Acoustic Wave Filter IDT Layout for Out-of-Band Wave Suppression
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Solution Overview
Problem
The increase in Q-value of acoustic wave filters leads to unnecessary resonance and excitation waves outside the pass band, making it impossible to ensure attenuation characteristics in those bands.
Innovation Solution
An acoustic wave filter design incorporating a filter circuit with an additional circuit containing interdigital transducers (IDTs) and reflectors, where the outermost IDT has specific electrode finger arrangements and pitch ratios to curb excitation waves outside the pass band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the Q-value of the acoustic wave element is increased by providing a reflector, then the Q-value is improved, but unnecessary resonance and excitation waves occur in bands outside the pass band
Solution Approach 1:
The outermost IDT is segmented into two excitation portions (first and second excitation portions) with different electrode finger arrangement pitches. This segmentation allows each portion to contribute differently to wave generation, enabling the first portion to work with the reflector for high Q-value while the second portion suppresses unwanted excitation waves through its specifically designed pitch ratio relationship with the reflector.
Solution Approach 2:
Different portions of the outermost IDT are given different local qualities through varying electrode finger arrangement pitches. The first excitation portion has a pitch designed for optimal reflector interaction, while the second excitation portion has a pitch specifically designed to curb excitation waves. This local differentiation resolves the contradiction by allowing each region to perform its specialized function.
2Reliability
If the Q-value is increased, then filter selectivity is improved, but attenuation characteristics in bands outside the pass band deteriorate
Solution Approach 1:
The outermost IDT is divided into two excitation portions with different pitch characteristics. The first excitation portion maintains high Q-value for improved filter selectivity, while the second excitation portion with its specific pitch ratio (0.5-0.97 or 1.15-2.0 times the reflector pitch) actively suppresses excitation waves, thereby ensuring good attenuation characteristics in bands outside the pass band.
Solution Approach 2:
The invention changes the parameter of electrode finger arrangement pitch in the outermost IDT by creating two distinct portions with different pitch values. This parameter differentiation allows the filter to achieve both high Q-value (for selectivity) and effective suppression of out-of-band excitation waves (for attenuation), resolving the contradiction between these two performance aspects.
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 design effectively suppresses excitation waves in bands outside the pass band, ensuring improved attenuation characteristics.
Implementation Method 1
an acoustic wave filter includes: a plurality of input-output terminals; a filter circuit provided in a first path linking the plurality of input-output terminals; and an additional circuit provided in a second path connected in parallel to at least a portion of the first path. The additional circuit includes a resonator including a plurality of interdigital transducers (IDTs)
Implementation Method 2
The reflector is placed adjacent to an outermost IDT that is located in an outermost side portion of the plurality of IDTs with respect to the first direction and includes a plurality of reflection electrode fingers extending in the second direction and arranged along the first direction
Data Source
AI summary
An acoustic wave filter 1 includes a filter circuit 10 and an additional circuit 20. The additional circuit 20 includes a plurality of IDTs 31, 32 and a reflector 41. A first excitation portion 51x of the IDT 31 is placed adjacent to the reflector 41 and, among a plurality of electrode fingers 35 included in the outermost IDT 31, includes electrode fingers numbered in ((N/2)+1) on condition that a total number N of the electrode fingers included in the pertinent IDT is an even number or includes electrode fingers numbered in ((N+1)/2) on condition that the total number N is an odd number. A second excitation portion 51y of the IDT 31 includes two or more electrode fingers, except the electrode fingers of the first excitation portion 51x. Provided an average arrangement pitch of the electrode fingers included in the first excitation portion 51x is defined as px and provided an average arrangement pitch of a plurality of reflection electrode fingers 45 is defined as pr, pr/px has a value in a range from 0.5 to 0.97 inclusive or a range from 1.15 to 2.0 inclusive.