IEEE ANSI C63.12-1999 pdf download American National Standard Recommended Practice for Electromagnetic Compatibility Limits
3.8 noise amplitude distribution (NAD): A distribution showing the pulse amplitude that is equaled or exceeded as a function of pulse repetition rate.
3.9 random noise: Electromagnetic noise, the values of which at given instants are not predictable.
NOTE—The part of the noise that is unpredictable except in a statistical sense. The term is most frequently applied to the limiting case in which the number of transient disturbances per unit time is large, so that the spectral characteristics are the same as those of thermal noise. Thermal noise and shot noise are special cases of random noise.
4. Description of environmental radio noise
The minimum level required for satisfactory reception of desired radiated signals is determined by the level of environmental radio noise or undesired signals with which the desired signal must compete. Several types of radio noise may influence reception and consequent equipment operation; however, with a particular sys- tem and environment one type will generally predominate at a given time, especially if the receiving equipment is located physically near a specific source.
Sources of radio noise usually are divided into two general groups, those producing wide bandwidth and those producing narrow bandwidth noise, in which the distinction is usually based on comparison with the bandwidth of a typical receiver. Wide bandwidth noise is frequently impulsive and can be divided further into two groups, natural and man-made. Narrow bandwidth noise is usually generated by a variety of restricted radiation devices. These include industrial, scientific, and medical (ISM) equipments, licensed radio transmitters, and digital devices that produce line spectra at harmonics of the clock frequency. Restricted radiation devices generally radiate radio frequency energy over a limited portion of the spectrum clustered around discrete frequencies. Licensed radio transmitters radiate a noise spectrum near their carrier frequencies. The rapid proliferation of low-power portable transmitting sources has increased the need for consideration of the immunity (susceptibility) of electronic systems that must operate satisfactorily in close proximity to these sources.
To the extent that radio noise varies, a time domain statistical description is necessary to characterize it. Just how much detail is needed in the description depends upon the nature of the source, the desired accuracy of predicting degradation, and the information bandwidth of the system with which it may interfere. For many man-made sources the noise can be characterized as stationary whereas for natural sources, the noise may occur with variations having time periods ranging from fractions of a second to a year or more.