Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 55

required magnitude. In full/half duplex systems the maximum power consumption
of the chip is fixed by the power matching point (i.e. by the coil geometry and
field strength H ).

3.3.1.3 Data transmission transponder reader

→

In sequential systems (Figure 3.28) a full read cycle consists of two phases, the charging
phase and the reading phase (Figure 3.29).
The end of the charging phase is detected by an end of burst detector , which mon-
itors the path of voltage at the transponder coil and thus recognises the moment when
the reader field is switched off. At the end of the charging phase an on-chip oscillator,
which uses the resonant circuit formed by the transponder coil as a frequency deter-
mining component, is activated. A weak magnetic alternating field is generated by the
transponder coil, and this can be received by the reader. This gives an improved signal-
interference distance of typically 20 dB compared to full/half duplex systems, which
has a positive effect upon the ranges that can be achieved using sequential systems.

End of
burst
detector

Oscill-
ator

Clock
divider

Vcc

Discharge

Transponder
coil in
resonance

Shift
register

Data
EEPROM

Charging
C

Tuning Modulation

Figure 3.28 Block diagram of a sequential transponder by Texas Instruments TIRIS Systems,
using inductive coupling

Charging phase:
Reading phase:
Discharging
phase:

Graph of U CHARGE

t

Figure 3.29 Voltage path of the charging capacitor of an inductively coupled SEQ transponder
during operation