Composite SAW-LC Filter Layout for Capacitor Area Reduction
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Solution Overview
Problem
Conventional composite filters in portable wireless communication devices, such as mobile phones, are limited in downsizing due to the large area occupied by capacitors, which are proportional to their capacitance, hindering further miniaturization without degrading filter characteristics.
Innovation Solution
The capacitors' comb-shaped electrodes are placed oppositely on a piezoelectric substrate, differing in orientation from surface acoustic wave resonator electrodes, allowing for reduced area occupation and preventing excitation, thus enabling a downsized composite filter without compromising performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the low-pass filter is embedded in laminated substrate with planar capacitor electrodes, then the filter structure is stable and easy to manufacture, but the capacitor occupies large area proportional to its capacitance, preventing further downsizing
Solution Approach 1:
The patent transitions from planar capacitor electrodes to comb-shaped electrodes that extend in the depth direction (z-axis) of the piezoelectric substrate. This dimensional change allows the capacitor to achieve required capacitance values without proportionally increasing the surface area, as capacitance is now contributed by the extended electrode length in the vertical dimension rather than only by planar surface area.
Solution Approach 2:
The patent utilizes a piezoelectric substrate with high dielectric constant (such as lithium tantalate or lithium niobate) to enhance the capacitance density of the comb-shaped electrodes. The combination of the piezoelectric material's high dielectric properties and the comb-shaped geometry creates a composite structure that achieves high capacitance in a compact footprint.
2Device complexity
If the comb-shaped electrodes of capacitors are placed in the same orientation as surface acoustic wave resonator electrodes, then the capacitor structure is simple, but the electrodes become excited and produce loss, degrading filter characteristics
Solution Approach 1:
The patent introduces asymmetry in the orientation of capacitor electrodes relative to resonator electrodes. The comb-shaped capacitor electrodes are rotated by a specific angle (such as 90 degrees or other non-zero angle) relative to the resonator electrodes, creating an asymmetric configuration that prevents the capacitor electrodes from being excited by the surface acoustic waves while maintaining the simplicity of the comb-shaped structure.
3Ease of manufacture
If conventional planar capacitor electrodes are used in laminated substrate, then the manufacturing process is straightforward, but the filter cannot be further downsized due to large capacitor area
Solution Approach 1:
The patent employs comb-shaped electrodes that extend in the depth direction of the piezoelectric substrate, utilizing the vertical dimension to increase capacitance without proportionally increasing the horizontal footprint. This allows significant reduction in the overall filter area while maintaining the required capacitance values for the low-pass filter function.
Solution Approach 2:
The use of piezoelectric substrate with high dielectric constant enhances the capacitance density, allowing the capacitor to achieve the required electrical performance in a much smaller volume compared to conventional planar capacitors in laminated ceramic substrates.
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 solution results in a significantly smaller composite filter, with the new design achieving approximately 1/18 the area of conventional filters while maintaining effective frequency attenuation and waveform integrity, capable of attenuating harmonics by 35 dB at 4 GHz and 20 dB at 6 GHz without degrading pass band waveforms.
Implementation Method 1
a piezoelectric substrate (10) made from LiTaO3
Implementation Method 2
elastic surface-wave filter (12) formed by combining surface acoustic wave resonators (14)
Data Source
AI summary
A composite filter downsized without degrading its characteristics is disclosed. The filter includes a surface acoustic wave filter and an LC filter coupled to the surface acoustic wave filter. The LC filter is formed by combining a π-shaped LC filter formed of two capacitors and an inductor coupled together in a π-shape with a capacitor coupled in parallel to the inductor. The two capacitors of the π-shaped LC filter are placed on a piezoelectric substrate with their comb-shaped electrodes opposed to each other. This opposing direction differs from an opposing direction of comb-shaped electrodes of a surface acoustic wave resonator.