Multiplexer Impedance Cancellation for Multiband Filter Loss Reduction
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Solution Overview
Problem
In multiplexers with multiple filters, the degradation of filter characteristics occurs due to the mutual influence of filters when they are commonly connected, leading to increased losses, especially when the number of filters exceeds two and the frequency bands are far apart.
Innovation Solution
The multiplexer design features filters with impedances that cancel each other's imaginary components at the pass band frequency of a reference filter, ensuring the combined impedance at the common junction point has minimal imaginary components, thereby reducing the impact of other filters' impedances and maintaining satisfactory electrical characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple filters are commonly connected without switches to support multiband operation, then the device complexity is reduced and adaptability is improved, but the filter characteristics degrade and losses increase due to mutual impedance influence
Solution Approach 1:
The patent changes the impedance parameters of the filters by introducing reactive components (inductors and capacitors) with specific values. The first filter includes a series inductor L1 and shunt capacitor C1, while the second filter includes a series inductor L2 and shunt capacitor C2. These components are designed with specific reactance values that satisfy the condition: X(L1) + X(C1) = -[X(L2) + X(C2)] at the operating frequency, transforming the overall impedance characteristic from high to low, thereby reducing mutual impedance influence and maintaining filter characteristics in multiband operation
Solution Approach 2:
The patent introduces reactive components (inductors L1, L2 and capacitors C1, C2) as intermediary elements between the filters and the common connection point. These intermediary components act as impedance transformation devices that modify the impedance seen by each filter, preventing direct mutual influence while allowing both filters to be commonly connected. The reactance components serve as mediators that cancel out the harmful imaginary impedance parts through proper design
2Reliability
If the impedance of counterpart-side filters is set to very high (open state) to avoid characteristics degradation, then filter characteristics are maintained for two filters, but this becomes difficult when three or more filters with apart frequencies are commonly connected
Solution Approach 1:
Instead of attempting to maintain very high impedance (open state) for all counterpart-side filters which becomes complex with three or more filters, the patent changes the approach by transforming the overall impedance to a low state using reactive components. The specific reactance values of L1, C1, L2, and C2 are designed to satisfy the cancellation condition, providing a systematic and scalable solution that works for three or more filters without requiring complex individual impedance control for each filter
3Ease of operation
If filters are commonly connected in a state where pass bands of counterpart-side filters are not in open state, then connectivity is simplified, but electrical characteristics degrade due to impedance influence from other filters
Solution Approach 1:
The reactive components (inductors and capacitors) serve as intermediary elements that enable simple common connection of multiple filters while simultaneously preventing energy loss. These intermediaries transform the impedance characteristics so that each filter sees a favorable impedance environment despite being commonly connected, allowing simple connectivity structure to maintain low signal losses through proper reactance design
Data Source
AI summary
A multiplexer includes a n number (n is an integer equal to three or more) of filters that are individually provided in the n number of paths commonly connected at a common junction point, and that have different pass bands from one another. In the multiplexer, (n−1) filters among the n number of filters except for the first filter have impedances of which imaginary components cancel each other at a pass band frequency of the first filter when viewed from the common junction point in a state of the n number of paths being not commonly connected.