LiNbO3 SAW Filter Tuning to Suppress Rayleigh-Wave Spurs
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Solution Overview
Problem
Existing SAW filter devices with 41°-rotated Y-cut X-propagation LiNbO3 substrates experience increased insertion loss in the pass band lower than the trap band and high frequency end portion, due to spurious component generation from Rayleigh waves, which deteriorates the filter's performance.
Innovation Solution
A SAW filter device using a θ-rotated Y-cut X-propagation LiNbO3 substrate with a cutting angle between 50° and 55°, one-port SAW resonators with specific IDT film thickness and duty ratio, and integrated inductors to reduce spurious components and minimize insertion loss, while achieving a wide trap band and low-loss pass band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a 41°-rotated Y-cut X-propagation LiNbO3 substrate is used to achieve a wide trap band, then the trap band width is improved, but insertion loss increases in the pass band lower than the trap band and in the high frequency end portion
Solution Approach 1:
The patent changes the cutting angle parameter of the LiNbO3 substrate from the conventional 41° to a new range of 50° to 55°. This parameter change fundamentally alters the wave propagation characteristics, eliminating Rayleigh wave generation while maintaining leaky surface acoustic wave functionality, thereby resolving the contradiction between trap band width and insertion loss
Solution Approach 2:
The patent converts the harmful effect of Rayleigh waves (which cause spurious components and increase insertion loss) into a beneficial design feature by selecting a cutting angle range where Rayleigh waves do not propagate. This transforms the problem of Rayleigh wave interference into an opportunity to achieve clean signal transmission with reduced insertion loss
2Power
If the IDT film thickness is increased to improve the electromechanical coupling coefficient, then the coupling coefficient is improved, but spurious component generation increases due to Rayleigh wave influence
Solution Approach 1:
The patent optimizes the IDT film thickness parameter to a specific range (2.5% to 7.5% of the electrode cycle) that maximizes electromechanical coupling while operating on a substrate cutting angle (50° to 55°) that eliminates Rayleigh wave generation, thereby achieving high coupling coefficient without spurious component generation
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 effectively reduces spurious component generation and insertion loss, achieving a wide trap band and low-loss pass band by optimizing the cutting angle, IDT film thickness, and duty ratio, thereby improving the filter's frequency characteristics.
Implementation Method 1
a series arm surface acoustic wave resonator, an output electrode, a parallel arm surface acoustic wave resonator, and a ground electrode are disposed on a 41°-rotated Y-cut X-propagation LiNbO3 substrate
Implementation Method 2
changes in phase velocity and electromechanical coupling coefficient of a Rayleigh wave and a leaky surface acoustic wave which propagate on a LiNbO3 substrate
Implementation Method 3
a SAW filter device having a trap band and a pass band lower than the trap band
Data Source
AI summary
A SAW filter device includes a SAW filter chip in which one-port surface acoustic wave resonators each including an IDT made of Al or an Al alloy, are provided on a θ-rotated Y-cut X-propagation LiNbO3 substrate. The cutting angle θ of the θ-rotated Y-cut X-propagation LiNbO3 substrate is in the range between about 50° and about 55°. The normalized film thickness of the IDT 100h/λ (%) (h denotes the thickness of the IDT and λ denotes the wavelength of a surface acoustic wave) is in the range between about 2% and about 4%. The duty ratio of the IDT is equal to or less than about 0.4.