Acoustic filter with improved reflectivity
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Solution Overview
Problem
Acoustic filters, particularly TC-SAW filters, face challenges with spurious modes that affect reflectivity and selectivity due to the excitation of secondary modes like plate modes, which are exacerbated by the need for thick dielectric layers to compensate for temperature variations, leading to performance impairments and interference with other LTE bands.
Innovation Solution
A filter circuit design that includes a series signal line with a micro-acoustic series resonator and a final series capacitance between the last resonator and the antenna terminal, optionally with additional parallel branches and impedance elements, effectively suppresses spurious modes by reducing their excitation and minimizing their impact on reflectivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a thick dielectric layer is deposited to compensate for temperature variations, then the temperature coefficient specification is fulfilled, but the excitation of plate mode is strongly increased
Solution Approach 1:
The harmful plate mode is extracted and targeted for suppression by adding a dedicated suppression resonator tuned to the plate mode frequency, separating the temperature compensation function (thick dielectric layer) from the plate mode excitation problem
Solution Approach 2:
A suppression resonator is introduced as an intermediary element between the main filter circuit and the plate mode, acting as a selective damper that targets only the harmful plate mode frequencies without affecting the main filter operation or temperature compensation
2Manufacturing precision
If the resonator is modified to move spurious modes out of the band, then the selectivity is improved, but the quality factor of the resonator is reduced
Solution Approach 1:
The filter system is segmented into two independent parts: the main filter resonators that provide selectivity and quality factor, and a separate suppression resonator that specifically targets plate modes, allowing each to be optimized independently without compromising the other
3Object-generated harmful factors
If a shunt line is inserted to produce a resonance at the spurious mode frequency, then the spurious mode is addressed, but the selectivity is reduced due to limited Q-factor of passive elements
Solution Approach 1:
A suppression resonator is introduced as an intermediary element between the main filter circuit and the plate mode, acting as a selective damper that targets only the harmful plate mode frequencies without affecting the main filter operation or temperature compensation
Solution Approach 2:
The suppression resonator is designed with specific parameters (high Q-factor, specific resonant frequency matching the plate mode) that differentiate it from the main filter resonators, enabling selective suppression of plate modes while preserving main filter selectivity
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 filter circuit successfully suppresses spurious modes, maintaining filter performance and selectivity while avoiding the need to move these modes out of the band, thus preserving the quality factor and reducing interference with other frequency bands.
Implementation Method 1
A main operating unit is arranged in the series signal line providing most of the filter function of the filter circuit thereby creating a pass band. A micro acoustic series resonator is a last element arranged in the series signal and part of the main operating unit.
Implementation Method 2
a final series capacitance is circuited between the last element and the antenna terminal as an additional element
Implementation Method 3
In the so-called TC-SAW filters (temperature compensated SAW filters), the resonators are commonly covered with a dielectric layer to reduce the sensitivity of the filters to temperature variations.
Implementation Method 4
the filter circuit may be formed on a substrate being a carrier only or provide with a functional material like a piezoelectric layer
Data Source
AI summary
A filter circuit comprises a main operating unit (MU) arranged in the series signal line providing most of the filter function of the filter circuit. A micro acoustic last series resonator (RLs) as a last element of the main operating unit in the series signal line is prone to excite a spurious mode that is damped with a final series capacitance (CEs) circuited between the last element and the antenna terminal (AT).