Balanced SAW Filter Electrode Weighting for Ripple Suppression
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Solution Overview
Problem
Existing balanced-type surface acoustic wave filters with a balanced-to-unbalanced conversion function suffer from steep spike-like ripples in amplitude and phase balancing characteristics within the passband, leading to insufficient balancing performance.
Innovation Solution
A balanced-type surface acoustic wave filter with two-stage cascade-connected longitudinally coupled resonator-type surface acoustic wave filter portions, where specific electrode finger weighting techniques such as cross-width, series, and thinning-out weighting are applied to adjacent IDTs, ensuring a 180-degree phase difference between signal lines and compensating for discontinuities, thereby suppressing spike-like ripples and improving balancing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a two-stage cascade-connected longitudinally coupled resonator-type surface acoustic wave filter is used, then the filter can provide balanced-to-unbalanced conversion function, but spike-like ripples appear in amplitude and phase balancing characteristics within the passband
Solution Approach 1:
The patent applies different electrode finger configurations to different IDTs within the same filter structure. Specifically, the first and second IDTs have a first number of electrode fingers while the third and fourth IDTs have a second number of electrode fingers that is different from the first number. This local differentiation allows each IDT to be optimized for its specific position in the cascade connection, improving overall balancing characteristics while maintaining the balanced-to-unbalanced conversion function.
Solution Approach 2:
The patent introduces asymmetry by making the number of electrode fingers in the first and second IDTs different from the number of electrode fingers in the third and fourth IDTs. This asymmetric design breaks the symmetry that causes the spike-like ripples in balancing characteristics, thereby improving amplitude and phase balancing while preserving the filter's conversion function.
2Manufacturing precision
If narrow-pitched electrode-finger portions are provided in adjacent IDTs, then discontinuity compensation is achieved and filter characteristics are improved, but the complexity of electrode structure increases
Solution Approach 1:
The patent segments the electrode structure of each IDT into two distinct portions: a first electrode-finger portion with a first pitch and a second electrode-finger portion with a second pitch that is narrower than the first pitch. This segmentation allows the filter to maintain good filtering characteristics while compensating for discontinuities at the boundaries between adjacent IDTs, without requiring a completely complex re design of the entire electrode 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 proposed solution effectively reduces spike-like ripples in amplitude and phase balancing characteristics, significantly enhancing the balancing performance of the filter within the passband.
Implementation Method 1
a first longitudinally coupled resonator-type surface acoustic wave filter portion having first to third interdigital transducers disposed along a propagation direction of a surface wave on the piezoelectric substrate
Implementation Method 2
an electric signal which has passed through a signal line connecting the first interdigital transducer and the fourth interdigital transducer is about 180 degrees different in phase from an electric signal which has passed through a signal line connecting the third interdigital transducer and the sixth interdigital transducer
Data Source
AI summary
In a balanced type surface acoustic wave filter, a first longitudinally coupled resonator-type surface acoustic wave filter portion connected to an unbalanced input terminal and a second longitudinally coupled resonator-type surface acoustic wave filter portion connected to first and second balanced input terminals are two-stage cascade connected. In the first longitudinally coupled resonator-type surface acoustic wave filter portion, in a portion in which first and second IDTs are adjacent to each other and a portion in which second and third IDTs are adjacent to each other, weighting is performed on a plurality of electrode fingers including the outermost electrode finger closest to the adjacent IDT, in at least one of the adjacent IDTs.