LiTaO3 SAW Filter Layout for Wide Passband and Low Insertion Loss
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Solution Overview
Problem
Existing surface acoustic wave (SAW) filters face challenges in achieving low loss across a wide passband, as the propagation performance is not solely determined by propagation loss in the range from resonance to anti-resonance points, especially when the passband width exceeds 3% of the center frequency.
Innovation Solution
A SAW filter design utilizing a LiTaO3 piezoelectric substrate with interdigital transducer electrodes, featuring a ladder-type or longitudinally-coupled-resonator configuration, which shifts the cutoff frequency due to bulk wave radiation to a range higher than the passband, thereby reducing insertion loss on the high frequency side by optimizing the normalized film thickness and duty ratio of the electrodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the passband width is increased to 3% or greater of the center frequency, then the filter covers a wider frequency range, but the propagation performance deteriorates and insertion loss increases
Solution Approach 1:
The patent applies parameter changes by optimizing the normalized film thickness (h/λ) to be 7.5% to 8.0% and the duty ratio (line width/pitch) to be 0.55 to 0.65. These specific parameter ranges are designed to control bulk wave radiation and maintain low propagation loss across the widened passband, resolving the contradiction between passband width and insertion loss.
2Loss of energy
If the normalized film thickness and duty ratio are optimized to decrease propagation loss, then insertion loss is reduced in narrow band specifications, but the benefit diminishes in wide band specifications
Solution Approach 1:
The patent identifies specific parameter ranges (normalized film thickness: 7.5%-8.0%, duty ratio: 0.55-0.65) that simultaneously achieve low propagation loss and support wide passband operation. This resolves the contradiction by finding parameter values that work effectively across both narrow and wide band specifications.
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 proposed design successfully reduces insertion loss on the high frequency side within the passband, maintaining low return loss and preserving fundamental characteristics such as resonance Q values and temperature stability, even with a bandwidth ratio of 2.5% or greater.
Implementation Method 1
surface acoustic wave (SAW) filters that utilize leaky waves
Implementation Method 2
utilizes a leaky wave that propagates on the LiTaO3 piezoelectric substrate
Implementation Method 3
interdigital transducer electrodes disposed on the LiTaO3 piezoelectric substrate
Implementation Method 4
a cutoff frequency at which bulk wave radiation of a parallel resonator, among the parallel resonators, increases is in a frequency range higher than the passband
Data Source
AI summary
A ladder-type surface acoustic wave filter includes interdigital transducer electrodes disposed on a LiTaO3 piezoelectric substrate, and series resonators and parallel resonators defined by the interdigital transducer electrodes, and utilizes a leaky wave that propagates on the LiTaO3 piezoelectric substrate. A bandwidth ratio indicating a bandwidth of a passband of the ladder-type surface acoustic wave filter is about 2.5% or greater, and a cutoff frequency due to bulk wave radiation of one of the parallel resonators, is in a frequency range higher than the passband.