Edge-Coupled Filter With Quarter-Wavelength Open Stub

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Solution Overview

Problem

Edge-coupled filters face limitations in implementing wide band filters due to tight coupling requirements, leading to unwanted spurious pass-bands around twice the center frequency, which affects attenuation and selectivity.

Innovation Solution

The design incorporates open stubs with lengths equal to a quarter of the wavelength at twice the passband center frequency, and matching impedances to reduce spurious pass-bands and enhance filter characteristics, allowing for improved attenuation and selectivity without additional impedance matching efforts.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If tight coupling is used in the first and last coupled-line sections to achieve proper filter implementation, then the filter structure is properly implemented, but the realization of wide band filters is prevented and unwanted spurious pass-bands appear around twice the center frequency

Engineering Contradiction:

Improvefilter implementationVSAvoidwide band filter realization

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

An open stub is introduced as an intermediary element connected to the first coupled-line section. This stub acts as a mediator that modifies the coupling characteristics, enabling wide band operation while maintaining proper filter implementation. The open stub transforms the tight coupling requirement into a more flexible coupling arrangement that suppresses spurious pass-bands.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The electrical length of the open stub is specifically designed to be one-quarter of the wavelength at twice the passband center frequency. By changing the electrical length parameter of the stub to target the spurious frequency, the coupling characteristics are modified to suppress unwanted pass-bands while enabling wide band operation.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If tight coupling is used in the first and last coupled-line sections, then the filter structure is properly implemented, but attenuation and selectivity are degraded due to spurious pass-bands

Engineering Contradiction:

Improvefilter structure implementationVSAvoidattenuation and selectivity

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The open stub serves as an intermediary that improves attenuation and selectivity by suppressing spurious pass-bands. It mediates between the coupled-line sections to eliminate unwanted frequency responses while maintaining the proper filter structure implementation.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

By setting the electrical length of the open stub to one-quarter wavelength at twice the passband center frequency, the parameter is optimized to create transmission zeros at the spurious frequency. This parameter change directly improves attenuation and selectivity by eliminating the unwanted pass-bands.

Inventive Principle:

Principle #35Parameter changes

3Reliability

If additional impedance matching efforts are made to reduce spurious pass-bands, then filter characteristics are improved, but design complexity and costs increase

Engineering Contradiction:

Improvefilter characteristicsVSAvoiddesign complexity and costs

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The open stub provides an elegant intermediary solution that improves filter characteristics without requiring complex impedance matching networks. Instead of adding multiple matching elements, a single stub serves as the intermediary to suppress spurious pass-bands, thereby reducing design complexity and costs.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The solution relies on a simple parameter specification (electrical length equals one-quarter wavelength at twice the passband center frequency) rather than complex impedance matching calculations. This parameter-based approach simplifies the design process and reduces costs while effectively improving filter characteristics.

Inventive Principle:

Principle #35Parameter changes

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 approach effectively reduces unwanted spurious pass-bands, increases attenuation, and improves filter selectivity, providing better filter characteristics and reduced design complexity and costs.

Implementation Method 1

A length of the open stub is chosen such that an electrical length of the open stub is equal, within a tolerance of +/−20 percent, to a fourth of a wavelength of a signal having a frequency of twice a passband center frequency of the filter

Methodology Applied

Scientific EffectResonance: Resonance

Data Source

PatentUS11018647B2Electrical filter structure

Publication Date: 2021.05.25 ADVANTEST CORP

AI summary

An electrical filter structure for forwarding an electrical signal from a first port to a second port in a frequency selective manner, wherein the filter is an edge-coupled filter, the filter comprising: a plurality of coupled line sections coupled in a series, comprising at least a first coupled line section and a last coupled line section; wherein the first port is connected with the first of the coupled line sections using a first transmission line; wherein the second port is connected with the last of the coupled line sections using a second transmission line; wherein the electrical filter comprises an open stub; wherein a length of the open stub is chosen such that an electrical length of the open stub is equal, within a tolerance of +/−20 percent, to a fourth of a wavelength of a signal having a frequency of twice a passband center frequency of the filter.