CHAPTER 2. TYPES AND CAUSES OF EMC PROBLEMS

TM 11-490-5

CHAPTER 2

TYPES AND CAUSES OF EMC PROBLEMS

traceable to either "thermal agitation" or "shot effect."

2-1. General

Thermal-agitation noise is caused by the unequal flow of

a. Electromagnetic interference (EMI), commonly

electrons across a circuit element from the effects of

called "interference," is defined herein as any

heat, resulting in a net voltage of random variation.

electromagnetic

energy

which

degrades

the

Shot effect is caused by the random emission of

performance of C-E equipment.

electrons from an electron tube cathode. Noise in a

b. EMC problems resulting from interference are

semiconductor device is primarily caused by the random

directly related to three factors: (1) interference sources,

motion of majority carriers crossing a junction and by

(2) transfer media, and (3) equipment susceptibility.

fluctuations in the relative amounts of current flowing

from the emitter to the collector and base of a transistor.

2-2. Interference Sources

The noise power level of a semiconductor device is

a. General. Interference may result from either

greater at the lower frequencies and depends upon the

natural or manmade causes (fig 2-1). While the effects

material of the semiconductors used. The types of

of interference from natural sources are geographically

noise described are strictly-speaking man-made,

widespread, manmade interference (intentional or

because the equipment is man-made. However, they

unintentional) usually affects only limited areas near the

are described here as "natural" to make a point. They

specific sources. An exception is high frequency (HF)

do-not arise from fault conditions or from equipment

interference which may emanate from distant radiations

maladjustment or any other conditions which can be

which have been refracted from the ionosphere.

remedied by operating personnel.

Nor are they

b. Natural Interference Sources. These sources

intentionally generated. The types of noise described

may be as close to the operator as his own equipment

are "natural" in the sense that they are always present to

and the earth's atmosphere, or as distant as the sun and

varying degrees in all energized electronic equipment

the outer reaches of the galaxy.

and the extent to which they can be minimized in

practice, is dependent on the funds available during the

design and construction stages of the equipment.

(1) Equipment noise.

This noise, self-

generated within electronic equipment, is generally

2-1