schematic diagram of frequency response and spurious signals provided for this application;

[0023] Figure 5 The resonant frequency of the fifth surface acoustic wave resonator E and the anti-resonator frequency of the sixth surface acoustic wave resonator F provided in the present application;

[0024] Figure 6 Schematic diagram of optimized spurious signals provided for this application.

[0025] Description of reference numerals:

[0026] 1. First reflection grating; 2. IDT; 201. First bus bar; 202. Second bus bar; 203. First metal interdigital strip; 204. Second metal interdigital strip; 3. Second reflection grating. DETAILED DESCRIPTION

[0027] The present application is further described below in detail through specific embodiments in conjunction with the accompanying drawings.

[0028] In the description of this application: unless otherwise specified, the meaning of "plurality" is two or more. The terms "first", "second", "third", etc. in this application are intended to distinguish the objects referred to, and do not have special meanings in terms of technical connotations (for example, they should not be understood as emphasizing the importance or order, etc.). Expressions such as "including", "comprising", "having", etc. also mean "not limited to" (certain units, components, materials, steps, etc.).

[0029] The terms such as "upper", "lower", "left", "right", "middle", etc. cited in this application are usually used to indicate the general relative position relationship for the purpose of intuitive understanding by referring to the drawings, and are not absolute limitations on the position relationship in the actual product.

[0030] After analysis, it is found that for the surface acoustic wave filter designed with LN41 and a trapezoidal impedance element (IEF) design structure, the surface wave excited by the interdigital transducer with LN41 as the substrate material is a leakage wave, but the tangent cannot completely suppress the Rayleigh wave, so the stray parasitic signal is generated.

[0031] See also Figure 1 The impedance element (IE) of the trapezoidal impedance element filter (IEF) is a surface acoustic wave resonator, which contains an interdigital transducer (IDT) and two reflection gratings deposited on the surface of the piezoelectric substrate on both sides. The IDT includes two groups of metal interdigital strips, which are deposited on the surface of the piezoelectric substrate by coating and photolithography. The electrode fingers in each group are connected together by a bus bar. If voltage is applied to the bus bar, due to the piezoelectric effect, this structure will excite surface acoustic waves in two directions. The reflection grating is parallel to the interdigital transducer electrode. The excited surface acoustic wave forms a standing wave under the action of the reflection grating, thereby realizing the basic function of the resonator.

[0032] See also Figure 2 , Figure 2It is the simplest impedance element filter, which contains two surface acoustic wave resonators, which are electrically connected in a trapezoidal arrangement in the Gamma part. The series X and parallel Y impedance elements are given different resonant and anti-resonant frequencies. In the required frequency band at the minimum insertion loss point, the series resonator has a low impedance Z and the parallel resonator Y has a high impedance. In this case, the input signal is transmitted between the input (INPUT) and the output (OUTPUT) with low insertion loss. In order to improve the out-of-band suppression of the filter, multiple Gamma structures are generally cascaded. At the same time, since weak Rayleigh waves are also excited when the leakage wave is excited, a spurious signal appears outside the low-end band of the filter.

[0033] Based on the above analysis, the present application provides a surface acoustic wave filter capable of reducing stray signals. The surface acoustic wave filter adopts an impedance element structure and is a single-ended input and output structure.

[0034] The surface acoustic wave filter includes a plurality of surface acoustic wave resonators, which are connected together in series and in parallel at the same time. Among the plurality of parallel surface acoustic wave resonators, the Rayleigh wave resonance frequency of one surface acoustic wave resonator in two surface acoustic wave resonators located at symmetrical positions is equal to the Rayleigh wave anti-resonance frequency of the other surface acoustic wave resonator. In order to suppress the stray signals caused by the Rayleigh waves excited by the parallel resonators, the half wavelength of the two parallel resonators at symmetrical positions is fine-tuned to stagger the frequencies of the two, so that the frequency of one of the resonators falls exactly on the anti-resonance frequency of the other resonator, and the positive and negative superposition is used to reduce the stray signals.

[0035] For details, please continue to refer to Figure 1 The surface acoustic wave resonator includes an interdigital transducer 2, a first reflection grating 1 and a second reflection grating 3. The interdigital transducer 2, the first reflection grating 1 and the second reflection grating 3 are all deposited on the surface of the piezoelectric substrate, wherein the first reflection grating 1 is located on the left side of the interdigital transducer 2, and the second reflection grating is located on the right side of the interdigital transducer 2.

[0036] The interdigital transducer 2 includes a first metal interdigital bar 203, a second metal interdigital bar 204, a first bus bar 201 and a second bus bar 202. The first bus bar 201 and the second bus bar 202 are deposited on the surface of the piezoelectric substrate in parallel with each other. The first metal interdigital bar 203 and the second metal interdigital bar 204 are cross-deposited on the surface of the piezoelectric substrate and are located between the first bus bar 201 and the second bus bar 202.

[0037] The interdigital transducer 2 is made of one or more of aluminum, aluminum-copper alloy, copper or gold; the interdigital transducer 2 is deposited on the surface of the piezoelectric substrate by coating and photolithography, that is, the interdigital transducer 2 is made of a metal film deposited on the piezoelectric substrate material by photolithography patterning; the interdigital transducer 2 is a single-layer structure or a multi-layer structure stacked up and down.

[0038] Specifically, the material of the piezoelectric substrate is lithium niobate, lithium tantalate, quartz, depyroelectric lithium niobate, lithium tantalate wafer or composite wafer.

[0039] In order to more clearly illustrate a surface acoustic wave filter capable of reducing stray signals provided by the present application, a commonly used design structure (2T design structure) of four resonators in series and two resonators in parallel is taken as an example:

[0040] See also Figure 3 The surface acoustic wave filter comprises a first surface acoustic wave resonator A, a second surface acoustic wave resonator B, a third surface acoustic wave resonator C, a fourth surface acoustic wave resonator D, a fifth surface acoustic wave resonator E and a sixth surface acoustic wave resonator F. The first surface acoustic wave resonator A, the second surface acoustic wave resonator B, the third surface acoustic wave resonator C and the fourth surface acoustic wave resonator D are sequentially connected in series between an input end and an output end. One end of the fifth surface acoustic wave resonator E is connected in parallel between the first surface acoustic wave resonator A and the second surface acoustic wave resonator B, and the other end is grounded. One end of the sixth surface acoustic wave resonator F is connected in parallel between the third surface acoustic wave resonator C and the fourth surface acoustic wave resonator D, and the other end is grounded. The first surface acoustic wave resonator A and the fourth surface acoustic wave resonator D have the same structure, the second surface acoustic wave resonator B and the third surface acoustic wave resonator C have the same structure, and the fifth surface acoustic wave resonator E and the sixth surface acoustic wave resonator F have the same structure.

[0041] Specifically, in the actual production process of the surface acoustic wave filter, it was found that there was a spurious signal at the low end, such as Figure 4 As shown, analysis shows that the stray signal is generated by the fifth surface acoustic wave resonator E and the sixth surface acoustic wave resonator F working together to excite the Rayleigh wave, and it cannot be eliminated.

[0042] Taking the 1650MHz product as an example, the center frequency is 1650MHz, and some structural parameters are shown in Table 1:

[0043] Serial number Half wavelength (um) Number of fingers 1 1.355 232 2 1.471 380 3 1.343 179 4 1.471 380 5 1.355 232

[0044] The first surface acoustic wave resonator A and the fourth surface acoustic wave resonator D have the same structure, and the fifth surface acoustic wave resonator E and the sixth surface acoustic wave resonator F have the same structure, so the corresponding resonant frequencies are also the same. The spurious signal is mainly generated by the superposition of the Rayleigh waves of the parallel surface acoustic wave resonator E and the sixth surface acoustic wave resonator F. In this embodiment, the Rayleigh wave resonant frequency of the fifth surface acoustic wave resonator E is exactly the Rayleigh wave anti-resonant frequency of the sixth surface acoustic wave resonator F by changing the half wavelength of the fifth surface acoustic wave resonator E and the sixth surface acoustic wave resonator F, such as Figure 5 shown.

[0045] After calculation, the half wavelength of the fifth surface acoustic wave resonator E is designed to be 1.470um, and the half wavelength of the sixth surface acoustic wave resonator F is designed to be 1.472um. In this way, the resonant frequency of the Rayleigh wave excited by the fifth surface acoustic wave resonator E is consistent with the anti-resonant frequency of the Rayleigh wave excited by the sixth surface acoustic wave resonator F. At the same time, since the electromechanical coupling coefficient of the leakage wave of LN41 is more than 100 times that of the Rayleigh wave, the fine-tuning of the half wavelength of the fifth surface acoustic wave resonator E and the half wavelength of the sixth surface acoustic wave resonator F has basically no effect on the entire passband. Figure 6 shown.

[0046] In order to suppress the stray signals caused by the Rayleigh waves excited by the parallel resonators, the present application fine-tunes the two parallel resonators by half a wavelength to stagger the frequencies of the two, so that the frequency of one resonator falls exactly on the anti-resonator frequency of the other resonator, and the positive and negative superposition is used to reduce the stray signals, effectively suppressing the stray parasitic signals, thereby improving the low-end out-of-band suppression of the surface acoustic wave filter and better adapting to the needs of the market.

[0047] It should be noted that in order to ensure the suppression of input and output impedances, the structure of a surface acoustic wave filter capable of reducing stray signals provided by the present application is a symmetrical structure. For example, when the number of parallel surface acoustic wave resonators is an odd number (for example, three), the middle resonator can be replaced by two parallel resonators through transformation, and then the frequencies of the resonators on both sides can be adjusted; when the number of parallel surface acoustic wave resonators is an even number (for example, four), the frequencies of the two symmetrical resonators can be adjusted.

[0048] The technical features of the above embodiments may be arbitrarily combined (as long as there is no contradiction in the combination of these technical features). To make the description concise, not all possible combinations of the technical features in the above embodiments are described; these embodiments that are not explicitly written should also be considered to be within the scope of this specification.

Claims

1. A surface acoustic wave filter capable of reducing stray signals, characterized in that: The surface acoustic wave filter adopts an impedance element structure and is a single-ended input and output structure; The surface acoustic wave filter comprises a plurality of surface acoustic wave resonators, wherein the plurality of surface acoustic wave resonators are connected together in series and in parallel at the same time, and in the plurality of surface acoustic wave resonators connected in parallel, the Rayleigh wave resonance frequency of one surface acoustic wave resonator in two surface acoustic wave resonators located at symmetrical positions is equal to the Rayleigh wave anti-resonance frequency of the other surface acoustic wave resonator; The surface acoustic wave resonator includes an interdigital transducer, a first reflection grating and a second reflection grating, wherein the interdigital transducer, the first reflection grating and the second reflection grating are all deposited on the surface of a piezoelectric substrate, wherein the first reflection grating is located on the left side of the interdigital transducer, and the second reflection grating is located on the right side of the interdigital transducer.

2. The surface acoustic wave filter capable of reducing stray signals according to claim 1, characterized in that: The interdigital transducer includes a first metal interdigital bar, a second metal interdigital bar, a first bus bar and a second bus bar, the first bus bar and the second bus bar are deposited parallel to each other on the surface of the piezoelectric substrate, the first metal interdigital bar and the second metal interdigital bar are cross-deposited on the surface of the piezoelectric substrate, and are located between the first bus bar and the second bus bar.

3. The surface acoustic wave filter capable of reducing stray signals according to claim 1, characterized in that: The surface acoustic wave filter comprises a first surface acoustic wave resonator, a second surface acoustic wave resonator, a third surface acoustic wave resonator, a fourth surface acoustic wave resonator, a fifth surface acoustic wave resonator and a sixth surface acoustic wave resonator, wherein the first surface acoustic wave resonator, the second surface acoustic wave resonator, the third surface acoustic wave resonator and the fourth surface acoustic wave resonator are sequentially connected in series between an input end and an output end, one end of the fifth surface acoustic wave resonator is connected in parallel between the first surface acoustic wave resonator and the second surface acoustic wave resonator, and the other end is grounded, one end of the sixth surface acoustic wave resonator is connected in parallel between the third surface acoustic wave resonator and the fourth surface acoustic wave resonator, and the other end is grounded, wherein the first surface acoustic wave resonator and the fourth surface acoustic wave resonator have the same structure, the second surface acoustic wave resonator and the third surface acoustic wave resonator have the same structure, and the fifth surface acoustic wave resonator and the sixth surface acoustic wave resonator have the same structure; The Rayleigh wave resonance frequency of the fifth surface acoustic wave resonator is the Rayleigh wave anti-resonance frequency of the sixth surface acoustic wave resonator or the Rayleigh wave anti-resonance frequency of the fifth surface acoustic wave resonator is the Rayleigh wave resonance frequency of the sixth surface acoustic wave resonator.

4. The surface acoustic wave filter capable of reducing stray signals according to claim 3, characterized in that: The half wavelength of the fifth surface acoustic wave resonator and the half wavelength of the sixth surface acoustic wave resonator deviate by 0.13%, so that the Rayleigh wave resonance frequency of the fifth surface acoustic wave resonator is the Rayleigh wave anti-resonance frequency of the sixth surface acoustic wave resonator.

5. The surface acoustic wave filter capable of reducing stray signals according to claim 1, characterized in that: The interdigital transducer is made of one or more of aluminum, aluminum-copper alloy, copper or gold.

6. The surface acoustic wave filter capable of reducing stray signals according to claim 1, characterized in that: The interdigital transducer is deposited on the surface of the piezoelectric substrate by means of coating and photolithography.

7. The surface acoustic wave filter capable of reducing stray signals according to claim 1, characterized in that: The interdigital transducer is a single-layer structure or a multi-layer structure.

8. The surface acoustic wave filter capable of reducing stray signals according to claim 1, characterized in that: The material of the piezoelectric substrate is lithium niobate, lithium tantalate, quartz, depyroelectric lithium niobate, lithium tantalate wafer or composite sheet.

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