Understanding and preventing radio frequency interference
Radio-based devices such as walkie-talkies and pagers have been used in our plants for many years but in recent years there has been an increased number of radio frequency sources both inside and outside our facilities. This combined with the greater clock speed of our microprocessor-based systems and increased use of digital communication systems complicates the electromagnetic environment in our facilities.
Our instrumentation systems must be able to function in the presence of high-frequency interference which is commonly referred to as radio frequency interference (RFI). It is also sometimes referred to as electromagnetic interference (EMI), though this is somewhat of a misnomer as EMI covers a wider range of frequencies.
RFI can be defined as objectionable high-frequency electromagnetic radiation where the source is further away than the radiation's wavelength (l) divided by 2p, i.e. l/2p.
The area past this distance is called the far field and the radiation is called a plane wave. Shorter than this distance is called the near field and electric or magnetic fields will dominate the interference coupling mechanism. The effectiveness of coupling RFI into a system is a function of the radiating source, its strength, the characteristics of the transmission path, the distance involved, and the sensitivity of the receiver.
Four basic methods can minimize RFI effects:
Eliminate the radiating source;
Shield either the source or the receiver;
Separate distance; and
Improve circuit design.
The basic method for getting rid of a RFI source is to remove the radiating mechanism. Conversion of electromechanical contacts, for example, to solid state would remove the arc-generated RFI. Electrostatic discharge (ESD) generated RFI could be reduced by removing the charge generating mechanism or by providing a method to bleed off the charge.
Shielding is probably the most common means used to reduce the effects of RFI. Shielding can work both to prevent RFI from radiating out or to prevent RFI from getting in. The effectiveness of a shield is a function of the material, the frequency, the angle of incidence, coverage, and the thickness of the material. Metal is commonly used to shield RFI. Plastic materials used as shields are coated or impregnated with reflective and adsorptive materials or have embedded screens.
Often it is the enclosure openings, seams, and joints that are the limiting factors of the shield's effectiveness. The longest dimension of any opening should be less than l/20. Reduction in opening dimensions, screens, coatings, and special gasketing are some of the methods used to prevent RFI from getting into or out of enclosures. Cables in metal conduit are generally protected against RFI, but cables in a cable tray may open up windows of exposure. The effectiveness of cable shields such as aluminum foil, braided, and coaxial is a function of the material, frequency, thickness, and the shield coverage.
Distance can be used to provide separation between RFI generating equipment and the sensitive equipment by reducing the field strength of the RFI at the receiver. Administrative controls can also be used to prohibit RFI sources from being operated near sensitive equipment.
Circuit design can also help. Some common techniques used to minimize the effects of RFI include component location, conductor lengths, and component selection as well as the use of differential inputs, twisted pair cabling, common mode chokes, and ferrite beads.
RFI will be an increasing concern in the future. A good reference in this area is the book: Noise Reduction Techniques in Electronic Systems by Henry W. Ott.
EMI: Electromagnetic interference, electrical interference from electric, magnetic, or plane wave fields.
ESD: Electrostatic discharge, the discharge of free electrons from an insulator to a conductor or to another insulator.
Far field: The area away from a receiving device greater than the incident radiation’s wavelength divided by 2p.
Near field: The area nearer to a receiving device less than the incident radiation’s wavelength divided by 2p.
Plane wave: Electromagnetic radiation where the source is further away than the radiation’s wavelength divided by 2p.
RFI: Radio Frequency Interference, electromagnetic radiation where the source is further away than the radiation’s wavelength divided by 2p, which is generally in the range of a few 100 kHz to the GHz.
Shield: A material placed between unwanted electromagnetic fields and a receiving or transmitting device with the purpose of minimizing the transmission of the electromagnetic fields past the shield.