11

04/00

s

Unsymmetrical interference:

– This term is used to describe interference on a single line, relative to the reference potential

(fig. 1c)

1.4.2

Characteristics of interferences

In order to be able to choose the correct EMC measures, we need to know the characteristics of the

interferences, how they are propagated and the mechanisms by which they are coupled into the cir-

cuit. In principle, the interferences can also be classified according to their range (fig. 2). At low fre-

quencies, it can be assumed that the interference only spreads along conductive structures, at high

frequencies only by means of electromagnetic radiation. In the MHz frequency range, the term cou-

pling is generally used to describe the mechanism.

Analogously, conducted interference on lines at frequencies of up to several hundred kHz are main-

ly symmetrical (

differential mode), at higher frequencies, they are asymmetrical (common mode).

This is because the coupling factor and the effects of parasitic capacitance and inductance between

the conductors increase with frequency.

X capacitors and single chokes are suitable as suppression measures for the differential mode com-

ponents. Where asymmetrical, i.e. common-mode interference has to be eliminated, current-com-

pensated chokes and Y capacitors are mainly used, the prerequisite for this being, however, a well-

designed, EMC-compliant grounding and wiring system.

The categorization of types of interference and suppression measures and their relation to the fre-

quency ranges is reflected in the frequency limits for interference voltage and interference field

strength measurements.

Fig. 2

Frequency range overview

Pc ch. = Iron powder core chokes, but also all single chokes/ X cap = X capacitors

Cc ch. = Current-compensated chokes / Y cap = Y capacitors

Y cap

Cc ch.

Interference voltage

10

Max. ratings

_

_

characteristic

Interference

propagation

Interference

Remedies

Pc ch.

Line

Differential mode

X cap

Field strength

0

SSB1558-D

Shielding

Field

Field

Coupling

Ground

Common mode

f

General Technical Information