Part 2: EMI-RFI Transfer Mechanisms
Electromagnetic Interference is caused by undesireable
radiated electromagnetic fields or conducted voltages and currents. The interference is
produced by a source emitter and is detected by a susceptible victim via a coupling path.
The coupling path may involve one or more of the following coupling mechanisms:
1. Conduction - electric current
2. Radiation - electromagnetic field
3. Capacitive Coupling - electric field
4. Inductive Coupling - magnetic field
If either the electromagnetic source, coupling means, or victim is missing, EMI cannot occur. Although shielding or distance separation may solve some interference
problems by reducing the coupling and susceptibility mechanisms, the most effective approach is to avoid the problem at the source through proper circuit design.
Conducted noise is coupled between components through interconnecting wires such as through power supply and ground wires. Common impedance coupling is caused when currents from two or more circuits flow through the same impedance such as in power supply and ground wires.
Radiated electromagnetic field coupling may be treated as two cases. In the near field, E and H field coupling are treated separately. In the far field, coupling is treated as a plane wave coupling.
Electric field coupling is caused by a voltage difference between conductors. The coupling mechanism may be modeled by a capacitor.
Magnetic field coupling is caused by current flow in conductors. The coupling mechanism may be modeled by a transformer.
Some typical external noise sources into a radio receiver include radiated electric field coupling from: high-voltage power lines, broadcast antennas, communications transmitters, vehicle ignition systems and electric machinery.
Most conducted coupling from external sources occurs through the ac power lines.
Typical radio interference to other equipment includes radiated electric field coupling to: TV sets, broadcast receivers, telephone lines, appliances, and communications receivers.
Most conducted coupling to other equipment occurs through the ac power lines.
The most common methods of noise reduction include proper equipment circuit design, shielding, grounding, filtering, isolation, separation and orientation, circuit impedance level control, cable design, and noise cancellation techniques.
Electromagnetic radiation involves electric (E) and magnetic (H) fields. Any change in the flux density of a magnetic field will produce an electric field change in time and space (Faraday's Law). This change in an electric field causes another change in
the magnetic field due to the displacement current (Maxwell). A time-varying magnetic field produces an electric field and a time-varying electric field results in a magnetic field.
This forms the basis of electromagnetic waves and time-varying electromagnetics (Maxwell's Equations). Wave propagation occurs when there are two forms of energy and the presence of a change in one leads to a change in the other.
Energy interchanges between electric and magnetic fields as the wave progresses.
Electromagnetic waves exist in nature as a result of the radiation from atoms or molecules when they change from one energy state to another and by natural fluctuations such as lightning. The technology of generating and processing electromagnetic waves forms the basis of telecommunications.
Electromagnetic Effects (EME) includes many electromagnetic environmental disciplines such as Electromagnetic Compatibility (EMC), Electromagnetic Interference (EMI), and Electromagnetic
Electromagnetic Interference (EMI) is electromagnetic energy that adversely affects the performance of electrical/electronic equipment
by creating undesirable responses or complete operational failure. The interference sources may be external or internal to the electrical or electronic equipment and they may propagate by radiation or conduction. This discipline includes Radio Frequency Interference (RFI), the term which was originally used to describe most electrical interference. EMI is usually divided into two general categories to help in analyzing conducted and radiated interference effects: narrowband and broadband.
Narrowband Emissions - a narrowband signal occupies a very small portion of the radio spectrum. The magnitude of narrowband radiated emissions is usually expressed in terms of volts per meter (V/m). Such signals are usually continuous sine waves (CW) and may be continuous or
intermittent in occurrence. Communication transmitters such as single-channel AM, FM and SSB fall into this category. Spurious emissions, such as harmonic outputs of narrowband communication transmitters, power-line hum, local oscillators, signal generators, test equipment, and
many other man made sources are narrowband emissions.
Broadband Emissions - a broadband signal may spread its energy across hundreds of megahertz or more. The magnitude of broadband radiated emissions is usually expressed in terms of volts per meter per MHz (V/m/MHz). This type of signal is composed of narrow pulses having relatively short rise and fall times.
Broadband signals are further divided into random and impulse sources. These may be transient, continuous or intermittent in occurrence. Examples include unintentional emissions from communication and radar transmitters, electric switch contacts, computers, thermostats, motor speed controls,
thyratron circuits, ignition systems, voltage regulators, pulse generators, arc/vapor lamps, and intermittent ground connections. They may also result from galactic and solar noise, lightning electromagnetic pulses, and by radio frequency pulses associated with electrostatic discharges.
Electromagnetic Compatibility (EMC) is the ability of electrical or electronic equipment/systems to function in the intended operating environment without causing or experiencing performance degradation due to unintentional EMI.
It is recommended that the performance be tested or qualified to insure operation within a defined margin of safety for the required design levels of performance. The EMI source minus the coupling mechanism path losses should result in an emission level that is less than the victim's susceptibility threshold minus a predetermined safety margin.
The goal of EMC is to minimize the influence of electrical noise.
Electronic equipment can malfunction or become totally inoperable if not designed to properly minimize the effects of interference from the internal and external
electromagnetic environments. Proper equipment and system designs are also necessary for minimizing potential electromagnetic emissions into the operating environment.