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Active Filters

Active Filters

Active filters are electronic circuits that use operational amplifiers (op-amps), resistors, and capacitors to filter out unwanted frequency components from a signal. A first-order low-pass active filter is designed to pass signals with a frequency lower than a certain cutoff frequency and attenuate frequencies higher than that cutoff frequency. The transfer function for a first-order low-pass active filter is:

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Updated: Jun 20, 2026

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Published on: August 15, 2018

Ferroelectric liquid-crystal tunable filter.

H J Masterson, G D Sharp, K M Johnson

|September 18, 2009

Summary

This summary is machine-generated.

A novel tunable Lyot filter using ferroelectric liquid-crystal (FLC) wave plates was developed. This FLC tunable filter offers rapid tuning and low voltage operation for advanced optical filtering applications.

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Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

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Area of Science:

Optics and Photonics
Materials Science

Background:

Lyot filters are crucial optical components for wavelength selection.
Existing tunable filters often face limitations in speed, voltage, or aperture size.

Purpose of the Study:

To introduce and demonstrate a new discrete-channel tunable Lyot filter.
To utilize ferroelectric liquid-crystal (FLC) smectic C* wave plates for tunability.

Main Methods:

The device employs FLC wave plates whose retardation is electrically tuned.
Rotation of FLC wave plates by 45 degrees electrically modifies the filter's design wavelength.
A three-stage filter was constructed to demonstrate transmission characteristics.

Main Results:

Demonstrated a discrete-channel tunable Lyot filter with FLC wave plates.
Transmission characteristics showed good agreement with theoretical predictions.
The FLC tunable filter exhibited low switching voltages (+/-10 V) and rapid tunability (~100 kHz).

Conclusions:

The developed FLC tunable Lyot filter presents a viable alternative to existing filter technologies.
Key advantages include low switching voltage, high speed, potential for high transmission, wide field of view, and large entrance aperture.