Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 41

Magnetic field H

~

Chip

C r

C 1

C 2

R i

Transponder

Reader

Figure 3.13 Power supply to an inductively coupled transponder from the energy of the mag-
netic alternating field generated by the reader

A small part of the emitted field penetrates the antenna coil of the transponder,
which is some distance away from the coil of the reader. A voltage U i is generated in
the transponder's antenna coil by inductance. This voltage is rectified and serves as the
power supply for the data-carrying device (microchip). A capacitor C r is connected in
parallel with the reader's antenna coil, the capacitance of this capacitor being selected
such that it works with the coil inductance of the antenna coil to form a parallel resonant
circuit with a resonant frequency that corresponds with the transmission frequency of
the reader. Very high currents are generated in the antenna coil of the reader by
resonance step-up in the parallel resonant circuit, which can be used to generate the
required field strengths for the operation of the remote transponder.
The antenna coil of the transponder and the capacitor C 1 form a resonant circuit
tuned to the transmission frequency of the reader. The voltage U at the transponder
coil reaches a maximum due to resonance step-up in the parallel resonant circuit.
The layout of the two coils can also be interpreted as a transformer ( transformer
coupling ), in which case there is only a very weak coupling between the two wind-
ings (Figure 3.14). The efficiency of power transfer between the antenna coil of the
reader and the transponder is proportional to the operating frequency f , the number
of windings n , the area A enclosed by the transponder coil, the angle of the two coils
relative to each other and the distance between the two coils.
As frequency f increases, the required coil inductance of the transponder coil,
and thus the number of windings n decreases (135 kHz: typical 100 - 1000 windings,
13.56 MHz: typical 3 - 10 windings). Because the voltage induced in the transponder
is still proportional to frequency f (see Chapter 4), the reduced number of windings
barely affects the efficiency of power transfer at higher frequencies. Figure 3.15 shows
a reader for an inductively coupled transponder.

3.2.1.2 Data transfer transponder reader →

Load modulation As described above, inductively coupled systems are based upon
a transformer-type coupling between the primary coil in the reader and the secondary
coil in the transponder. This is true when the distance between the coils does not exceed