Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 44

Magnetic field H

∼

Chip

R i

C 1 C 2

f rdr + f s

Transponder

BP

Binary code signal

DEMOD

Reader

Figure 3.16 Generation of load modulation in the transponder by switching the drain-source
resistance of an FET on the chip. The reader illustrated is designed for the detection of a
subcarrier

f T = 13.560 MHz

0 dB

Carrier signal of the reader,
measured at the antenna coil

Modulation product by
load modulation with subcarrier

13.348 MHz 13.772 MHz

− 80 dB

f

f S = 212 kHz

Figure 3.17 Load modulation creates two sidebands at a distance of the subcarrier frequency
f S around the transmission frequency of the reader. The actual information is carried in the
sidebands of the two subcarrier sidebands, which are themselves created by the modulation of
the subcarrier

the significantly stronger signal of the reader by bandpass (BP) filtering on one of the
two frequencies f READER ± f S . Once it has been amplified, the subcarrier signal is now
very simple to demodulate.
Because of the large bandwidth required for the transmission of a subcarrier, this
procedure can only be used in the ISM frequency ranges for which this is permitted,
6.78 MHz, 13.56 MHz and 27.125 MHz (see also Chapter 5).

Example circuit-load modulation with subcarrier Figure 3.18 shows an example
circuit for a transponder using load modulation with a subcarrier. The circuit is designed
for an operating frequency of 13.56 MHz and generates a subcarrier of 212 kHz.