Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 149

thus reaches its maximum value for the associated frequency, the so-called Bragg
frequency f B . See Figure 4.94.

v
2 p

f B = ( 4 . 117 )

4.3.3 Functional diagram of SAW transponders
(Figure 4.95)

A surface wave transponder is created by the combination of an interdigital transducer
and several reflectors on a piezoelectric monocrystal, with the two busbars of the
interdigital transducer being connected by a (dipole) antenna.
A high-frequency interrogation pulse is emitted by the antenna of a reader at periodic
intervals. If a surface wave transponder is located in the interrogation zone of the reader
part of the power emitted is received by the transponder's antenna and travels to the
terminals of the interdigital converter in the form of a high-frequency voltage pulse.
The interdigital transducer converts part of this received power into a surface acoustic
wave, which propagates in the crystal at right angles to the fingers of the transducer. 8

Reflector period p

Figure 4.94 Geometry of a simple reflector for surface waves (reproduced by permission of
Siemens AG, ZT KM, Munich)

Antenna

High frequency
interrogation
pulse

Interdigital transducer

HF response

Reflectors

Piezoelectric monocrystal

Figure 4.95 Functional diagram of a surface wave transponder (reproduced by permission of
Siemens AG, ZT KM, Munich)

8 To convert as much of the received power as possible into acoustic power, firstly the transmission frequency
f 0 of the reader should correspond with the mid-frequency of the interdigital converter. Secondly, however,
the number of transducer fingers should be matched to the coupling coefficient k 2 .