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Passive Filters

Passive Filters

Passive filters are utilized to shape the frequency spectrum of signals across a diverse array of applications. These filters, using only passive elements like resistors (R), inductors (L), and capacitors (C), are capable of selectively allowing or blocking certain frequency ranges without the need for external power sources.
Low-Pass Filters
Low-pass filters are designed to transmit signals with frequencies lower than the cutoff frequency, ωc, and attenuate those above it. The cutoff...

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:

Design Example

Design Example

The innovation of touch-tone telephony revolutionized the telecommunications industry by replacing the traditional rotary dial with a dual-tone multi-frequency (DTMF) signaling system. This system uses a matrix-style keypad with buttons arranged in four rows and three columns, creating 12 distinct signals each assigned to a pair of frequencies. Each button press results in a simultaneous generation of two sinusoidal tones – one from a low-frequency group (697 to 941 Hz) and one from a...

Bandpass Sampling

Bandpass Sampling

In signal processing, bandpass sampling is an effective technique for sampling signals that have most of their energy concentrated within a narrow frequency band. This type of signal is known as a bandpass signal. The key principle of bandpass sampling involves sampling the signal at a rate that is greater than twice the signal's bandwidth to prevent aliasing.
A bandpass signal has a spectrum with a lower frequency limit, denoted as ω1, and an upper frequency limit, denoted as ω2....

Design Example: Capacitance Multiplier Circuit

Design Example: Capacitance Multiplier Circuit

In integrated circuit technology, a capacitance multiplier is often utilized to produce a larger capacitance value when a small physical capacitance falls short. This is achieved by a circuit that multiplies capacitance values by a factor of up to 1000, such that a 10-pF capacitor can replicate the performance of a 100-nF capacitor.
The circuit illustrated in Figure 1 below incorporates two op-amps, with the first operating as a voltage follower and the second acting as an inverting amplifier.

Upsampling

Upsampling

Managing signal sampling rates is essential in digital signal processing to maintain signal integrity. A decimated signal, characterized by a reduced frequency range due to its lower sampling rate, can be upsampled by inserting zeros between each sample. This upsampling process expands the original spectrum and introduces repeated spectral replicas at intervals dictated by the new Nyquist frequency. To refine this zero-inserted sequence, it is passed through a lowpass filter with a cutoff...

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Related Experiment Video

Updated: Aug 19, 2025

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

Published on: February 4, 2018

Reconfigurable high-order mode pass filter for mode-division multiplexing.

Prapty Saha¹, Oruni Aminul¹, Md Atiqur Rahman¹

¹Department of Electrical and Electronic Engineering, University of Chittagong, Chittagong, 4331, Bangladesh.

|November 28, 2022

Summary

This study presents a reconfigurable mode filter for mode division multiplexing (MDM) optical systems. The novel device acts as a higher-order mode pass filter or a fundamental mode (TE0) pass filter, enabling dynamic signal control.

Keywords:

MZI device Mode converter Mode filter Planar waveguide

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Area of Science:

Photonics and Optical Engineering
Integrated Optics
Waveguide Technology

Background:

Mode division multiplexing (MDM) optical systems require effective mode filters to reduce modal crosstalk.
Filtering lower-order modes in optical systems presents a significant challenge.

Purpose of the Study:

To propose and demonstrate a reconfigurable higher-order mode pass filter for MDM systems.
To enable dynamic switching between higher-order and fundamental mode filtering functionalities.

Main Methods:

A reconfigurable filter structure comprising two tunable mode converters and a directional coupler (DC) was designed.
The structure was implemented on a three-mode planar waveguide platform.
Simulations were performed to evaluate filter performance across the C-band.

Main Results:

The proposed filter achieved high performance for second-order mode (TE2) transmission with low excess loss (~0.61 dB at 1.550 μm).
Extinction ratios of ≥24 dB (TE0 & TE2) and ≥25 dB (TE1 & TE2) were maintained across the entire C-band (1.530-1.565 μm).
The device demonstrated negligible polarization dependence, with similar results for TM polarization.

Conclusions:

The reconfigurable mode filter offers a versatile solution for MDM systems, capable of filtering specific higher-order modes or the fundamental mode.
The demonstrated performance, including high extinction ratios and low loss, supports its application in advanced optical communication systems.
The dynamic tunability of the filter opens possibilities for flexible optical signal processing and routing.