Sources of interference

Since natural interference phenomena do not really affect the modern equipment, the actual problems today are generated accidentally by the actual operation of the equipment. Basically, the radiated interfering signals are classified in a narrow band and broad band. Narrow band signals occupy a small portion of the radio spectrum and have the energy concentrated in a single frequency wave. However, when modulation is introduced, the narrow band signal may generate side bands of energy which may cover hundreds of kilohertz. Examples of sources for narrow band signals are radio & TV transmitters, radio transceivers, cellular telephonic equipment and Doppler radar. These sources start out with very low harmonic frequency outputs but can cause harmonic frequencies to be generated if they are used in areas which can present non-linear conditions to the RF energy through secondary transmission of these signals.

Broad band signals are those whose energy is spread over tens of hundreds of Megahertz. They are generated by narrow pulses with sharp rise times, characteristic of radars, gas discharge tubes, engine ignition systems, power line discharges, computer clocking pulses, motor brushes and switching regulators. The steepness of the pulses causes problems because the short rise times mean very high frequencies and that which may appear to be a low impedance can actually have a high inductive component and be a high impedance for the rise time.

Oscillator circuits on printed circuit boards can cause energy to be both conducted and radiated. These oscillator sources may be part of the power supply (as in switching regulators) or they may be part of the logic clock circuits. Other source of radiation could be amplifier circuits with a high slew rate, which can have very fast rise times and, if not terminated properly, can cause a large spectrum of noise. Actually, every frequency source could be a potential source of interference and all interference signals can be coupled to the power line and conducted to the power mains, thus creating problems to other equipments connected close to the signal source

Using ferrite cores for EMI suppression

Based on the specific magnetic properties of the Nickel-Zinc ceramic ferrite materials, molded cores, when used as EMI filters, absorb the energy of the high frequency noise on the line and dissipate it as quantities of heat. Since the electrical resistance component of the material is reduced at low levels of frequency, the ferrite cores provide very low series impedance and do not affect the normal data signals on the line.