Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 98

M R 2 C mod

i 1 i 2

L 1 L 2 u 2

C 2 R L

S

Figure 4.40 Equivalent circuit diagram for a transponder with capacitive load modulator. To
transmit data the switch S is closed in time with the data stream — or a modulated subcar-
rier signal

Re in Ω

0 10 20

Z ′ T (C 2 )

Z ′ T (C 2 + C mod )

330

Im in Ω

300

270

Figure 4.41 Locus curve of transformed transponder impedance for the capacitive load mod-
ulation ( C 2 || C mod = 40 - 60 pF) of an inductively coupled transponder. The parallel connection
of a modulation capacitor C mod results in a modulation of the magnitude and phase of the
transformed transponder impedance Z T

through a segment of the circle in the complex Z plane that is typical of the parallel
resonant circuit.
Demodulation of the data signal is similar to the procedure used with ohmic load
modulation. Capacitive load modulation generates a combination of amplitude and
phase modulation of the reader antenna voltage u RX and should therefore be processed
in an appropriate manner in the receiver branch of the reader. The relevant vector
diagram is shown in Figure 4.42.

Demodulation in the reader For transponders in the frequency range < 135 kHz the
load modulator is generally controlled directly by a serial data stream encoded in the
baseband, e.g. a Manchester encoded bit sequence. The modulation signal from the
transponder can be recreated by the rectification of the amplitude modulated voltage
at the antenna coil of the reader (see Section 11.3).
In higher frequency systems operating at 6.78 MHz or 13.56 MHz, on the other
hand, the transponder's load modulator is controlled by a modulated subcarrier signal