What Is a Line Filter?
A line filter is a filter that is inserted into the power supply line of a device to eliminate noise generated by the device and noise coming in from external devices. They are also called noise filters because they are used to remove noise.
Generally, noise emission is called EMI (Electromagnetic Interference), and noise radiation from external devices is called EMS (Electro Magnetic Susceptibility: EMS). In addition, various types of equipment must be designed to suppress their noise generation and not be affected by noise from peripheral equipment. This is called EMC (Electro Magnetic Compatibility).
Line filters are used to achieve this EMC and are used either in the power supply line inside the equipment or outside the equipment.
Uses of Line Filters
Line filters are a type of low-pass filter. This is because the main component of noise in digital equipment is the high-frequency waves generated by the various digital ICs mounted on the printed circuit boards of the equipment.
It is effective to install line filters as close as possible to noise-generating equipment. The greater the distance, the more likely it is that noise will propagate through the space between them.
The ground wire coming from the line filter should be as short as possible. If it is longer, the noise rejection effect may be reduced by half.
Principle of Line Filters
Noise generated by electronic and electric circuits is becoming increasingly higher in operating frequency as the scale of circuits grows larger and more sophisticated. As a result, the noise generated is also becoming higher frequency, and its removal is becoming more and more difficult.
There are two types of noise generated by devices: radiation noise and propagation noise. Radiated noise is a type of noise in which high-frequency signals generated in circuits propagate through the air and interfere with other equipment.
Propagation noise is the type of noise that interferes with other devices via the power cables and signal cables of devices. These types of noise can lead to malfunctions of electronic and electrical equipment or, in the worst case, to equipment destruction.
Line filters to eliminate such noise consist of coils, capacitors, and ferrite cores.
Coils and ferrite cores allow low-frequency signals to pass through these devices unconditionally. However, the higher the frequency of the signal, the more difficult it is to pass through. By directly connecting these coils and ferrite cores to the signal line, noise radiation is suppressed.
A capacitor, on the other hand, does not allow low-frequency signals to pass through but has the characteristic of allowing signals to pass through more easily as the frequency increases. By placing this capacitor between the signal line and ground, high-frequency signals are discarded on the ground side, thereby suppressing noise radiation together with the coil and ferrite core.
Furthermore, by connecting multiple low-pass filters consisting of the coil, ferrite core, and capacitor in a vertical connection, the frequency selectivity can be further increased. In other words, the desired low frequencies can be passed through more easily and unwanted high frequencies can be eliminated.