Elastic Wave Filter Layout for High-Impedance Balanced Conversion
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Solution Overview
Problem
Existing surface acoustic wave filter devices and boundary elastic wave filters struggle to achieve a high impedance ratio of at least 4:1 for balanced-unbalanced conversion, which is necessary to accommodate the increasing input impedance of balanced mixer ICs, particularly when the impedance exceeds 200Ω, as they cannot effectively match the impedance of balanced mixer ICs with higher input impedances.
Innovation Solution
The design of a balanced elastic wave filter device with specific configurations of IDTs and bus bars, including division of IDTs into sub-IDT portions and the use of common bus bars to connect these portions, allows for a significant increase in impedance on the balanced terminal side relative to the unbalanced terminal side, achieving an impedance ratio of up to 12:1, enabling effective impedance matching with higher input impedance balanced mixer ICs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional surface acoustic wave filter devices are used, then the basic balanced-unbalanced conversion function is provided, but the impedance ratio is limited to about 4:1 and cannot accommodate higher input impedances of modern balanced mixer ICs
Solution Approach 1:
The IDTs are divided into multiple sub-IDT portions (first, second, third sub-IDT portions) along the wave propagation direction. This segmentation allows independent impedance control of different sections, enabling the achievement of high impedance ratios by optimizing each sub-IDT's contribution to the overall impedance transformation from 50Ω to 200Ω or higher.
Solution Approach 2:
Different sub-IDT portions are designed with different characteristics (number of finger pairs, electrode patterns, connecting methods) to create localized impedance variations. The first, second, and third sub-IDT portions have different configurations that collectively achieve the desired high impedance ratio, with each portion contributing differently to the overall impedance transformation.
2Reliability
If the impedance ratio is increased to match high input impedance balanced mixer ICs (200Ω or higher), then impedance matching is improved, but maintaining sharp filter characteristics without passband ripples becomes more difficult
Solution Approach 1:
Dividing IDTs into multiple sub-IDT portions allows precise control of impedance transformation while maintaining filter characteristics. Each sub-IDT can be optimized for its specific function in the impedance transformation chain, enabling achievement of 200Ω or higher output impedance while keeping passband ripples below -20dB through careful design of each segment's electrical length and impedance.
Solution Approach 2:
The patent employs parameter optimization including number of finger pairs (e.g., 11, 13, 15 pairs in different sub-IDTs), electrode thickness ratios, and connecting methods (series/parallel combinations) to simultaneously achieve high impedance ratio and sharp filter characteristics. The electrical lengths of sub-IDTs are precisely controlled to maintain phase relationships that suppress passband ripples.
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 configuration ensures easy impedance matching with balanced mixer ICs having higher input impedances, providing a balanced-unbalanced conversion function while maintaining sharp filter characteristics and avoiding ripples in the passband, thus enhancing the performance of surface acoustic wave and boundary elastic wave filters.
Implementation Method 1
a first elastic wave filter unit including first to third IDTs (11 to 13) arranged in this order in an elastic wave propagation direction
Implementation Method 2
The present invention relates to an elastic wave filter device using elastic waves, such as surface acoustic waves and boundary elastic waves
Data Source
AI summary
An elastic wave filter device includes first and second elastic wave filter units connected to an unbalanced terminal. The first and second elastic wave filter units include first to third IDTs and fourth to sixth IDTs, respectively. One end of the first IDT, one end of the second IDT, one end of the fourth IDT, and one end of the sixth IDT are commonly connected together and are connected to the unbalanced terminal. The second IDT and the fifth IDT are each divided into first to third sub-IDT portions in the elastic wave propagation direction, respectively. The second sub-IDT portions of the second and fifth IDTs are connected to first and second balanced terminals, respectively.