How to Eliminate Electromagnetic Interference

EMI comes from the radio part of the electromagnetic spectrum. Electronic equipment tends to pick up undesirable signals such as radio and television broadcasts as well as those produced by fluorescent lights and other nearby appliances. Wires in the equipment act like antennas that induce small currents from these signals; these currents are strong enough to interfere with audio gear, televisions, computers and other devices. Equipment design specialists spend considerable time ensuring that EMI does not become a problem.

A key tactic in eliminating EMI is the use of earth ground, such as a direct connection to an electrical outlet ground or a cold-water pipe. A grounding point acts like a sink into which the energy in EMI drains. Because EMI follows the path of least resistance, it flows preferentially though a heavy wire leading from a metal equipment chassis to ground rather than through electronic circuits to a weaker ground connection. This is one reason why professional electronic equipment has a three-wire power plug -- the grounding wire helps reduce EMI.

A metal shield or screen will block EMI, especially if the barrier is connected to ground. Audio and video cables, for example, consist of an inner conductor surrounded by a braided outer shield; EMI enters the shield and flows directly to ground instead of interfering with the signals the cable is carrying. A grounded metal equipment chassis also acts as an EMI shield for circuits inside the cabinet. The ultimate form of EMI shield is a box called a Faraday cage. The cage consists of a grounded metal sheet, hardware cloth or wire; it completely surrounds electronic devices, effectively blocking any external EMI. Faraday cages see use in science, medicine and industry for carrying out sensitive electronic measurements.

Long audio cables, such as those for guitars and microphones, tend to pick up buzzing noises from EMI. A connection called a "balanced line" solves this problem by using two wires carrying signals equal in strength but of opposite polarity. Any EMI entering the cable appears as a positive signal on both wires. At the receiving end, a circuit subtracts the two signals, substantially reducing EMI noise.

Optical fiber cables are completely immune to EMI because they carry pulses of laser light instead of electrical signals. Radio and electrical interference have no effect on optical signals. Although fiber optics can be expensive relative to copper wire, many audio and data devices now have light-based inputs and outputs.