Ladder-Type Filter Electrode Layout for Transverse-Mode Suppression
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Solution Overview
Problem
Conventional ladder-type filters face challenges in suppressing spurious emission by the transverse mode while maintaining a compact size, especially for low frequency bands like UMTS, where increased electrode finger pairs and pitch lead to larger filter sizes.
Innovation Solution
The design incorporates interdigital electrodes on both series and parallel arms of a piezoelectric substrate with electrode cross widths 23 times or more than the wavelength of excited elastic waves, reducing spurious emission and allowing for downsizing without increasing the number of electrode finger pairs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the number of electrode finger pairs is increased to secure enough capacitance, then the capacitance of elastic-wave resonator is improved, but the filter size becomes larger
Solution Approach 1:
The patent changes the geometric parameters of the interdigital electrodes, specifically making the electrode cross width D1 of interdigital electrode 5a different from the electrode cross width D2 of interdigital electrode 5b. This parameter variation allows optimization of capacitance without proportionally increasing the filter area, as the differential cross widths create more efficient capacitance distribution.
Solution Approach 2:
The patent applies different cross widths to different sections of the interdigital electrodes. By making D1 different from D2, the electrode structure achieves non-uniform local properties that optimize capacitance in specific regions, allowing sufficient total capacitance with fewer electrode finger pairs and thus reducing overall filter size.
2Measurement precision
If the distance between electrode fingers is increased for low frequency band, then the resonance frequency characteristics are improved, but the filter size becomes extremely large
Solution Approach 1:
The patent varies the electrode cross widths (D1 and D2) to compensate for the increased finger pitch required for low frequency operation. This parameter modification allows maintaining proper resonance frequency characteristics while reducing the number of finger pairs needed, thereby preventing extreme size increase.
Solution Approach 2:
By creating sections with different cross widths along the electrode length, the patent optimizes the local capacitance distribution. This allows the filter to achieve correct resonance characteristics for low frequency bands without requiring uniformly large dimensions across the entire structure.
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 suppresses transverse-mode spurious emission, reduces insertion loss, and enables a more compact filter size suitable for communication systems using passbands below 1 GHz, particularly for UMTS.
Implementation Method 1
ladder-type filter having plural elastic-wave resonators provided on the series and parallel arms in a ladder shape on a piezoelectric substrate
Implementation Method 2
Each elastic-wave resonator has an interdigital electrode, which has plural electrode finger pairs formed of electrode fingers extending from first and second bus bars
Data Source
AI summary
A ladder-type filter having plural elastic-wave resonators provided on a series arm and a parallel arm in a ladder shape on a piezoelectric substrate. Each of the elastic-wave resonators has an interdigital electrode. The interdigital electrode has plural electrode finger pairs formed of electrode fingers extending from first and second bus bars. The interdigital electrode included in the elastic-wave resonator provided on the parallel arm has an electrode cross width of the electrode fingers extending from the first and second bus bars 23 times or more of the wavelength of elastic waves excited by the interdigital electrodes.