Coupled-Cavity SAW Filter Structure for Compact Stable Passbands
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Solution Overview
Problem
Surface acoustic wave (SAW) filter devices face challenges with compactness and performance due to sensitivity to temperature and weak electromechanical coupling in traditional piezoelectric substrates, leading to suboptimal pass-band characteristics and large footprints in existing filter structures.
Innovation Solution
A coupled cavity filter structure using a composite substrate with a piezoelectric layer and inter-digitated comb electrodes, employing guided shear or longitudinal waves to achieve higher electromechanical coupling and thermal compensation, along with Bragg mirrors and multiple reflecting structures to optimize filter performance and reduce size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If traditional piezoelectric substrates (monolithic Quartz, LiNbO3, or LiTaO3) are used, then the device can be manufactured with existing processes, but the filter performance deteriorates due to high temperature sensitivity or weak electromechanical coupling
Solution Approach 1:
The patent uses a composite substrate consisting of a piezoelectric layer deposited on a non-piezoelectric substrate. This composite structure combines the advantages of piezoelectric materials (for SAW generation) with the thermal stability of non-piezoelectric substrates, resolving the contradiction between manufacturability and filter performance by achieving both electromechanical coupling and temperature compensation.
2Ease of manufacture
If classical filter structures are used, then the device can be implemented with conventional designs, but the footprint increases and compactness is reduced
Solution Approach 1:
The patent transitions from planar filter structures to three-dimensional cavity structures by utilizing the thickness dimension of the piezoelectric layer. The cavities are formed within the piezoelectric layer itself, allowing the filter to achieve compactness in the vertical dimension while maintaining conventional planar electrode layouts, thus reducing overall footprint without sacrificing design simplicity.
3Reliability
If more poles are placed in the passband to improve filter characteristics, then the filter performance improves, but the device footprint increases
Solution Approach 1:
The patent nests multiple acoustic cavities within the thickness of the piezoelectric layer, allowing multiple poles to be placed in the passband without increasing the planar footprint. The cavities are stacked or arranged in the vertical dimension, enabling complex filter responses with multiple resonant frequencies while maintaining a compact overall structure.
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 solution enables adjustable passband, reduced footprint, and improved filter performance with enhanced electromechanical coupling, leading to lower insertion loss and sharper transition bands, while maintaining thermal stability.
Implementation Method 1
The synthesis of SAW filters requires different types of tools and allows for different types of structures to be implemented... SAW filter devices commonly use wafers made from a monolithic Quartz, LiNbO3 or LiTaO3 crystal as piezoelectric materials
Implementation Method 2
A coupled cavity filter structure using a surface acoustic wave, in particular, a guided surface acoustic wave, comprising an acoustic wave propagating substrate
Implementation Method 3
The use of cavities for generating poles and zeros in filter transfer function is a well-known technique used systematically when developing microwave filters operating at several GHz
Data Source
AI summary
A coupled cavity filter structure that uses a surface acoustic wave, in particular, a guided surface acoustic wave, comprises an acoustic wave propagating substrate, at least one input transducer structure and one output transducer structure, provided over the substrate, each comprising inter-digitated comb electrodes, at least one reflecting structure comprising at least one or more metallic strips positioned at a distance and in between the input and output transducer structures, in the direction of propagation of an acoustic wave. The acoustic wave propagating substrate is a composite substrate comprising a base substrate and a piezoelectric layer. In additional embodiments, a coupled cavity filter structure comprises a groove. In additional embodiments, a SAW ladder filter device comprises at least two coupled cavity filter structures as described herein, wherein the at least two coupled cavity filter structures are positioned on a single line.