Ladder-Type Elastic Wave Filter Layout for Passband Ripple Suppression
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Solution Overview
Problem
Longitudinally-coupled-resonator-type surface acoustic wave filter devices with series-weighted IDTs experience high ripple levels in the pass band due to discontinuity in acoustic velocity caused by large differences in metallization ratios between adjacent acoustic tracks.
Innovation Solution
Incorporating metallization ratio increasing and reducing portions, such as protrusions and electrode cutout portions, in the IDTs to reduce the difference between the metallization ratios of adjacent acoustic tracks, thereby minimizing ripple generation in the pass band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If series weighting is performed by disposing a dummy electrode in an IDT, then the balance between balanced terminals is improved, but a high level of ripple is generated in the pass band
Solution Approach 1:
The patent applies local quality by making the electrode fingers in the series-weighted portion have different lengths compared to other electrode fingers. Specifically, the outermost electrode fingers (first and second electrode fingers) are made shorter than the inner electrode fingers, creating a local variation in electrode geometry. This local structural differentiation enables series weighting to improve terminal balance while the gradual transition of electrode lengths minimizes acoustic velocity discontinuity, thereby suppressing ripple generation in the pass band.
2Reliability
If the length of outermost electrode fingers is made shorter for series weighting, then terminal balance is improved, but discontinuity in acoustic velocity increases causing ripple
Solution Approach 1:
The patent applies parameter changes by systematically varying the length parameter of electrode fingers across different positions. The outermost electrode fingers are made shorter than inner electrode fingers, creating a gradual parameter transition. This parameter variation achieves series weighting for improved terminal balance while the controlled, gradual change in electrode dimensions maintains acoustic velocity continuity, preventing ripple generation.
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 suppresses ripple generation in the pass band, improving filter characteristics by ensuring continuity in acoustic velocity and reducing insertion loss, as demonstrated in the boundary acoustic wave filter device.
Implementation Method 1
The IDT 1001 is disposed on a piezoelectric substrate 1000... excitation of a surface acoustic wave is weakened by the dummy electrode 1006
Implementation Method 2
various surface acoustic wave filter devices have been used as band-pass filters for mobile telephones
Data Source
AI summary
In a longitudinally-coupled-resonator-type elastic wave filter device, at least one of two IDTs which are adjacent to each other is provided with a dummy electrode for series weighting in a portion in which the IDTs are adjacent to each other, and at least one of series-weighted portions includes at least one of metallization ratio reducing portion in a first acoustic track and metallization ratio increasing portion in a second acoustic track so as to reduce the difference between the metallization ratio of the first acoustic track passing through a connecting portion of the dummy electrode and the metallization ratio of each of second and third acoustic tracks each placed on either side of the first acoustic track in the direction of propagation of an elastic wave.