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An interference source which is at a floating potential primarily emits differential-mode, i.e. symmet-

rical interference which is propagated along the connected lines. On power lines, the interference

current will flow towards the disturbed equipment on one wire and away from it on the other wire,

just as the mains current does.

Symmetrical or differential-mode interference occurs mainly at low frequencies (up to several

hundred kHz).

However, parasitic capacitances in interference sources and disturbed equipment or intended

ground connections, also lead to an interference current in the ground circuit. This interference cur-

rent flows towards the disturbed equipment through both the connecting lines and returns to the in-

terference source through the ground lines.The currents on the connecting lines are in common

mode and the interference is thus designated as common-mode or asymmetrical interference.

Since the parasitic capacitances will tend towards representing a short-circuit with increasing fre-

quencies and the coupling to the connecting cables and the equipment itself will increase corre-

spondingly, common-mode interference becomes dominant at multiple-MHz frequencies.

In European usage, the concept of an “unsymmetrical interference” is used, in addition to the two

components described above, to describe interference. This term is used to describe the interfer-

ence voltage between a line and reference ground potential.

2.6

Filter circuits and line impedance

Interference suppression filters are virtually always designed as reflecting lowpass filters, i.e. they

reach their highest insertion loss when they are - on the one hand - mismatched to the impedance

of the interference source or disturbed equipment and - on the other hand - mismatched to the im-

pedance of the line. Possible filter circuits for various line, interference source and disturbed equip-

ment impedance conditions are shown in fig. 9.

Fig. 9

Filter circuits and impedance relationships

Line

Impedance of

impedance

source of interference / disturbed equipment

low

high

high

high

high

high

unknown

unknown

low

low

low

low

unknown

unknown

General Technical Information