Composite RF Filter Circuit for Low-Loss Multiband Isolation
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Solution Overview
Problem
Composite filter apparatuses face signal leakage issues due to impedance mismatch between filters with different pass bands, leading to increased loss in the pass band of lower-frequency filters when connected to higher-frequency filters.
Innovation Solution
Incorporating a first inductor between the common terminal and the second filter, and a parallel-arm resonator with a resonant frequency lower than the high end frequency of the first pass band, to adjust the impedance and reduce loss by advancing the phase to an open state, thereby minimizing signal leakage and reducing the size of the apparatus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the inductance component of the phase shifter is increased to increase the impedance of the second filter in the pass band of the first filter, then signal leakage is prevented, but the insertion loss of the phase shifter increases and the loss in the pass band of the second filter increases
Solution Approach 1:
The patent changes the resonant frequency parameter of the parallel-arm resonator to be lower than the high end frequency of the first pass band. This parameter adjustment allows the resonator to provide appropriate impedance transformation without requiring excessive inductance, thereby reducing insertion loss while still preventing signal leakage.
Solution Approach 2:
The parallel-arm resonator acts as an intermediary element between the first and second filters. By positioning its resonant frequency below the first pass band's high end frequency, it mediates the impedance interaction between the two filters, enabling the second filter to present high impedance in the first pass band without the phase shifter introducing excessive loss.
2Adaptability or versatility
If multiple filters with different pass bands are connected to one antenna to support multiple frequency bands, then multiband communication capability is achieved, but signal leakage from lower frequency filter to higher frequency filter occurs due to impedance mismatch
Solution Approach 1:
The patent applies local quality by making the parallel-arm resonator's characteristics specific to the frequency separation between first and second pass bands. Its resonant frequency is deliberately set below the high end of the first pass band, creating a localized impedance effect that specifically addresses signal leakage from the first to second filter while maintaining the multiband capability.
Solution Approach 2:
The parallel-arm resonator serves as an intermediary structure between the series-arm resonators and ground, providing frequency-selective impedance transformation. This intermediary element enables multiple filters to coexist on a single antenna by preventing harmful signal leakage while preserving multiband operation.
3Object-affected harmful factors
If the resonant frequency of the parallel-arm resonator is set lower than the high end frequency of the first pass band, then the impedance of the second filter is increased in the first pass band reducing signal leakage, but the anti-resonant frequency must be within the second pass band to maintain filter performance
Solution Approach 1:
The patent systematically optimizes two critical parameters: the resonant frequency set below the first pass band's high end to increase impedance and reduce leakage, and the anti-resonant frequency positioned within the second pass band to maintain filter performance. This dual parameter optimization resolves the contradiction between preventing leakage and maintaining device functionality.
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 the loss in the pass band of each filter while maintaining low loss in higher-frequency filters, improving bandpass characteristics and reducing the overall size of the composite filter apparatus.
Implementation Method 1
a resonant frequency of the first parallel-arm resonator is lower than a high end frequency of the first pass band
Implementation Method 2
a first inductor that is provided between the common terminal and the second filter
Data Source
AI summary
A composite filter apparatus includes a common terminal, a transmission-side terminal, a reception-side terminal, a transmission filter, and a reception filter with a pass band higher than the pass band of the transmission filter. The transmission filter is connected between the common terminal and the transmission-side terminal and the reception filter is connected between the common terminal and the reception-side terminal. An inductor is provided between the common terminal and the reception filter in a path between the common terminal and the reception-side terminal. The resonant frequency of a parallel-arm resonator closest to the common terminal in the reception filter is lower than a high end frequency of the pass band of the transmission filter and is lower than the resonant frequencies of the other parallel-arm resonators in the reception filter.