lowpass

A lowpass filter is a system that passes signals with frequencies below a chosen cutoff and attenuates signals above it. The cutoff frequency, fc, separates passband from stopband. Signals well below fc experience little attenuation, while higher-frequency components are increasingly suppressed in the transition region, whose width depends on the filter order and design.

Lowpass filters can be built as analog networks using resistors, capacitors, inductors, and sometimes active amplifiers, or implemented digitally as finite impulse response (FIR) or infinite impulse response (IIR) filters. Analog implementations rely on reactive components, whereas digital designs perform frequency-domain or time-domain processing by computing sums of past samples.

Common frequency responses include Butterworth (maximally flat passband), Chebyshev (ripple in passband or stopband), and Bessel (linear phase). Higher-order filters have steeper roll-off but may introduce greater phase distortion or delay. FIR filters can be designed for linear phase; IIR filters achieve sharper attenuation for a given order.

Applications include anti-aliasing for analog-to-digital conversion, anti-imaging for digital-to-analog conversion, audio and video processing, and measurement systems to remove high-frequency noise. Important concepts include cutoff frequency, transition band, stopband attenuation, roll-off, group delay, and sampling rate.