When energy emitted by a device, usually radio frequency energy, travels through space to reach a sensitive device, it is called radiated interference. The source of the interference can either be part of the disturbed system or a completely electrically isolated unit. Conducted interference arises because there is an electromagnetic or signal cable connection between the source and the sensitizer, and the interference travels along the cable from one unit to another. Conducted interference often affects the power supply to the unit, which can be controlled by filters. Radiated interference can affect any signal path in a device and is more difficult to shield.
The mechanism by which radiated electromagnetic energy becomes electromagnetic interference can be explained by Faraday's law. This law states that when a varying electric field is applied to a conductor, a current is induced in that conductor. This current is independent of the operating current, but the circuit will receive the current and respond as if it were the operating current. In other words, random RF signals can send commands to a computer to make program changes.
Technical DriversThere are a number of factors that make EMC an important element in the design of electronic equipment.
First, the growing number of electronic devices brings with it many sources of electromagnetic interference and sensitizers, which increase the potential for interference.
The miniaturization of equipment has brought sources and sensitizers into close proximity. This shortens the propagation path and increases the chance of interference. The miniaturization of devices increases their sensitivity to interference. As devices become smaller and more portable, devices such as car phones and laptops are available everywhere and are not necessarily confined to a controlled environment such as an office. This also raises compatibility issues. For example, many automobiles have extensive electronic circuitry, including antilock control systems, and if a car phone is not compatible with this control system, it can cause malfunctions.
Developments in interconnect technology have lowered the threshold for electromagnetic interference. For example, the lower supply voltage of large-scale integrated circuit chips lowers the internal noise threshold, and the lower levels of their fine geometry are damaged by arcing. Their faster synchronous operation produces sharper current pulses, which can create problems with broadband emissions from I/O ports. In general, high-speed digital circuits generate more interference than analog circuits.
Traditionally, electronic circuits were housed in metal boxes that provided shielding by cutting off the path of electromagnetic energy. Now, in order to reduce the weight and cost, more and more use of plastic chassis. Plastic cases are transparent to electromagnetic interference, so sensitive devices are left unprotected.
Changes in legislation are also driving forces. The implementation of mandatory standards to control electromagnetic emissions and sensitivities is forcing manufacturers to implement EMC programs. Product reliability regulations will make reliability a top priority, as manufacturers will be held liable if equipment malfunctions due to interference and causes injury. This is particularly important for medical devices.
Reliability has become an important market characteristic for electronic equipment in an increasingly competitive industry. Automated equipment, especially medical equipment, must work continuously, and this is where EMI shielding technology within the equipment improves reliability.
The requirement for data confidentiality is an important driver of the shielding market. There have been reports of information being stolen from the U.S. Embassy in Moscow by the former Soviet Union through the reception of electromagnetic energy generated by equipment within the Embassy. The same technique was used to intercept passwords and then attack bank computer systems. By shielding, the electromagnetic emissions from the equipment were able to be reduced, improving the security of the system.
Nowadays, people are becoming more aware of the health effects of various types of radiation. The dangers of excessive X-ray and ultraviolet exposure have been well documented.