Surface Acoustic Wave Filter Spacing for Lower Spurious Response
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Solution Overview
Problem
Longitudinally coupled resonator type surface acoustic wave filters often experience spurious responses in the frequency region at approximately 1.1 times the high-frequency end of the pass band due to narrow-pitch portions, which can lead to energy leakage and increased insertion loss.
Innovation Solution
A surface acoustic wave filter with a multilayer body comprising a high-acoustic-velocity member, a low-acoustic-velocity film, and a piezoelectric film, where interdigital transducers have uniform electrode finger pitches and an inter-electrode finger center distance between 0.25λ and 0.37λ, preventing spurious responses and reducing energy leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a narrow-pitch portion having a small electrode finger pitch is provided at a portion where interdigital transducers are adjacent to each other, then insertion loss is reduced, but a spurious response occurs in a frequency region at approximately 1.1 times of the high-frequency-side end portion of the pass band
Solution Approach 1:
The patent applies local quality by providing a narrow-pitch portion only at specific locations where interdigital transducers are adjacent to each other, while maintaining uniform pitch in other regions. This localized modification reduces insertion loss at critical interfaces without causing spurious responses across the entire structure, as the narrow pitch is confined to areas where it is most beneficial for energy coupling.
Solution Approach 2:
The patent changes the electrode finger pitch parameter locally by setting it to a smaller value (narrow pitch) at specific portions where interdigital transducers are adjacent, while maintaining a standard pitch elsewhere. This parameter modification optimizes energy transmission at interfaces without generating harmful spurious responses in the frequency region.
2Loss of energy
If a multilayer body with high-acoustic-velocity film and low-acoustic-velocity film is used, then leakage of energy of surface acoustic wave is reduced, but spurious response occurs when narrow-pitch portion is provided
Solution Approach 1:
The patent combines the multilayer structure with localized narrow-pitch portions, applying the quality modification only where interdigital transducers are adjacent. This approach allows the multilayer body to prevent energy leakage throughout the structure while the localized narrow pitch reduces insertion loss without causing spurious responses.
Solution Approach 2:
The patent uses a composite multilayer structure combining high-acoustic-velocity film and low-acoustic-velocity film with piezoelectric film. This composite structure provides superior energy confinement and reduces both energy leakage and spurious responses when combined with the uniform or substantially uniform electrode finger pitch design.
3Object-generated harmful factors
If electrode finger pitch is uniform or substantially uniform in each interdigital transducer, then spurious response is prevented, but insertion loss may increase
Solution Approach 1:
The patent resolves this contradiction by maintaining uniform electrode finger pitch as the general rule while introducing narrow-pitch portions only at specific locations where interdigital transducers are adjacent. This localized exception allows the structure to prevent spurious responses overall while reducing insertion loss at critical interfaces through the narrow pitch portions.
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 effectively prevents spurious responses and ensures a sufficient pass band width, reducing insertion loss and maintaining a required band width ratio, making it suitable for use in cellular phone duplexers.
Implementation Method 1
a piezoelectric film provided on the low-acoustic-velocity film; a plurality of interdigital transducers provided on the piezoelectric film
Implementation Method 2
a multilayer body including a high-acoustic-velocity member, a low-acoustic-velocity film, and a piezoelectric film; leakage of energy of a surface acoustic wave is reduced
Data Source
AI summary
A longitudinally coupled resonator type surface acoustic wave filter includes a high-acoustic-velocity member, a low-acoustic-velocity film provided on the high-acoustic-velocity member, a piezoelectric film provided on the low-acoustic-velocity film, a plurality of interdigital transducers provided on the piezoelectric film and along a propagation direction of a surface acoustic wave and each including a plurality of electrode fingers, and reflectors arranged such that the interdigital transducers are interposed therebetween from both sides in the propagation direction of the surface acoustic wave. An electrode finger pitch is uniform or substantially uniform in each of the interdigital transducers. When a wavelength determined by the electrode finger pitch in the reflector is defined as λ, an inter-electrode finger center distance that is an interval between each of the interdigital transducers and the interdigital transducer adjacent thereto is not shorter than about 0.25λ and not longer than about 0.37λ.