Surface Acoustic Wave Filter Cavities for Secondary Lobe Suppression
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Solution Overview
Problem
Existing surface elastic wave filters with resonant cavities suffer from significant secondary lobes in the rejection or transition band, which increase in amplitude as the relative bandwidth is widened.
Innovation Solution
A surface elastic wave filter with resonant cavities is designed using a composite substrate, featuring internal reflective structures with distinct periods and separation distances, and optionally external mirrors, to reduce the amplitude of lobes in the rejection or transition band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the relative bandwidth of the filter is widened, then the filter's bandwidth increases, but the amplitude of secondary lobes in the rejection or transition band increases
Solution Approach 1:
The internal reflective grating is divided into multiple distinct reflective structures (first reflective structure with period P1, second reflective structure with period P2, etc.) with different periods and spacing. This segmentation allows each structure to contribute differently to the overall filter response, enabling bandwidth control while suppressing secondary lobes through constructive and destructive interference patterns.
Solution Approach 2:
Different regions of the filter have different local properties - the first reflective structure has period P1 with spacing d1, the second has period P2 with spacing d2, where P1 ≠ P2 and d1 ≠ d2. This local variation in periodicity and spacing creates position-dependent phase shifts that cancel secondary lobes while maintaining the desired passband characteristics.
2Ease of manufacture
If traditional filter synthesis methods are used, then the filter design is simple, but secondary lobes appear in the rejection or transition band
Solution Approach 1:
The invention modifies the periodicity parameters of the reflective structures - using multiple periods (P1, P2, ...) instead of a single period, and varying the spacing between structures (d1, d2, ...). These parameter changes transform the filter's frequency response by creating multiple interference conditions that suppress secondary lobes while maintaining design feasibility through systematic parameter selection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed filter structure effectively reduces lobe amplitudes by 5-10 dB even for high bandwidths, improving the filter's performance and conforming to predetermined templates.
Implementation Method 1
arranged on a piezoelectric substrate, at least two electroacoustic transducers 2a and 2b, respectively input and output. The two transducers define a path along which elastic surface waves propagate.
Implementation Method 2
The filter 1 may include two external reflective gratings 3a, 3b (and preferably only reflective), typically Bragg mirrors, arranged on either side of the pair of input transducers 2a and output transducers 2b, along the X axis of propagation of elastic waves.
Implementation Method 3
We thus define a plurality of resonant cavities C1, C2 along the wave propagation path, these resonant cavities forming as many poles of filter 1.
Data Source
Figure 1~2a
Figure 2b~3b
Figure 3c~3d
AI summary
The invention relates to a surface elastic wave filter with resonant cavities comprising a composite substrate (S) formed of a base substrate (S3) and a piezoelectric top layer (S1); at least one input electroacoustic transducer (2a) and one output electroacoustic transducer (2b), disposed on the top layer, and at least one internal reflective structure, disposed between the input electroacoustic transducer (2a) and the output electroacoustic transducer (2b). According to the invention, the internal reflective structure comprises a first structure (Pright) comprising at least one reflective grating (R1) having a first period and a second structure (Pleft) comprising at least one reflective grating (R2) having a second period, the first period being longer than the second period.