Digital Signal Processing Reference
In-Depth Information
Interrogation field strength H
min
3
2.5
2
1.5
1
0.5
10
7
10
7
10
7
10
7
10
7
10
7
1
×
1.2
×
1.4
×
1.6
×
1.8
×
2
×
Resonant frequency (MHz)
R
L
=
1500 Ohm
Figure 4.21
Interrogation sensitivity of a contactless smart card where the transponder res-
onant frequency is detuned in the range 10 - 20 MHz (
N
=
4,
A
=
0
.
05
×
0
.
08 m
2
,
u
2
=
5V,
L
2
=
3
.
5
µ
H,
R
2
=
5
,
R
L
=
1
.
5k
). If the transponder resonant frequency deviates from the
transmission frequency (13.56 MHz) of the reader an increasingly high field strength is required
to address the transponder. In practical operation this results in a reduction of the read range
we obtain the following relationship between the energy range and interrogation field
H
min
of a transponder for a given reader:
3
I
·
N
1
·
R
2
2
2
x
=
−
R
2
(
4
.
39
)
·
H
min
As an example (see Figure 4.22), let us now consider the energy range of a transpon-
der as a function of the power consumption of the data carrier (
R
L
=
u
2
/i
2
). The reader
in this example generates a field strength of 0.115 A/m at a distance of 80 cm from the
transmitter antenna (radius
R
of transmitter antenna: 40 cm). This is a typical value
for RFID systems in accordance with ISO 15693.
As the current consumption of the transponder (lower
R
L
) increases, the interroga-
tion sensitivity of the transponder also increases and the energy range falls.
The maximum energy range of the transponder is determined by the distance
between transponder and reader antenna at which the minimum power supply
u
2min
required for the operation of the data carrier exists even with an unloaded transponder
resonant circuit (i.e.
i
2
→
0,
R
L
→∞
). Where distance
x
=
0 the maximum current
i
2
represents a limit, above which the supply voltage for the data carrier falls below
u
2min
, which means that the reliable operation of the data carrier can no longer be
guaranteed in this operating state.
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