Parallel SAW Resonator Layout for Transverse Mode Suppression
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Solution Overview
Problem
Conventional surface acoustic wave resonators with LiNbO3 substrates suffer from spurious responses due to resonance in the transverse mode, especially when the number of comb electrode fingers is increased, which degrades in-band frequency characteristics and capacitance, making it difficult to achieve wide frequency band filters with suppressed spurious responses.
Innovation Solution
A surface acoustic wave resonator design featuring two apodized comb electrode resonators connected in parallel, each with a smaller number of electrode fingers, which suppresses undesired transverse mode resonance and allows for a desired electrostatic capacitance, using a rotated Y-cut LiNbO3 substrate with a silicon dioxide film for improved frequency bandwidth and temperature characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the number of comb electrode fingers is increased to increase capacitance, then the capacitance increases, but spurious responses occur due to resonance in the transverse mode
Solution Approach 1:
The patent divides the single comb electrode structure into multiple separate comb electrodes (first comb electrode and second comb electrode). Each comb electrode has a reduced number of fingers compared to a single large comb electrode, which suppresses transverse mode resonance. By connecting multiple comb electrodes in parallel, the total capacitance is maintained or increased while avoiding spurious responses that would occur with a single large comb electrode.
2Adaptability or versatility
If LiNbO3 substrates are used to widen frequency bands, then the electromechanical coupling coefficient increases, but spurious responses occur due to resonance in the transverse mode
Solution Approach 1:
The patent uses LiNbO3 substrate to achieve wide frequency band operation through high electromechanical coupling coefficient, but divides the comb electrode into multiple segments. This segmentation suppresses the transverse mode resonance that would otherwise cause spurious responses on LiNbO3 substrate, thereby maintaining both wide bandwidth and clean frequency characteristics.
3Object-generated harmful factors
If apodization is applied to suppress transverse mode resonance, then spurious responses are reduced for small number of electrode fingers, but it is not effective when comb electrodes have a large number of electrode fingers
Solution Approach 1:
Instead of applying complex apodization to a large number of electrode fingers, the patent segments the comb electrode into multiple smaller comb electrodes. Each segment has fewer fingers, making apodization more effective and simpler to implement. The parallel connection of segments maintains the required electrical performance while reducing spurious responses.
4Quantity of substance
If the number of electrode fingers is increased to obtain large capacitance for ladder type filters, then the capacitance increases, but spurious responses are liable to occur
Solution Approach 1:
The patent achieves the required large capacitance for ladder type filters by connecting multiple comb electrodes in parallel rather than using a single comb electrode with many fingers. Each comb electrode has a manageable number of fingers that suppresses transverse mode resonance, while the parallel configuration provides the necessary total capacitance without spurious responses.
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 design effectively suppresses spurious responses and maintains low insertion loss, enabling a wide frequency band filter with increased capacitance, enhancing the flexibility in designing ladder type filters and improving the performance of telecommunications devices.
Implementation Method 1
a piezoelectric substrate, a first surface acoustic wave resonator having a comb electrode provided on the piezoelectric substrate
Implementation Method 2
Both of the first surface acoustic wave resonator and the second surface acoustic wave resonator have an apodized configuration... suppresses the resonance in the transverse mode
Implementation Method 3
surfaces of the comb electrodes and the LiNbO3 substrate are covered with a thin film of silicon dioxide
Data Source
Figure 1~2B
Figure 3~4
Figure 5A~6
AI summary
A surface acoustic wave resonator, comprises a piezoelectric substrate (10); a first surface acoustic wave resonator (22) having a first comb electrode (22a) provided on the piezoelectric substrate (10) and being designed to obtain a desired frequency characteristic by varying first overlapping lengths (L1) of the first comb electrode fingers; and a second surface acoustic wave resonator (21) having a second comb electrode (21a) provided on the piezoelectric substrate (10), being connected in parallel to the first surface acoustic wave resonator (22), and being designed to obtain a desired frequency characteristic by varying second overlapping lengths (L2) of the second comb electrode fingers smaller than the first overlapping lengths (L1) in order to differentiate the resonance frequencies in a transverse mode between the first and second surface acoustic wave resonator (22; 21), so as to separate spurious responses occurring within the passing frequency band due to standing waves developed in a direction orthogonal to the propagating direction, with the frequency of the spurious responses to the transverse mode resonances in the transverse mode being dependent upon the first and second overlapping lengths (L1; L2) of the first and second comb electrode (22a; 21a) fingers.