Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 49

Directional
coupler

P 1 P 1 ′

TX D 1 D 2

ISD
6408

Transceiver
receiver

C 1 C 2

D 3

P 2 ′ P 2 ′

RX

Reader R L

Dipole Transponder

Figure 3.21 Operating principle of a backscatter transponder. The impedance of the chip is
'modulated' by switching the chip's FET (Integrated Silicon Design, 1996)

Reader

Transponder

Chip

Ferrite core

Reader coil

Figure 3.22 Close coupling transponder in an insertion reader with magnetic coupling coils

Because the voltage U induced in the transponder coil is proportional to the fre-
quency f of the exciting current, the frequency selected for power transfer should be
as high as possible. In practice, frequencies in the range 1 - 10 MHz are used. In order
to keep the losses in the transformer core low, a ferrite material that is suitable for this
frequency must be selected as the core material.
Because, in contrast to inductively coupled or microwave systems, the efficiency
of power transfer from reader to transponder is very good, close coupling systems
are excellently suited for the operation of chips with a high power consumption. This
includes microprocessors, which still require some 10 mW power for operation (Sickert,
1994). For this reason, the close coupling chip card systems on the market all contain
microprocessors.
The mechanical and electrical parameters of contactless close coupling chip cards are
defined in their own standard, ISO 10536. For other designs the operating parameters
can be freely defined.

3.2.3.2 Data transfer transponder reader

→

Magnetic coupling Load modulation with subcarrier is also used for magnetically
coupled data transfer from the transponder to the reader in close coupling systems.