Elastic Wave Filter Electrode Layout for Balanced Signal Matching
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Solution Overview
Problem
Surface acoustic wave filters with balanced-unbalanced conversion functions face issues with signal balance, as simply making one side of the virtual central axis different from the other does not sufficiently improve balance, leading to deteriorated performance in 5-IDT-type filters compared to 3-IDT-type filters.
Innovation Solution
A 5-IDT-type longitudinally coupled resonator-type elastic wave filter is designed with specific configurations, including divided comb-shaped electrodes and varying electrode finger pitches or duty ratios between IDTs, to enhance signal balance between balanced signal terminals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a 5-IDT-type longitudinally coupled resonator-type surface acoustic wave filter is configured with symmetric design parameters on both sides of the virtual central axis, then the device complexity is reduced and manufacturing is simplified, but the signal balance between first and second balanced signal terminals deteriorates
Solution Approach 1:
The patent applies asymmetry by making the electrode finger pitch of the first IDT different from that of the fifth IDT, while maintaining symmetry in other parameters. Specifically, the first IDT has a smaller electrode finger pitch than the fifth IDT, creating an asymmetric configuration that improves signal balance between the balanced signal terminals. This selective asymmetry resolves the contradiction by achieving better signal balance without excessively complicating the manufacturing process.
Solution Approach 2:
The patent applies local quality by varying the electrode finger pitch only in specific IDTs (first and fifth IDTs) while keeping other parameters symmetric. The second and fourth IDTs maintain symmetric electrode finger pitches, and the third IDT remains symmetric. This localized variation in specific regions achieves the desired signal balance improvement without requiring asymmetric changes throughout the entire device.
2Manufacturing precision
If the electrode finger pitch of the first IDT is made smaller than that of the fifth IDT, then the signal balance between balanced signal terminals is improved, but the device complexity increases due to additional design parameters
Solution Approach 1:
The patent implements asymmetry by setting the electrode finger pitch of the first IDT to be smaller than that of the fifth IDT. This asymmetric configuration is specifically designed to improve signal balance between the balanced signal terminals. The asymmetry is limited to this single parameter difference, avoiding excessive complexity while achieving the desired performance improvement.
Solution Approach 2:
The patent applies parameter changes by modifying the electrode finger pitch parameter of the first and fifth IDTs. By changing this specific geometric parameter, the patent achieves improved signal balance without requiring changes to other design parameters such as IDT lengths, electrode widths, or reflector configurations, thereby limiting the increase in device complexity.
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 configuration effectively improves signal balance between balanced signal terminals, achieving better amplitude and phase balance compared to comparative examples, while maintaining suitable phase balance within required ranges.
Implementation Method 1
a piezoelectric substrate P; a longitudinally coupled resonator-type surface acoustic wave filter unit 200A provided on the piezoelectric substrate P
Implementation Method 2
first to fifth interdigital electrode units (IDTs) 201 to 205 which are arranged in the surface acoustic wave propagation direction
Data Source
AI summary
An elastic wave filter includes at least a piezoelectric substrate and a longitudinally coupled resonator-type elastic wave filter unit provided on the piezoelectric substrate. The longitudinally coupled resonator-type elastic wave filter unit includes first to fifth IDTs and first and second reflectors arranged on either side of the first to fifth IDTs. The third IDT includes first and second divided comb-shaped electrodes. The second and fourth IDTs are connected to an unbalanced signal terminal, the first IDT and the first divided comb-shaped electrode of the third IDT are connected to a first balanced signal terminal, and the fifth IDT and the second divided comb-shaped electrode of the third IDT are connected to a second balanced signal terminal. The first and fifth IDTs each have a constant electrode finger pitch, and the electrode finger pitch of the first IDT is less than that of the fifth IDT.