Overview of EMC/RFI Issues
The phenomenon of electromagnetic interference (EMI) is familiar to virtually everyone, even if they do not understand the underlying principles. Most people have witnessed first-hand the effects of interference. To control EMI, government organizations, such as the FCC, CSA, and EEC, mandate that manufacturers may not design, produce or sell electronic equipment that jams the public broadcast services. In other instances, however, EMI can constitute more than a mere nuisance. The military and medical communities, for example, require trouble-free operation of their electronic equipment in adverse electromagnetic environments since malfunctions could jeopardize missions and personnel. The European Union’s EMC directive also mandates that “the apparatus has an adequate level of intrinsic immunity to electromagnetic disturbance to enable it to operate as intended”.
EMC Design of High Speed Systems
The interference and susceptibility (immunity) effects of electronic apparatus are created by time-variant electromagnetic fields which may be propagated along a conducting medium or by radiation through space. Because the source of the conducted and radiated interference energy levels may be related, a coordinated systems design effort is required to reduce these effects. A design program for an equipment item that must meet both an emission and an immunity requirement consists of:
- Suppression: Reducing the interference at its source.
- Isolation: Isolating the offending circuits by filtering, grounding and shielding.
- Desensitization: Increasing the immunity of any susceptible circuits.
These three steps should be carried on throughout the entire equipment design and implemented as early as possible within the design program.
Effects of Logic Speed
The trend in today’s electronic devices is faster, smaller, and digital rather than analogue. Most equipment (95%) of today contains digital circuits. Today’s digital designer must create a circuit board that has the lowest possible EMI, combined with the highest possible operating/processing speeds. Design of the PCB is the most critical EMC influencing factor for any system, since virtually all active devices are located on the board. It is the changing current (accelerated electron movement) produced by the active devices that result in EMI.
There are two approaches that can be used to reduce the emission from the PC board. The first approach is to operate the circuit at the slowest speeds consistent with the functionality of the system, lay out the PCB with the smallest possible loop areas (especially the high speed devices), and insert suppression components such as filters, ferrite beads, and bypass capacitors into the circuit to reduce its bandwidth. These techniques will result in a desired decrease in the high frequency harmonic amplitudes and circuit bandwidth and a corresponding undesired decrease in both the operating speed and system reliability. The use of slower speeds with reduced bandwidth will help to desensitize the circuit to external susceptibility fields. The second is to use shielding. Shielding is the only non-invasive suppression technique. Since the shielding is not inserted into the circuit, it does not affect the high frequency operating speed of the system, nor does it affect the operation of the system should changes be made to the design in the future. In addition, shielding does not create timing problems and waveform distortion; it does not decrease system reliability; and it reduces crosstalk. Plus, shielding works for both emission suppression as well as susceptibility (immunity) problems. Even with the overall advantages of shielding, the most cost-effective approach is to use a combination of circuit suppression/hardening and shielding.
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