SH-Wave Ladder Filter Layout for Rayleigh Ripple Suppression
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Solution Overview
Problem
Filter devices with bandpass filters connected in common experience degradation in filter characteristics due to unwanted Rayleigh wave responses, particularly when using piezoelectric substrates with high acoustic velocity layers, leading to increased insertion loss and ripple in pass bands.
Innovation Solution
Incorporating an inductor connected in series with the acoustic wave resonator having the shortest electrode finger pitch in the bandpass filter, which reduces or prevents the effect of Rayleigh waves on the pass band of another filter without degrading the filter characteristics, by positioning the ripple outside the pass band and maintaining lower insertion loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple bandpass filters are connected in common to an input/output terminal, then the filter device can handle multiple frequency bands, but the Rayleigh wave response in one filter causes ripple in the pass band of another filter, degrading filter characteristics
Solution Approach 1:
A damping layer is introduced as an intermediary between the piezoelectric layer and the acoustic wave resonators. This damping layer specifically suppresses the Rayleigh wave response without affecting the desired acoustic wave operation, thereby eliminating the harmful interference between connected filters while maintaining multi-frequency band capability
2Speed
If a piezoelectric substrate with a high acoustic velocity layer is used, then the acoustic wave resonators can operate at higher frequencies, but the Rayleigh wave response becomes stronger, causing larger ripple and degrading filter characteristics
Solution Approach 1:
The high acoustic velocity layer is retained to enable high-frequency operation, but the unwanted Rayleigh wave energy is redirected and dissipated by the damping layer. The damping layer converts the harmful Rayleigh wave into beneficial thermal energy, allowing the high acoustic velocity substrate to be used without its detrimental effects
3Object-affected harmful factors
If one tries to reduce or prevent the response caused by the Rayleigh wave, then the ripple in pass bands is reduced, but the filter characteristics of the bandpass filter itself may degrade
Solution Approach 1:
The damping layer is designed with specific local properties - it is positioned only where needed to suppress Rayleigh waves, and its material characteristics are optimized to dampen surface waves while being transparent to the bulk acoustic waves used by the resonators. This selective damping preserves filter characteristics while eliminating Rayleigh wave interference
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 effectively reduces the impact of Rayleigh wave responses on connected bandpass filters, maintaining excellent filter characteristics and minimizing insertion loss, thereby enhancing the overall performance of filter devices.
Implementation Method 1
there is a response caused by a Rayleigh wave, which is an unwanted wave, in the acoustic wave resonator of the bandpass filter
Implementation Method 2
In the case where shear horizontal (SH) waves are used as a major mode of a bandpass filter
Implementation Method 3
an inductor is connected in series to the parallel arm resonator having a shortest electrode finger pitch of the interdigital transducer electrode
Data Source
AI summary
A filter device includes a common connection terminal, a first bandpass filter connected to the common connection terminal and including an inductor, and a second bandpass filter connected to the common connection terminal and having a pass band lower in frequency than a pass band of the first bandpass filter. The filter device uses SH waves. The first bandpass filter is a ladder filter. Each of series arm resonators and parallel arm resonators includes an interdigital transducer electrode. Of the parallel arm resonators of the first bandpass filter, the inductor is connected in series to the parallel arm resonator with a shortest electrode finger pitch of the interdigital transducer electrode.