Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 34

Receiver Transmitter

ASK

1 kHz
generator

Alarm 1 kHz detector

2.45 GHz

2nd harmonic
4.90 GHz 1-bit transponder

Figure 3.6 Microwave tag in the interrogation zone of a detector

In the example above, the amplitude of the carrier wave is modulated with a signal
of 1 kHz (100% ASK). The second harmonic generated at the transponder is also
modulated at 1 kHz ASK. The signal received at the receiver is demodulated and
forwarded to a 1 kHz detector. Interference signals that happen to be at the reception
frequency of 4.90 GHz cannot trigger false alarms because these are not normally
modulated and, if they are, they will have a different modulation.

3.1.3 Frequency divider

This procedure operates in the long wave range at 100 - 135.5 kHz. The security tags
contain a semiconductor circuit (microchip) and a resonant circuit coil made of wound
enamelled copper. The resonant circuit is made to resonate at the operating frequency
of the EAS system using a soldered capacitor. These transponders can be obtained in
the form of hard tags (plastic) and are removed when goods are purchased.
The microchip in the transponder receives its power supply from the magnetic field
of the security device (see Section 3.2.1.1). The frequency at the self-inductive coil is
divided by two by the microchip and sent back to the security device. The signal at
half the original frequency is fed by a tap into the resonant circuit coil (Figure 3.7).

f/2

+

~

DIV 2

C r

C 1

C 2

−

f 1/2

R i

Security tag

f/2 bandpass
analysis
electronics

Power, clock f

Magnetic field H

Security device

Figure 3.7 Basic circuit diagram of the EAS frequency division procedure: security tag (trans-
ponder) and detector (evaluation device)