SAW Filter Piston Mode Edge Region Design
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Solution Overview
Problem
Narrow band SAW filters face challenges with transverse modes that introduce undesirable responses and spurious rejection due to multiple guided modes, which existing solutions like reducing aperture or apodization only partially address, leaving parasitics and limited improvements.
Innovation Solution
The design incorporates a SAW transducer with a pair of opposing bus bars and interdigital electrodes, where the edge regions have lower wave velocity than the center, achieved through varying electrode widths, metal thickness, and dielectric layers, to excite a piston mode with a flat amplitude distribution, enhancing directionality and reducing spurious modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the transducer aperture is reduced to reduce the number of guided modes, then the number of transverse modes is reduced, but the impedance increases
Solution Approach 1:
The patent applies apodization to create a non-uniform amplitude distribution across the transducer aperture, with the center region having higher amplitude and edge regions having lower amplitude. This local variation in excitation strength suppresses transverse modes while maintaining a larger overall aperture, thereby avoiding the high impedance problem associated with uniformly small apertures.
2Object-generated harmful factors
If apodization is applied to reduce coupling to other modes, then the coupling to transverse modes is reduced, but the improvement is limited by the main transverse mode extending outside the acoustic aperture
Solution Approach 1:
The patent divides the transducer aperture into distinct regions: a center region with higher amplitude and edge regions with lower amplitude. This segmentation allows the apodization function to be implemented in a controlled manner, effectively suppressing transverse modes that extend beyond the main aperture while maintaining manufacturing feasibility.
3Shape
If a lower wave velocity region is added to the edge of the transducer to produce a mode constant in the aperture, then the mode shape is improved, but parasitics remain
Solution Approach 1:
The patent modifies the wave velocity parameter in the edge regions by varying electrode widths, metal thickness, and dielectric layer properties. This parameter change creates a gradual transition that maintains the desired mode shape while minimizing parasitic effects through optimized geometric and material parameters.
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
This approach significantly reduces spurious mode amplitudes and improves filter response by directing acoustic energy and using acoustic reflectors in the edge regions, resulting in a more efficient and effective SAW filter performance.
Implementation Method 1
a piezoelectrically excited strongest mode is a piston mode
Data Source
AI summary
A SAW transducer includes a pair of opposing bus bars formed on a surface of a piezoelectric substrate with a plurality of interdigital electrodes extending from each of the opposing bus bars. A center region and opposing edge regions are formed, wherein each edge region is proximate each bus bar and includes free end portions of the electrodes such that a wave velocity in each edge region is substantially lower than a wave velocity in the bus bars and in the center region, and wherein electro-acoustic sources extend into the edge regions for providing a SAW transducer having the piezoelectrically excited strongest mode is a piston mode with an amplitude distribution generally flat across most of the resonator area.