Elements of the Electromagnetic Environment

When an EMI problem occurs, the test or design engineer needs to approach the situation logically. A simple EMI model has three elements:

1. There must be a source of energy.
   
   
2. There must be a receptor that's upset by this energy when the intensity of the electromagnetic interference is above a tolerable limit.
   
   
3. There must be a coupling path between the source and receptor for the unwanted energy transfer. For interference to exist, all three elements have to be present.

If one of the three elements is removed, there can be no interference. It therefore becomes the engineer's task to determine which is the easiest element to remove. Generally, designing a PCB (printed circuit board) that eliminates most sources of RF interference is the most cost-effective approach (called suppression). 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 of the assembly that require this energy. The second and third elements tend to be addressed with containment techniques. Illustration 1 (below) provides a breakdown of the relationship between these three elements and presents a list of items associated with each element.

NATURAL Terrestrial Atmospheric Triboelectric Extraterrestial Sun Cosmic Radio stars HUMAN-MADE Communications Broadcast Navigation Radar Two-way radio Industrial Arc welders Ultra cleaners RF induction heaters Fluorescent lights Medical CAT scanners Diathermy Home Shavers HV insect killers Microwave oven Computing devices	RADIATION Far-field Plane wave Near-field Capacitive crosstalk Inductive crosstalk Forward crosstalk Backward crosstalk CONDUCTION Power distribution Signal distribution Ground loops	BIOLOGICAL Human Animal Plants HUMAN-MADE Communications Broadcast receivers Navigation receivers Radar receivers Two-way receivers Industrial Controllers Amplifiers Medical Biomedical sensors Ordinance EEDs Dynamic caps Computing devices Line receivers Power supplies Disk drives Video amplifiers

With respect to PCBs, note the following:

• Noise sources are frequency generation circuits, component radiation within a plastic package, incorrect trace routing, ground bounce from digital logic, and common-mode currents developed within an assembly.
  
  
• The propagation path is the medium that carries the RF energy, such as free space or interconnects (common impedance coupling).
  
  
• Receptors are devices that easily accept interference from I/O cables, or by radiated means, transferring this harmful energy to circuits and devices susceptible to disruption.

Generally speaking, a product must be designed for two levels of performance: one to minimize RF energy exiting an enclosure (emissions), and the other to minimize the amount of RF energy entering (susceptibility or immunity). Both emissions and immunity are transmitted by radiated or conductive means. This relationship is shown in Illustration 2 (below). In addition, the engineer should realize that a product must be compatible within itself, i.e., emission levels must not compromise the performance of sensitive segments within the product. When dealing with emissions, a general rule-of-thumb is:

The higher the frequency, the greater the efficiency of a radiated coupling path; the lower the frequency, the greater the efficiency that a conducted coupling path will cause EMI. The extent of coupling depends on the frequency of the circuit and edge rate transition of digital components switching ogic states and transfer mechanism.

above: (Illustration 2) Variants of EMI coupling paths

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