Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 76

an inductively coupled RFID system. However, we must also bear in mind that the
influence of component tolerances in the system also reaches a maximum in the Q max
range. This is particularly important in systems designed for mass production. Such
systems should be designed so that reliable operation is still possible in the range
Q Q max at the maximum distance between transponder and reader.
R L should be set at the same value as the input resistance of the data carrier after
setting the 'power on' reset, i.e. before the activation of the voltage regulator, as is the
case for the maximum energy range of the system.

4.1.8 Practical operation of the transponder

4.1.8.1 Power supply to the transponder

Transponders are classified as active or passive depending upon the type of power
supply they use.
Active transponders incorporate their own battery to provide the power supply to the
data carrier. In these transponders, the voltage u 2 is generally only required to generate
a 'wake up' signal. As soon as the voltage u 2 exceeds a certain limit this signal is
activated and puts the data carrier into operating mode. The transponder returns to the
power saving 'sleep' or 'stand-by mode' after the completion of a transaction with the
reader, or when the voltage u 2 falls below a minimum value.
In passive transponders the data carrier has to obtain its power supply from the
voltage u 2 . To achieve this, the voltage u 2 is converted into direct current using a low
loss bridge rectifier and then smoothed. A simple basic circuit for this application is
shown in Figure 3.18.

4.1.8.2 Voltage regulation

The induced voltage u 2 in the transponder coil very rapidly reaches high values due to
resonance step-up in the resonant circuit. Considering the example in Figure 4.14, if
we increase the coupling coefficient k — possibly by reducing the gap between reader
and transponder — or the value of the load resistor R L , then voltage u 2 will reach
a level much greater than 100 V. However, the operation of a data carrier requires a
constant operating voltage of 3 - 5 V (after rectification).
In order to regulate voltage u 2 independently of the coupling coefficient k or other
parameters, and to hold it constant in practice, a voltage-dependent shunt resistor R S
is connected in parallel with the load resistor R L . The equivalent circuit diagram for
this is shown in Figure 4.17.

M R 2

i 1 i 2

L 1 L 2 u 2

R s

C 2

R L

Figure 4.17 Operating principle for voltage regulation in the transponder using a shunt

regulator