Digital Signal Processing Reference
In-Depth Information
50.0
25.0
100.0
200.0
10.0
3
500.0
4
500.0
200.0
1
100.0
50.0
2
0
Z
0
=
50.00
Ω
f_main
=
868.000 MHz
Figure 4.84
Matching of a Schottky detector (point 1) to a dipole antenna (point 4) by means
of the series connection of a coil (point 1 - 2), the parallel connection of a second coil (point
2 - 3), and finally the series connection of a capacitor (point 3 - 4)
case of labels, in which the transponder chip is mounted directly upon foil, additional
components are avoided where possible.
If the elements of a dipole are shortened or lengthened (i.e. operated at above or
below their resonant frequency), then the impedance
Z
A
of the antenna contains an
inductive or capacitive component
X
T
=
0. Furthermore, the radiation resistance
R
s
can be altered by the construction format. By a suitable antenna design it is thus
possible to set the input impedance of the antenna to be the complex conjugate of the
input impedance of the transponder, i.e.
Z
T
=
Z
A
(Figure 4.85). The power matching
between transponder chip and antenna is thereby only realised by the antenna.
4.2.6.3 Power supplyof active transponders
In active transponders the power supply of the semiconductor chip is provided by
a
battery
. Regardless of the distance between transponder and reader the voltage is
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