Ladder Acoustic Wave Filter Tuning for Steep Skirts and Low Loss
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Solution Overview
Problem
Surface acoustic wave filter devices with ladder circuit configurations face challenges in achieving a steep filter characteristic while maintaining low loss within the passband, particularly due to abrupt changes in filter characteristics at the end of the passband and increased loss on the lower-frequency side when steepness is increased.
Innovation Solution
The acoustic wave filter device incorporates a ladder circuit configuration with series and parallel arm resonators, where the anti-resonant frequency of at least one series arm resonator is lower than the others and located within the passband, and the electromechanical coupling coefficient is adjusted by varying the propagation direction ψ of the piezoelectric substrate, allowing for increased steepness on both higher and lower-frequency sides of the passband without significant insertion loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the electromechanical coupling coefficient of the parallel arm resonator is reduced to increase filter characteristic steepness, then the steepness of filter characteristic is improved, but the loss in the passband is increased
Solution Approach 1:
The invention changes the parameter of electromechanical coupling coefficient by selecting resonators with different coupling coefficients at different positions in the ladder circuit. Specifically, series arm resonators have larger coupling coefficients while parallel arm resonators have smaller coupling coefficients, creating a parameter gradient that simultaneously achieves steep filter characteristics and low passband loss
Solution Approach 2:
The invention applies different electromechanical coupling coefficients to different parts of the circuit according to their functional requirements. Series arm resonators use larger coupling coefficients for steepness on the higher-frequency side, while parallel arm resonators use smaller coupling coefficients to maintain low loss in the passband, particularly on the lower-frequency side
2Manufacturing precision
If the anti-resonant frequency of series arm resonator is lowered to increase steepness on higher-frequency side, then the steepness is improved, but the passband width may be reduced
Solution Approach 1:
The invention optimizes the anti-resonant frequency parameter of series arm resonators to be lower than that of parallel arm resonators. This parameter configuration creates a frequency separation that enables steep filter characteristics on the higher-frequency side while preserving adequate passband width through proper frequency positioning
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 enhances the steepness of the filter characteristic at the boundary between the passband and attenuation band while maintaining low insertion loss within the passband, improving the frequency temperature characteristic and allowing for a wider passband width.
Implementation Method 1
Surface acoustic wave filter devices having a ladder circuit configuration are widely used as bandpass filters for mobile telephones
Implementation Method 2
an acoustic wave filter device having a configuration in which an Au electrode is formed on an LiNbO3 substrate with Euler angles (0°, 105°, ψ)
Data Source
AI summary
An acoustic wave filter device is capable of increasing the steepness of a filter characteristic at a boundary between a passband and an attenuation band and achieving a low loss in the passband. The acoustic wave filter device has a ladder circuit configuration including a plurality of series arm resonators and at least one parallel arm resonator. The anti-resonant frequency of the series arm resonator is different from that of the series arm resonator. The series arm resonator having the lowest anti-resonant frequency has a resonant frequency located in the passband and an electromechanical coupling coefficient less than an average of electromechanical coupling coefficients of the series arm resonators.