The Nature of Electromagnetic Interference

EMC (electromagnetic compatibility) is grouped into two categories: internal and external. The internal category is the result of signal degradation along a transmission path, including parasitic coupling between circuits in addition to field coupling between internal subassemblies, such as a power supply to a disk drive. Stated more specifically, the problems are signal losses and reflections along the path, along with crosstalk between adjacent signal traces.

External problems are divided into emissions and immunity. Emissions derive primarily from harmonics of clocks or other periodic signals. Remedies concentrate on containing the periodic signal to as small an area as possible, blocking parasitic coupling paths to the outside world.

Susceptibility to external influences, such as ESD (electrostatic discharge) or RFI (radio frequency interference), is related initially to propagated fields that couple into I/O (input/output) lines, which then transfer to the inside of the unit, and secondarily to case shielding. The principal recipients are high-speed transmission lines and sensitive adjacent traces, particularly those terminated with edge-triggered components.

There are five major considerations when performing EMC analysis on a product or design:

1. Frequency: Where in the frequency spectrum is the problem observed?
   
   
2. Amplitude: How strong is the source energy level, and how great is its potential to cause harmful interference?
   
   
3. Time: Is the problem continuous (periodic signals), or does it exist only during certain cycles of operation (e.g., disk drive write operation or network burst transmission)?
   
   
4. Impedance: What is the impedance of both the source and receptor units, and the impedance of the transfer mechanism between the two?
   
   
5. Dimensions: What are the physical dimensions of the emitting device that cause emissions to occur? RF currents will produce electromagnetic fields that will exit an enclosure through chassis leaks that equal significant fractions of a wavelength or significant fractions of a "rise-time distance." Trace lengths on a PCB have a direct relationship as transmission paths for RF currents.

Whenever an EMI problem is approached, it's helpful to review the list above based on product application. Understanding these five considerations will clarify much of the mystery of how EMI exists within a PCB. Applying these five items informs us that design techniques make sense in certain contexts but not in others. For example, single-point grounding is excellent when applied to low-frequency applications, but it's completely inappropriate for radio frequency signals, which is where most of the EMI problems occur. Many engineers blindly apply single-point grounding for all product designs, without realizing that additional and more complex problems are created using this grounding methodology.

When designing a PCB, we are concerned with current flow within the assembly. Current is preferable to voltage for a simple reason: current always travels around a closed-loop circuit following one or more paths. It's to our advantage to direct or steer this current in the manner that's desired for proper system operation. To control the path that the current flows, we must provide a low-impedance, RF return path back to the source of the energy. We must also divert interference current away from the load or victim circuit. For those applications that require a high-impedance path from source to the load, consider all possible paths through which the return current may travel.

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