Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 35

Table 3.3 Typical system parameters (Plotzke et al. , 1994)

Frequency 130 kHz

Modulation type: 100% ASK

Modulation frequency/modulation signal: 12.5 Hz or 25 Hz, rectangle 50%

The magnetic field of the security device is pulsed at a lower frequency (ASK
modulated) to improve the detection rate. Similarly to the procedure for the generation
of harmonics, the modulation of the carrier wave (ASK or FSK) is maintained at half
the frequency ( subharmonic ). This is used to differentiate between 'interference' and
'useful' signals. This system almost entirely rules out false alarms.
Frame antennas, described in Section 3.1.1, are used as sensor antennas.

3.1.4 Electromagnetic types

Electromagnetic types operate using strong magnetic fields in the NF range from 10 Hz
to around 20 kHz. The security elements contain a soft magnetic amorphous metal strip
with a steep flanked hysteresis curve (see also Section 4.1.12). The magnetisation of
these strips is periodically reversed and the strips taken to magnetic saturation by
a strong magnetic alternating field. The markedly nonlinear relationship between the
applied field strength H and the magnetic flux density B near saturation (see also
Figure 4.50), plus the sudden change of flux density B in the vicinity of the zero
crossover of the applied field strength H, generates harmonics at the basic frequency
of the security device, and these harmonics can be received and evaluated by the
security device.
The electromagnetic type is optimised by superimposing additional signal sections
with higher frequencies over the main signal. The marked nonlinearity of the strip's
hysteresis curve generates not only harmonics but also signal sections with summation
and differential frequencies of the supplied signals. Given a main signal of frequency
f S =

20 Hz and the additional signals f 1 = 3 . 5and f 2 = 5 . 3 kHz, the following signals

are generated (first order):

f 1 + f 2 = f 1 + 2 = 8 . 80 kHz

f 1 − f 2 = f 1 − 2 = 1 . 80 kHz

f S + f 1 = f S + 1 = 3 . 52 kHz and so on

The security device does not react to the harmonic of the basic frequency in this case,
but rather to the summation or differential frequency of the extra signals.
The tags are available in the form of self-adhesive strips with lengths ranging from
a few centimetres to 20 cm. Due to the extremely low operating frequency, electromag-
netic systems are the only systems suitable for products containing metal. However,
these systems have the disadvantage that the function of the tags is dependent upon
position: for reliable detection the magnetic field lines of the security device must run
vertically through the amorphous metal strip. Figure 3.8 shows a typical design for a
security system.