Cascaded LBAW Filter with Capacitive Coupling for Sideband Suppression
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Solution Overview
Problem
Existing radio-frequency (RF) band pass filters, particularly Lateral Bulk Acoustic Wave (LBAW) filters, suffer from unwanted sidebands that degrade their performance, and there is a need to improve the stop band rejection and band pass response.
Innovation Solution
The solution involves cascading two or more LBAW filters in series, with a common electrode connecting the output of one filter to the input of the next, to suppress spurious transmission peaks and enhance the band pass response by optimizing the piezoelectric layer thickness and electrode geometry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single LBAW filter is used, then the device complexity is low, but the stop band rejection is insufficient due to unwanted sidebands
Solution Approach 1:
The filter is divided into multiple LBAW filter elements (first, second, and third elements) with distinct electrode configurations. Each element contributes to different aspects of the frequency response, with the first and second elements forming a series connection and the third element providing parallel coupling, collectively achieving superior stop band rejection while maintaining manageable complexity through modular design.
Solution Approach 2:
The filter employs a composite electrode structure with multiple counter electrodes (first counter electrode, second counter electrode, and third counter electrode) coupled to different portions of the piezoelectric layer. This composite configuration creates multiple coupling paths that enhance stop band rejection by suppressing unwanted sidebands through destructive interference, while the integrated piezoelectric layer maintains fabrication simplicity.
2Reliability
If conventional BAW filters are used, then stop band rejection can be achieved, but the manufacturing complexity increases significantly requiring multiple lithographic patterning steps
Solution Approach 1:
Multiple filter elements sharing a common piezoelectric layer are merged into a single integrated structure. The first, second, and third LBAW filter elements are formed on the same piezoelectric layer with shared electrodes and coupling mechanisms, eliminating the need for separate fabrication processes for each filter element and reducing the total number of lithographic patterning steps compared to conventional stacked BAW filters.
Solution Approach 2:
The piezoelectric layer serves multiple functions simultaneously: it acts as the acoustic wave propagation medium for all filter elements, provides the substrate for all electrode structures, and enables both series and parallel coupling mechanisms. This multi-functionality reduces manufacturing complexity by eliminating the need for separate piezoelectric layers and associated fabrication steps for each filter element.
3Adaptability or versatility
If LBAW filters are designed for wider bandwidth, then the communication performance improves, but the sideband suppression becomes more challenging
Solution Approach 1:
The filter employs dynamic coupling mechanisms where the third counter electrode is capacitively coupled to both the first and second counter electrodes, creating adjustable coupling paths that can be optimized for different bandwidth requirements. The capacitive coupling strength can be tuned by adjusting the geometry and positioning of the third counter electrode relative to the other electrodes, enabling dynamic optimization of both bandwidth and sideband suppression characteristics.
Solution Approach 2:
The filter design allows independent optimization of multiple parameters including the spacing between counter electrodes, the dimensions of the third counter electrode, and the capacitive coupling strength. By adjusting these parameters, the filter can be tuned to achieve wider bandwidth while maintaining effective sideband suppression through optimized destructive interference patterns across the extended frequency range.
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 approach effectively suppresses sidebands, improves stop band rejection, and allows for wider bandwidth operation, simplifying fabrication and enabling higher frequency performance compared to conventional filters.
Implementation Method 1
By applying an alternating voltage across the piezoelectric layer at the input resonator, a mechanical resonance is formed in the piezoelectric layer below the input electrode
Implementation Method 2
The piezoelectric layer thickness and the gap between electrodes can be designed such that this mechanical resonance is coupled across the gap to the output resonator
Implementation Method 3
The common electrode electrically connects the first output electrode to the second input electrode
Data Source
AI summary
An acoustic wave filter device with two-stage acoustic wave filters is provided. Each of the two stages includes a respective acoustic wave filter element. A first acoustic wave filter element (100a) includes a first input electrode (150a), a first output electrode (174a), and a first counter electrode (120a). The first input electrode and the first output electrode are located on a top surface of piezoelectric layer (650), and the first counter electrode is located on a bottom surface of the piezoelectric layer. A second acoustic wave filter element (100b) includes a second input electrode (154b), a second output electrode (174b), and a second counter electrode (120b). The second input electrode and the second output electrode are located on the top surface of the piezoelectric layer, and the second counter electrode is located on a bottom surface of the piezoelectric layer. The two acoustic wave filter elements are connected in series through a common floating electrode (602).