Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 156

0

0

− 20

Ungated

− 20

− 40

Gated

− 60

− 40

− 80

− 100

− 60

0 5 10 15 20 25 30 432 433 434

Frequency (MHz)

435 436

Time ( µ s)

Figure 4.102 Left, measured impulse response of a surface wave transponder with two res-
onators of different frequency; right, after the Fourier transformation of the impulse response
the different resonant frequencies of the two resonators are visible in the line spectrum (here:
approx. 433.5 MHz and 434 MHz) (reproduced by permission of Siemens AG, ZT KM, Munich)

Interdigital transducer Transducer

HF interrogation
signal

Antenna

Z

r

HF response

Load impedance
(switch or
external sensor)

Piezoelectric crystal

Figure 4.103 Principal layout of a passive surface wave transponder connected to an external
sensor (reproduced by permission of Siemens AG, ZT KM, Munich)

"OFF"

"ON"

Figure 4.104 Passive recoding of a surface wave transponder by a switched interdigital trans-
ducer (reproduced by permission of Siemens AG, ZT KM, Munich)