hether the essential re- distribution may vary depending on the operating mode of the circuit or its resident software. quirements have been satisfied. Emissions from some classes of digital equipment, such as per- The essential requirements are that: sonal computers, have been regulated by some countries, again to control interference to broadcast and communication services. 'The apparatus shall be SO constructed that: Equipment emissions can be either conducted or radiated. Com- mercial emission standards differentiate between the two on the (a) Equipment shall not generate electromagnetic disturbances ex- basis of frequency; the breakpoint is universally accepted to be ceeding a level allowing radio and telecommunications equip- 30MHz since it has in fact been found empirically that the coupling ment and other apparatus to operate as intended. mechanisms are predominantly by conduction below 30MHz, and (b) Equipment shall have an adequate level of intrinsic immunity by radiation above it. from electromagnetic disturbances.' The regulation of emissions is intended only to reduce the threat to innocent 'victim' receivers. Some reasonable separation of the Thus protection is extended not only to radio and telecomms but offending emitter and the victim is assumed. If a personal computer also to other equipment such as information technology and medical is placed right next to a domestic radio set one can still expect there equipment, in fact any equipment which is susceptible to electro- to be interference. Regulations have nothing to say on the subject of magnetic (EM) disturbances. intra-system interference, SO that it is quite possible for two com- The second requirement states that the equipment should not puter systems, both of which meet the appropriate standard, to be malfunction in whatever hostile EM environment it may reasonably incompatible when they are placed together in the same rack or be expected to operate. cabinet. The range of EMC phenomena covered by the directive includes radiated emissions as