Elastic Wave Filter Resonator Layout for Steep Wideband Isolation
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Solution Overview
Problem
Existing surface acoustic wave filters struggle to achieve high steepness in bandpass characteristics outside the pass band while maintaining desired impedance inside the pass band, particularly for frequency standards with wide bandwidths and narrow intervals between adjacent bands, such as Band 25 of the LTE standard.
Innovation Solution
The implementation of an elastic wave filter configuration with four or more series resonators connected in series and a parallel resonator between input/output terminals, where the frequency differences between anti-resonant and resonant frequencies of all series resonators except the first and second resonators are smaller than those of the first and second resonators, and their anti-resonant frequencies are shifted towards the high-frequency side, maintaining impedance inside the pass band and enhancing steepness outside it.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional ladder surface acoustic wave device design is used, then the device can operate at high frequencies, but it cannot achieve high steepness in bandpass characteristics outside the pass band while maintaining desired impedance inside the pass band for wide bandwidth applications
Solution Approach 1:
The patent applies parameter changes by systematically varying the resonant frequencies of series resonators and anti-resonant frequencies of parallel resonators according to specific mathematical relationships. The series resonators are designed with resonant frequencies that increase from low to high frequency, while parallel resonators have anti-resonant frequencies that decrease from high to low frequency. This parameter optimization enables high steepness in bandpass characteristics outside the pass band while maintaining desired impedance inside the pass band, resolving the technical contradiction for wide bandwidth applications like Band 25 (65 MHz bandwidth, 15 MHz interval).
Solution Approach 2:
The patent implements local quality by assigning different frequency characteristics to different resonators within the filter structure. Specifically, each series resonator and parallel resonator is designed with unique resonant and anti-resonant frequencies that follow specific patterns. This localized frequency distribution allows different parts of the filter to contribute differently to the overall response, achieving both high steepness outside the pass band and proper impedance matching inside the pass band simultaneously.
2Adaptability or versatility
If the bandwidth is increased to support wide frequency bands, then the filter can handle more frequency resources, but the steepness of bandpass characteristic outside the pass band deteriorates
Solution Approach 1:
The patent resolves this contradiction by changing the frequency parameters of multiple resonators in a coordinated manner. The series resonators are designed with resonant frequencies that increase progressively, while parallel resonators have anti-resonant frequencies that decrease progressively. This creates a cumulative effect where the combined response of all resonators maintains high steepness even when the overall bandwidth is wide, enabling support for frequency standards like Band 25 with 65 MHz bandwidth.
Solution Approach 2:
The patent applies segmentation by dividing the wide bandwidth coverage into multiple resonators, each responsible for a specific frequency range. The series resonators cover increasing frequency ranges while parallel resonators cover decreasing frequency ranges, and their combined effect provides both wide bandwidth support and high steepness characteristics. This segmentation allows the filter to handle wide frequency resources while maintaining precise bandpass characteristics.
3Adaptability or versatility
If the interval between transmission and reception bands is narrowed, then the frequency resource utilization is improved, but the isolation between bands becomes difficult to achieve
Solution Approach 1:
The patent resolves this contradiction by optimizing the frequency parameters of series and parallel resonators to create sharp transition bands. The series resonators with increasing resonant frequencies and parallel resonators with decreasing anti-resonant frequencies work together to produce high steepness in the bandpass characteristics. This enables effective isolation between transmission and reception bands even when the interval is narrow (15 MHz in Band 25), while maintaining high frequency resource utilization.
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 achieves high steepness in insertion loss and isolation at the high-frequency side of the pass band while maintaining desired impedance inside the pass band, even for frequency standards with wide bandwidths and narrow intervals, as demonstrated by improved bandpass characteristics and impedance matching in the described examples.
Implementation Method 1
a parallel resonator that is connected between any of connection nodes between the input terminal, the output terminal and the four or more series resonators, and a reference terminal. Frequency differences that are differences between anti-resonant frequencies and resonant frequencies
Implementation Method 2
the parallel resonator and the series resonator are defined by interdigital transducer (IDT) electrodes provided on a piezoelectric substrate
Implementation Method 3
ladder surface acoustic wave device that is able to be applied to high frequency bands and wide bandwidths
Data Source
AI summary
A transmission filter includes five series resonators that include functional electrodes provided on a piezoelectric substrate, and connected in series with each other between a transmission input terminal and an antenna terminal, and parallel resonators. Frequency differences between the anti-resonant frequencies and the resonant frequencies of the series resonators other than the series resonator connected so as to be closest to the transmission input terminal and the series resonator connected so as to be closest to the antenna terminal are smaller than frequency differences of the series resonators, and anti-resonant frequencies of the series resonators are lower than the anti-resonant frequencies of the series resonators.