Detailed Explanation of the Principle and Function of Filters

A filter is a device used to filter waves, typically featuring two ports: one for input signals and one for output signals. As a critical electronic component, a filter transmits power supply energy to equipment while significantly attenuating electromagnetic interference (EMI) signals transmitted through the power supply, thereby protecting the equipment. Additionally, it effectively controls EMI signals generated by the equipment itself, preventing them from entering the power grid, contaminating the electromagnetic environment, and harming other devices. So, how does a filter work, and what are its functions? This article will introduce the principles, filtering concepts, and applications of filters.

Filters are primarily composed of low-pass filtering circuits made up of inductors and capacitors. These circuits allow the passage of current for useful signals while significantly attenuating higher-frequency interference signals. Since interference signals can be either differential-mode or common-mode, filters must attenuate both types. The basic principles of filtering include:

(1) Utilizing the property of capacitors to pass high frequencies and block low frequencies, high-frequency interference currents on the live and neutral wires can be directed to the ground (common-mode) or the live wire to the neutral wire (differential-mode).

(2) Leveraging the impedance characteristics of inductive coils to reflect high-frequency interference currents back to the interference source.

(3) Using interference suppression ferrites, which can absorb and convert interference signals within certain frequency bands into heat. Appropriate ferrite beads or rings can be directly placed on cables requiring filtering based on the frequency range of the interference signal.

Filtering is an important concept in signal processing. The role of a filtering circuit is to minimize the alternating current (AC) components in a pulsating direct current (DC) voltage, retaining its DC component, thereby reducing the output voltage ripple coefficient and smoothing the waveform.

Generally, filtering can be divided into classical filtering and modern filtering.

Classical filtering is an engineering concept based on Fourier analysis and transforms. According to advanced mathematical theory, any signal meeting certain conditions can be regarded as the superposition of an infinite number of sine waves. In other words, engineering signals are linear superpositions of sine waves of different frequencies. The different frequency sine waves that make up the signal are called its frequency components or harmonic components. A circuit that allows only signal components within a certain frequency range to pass normally while blocking others is called a classical filter or filtering circuit.

In both classical and modern filtering, the filter models are essentially the same (hardware-based filters have not seen significant advancements). However, modern filtering incorporates many concepts from digital filtering.

(1) Separating useful signals from noise to improve signal anti-interference capability and signal-to-noise ratio.

(2) Filtering out uninteresting frequency components to enhance analysis accuracy.

(3) Isolating a single frequency component from complex frequency components.

Filters are used to improve power quality, circuit linearity, reduce various types of clutter, nonlinear distortion interference, and harmonic interference. For weapon systems, filters are applied in the following scenarios:

(1) In addition to installing power filters on the main power distribution system and sub-distribution systems, filters should be installed on the power supply entering all equipment. It is preferable to use line-to-line filters rather than line-to-ground filters.

(2) For equipment sensitive to pulse interference and transient interference, when using an isolation transformer for power supply, a filter should be added at the negative terminal.

(3) When supplying power to weapon systems containing electro-explosive devices, filters should be installed. If necessary, filters should also be added to the leads of the electro-explosive devices.

(4) Filters should be placed at the interfaces between various subsystems or equipment to suppress interference and ensure compatibility.

(5) Control signals for equipment and subsystems should have filters or bypass capacitors added at both the input and output ends.

This article summarizes the principles, filtering concepts, parameters, functions, and application considerations of filters. With a wide variety of filters available, each with different performance characteristics, selecting the right filter requires a comprehensive consideration of the customer's actual operating environment and performance requirements to make a correct, effective, and reliable choice.