Handheld Terminal
CE Conformity, Directives and Standards
User's Manual, Version: 1.3
153
© KEBA 2007
fect is maintained even at higher frequencies, care must be taken to ensure
that the cable shield connects to the largest possible surface area and
thereby acts as a continuation of the device’s shielded housing. Pig tail
connections are not suitable.
It may be necessary to earth cable shields and shielded housings for safety
reasons but this is not an effective EMC measure.
14.3.2.2 Interference suppressors, filter components
Filtering is always necessary when unshielded signal and power supply
lines are brought into shielded areas. Alongside the wanted signals, these
lines often also carry interference signals that must not enter into shielded
areas. Filters should therefore guarantee the interference immunity of the
device but should also prevent the emission of interference from the device
via unshielded lines.
Unshielded lines are usually used when the wanted signals that are carried
are of a very low frequency. The normally high frequency interference sig-
nals are separated from the wanted signals by means of frequency selec-
tive filtering with the aid of low-pass filtering.
Low-pass filters must be sized in such a way that the lower frequency
wanted signals can pass through and the higher frequency interference
signals are filtered.
Multi-stage filters are often necessary for filtering. Nearly all filter combina-
tions contain Y-capacitors, i.e. capacitors that are connected to the filter
housing for the dissipation of interference currents. In order for these filters
to function correctly, the housing must be connected to a stable reference
potential.
Since the interference signals must not enter into the shielded area, the po-
tential of the shielded housing must also be the reference potential for the
filter circuits.
Filter circuits or filter components must therefore be placed precisely where
the lines enter the shielded area. If the filters are placed inside or outside
the shield wall, this can lead to a field coupling between the filtered and un-
filtered lines.
The connection to the reference potential must be as low impedance as
possible and therefore it is necessary for there to be contact over a large
surface area between the filter housing, which is usually metallic, and the
shield wall of the device. Pig tails may not be used to make such a contact.
Line filters
Typical line filters have a metal housing that has to be connected to the
earth wire (PE) for safety reasons and consist of a current-compensated
choke (see illustration) consisting of Y-capacitors, which are connected to