Acoustic Wave Filter Circuit for Leakage-Safe Bandpass Isolation
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Solution Overview
Problem
The integration of an additional circuit in filter devices leads to deterioration in bandpass characteristics due to signal leakage, compromising the attenuation and isolation performance.
Innovation Solution
Incorporating a phase shift circuit and capacitors with comb-shaped capacitance electrodes of smaller capacitance and area compared to IDT electrodes, these capacitors are strategically placed to minimize signal leakage and maintain bandpass characteristics by generating cancellation signals with opposite phases and identical amplitudes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an additional circuit is connected in parallel with the filter to improve attenuation characteristics, then isolation characteristics are improved, but bandpass characteristic deteriorates due to signal leakage
Solution Approach 1:
A capacitor is introduced as an intermediary element between the additional circuit and the first input/output terminal. This capacitor acts as a signal blocker for pass-band frequencies, preventing signal leakage into the additional circuit while allowing the additional circuit to function for improving isolation characteristics. The capacitor serves as a mediator that separates the functions of signal transmission and interference cancellation.
Solution Approach 2:
The additional circuit is designed with non-uniform electrode finger widths in the IDT (Interdigital Transducer), creating local variations in electrical characteristics. Specifically, electrode fingers have different widths in different regions, which allows the additional circuit to generate cancellation signals with appropriate amplitude and phase characteristics for specific frequency components without affecting the overall bandpass characteristic.
2Reliability
If the additional circuit is designed to generate cancellation signals, then attenuation characteristic is improved, but signal leakage to the additional circuit deteriorates bandpass characteristic
Solution Approach 1:
The capacitor functions as a frequency-selective intermediary that blocks pass-band signals from entering the additional circuit while allowing out-of-band signals to pass through. This enables the additional circuit to generate effective cancellation signals for improving attenuation without being contaminated by pass-band signals that would deteriorate bandpass characteristics.
3Manufacturing precision
If the capacitance of the capacitor is increased to reduce signal leakage, then bandpass characteristic is improved, but device area increases
Solution Approach 1:
The capacitance value is optimized to a specific range (0.5 pF to 5 pF) that provides sufficient signal blocking for pass-band frequencies while maintaining a compact device area. The electrode finger width is specifically designed to be smaller than that of the IDT electrode, which achieves the required capacitance value without increasing the overall device footprint.
Solution Approach 2:
The capacitor electrode has a localized structure with smaller finger width compared to the IDT electrode, concentrating the capacitive effect in a compact area. This local quality difference allows the capacitor to provide sufficient signal blocking without requiring a large device area.
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 reduces signal leakage, thereby preventing deterioration in bandpass characteristics and enhancing attenuation and isolation performance in filter devices.
Implementation Method 1
the at least two acoustic wave resonators are each defined by a piezoelectric body and an IDT electrode including a plurality of electrode fingers provided on the piezoelectric body
Implementation Method 2
the first capacitor is defined by the piezoelectric body and a comb-shaped capacitance electrode including a plurality of electrode fingers provided on the piezoelectric body
Data Source
AI summary
A filter device includes a filter between an input/output terminal and an input/output terminal, and an additional circuit connected in parallel with the filter. The filter includes at least two acoustic wave resonators defining a pass band of the filter. The additional circuit includes a longitudinally coupled resonator, and a capacitor connected between the longitudinally coupled resonator and the input/output terminal. A capacitance of a comb-shaped capacitance electrode of the capacitor is smaller than a capacitance of an IDT electrode of at least one acoustic wave resonator of the at least two acoustic wave resonators, and an area of the comb-shaped capacitance electrode of the capacitor is smaller than an area of the IDT electrode of the at least one acoustic wave resonator.