LAIRD EMI ESSENTIALS

LAIRD EMI INTRODUCTION

LAIRD 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 firsthand 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”.

LAIRD 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.

LAIRD Effects of Logic Speed

The trend in today’s electronic devices is faster, smaller, and digital rather than analog. 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.

LAIRD Design Approaches

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.