EMC (Electromagnetic compatibility)
Rationale of EMC
EMC is essential for the correct operation of various communication, computing and electrical equipment, in the real world environment with different noise sources both inside and outside the equipment. The engineer should realize that a product must be compatible within itself and with other equipment. Compatibility is achieved by: minimizing the noise sources, hardening the possible victim circuits, and blocking the coupling path between source of noise and victim. If EM interferences are suspected in a particular environment, EMC should address the risks to electronic equipment and human health. Electromagnetic interference can jam sensitive equipment, burn out electric circuits, and even prompt explosions. EMC can be implemented during the design stage or after production. Of course, designing in EMC is the best. Fig. 1 shows power absorption inside human head due to hand held device such as cellular phone, walkie-talkie, or cordless telephone.
Fig. 1: power absorption inside human head due to hand held device such as cellular phone, walkie-talkie, or cordless telephone.
Fig. 2: EMC test of a computer.
Importance of EMC
In modern society, EMC plays a central role for the digital circuitry, with their high switching speeds (increasing emissions) and lower circuit voltages (increasing susceptibility). So, nations became aware of EMC and issued regulations to the manufacturers of digital electronic equipment that should by applied before their equipment could be marketed. This regulatory environment led to a huge EMC industry that provides consultation services, testing devices, and analysis and design. A computer being tested for EMC compliance is shown in Fig. 2.
In addition, the increasing use of mobile communications and broadcast media channels has put huge pressure on the available frequency spectrum where sophisticated EMC design methods are increasingly needed, especially in digital communications, to control the cross-channel interference.
Examples of Interference
Digital systems, such as computers, tend to interfere with analog systems, such as voice and video communications. Computer clocks may have to be shielded and their output circuits may have to be filtered to prevent interference to communication equipment.
A solid shield is not practical since it does not allow light, air, and cables to pass through it, so shields with holes, as well as conductive glass may be needed. The widespread use of plastic enclosures has made thin film shields vital in achieving the needed shielding effectiveness. For interconnections, interference can result from improperly terminated or defective transmission lines.
Designing a PCB (printed circuit board) that eliminates most sources of RF interference (suppression) is the most cost-effective approach. The source of interference is the active element producing the original waveform. The PCB must be designed to keep the energy developed to only those sections that require this energy.
Few cases of electromagnetic interference are listed below.
- Five crashes of Blackhawk helicopters in USA shortly after their introduction into service due to electromagnetic interference, from very strong radar and radio transmitters, with the electronic flight control systems.
- Some passengers used electronic devices on board aircraft, including cellular phones, and pilots have reported anomalies with their navigation equipment that seem to correlate with the use of these personal electronics.
- Researchers in Netherlands conducted a study to assess incidents of electromagnetic interference by RFID on critical care equipment. RFID technology is used in hospitals to track medical supplies. Of the 123 tests conducted, researchers found 34 incidents of electromagnetic interference. The median distance at which the systems caused interference was 30 cm. Interference changed breathing machines' ventilation rates and caused syringe pumps to stop, among other things.
- Radio or TV interference occurs when a high frequency signal, from a common home appliance such as vacuum cleaner, distorts the incoming signal to the cable or antenna input of televisions and radios. Cordless telephones can also be affected.