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EMI Filters
EMI filters are passive electronic devices which block conducted electromagnetic interference generated by surrounding electronics. EMI, or electromagnetic interference, is the magnetic field which is created by electrical currents; EMI can travel along wires, electrical circuits and conductors, while radio frequencies (RF) are radiated electromagnetic “noise” which travels through the air as radio waves. EMI filters block the electromagnetic interference which is passed along through isolated power lines and power signals, diminishing electromagnetic “cross-talk” between various electrical devices and appliances running off the same power source. RF filters perform a similar function by blocking specific air-transmitted radio frequencies. Many power strips and high-use outlets have EMI filters built in, although EMI filters may be individually installed on any grounded electrical power source or circuit. Sound, lighting, medical and many other industries which rely heavily on electronic equipment use EMI filters to prevent equipment malfunction or lost communication signals.
Unlike all other types of EMI shielding products, EMI filters do not attempt to block EMI by enclosing electronic equipment in an EMI-absorbing material; instead, EMI filters block high electromagnetic frequencies, allowing normal low frequencies to pass. EMI filters are used to block electromagnetic interference which is conducted through power lines and signals and is not usually effective in blocking air-transmitted RFI. RF filters, on the other hand, are designed to block virtually any type of radio frequency which might interfere with electronic equipment performance; high-pass RF filters block all frequencies below a specific point; low-pass RF filters block all frequencies above a certain point, and band-pass filters are programmed to selectively filter all frequency bands other than those which are desired to pass. Usually attached or integrated into the power source or switch which is to be shielded, as with all types of EMI/RF shielding, EMI and RF filters work to both block incoming EMI/RF and outgoing EMI/RF. EMI filters can be an effective way to manage the EMI created on a specific power source or power line, although either RF filters or EMI shielding such as coating, enclosures and gaskets are still necessary to provide full protection from RFI, or electromagnetic interference conducted by radio waves.
Perforated or mesh EMI shielding materials are also referred to as EMI filters, although these are not the same as EMI and RF filters which block specific frequency transmissions. Often referred to as "honeycomb filters", mesh EMI filters are used in the same way coatings, enclosures, gaskets and various types of shielding are used to physically block EMI and RFI waves below a certain size; the size of the filter's holes determines which electromagnetic and radio frequencies are allowed to pass, and even large holes can block a considerable amount of EMI and RF. For this reason, magnetically permeable perforated metal, honeycomb configurations or screens are referred to as "filters".
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