Identifying the offending interference in a system can be difficult. To help in establishing a common language as a basis for understanding interference problems, a discussion of the effects of different types of interference and the possible methods of controlling them follows.

Electromagnetic Interference (EMI) sometimes seems to be the general catch-all term. In fact, it is a generic term for unwanted interference energies conducted as currents or radiated as electromagnetic fields. Radio frequency interference (RFI) is the older term used interchangeably in particular by the regulatory agencies. The Federal Communications Commission (FCC) and other agencies, such as West Germany's Verband Deutscher Elektrotechniker (VDE) have established rules governing EMI emissions.

The FCC Part 15 Subpart J (Docket 20780) deals with electronic data processing equipment, and applies to devices that generate pulse per second and use digital techniques. Equipment covered under Subpart J is broken down into two classes:

Class A: Covers computing devices used in commercial and industrial and business applications.

Class B: Applies to computing devices used in a residential environment.

As Figure 17 (see below) demonstrates Class B limits as much more stringent, requiring additional design effort to assure compliance. Every component and subsystem must be carefully scrutinized ahead of time to assure that the goals will be met.

The main design goal is to meet and, if possible, reduce emissions well below Class B, provided costs are within tolerable limits. Frequently, careful design and layout will yield these results at no extra cost. EMI currents in the fan power leads are referrer to as conducted EMI and are usually more of a problem than radiated EMI. In fact, when dealing with brushless DC fans, conducted EMI is normally the only concern. Different mode EMI is conducted on the power leads of the fan from the power switching transistors used for switching the motor windings.

Radiated EMI is generated by voltages and currents within the circuitry. If not properly suppressed or shielded, it may then be radiated by cables or structures which act as antennae. The FCC has placed limits on conducted EMI on power lines from .45 MHz through 30 MHz.

Equipment to measure and establish compliance is expensive and verification of compliance is best accomplished by outside facilities specializing in EMI and EMC testing. The engineer can get a fairly good idea of how well the EMI is suppressed in the fan by analyzing the ripple across the power leads with as oscilloscope.

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