Ladder-Type Duplexer Layout for High Isolation in Small SAW Filters
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Solution Overview
Problem
Conventional duplexers with ladder-type filters face challenges in achieving both small size and high isolation characteristics due to signal leakage between transmission and receiving frequency bands, particularly when using ladder-type filters with multiple resonators.
Innovation Solution
The duplexer design features ladder-type filter elements with comb-shaped electrode fingers on series and parallel arms, where resonators are arranged to avoid overlapping elastic wave propagating paths, ensuring that leaked elastic waves from series resonators are not received by parallel resonators, thereby enhancing isolation characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If resonators are placed closely to each other to achieve small size, then the device size is reduced, but signal leakage between filters increases and isolation characteristic deteriorates
Solution Approach 1:
The patent applies dimensionality change by arranging resonators in a three-dimensional configuration on the piezoelectric substrate. Specifically, resonators of the first filter and resonators of the second filter are positioned at different locations and orientations such that their elastic wave propagating paths do not overlap in the spatial domain. This spatial separation in multiple dimensions allows close placement for miniaturization while preventing signal leakage between filters, thus resolving the contradiction between small size and high isolation characteristic.
2Reliability
If comb-shaped electrodes are disposed with non-overlapped main propagating directions to reduce spurious characteristics, then spurious characteristics are reduced, but device complexity increases due to difficult layout
Solution Approach 1:
The patent employs asymmetry in the arrangement of comb-shaped electrodes between the two filters. The first filter and second filter are designed with different electrode configurations and orientations. Specifically, the main propagating directions of elastic waves in the first filter are made different from those in the second filter, creating an asymmetric layout. This asymmetric design naturally prevents overlapping of elastic wave paths, reducing spurious characteristics while simplifying the overall layout planning compared to symmetric designs that would require careful alignment.
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 design achieves superior isolation characteristics while allowing for a compact size, reducing signal leakage and improving communication quality in portable communication terminals.
Implementation Method 1
an elastic surface wave filter has been used. The elastic surface wave filter is an element which uses elastic surface waves that are excited by an IDT (Inter Digital Transducer) placed on a piezoelectric substrate
Implementation Method 2
each of the first and second filter elements has a ladder-type structure in which a plurality of resonators having comb-shaped electrode fingers is disposed
Data Source
AI summary
The present invention provides a duplexer using a ladder-type filter, which has a small size and is superior in isolation characteristic, and a communication apparatus using such a duplexer. A series resonator (R1) contained in a first filter element (F1) and a parallel resonator (R4) contained in a second filter element (F2) are disposed so as not to allow the elastic wave propagating paths to be overlapped with each other. By disposing them in this manner, it is possible to prevent an elastic wave, leaked from the series resonator (R1) and most influential to the isolation characteristic, from being received by the parallel resonator (R4). Consequently, it is possible to achieve a small-size duplexer that is superior in isolation characteristic.