ose to the bottom of the scale, whereas computers and instrumentation for Figure 23.16 Relative number of transients (percent) vs. control of critical systems such as fly by wire aircraft and nuclear maximum transient amplitude (volts) power systems come near the top. The purchasers and authorities 23/12 Safety and EMC responsible for such systems have recognised this for some years by specific Directives) have been Germany and the United States. and there is a well established raft of EMC requirements for them, In the US, emissions from digital devices are governed by FCC but in less critical sectors this is not the case and EMC performance (Federal Communications Commission) Rules part 15. A 'digital is mainly determined by market factors. device' is any electronic device that generates or uses timing signals Actually determining whether a piece of equipment has been or pulses exceeding 9kHz and uses digital techniques. There are affected by interference is not always easy. Interference may cause some quite broad exemptions from the rules depending on applica- a degradation in accuracy of measuring equipment, it may cause tion. Two classes are defined, depending on the intended market: noticeable deterioration in audio quality or it may corrupt a micro- class A for business, commercial or industrial use, and class B for processor program. If the processor circuit has a program recovery residential use. These classes are subject to different limits, class B mechanism this may correct the corruption before it is apparent, or being the stricter. Before being able to market his equipment in the the effects of the interference may be confused with a software US, a manufacturer must either obtain certification approval from glitch. Determining cause and effect in the operating environment the FCC if it is a personal computer, or must verify that the device is particularly difficult when the interference is transient or occa- complies with the applicable limits. sional in nature. When contemplating immunity testing, i