What is a filter? What are its classifications? How does it work?

A filter is a circuit or device that can selectively allow signals of a specific frequency to pass through and suppress signals of other frequencies. They play a vital role in signal processing, communication system, audio processing, image processing and other fields.

Filters are usually classified according to their frequency response and working mode. The following are some common filter classifications:

1. Classification by frequency response:

Low-pass filter: allows low-frequency signals to pass and suppresses high-frequency signals.

High-pass filter: allows high-frequency signals to pass and suppresses low-frequency signals.

Band-pass filter: only a certain range of frequency signals are allowed to pass.

Band-stop filter: Suppress frequency signals in a certain range.

2. Classification according to working methods:

Digital filter: filtering based on digital signals, often used in digital signal processing systems.

Analog filter: filtering based on analog signals, often used in analog circuits.

The working principle of the filter depends on its type. For example, a typical low-pass filter can be realized by using capacitance and inductance elements, and the combination of capacitance and inductance can produce frequency-dependent impedance, thus achieving selective transmission or suppression of signals with different frequencies. Digital filters usually use algorithms to process digital signals, such as finite impulse response (FIR) filters and infinite impulse response (IIR) filters.