Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 99

Im

u ZT

u RX

u RX-mod

u L1

u R1

Re

Figure 4.42 Vector diagram of the total voltage u RX available to the receiver of the reader.
The magnitude and phase of this voltage are modulated at the antenna coil of the reader ( L 1 )
by a capacitive load modulator

(see Section 6.2.4). The subcarrier frequency f H is normally 847 kHz (ISO 14443-2),
423 kHz (ISO 15693) or 212 kHz.
Load modulation with a subcarrier generates two sidebands at a distance of ± f H
to either side of the transmission frequency (see Section 6.2.4). The information to
be transmitted is held in the two sidebands, with each sideband containing the same
information. One of the two sidebands is filtered in the reader and finally demodulated
to reclaim the baseband signal of the modulated data stream.

The influence of theQ factor As we know from the preceding section, we attempt
to maximise the Qfactor in order to maximise the energy range and the retroactive
transformed transponder impedance. From the point of view of the energy range, a high
Q factor in the transponder resonant circuit is definitely desirable. If we want to transmit
data from or to the transponder a certain minimum bandwidth of the transmission path
from the data carrier in the transponder to the receiver in the reader will be required.
However, the bandwidth B of the transponder resonant circuit is inversely proportional
to the Q factor.

f RES
Q

B = ( 4 . 55 )

Each load modulation operation in the transponder causes a corresponding amplitude
modulation of the current i 2 in the transponder coil. The modulation sidebands of the
current i 2 that this generates are damped to some degree by the bandwidth of the
transponder resonant circuit, which is limited in practice. The bandwidth B determines
a frequency range around the resonant frequency f RES , at the limits of which the
modulation sidebands of the current i 2 in the transponder reach a damping of 3 dB
relative to the resonant frequency (Figure 4.43). If the Q factor of the transponder is