Composite Ladder Filter IDT Duty Tuning for Passband Ripple Control
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Solution Overview
Problem
Existing composite filter devices experience increased loss and deteriorated characteristics in the passband due to ripples caused by Rayleigh waves when bandpass filters are connected in common, particularly in mobile communication devices.
Innovation Solution
The composite filter device incorporates a ladder filter with specific duty ratios for its serial and parallel arm resonators, ensuring that the duty of the first serial arm resonator is less than the duty of the first parallel arm resonator, thereby shifting Rayleigh wave responses outside the passband to prevent ripples and improve filter characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bandpass filters are connected in common to a ladder filter, then filter functionality is achieved, but ripples appear in the passband causing loss increase and characteristic deterioration
Solution Approach 1:
The patent changes the physical parameters of the IDT electrodes by adjusting the duty ratio (width-to-period ratio) of the first serial arm resonator and first parallel arm resonator. Specifically, the duty ratio of the first serial arm resonator is set to be smaller than that of the first parallel arm resonator, which shifts the Rayleigh wave response frequency and eliminates ripples in the passband, thereby resolving the contradiction between maintaining filter functionality and preventing energy loss.
2Reliability
If the duty of the first serial arm resonator is made smaller than the first parallel arm resonator, then Rayleigh wave responses are shifted outside the passband, but this requires precise control of IDT electrode dimensions
Solution Approach 1:
The patent establishes specific parameter relationships between the IDT electrodes of different resonators. By defining that the duty ratio of the first serial arm resonator must be smaller than that of the first parallel arm resonator, the patent transforms a complex ripple suppression problem into a controllable parameter specification, making it easier to implement in manufacturing while achieving reliable passband ripple control.
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 prevents ripples in the passband, maintaining optimal filter characteristics with minimal impact on insertion loss or attenuation, enhancing the performance of bandpass filters in mobile communication devices.
Implementation Method 1
a leaky surface acoustic wave is utilized, and a ripple caused by a Rayleigh wave appears outside the passband
Implementation Method 2
an acoustic wave resonator including an IDT electrode
Data Source
AI summary
A composite filter device includes a ladder filter and at least one bandpass filter including one end connected in common to the ladder filter, the ladder filter including at least one serial arm resonator including a first serial arm resonator and at least one parallel arm resonator including a first parallel arm resonator. The first serial arm resonator is the closest serial arm resonator to a common terminal, and the first parallel arm resonator is the closest parallel arm resonator to the common terminal. Expression (1), Expression (2), or Expression (3) is satisfied, where a duty of an IDT of the first serial arm resonator is Sa, a duty of an IDT of the first parallel arm resonator is Pa, and a duty of an IDT of each of serial arm resonators other than the first serial arm resonator and of parallel arm resonators other than the first parallel arm resonator is Ta:Sa<Pa<Ta Expression (1)Ta<Sa<Pa Expression (2)Pa<Ta<Sa Expression (3).