Ladder-Type Filter Resonator Layout for Lower TCF
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Solution Overview
Problem
Existing ladder-type filters using acoustic wave resonators with comb-shaped electrodes and a piezoelectric layer bonded on a support substrate face challenges in reducing the temperature coefficient of frequency (TCF).
Innovation Solution
The proposed solution involves a ladder-type filter design that includes parallel and series resonators with specific electrode finger configurations. The parallel resonators have a first average pitch equal to or greater than twice the thickness of the piezoelectric layer, while the series resonators have a second average pitch and duty ratio that is less than the smallest first average duty ratio of the parallel resonators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the piezoelectric layer thickness is reduced to improve temperature stability, then the temperature coefficient of frequency is reduced, but the mechanical strength and structural integrity deteriorate
Solution Approach 1:
The patent uses a composite structure consisting of a piezoelectric layer bonded to a support substrate. This composite configuration provides mechanical strength through the support substrate while maintaining the thin piezoelectric layer necessary for low temperature coefficient of frequency. The support substrate acts as a mechanical reinforcement that compensates for the reduced thickness of the piezoelectric layer.
Solution Approach 2:
The patent introduces an insulating layer as an intermediary element between the support substrate and the piezoelectric layer. This insulating layer serves as a mediator that provides electrical isolation while allowing the thin piezoelectric layer to maintain its temperature stability characteristics without compromising the overall structural integrity through the support substrate.
2Temperature
If the duty ratio of series resonator electrode fingers is reduced to reduce TCF, then the temperature stability improves, but the manufacturing precision requirements increase
Solution Approach 1:
The patent applies parameter changes by optimizing the duty ratio of the series resonator electrode fingers to be less than the duty ratio of the parallel resonator electrode fingers. This parameter adjustment reduces the temperature coefficient of frequency while the patent addresses the manufacturing precision challenge through specific design constraints on the pitch relationship (largest pitch ≥ 2×thickness) that provide manufacturing tolerance.
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
This design effectively reduces the temperature coefficient of frequency (TCF) by adjusting the duty ratios and pitches of the resonators, thereby improving the temperature stability of the filter.
Implementation Method 1
acoustic wave resonators having a pair of comb-shaped electrodes formed on a piezoelectric layer
Implementation Method 2
acoustic wave resonators having a pair of comb-shaped electrodes formed on a piezoelectric layer
Data Source
AI summary
A ladder-type filter includes a support substrate, a piezoelectric layer provided on the support substrate, a parallel resonator including first electrode fingers provided on the piezoelectric layer and having a first average pitch and a first average duty ratio, a largest first average pitch being equal to or greater than two times a thickness of the piezoelectric layer, a first end of the parallel resonator being coupled to a path between input and output terminals, a second end of the parallel resonator being coupled to a ground, and a series resonator connected in series between the input and output terminals, the series resonator including second electrode fingers provided on the piezoelectric layer and having a second average pitch and a second average duty ratio, a second average duty ratio in at least one series resonator being less than a smallest first average duty ratio.