Digital Signal Processing Reference
In-Depth Information
300
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50
0
7.2
×
10
6
7.4
×
10
6
7.6
×
10
6
7.8
×
10
6
8
×
10
6
8.2
×
10
6
8.4
×
10
6
8.6
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10
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8.8
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Frequency (MHz)
|Z1|
Figure 3.3
The occurrence of an impedance 'dip' at the generator coil at the resonant frequency
of the security element (
Q
=
90,
k
=
1%). The generator frequency
f
G
is continuously swept
between two cut-off frequencies. An RF tag in the generator field generates a clear dip at its
resonant frequency
f
R
and vary in the presence of a metallic environment, no longer play a role as a result
of the 'scanning' of the entire frequency range.
Because the tags are not removed at the till, they must be altered so that they do not
activate the anti-theft system. To achieve this, the cashier places the protected product
into a device — the deactivator — that generates a sufficiently high magnetic field that
the induced voltage destroys the foil capacitor of the transponder. The capacitors are
designed with intentional short-circuit points, so-called
dimples
. The breakdown of the
capacitors is irreversible and detunes the resonant circuit to such a degree that this can
no longer be excited by the
sweep signal
.
Large area
frame antennas
are used to generate the required magnetic alternating
field in the detection area. The frame antennas are integrated into columns and com-
bined to form gates. The classic design that can be seen in every large department
store is illustrated in Figure 3.4. Gate widths of up to 2 m can be achieved using the
RF procedure. The relatively low detection rate of 70% (Gillert, 1997) is dispropor-
tionately influenced by certain product materials. Metals in particular (e.g. food tins)
affect the resonant frequency of the tags and the coupling to the detector coil and thus
have a negative effect on the detection rate. Tags of 50 mm
×
50 mm must be used to
achieve the gate width and detection rate mentioned above.
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