Acoustic Wave Filter TCF Compensation With Dielectric Film
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Solution Overview
Problem
Existing acoustic wave filters exhibit degraded attenuation and lower skirt slope at low temperatures due to a positive temperature coefficient of frequency (TCF) for resonant frequencies, which affects their temperature stability and rejection performance.
Innovation Solution
The acoustic filter device incorporates series and parallel resonators with specific configurations, including the use of negative or positive TCF materials and varying interdigital transducer (IDT) duty factors, to synchronize the TCF values of resonant and anti-resonant frequencies, thereby improving temperature stability and rejection performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing acoustic wave filters are used, then the filter allows frequencies in a specific band, but the resonant TCF has a positive value resulting in degraded attenuation and lower skirt slope at low temperature
Solution Approach 1:
The patent applies different TCF characteristics to different parts of the filter system. Specifically, it uses resonators with positive TCF for series resonators and resonators with negative TCF for parallel resonators, creating local quality differences that collectively achieve zero TCF for the overall filter response. This local differentiation allows the filter to maintain consistent performance across temperature variations.
Solution Approach 2:
The patent employs a composite approach by combining resonators with opposite TCF characteristics (positive and negative) in a unified filter structure. The series resonators use positive TCF materials while parallel resonators use negative TCF materials, creating a composite system where the TCF effects cancel each other out, achieving temperature compensation without requiring a single material with zero TCF.
2Reliability
If existing acoustic wave filters are used, then the filter structure is simple, but the rejection performance is degraded due to positive resonant TCF
Solution Approach 1:
The patent introduces local quality variations by assigning different TCF characteristics to specific resonator groups within the filter. Series resonators utilize positive TCF while parallel resonators utilize negative TCF, creating localized functional differences that improve rejection performance without requiring a complete redesign of the entire filter architecture.
Solution Approach 2:
The patent changes the TCF parameter of different resonator groups to achieve overall temperature compensation. By selecting resonators with opposite TCF signs and appropriate TCF magnitudes, the system achieves zero net TCF for the filter response, thereby improving rejection performance across temperature ranges while maintaining a relatively simple filter 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 enhances the temperature stability and rejection performance of acoustic wave filters by synchronizing the TCF values of resonant and anti-resonant frequencies, leading to improved attenuation and skirt slope across a wide temperature range.
Implementation Method 1
a piezoelectric layer over the substrate
Implementation Method 2
A surface acoustic wave resonator of a surface acoustic wave filter typically includes an interdigital transducer electrode on a piezoelectric substrate. A surface acoustic wave resonator is arranged to generate a surface acoustic wave.
Implementation Method 3
A layer of positive temperature coefficient of frequency dielectric material is disposed over one or more of the first plurality of acoustic wave resonators
Data Source
AI summary
An acoustic wave filter includes a substrate and a piezoelectric layer over the substrate. First acoustic wave resonators are disposed over the piezoelectric layer and arranged in series along a first branch, and second acoustic wave resonators are disposed over the piezoelectric layer, arranged in parallel, and connected to the first branch and to ground. The first and second acoustic wave resonators include an interdigital transducer electrode interposed between a pair of reflectors. A layer of positive temperature coefficient of frequency dielectric material is disposed over one or more of the first plurality of acoustic wave resonators to control the temperature coefficient of frequency and improve temperature stability of the acoustic wave filter.