Simply put, filtering refers to processing a signal. It selectively passes or suppresses signals of specific frequencies, thereby modifying the signal’s frequency spectrum characteristics.

We can view a filter as a frequency selector. It retains signal components within certain frequency ranges and removes components of other frequencies.

Its functions include noise removal, signal smoothing, extraction of useful signals, and more.

The concept of filtering originates from communication theory. It describes a technique for extracting useful signals from received signals that contain interference. The received signal is equivalent to an observed random process, while the useful signal corresponds to the estimated random process. For example, when radar tracks an aircraft, the measured aircraft position data contains measurement errors and other random interference.

Filtering is a core concept in signal processing and is divided into classical filtering and modern filtering.

Classical filtering is an engineering concept derived from Fourier analysis and transforms. According to advanced mathematical theory, any signal that meets certain conditions can be decomposed into infinitely many superimposed sine waves. In other words, engineering signals are formed by the superposition of sine waves of different frequencies. The sine wave components of different frequencies that make up a composite signal are called its frequency components or harmonic components.

Modern filtering uses analog electronic circuits to filter analog signals. Its basic principle leverages circuit frequency characteristics to select frequency components within a signal. When filtering by frequency, the signal is treated as an analog signal made up of superimposed sine waves of different frequencies. We achieve signal filtering by selecting different frequency components.

In electronic engineering, filtering is widely applied in signal processing, circuit design and communications. It can be implemented by adjusting the parameters of circuit components or adding dedicated circuits. In digital signal processing, filtering can also be realized via software.

Filtering is the operation of filtering specific frequency bands of a signal. It is a critical measure to suppress and prevent interference. Filtering is categorized into classical filtering and modern filtering.

Filtering is the operation of filtering specific frequency bands of a signal. It is a critical measure to suppress and prevent interference. It is a probability theory and method used to evaluate other related random processes based on observations of random process results.

Indeed, filter capacitors can reduce the AC resistance of a power supply. Adding a capacitor at the power supply output provides the current that rises and needs to be maintained in a very short time. The capacitor immediately reduces current and reverses the stored current to charge itself. During these brief periods, most current is exchanged directly within the capacitor instead of passing through the power supply’s internal resistance, which reduces the power supply’s AC resistance.

This is exactly how it works. Since power supplies have internal resistance and transmission lines also have impedance, modular capacitors allow part of the instantaneously varying current to be exchanged directly on the capacitor. This reduces the magnitude of current fluctuations on the circuit, which helps power the IC.

From this perspective, for an ideal power supply, the isolation capability cannot be ignored, as wires are not perfect.

In fact, all power sources have propulsion capability. As long as drive capacity is satisfied, output power will remain relatively stable. Output power requirements also impose certain limits on internal resistance, and these requirements vary. Internal resistance inevitably creates voltage drop. A regulated power supply can theoretically deliver stable rated output power within its output power range using a closed-loop control circuit.

The so-called AC resistance reduction mainly targets high-frequency AC resistance. Even if the closed-loop power supply is switched off, there are requirements for response speed and response. It cannot immediately track load current changes, leading to voltage fluctuations. Adding capacitors reduces instantaneous changes in internal power supply current and modifies the power supply characteristics.

When using a filtered power supply, we have discussed reducing AC resistance. In practice, lowering ripple at the output terminal is more important. For dry batteries, the output produces no ripple and filtering is unnecessary.

However, for complex circuits, adding capacitors is necessary to reduce AC resistance, especially close to IC power supply pins.

Filter circuits strip AC components from mixed AC and DC signals.

The AC power becomes pulsating DC after rectification. This type of DC power supply contains large AC ripple and cannot be directly used as a power supply for electronic circuits. A filter circuit can greatly reduce such AC ripple components and smooth the rectified voltage waveform.

Keywords

power supply filtering, DC filter circuit, ripple reduction, EMI filter, filter capacitor, pulsating DC, noise suppression, power supply noise, signal filtering, EMC filter