Evanescent-Mode Cavity Filter with Closed Loop Control

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Solution Overview

Problem

Existing tunable filters face challenges in responsiveness, accuracy, and stability, particularly in covering a wide frequency range from 1 to 110 GHz, while also needing to reduce Critical-Size, Weight, and Power (C-SWAP) and improve performance beyond switched fixed filter systems.

Innovation Solution

A tunable, evanescent-mode cavity filter with a closed loop control system that includes a capacitive post, a flexible diaphragm, and an actuator, coupled with a capacitive sense terminal, allowing for precise tuning through a digital signal conversion and high voltage generation, and featuring a dielectric barrier to prevent charge accumulation, enabling independent control and high-frequency responsiveness.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If a tunable filter system is designed to cover a wide frequency range (1-110 GHz), then the frequency coverage is improved, but the responsiveness, accuracy, and stability deteriorate

Engineering Contradiction:

Improvefrequency coverageVSAvoidresponsiveness, accuracy, and stability

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The filter system is divided into multiple independent resonant cavities, each capable of being tuned independently. This segmentation allows the system to maintain high reliability for each individual cavity while collectively covering a wide frequency range through coordinated operation of multiple cavities.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A closed-loop feedback control system is implemented with capacitive sensing that continuously monitors the actual frequency response and adjusts the tuning elements accordingly. This feedback mechanism maintains high accuracy and stability across the wide frequency range by compensating for drift and environmental variations.

Inventive Principle:

Principle #23Feedback

2Ease of manufacture

If switched fixed filter systems are used, then the implementation is simplified, but the C-SWAP (Critical-Size, Weight, and Power) increases and performance is limited

Engineering Contradiction:

Improvesystem implementationVSAvoidC-SWAP

Core Design Contradiction:

Ease of manufactureVSWeight of moving object

Solution Approach 1:

A single tunable filter system performs the function of multiple fixed filters across the 1-110 GHz range. By using continuously variable tuning elements (flexible diaphragms with capacitive posts) instead of switched fixed filters, the system achieves multi-frequency operation with reduced size, weight, and power compared to implementing separate fixed filters for each frequency band.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Measurement precision

If the gap between the flexible diaphragm and capacitive post is reduced to improve tuning precision, then the measurement precision is improved, but the device complexity increases

Engineering Contradiction:

Improvetuning precisionVSAvoidstructural complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

A flexible diaphragm made of thin film material is used as the tuning element. This flexible structure allows precise control of the gap distance between the diaphragm and capacitive post through applied voltage, achieving high tuning precision without requiring complex mechanical adjustment mechanisms. The flexibility enables fine positional control while maintaining structural simplicity.

Inventive Principle:

Principle #30Flexible shells and thin films

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 solution provides highly responsive, accurate, and stable tuning across a broad frequency range, reducing hysteresis effects and bias charge buildup, thereby enhancing filter performance with improved resolution and reduced C-SWAP.

Implementation Method 1

The ceiling of the cavity can be a movable structure, such as a flexible piezoelectric or MEMS electrostatically actuated membrane

Methodology Applied

Scientific EffectElectrostatic actuation: Electrostatics

Implementation Method 2

a capacitive sense terminal coupled to the actuator for measuring a capacitance associated with a notch frequency of the filter

Methodology Applied

Scientific EffectCapacitance measurement: Capacitance

Implementation Method 3

a dielectric spacer disposed adjacent to the electrical contact, thereby providing for a dielectric-barrier to prohibit the accumulation of sensing charge and bias charge in the actuator substrate

Methodology Applied

Scientific EffectDielectric barrier: Dielectric

Data Source

PatentUS9935349B2Tunable evanescent-mode cavity filter with closed loop control

Publication Date: 2018.04.03 BAE SYSTEMS INFORMATION ANDELECTRONIC SYSTEMS INTEGRATION INC

AI summary

An evanescent-mode cavity filter with an improved MEMS tuner design is disclosed. The MEMS tuner design allows for the independent control of individual poles in a multi-pole filter, which increases the adaptability of the filter in a crowded RF environment. The filter is further designed to minimize tuning voltages and hysteresis effects. A closed loop control system provides highly responsive tuning of the filter. The closed loop control allows for accurate and stable tuning that compensates for temperature and vibrational effects, while the tuner design enables fast tuning and significantly increases the resolution of the feedback measurement by eliminating charge buildup in the tuner substrate.