Page 107 - Power Electronics Handbook

, Narrowband emission

- Receptor bandwidth

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Figure 4.1 Typcs of emissions: (a) broadband; (b) narrowband

EMI can be radiated through space, as electromagnetic waves, or it can
be conducted as a current along a cable. Conduction can take the form of
common-mode or differential-mode currents. For differential mode the
currents are equal and opposite on the two wires and are caused primarily
by other users on the same lines. Common-mode currents are almost equal
in amplitude on the two lines, but travel in the same direction. These
currents are mainly caused by coupling of radiated EM1 to the power lines
and by stray capacitive coupling to the body of the equipment.
Emissions can be classed as broadband and narrowband. In broadband
emission the signal bandwidth is greater than the reference bandwidth and
the pulse-repetition frequency is less than that of the reference bandwidth.
The reference bandwidth, for EMC purposes, may be considered to be the
equipment being interfered with, or a test receiver. For a narrowband
emission the signal bandwidth is less than the reference bandwidth and the
pulse-repetition frequency is greater than that of the reference bandwidth.
Figure 4.1 illustrates the difference between broadband and narrowband
emissions. Broadband emissions are caused by low-frequency repetitive
pulses or individual impulses of electrical or magnetic state changes, such
as in switching or commutation. This results in many spectral lines
separated by a frequency less than the receptor bandwidth. The frequency
components may be coherent, Le. harmonically related in frequency, or
incoherent, i.e. random, such as noise. Narrowband emissions are caused
by high-frequency components separated in frequency by greater than the
receptor bandwidth.

4.3 Sources of EMI
EM1 sources can be broadly divided into two categories, natural and man
made. Naturally caused EM1 below 10MHz is mainly due to atmospheric