What’s a Line Filter?

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Line filters reduce electromagnetic interference (EMI) or radio frequency interference (RFI) in electronic devices. All electronic devices emit electromagnetic fields, which can interfere with communication signals. Line filters are usually capacitors that recognize and stop interfering signals, diverting them harmlessly into the device’s ground. Without line filters, interference can disrupt communication signals.

A line filter is an electronic device that is part of many electrically powered appliances and machines. Its purpose is to reduce, remove, or deflect electromagnetic interference (EMI) or radio frequency interference (RFI). Any electrical device can generate an electromagnetic field. Many devices can pick up these fields, creating interference with radio frequencies used by communications equipment. A line filter will eliminate this interference, allowing for a purer signal to be transmitted.

Communication devices, such as televisions, telephones, and radios, work by converting sound and visual information into electronic signals. These signals are then transmitted to receiving devices, which convert them back into understandable communications. While in electronic form, these signals are part of a larger energy field known as the electromagnetic spectrum. All electronic devices, including power lines, also emit electromagnetic fields as a result of their normal operation. A line filter must be used to separate these accidental electromagnetic signals.

When the first home entertainment and communication devices were introduced, there were generally few other electronic devices in the immediate environment. In modern times, every place will have more electrical devices, including computers, kitchen appliances, and power tools among others. Any of these machines can create electromagnetic interference. Most countries have regulations that limit the amount of interference a manufactured device can create. An examination of the device often reveals a notice describing applicable regulations.

Even a device that isn’t directly connected to a power line, such as a cell phone, is still connected to an internal battery. The signal arriving at the telephone, meanwhile, is often transmitted over the telephone lines before reaching a transmitting tower. Any of these sources or power lines will generate electromagnetic fields which can create interference. The most common type of line filter is used to prevent interference from device power connections. Another type prevents interference from outside power lines or other devices that may be nearby.

A line filter is usually a type of electronic component called a capacitor. A capacitor is a passive electronic circuit, integrated into the device, made up of conductors separated by an insulator. A line filter capacitor is designed to allow the desirable signal to pass unimpeded by recognizing the interfering signal and stopping it. It often diverts this interfering signal into the “ground” of the device, where it dissipates harmlessly. When a line filter fails or is not present, interference can be detected as audible or visual static, which, if severe enough, can completely disrupt the communication signal.

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