What Is an EMI/RFI Filter?
An EMI/RFI filter is a filter that blocks noise generated by electronic devices to protect other electronic devices from being adversely affected.
EMI/RFI filters eliminate noise when electromagnetic noise is present in the signals that are connected to and transmitted through the wiring of a circuit board or other device. Although EMI/RFI filters can be used by themselves to eliminate noise, they can also be used simultaneously with shields, common choke coils, and surge absorbers to ensure accurate signal transmission.
Note that EMI stands for “Eloctro Magnetic Interference”, which in Japanese means emission and electromagnetic interference radiation regulation.
Uses of EMI/RFI Filters
EMI/RFI filters are mainly used in the overall electrical circuits of devices that receive or transmit signals. They serve the purpose of eliminating noise from various sections, including the measuring and receiving components in measurement instruments and radars within production facilities, as well as the transmitting sections in base stations and satellites. They also eliminate noise from signals emitted by base stations and satellites.
Since there are many different types of noise, care must be taken to ensure that the EMI/RFI filter is compatible with that noise. Also, each EMI/RFI filter product differs in its noise rejection accuracy and method, so it is necessary to select the appropriate one.
Principle of EMI/RFI Filters
EMI/RFI filters use various electronic components to remove noise. Typical electronic components used in EMI filters are capacitors and inductors.
1. Capacitor
Capacitors function as low-pass filters when connected in parallel to the load of a circuit. The characteristic of the impedance of a capacitor is that it becomes smaller at higher frequencies.
In other words, the higher the frequency, the easier it is for current to flow through the capacitor and the harder it is for current to flow through the load. The capacitance of the capacitor also determines the frequency at which it is removed. The higher the impedance of the circuit used, the better the capacitor can function as a filter.
2. Inductor
An inductor functions as a low-pass filter when connected in series with a load in a circuit. The principle is based on the characteristic that the impedance of an inductor, contrary to the characteristic of a capacitor, increases as the frequency increases. The higher the frequency, the harder it is for current to flow through this circuit due to the impedance of the inductor.
Other Information on EMI/RFI Filters
1. How EMI/RFI Filters Work
When placed in the conduction path of a radio wave, a filter selects the signal and noise necessary for the operation of the circuit and removes only the noise. In selecting signals and noise, criteria are needed to separate the two.
EMI/RFI filters use the bias of the frequency distribution to separate noise. For the target radio noise, low-frequency waves are treated as signals, high-frequency waves are treated as noise, and low-frequency waves are passed through, thus functioning as a low-pass filter.
There are four types of filters that separate signal and noise by frequency distribution: low-pass filters, high-pass filters, band-pass filters, and band-elimination filters. Low-pass filters are often used in EMI/RFI filters because it is often difficult to narrow down the target noise frequency first.
In addition to frequency distribution, noise separation also uses propagation mode and voltage differences for common mode choke coils and voltage differences for surge absorbers.
2. Relationship Between EMI, EMS and EMC
EMS and EMC are similar terms to EMI. As we explained above, EMI suppresses noise emitted from equipment.
EMS refers to “Electromagnetic Susceptibility” that withstands noise emitted from other devices, and devices that have both EMI and EMS are called “Electro Magnetic Compatibility Electro Magnetic Compatibility Electromagnetic Compatibility