Digital Signal Processing Reference
In-Depth Information
25
Modulation products by
load modulation with modulated
subcarrier
20
15
10
5
0
1.2
×
10
7
1.3
×
10
7
1.4
×
10
7
1.5
×
10
7
1.6
×
10
7
1.7
×
10
7
f (Hz)
Q
=
10
Figure 4.44
If the transponder resonant frequency is markedly detuned compared to the trans-
mission frequency of the reader the two modulation sidebands will be transmitted at different
levels. (Example based upon subcarrier frequency
f
H
=
847 kHz)
rate/subcarrier frequency). Systems that require a short transaction time (that is, rapid
data transmission and large bandwidth) often only have a range of a few centimetres,
whereas systems with relatively long transaction times (that is, slow data transmission
and low bandwidth) can be designed to achieve a greater range. A good example of
the former case is provided by contactless smart cards for local public transport appli-
cations, which carry out authentication with the reader within a few 100 ms and must
also transmit booking data. Contactless smart cards for 'hands free' access systems that
transmit just a few bytes — usually the serial number of the data carrier — within 1 - 2
seconds are an example of the latter case. A further consideration is that in systems
with a 'large' transmission antenna the data rate of the reader is restricted by the
fact that only small sidebands may be generated because of the need to comply with
the radio licensing regulations (ETS, FCC). Table 4.4 gives a brief overview of the
relationship between range and bandwidth in inductively coupled RFID systems.
4.1.11 Measurement of system parameters
4.1.11.1 Measuring the coupling coefficient k
The coupling coefficient
k
and the associated mutual inductance
M
are the most
important parameters for the design of an inductively coupled RFID system. It is pre-
cisely these parameters that are most difficult to determine analytically as a result of
the — often complicated — field pattern. Mathematics may be fun, but has its limits.
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